Damage tolerance and defect accumulation in ultra-fine grained (UFG) materials

2017

Damage tolerance and defect accumulation in

ultra-fine grained (UFG) materials

Muchen Li Iowa State University

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Recommended Citation

Li, Muchen, "Damage tolerance and defect accumulation in ultra-fine grained (UFG) materials" (2017).Graduate Theses and Dissertations. 15568.

by

Muchen Li

A thesis submitted to the graduate faculty

in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE

Major: Material Science and Engineering Program of Study Committee: Peter Collins, Major Professor

Scott Chumbley Ashraf Bastawros

The student author and the program of study committee are solely responsible for the content of this thesis. The Graduate College will ensure this thesis is globally accessible and will not

permit alterations after a degree is conferred.

Iowa State University Ames, Iowa

2017

TABLE OF CONTENTS Page LIST OF FIGURES ... iv LIST OF TABLES ... vi NOMENCLATURE ... vii ACKNOWLEDGMENT... viii ABSTRACT ... ix CHAPTER 1 INTRODUCTION ... 1 1.1 Motivation ... 1

1.2 Framework of the Thesis ... 1

CHAPTER 2 LITERATURE REVIEW ... 2

2.1 Introduction of Magnesium and Alloys ... 2

2.2 Strengthening of Magnesium Alloys ... 5

2.2.1 Solid Solution Hardening ... 5

2.2.2 Precipitation Strengthening ... 6

2.2.3 Grain Refinement Strengthening ... 7

2.3 Equal Channel Angular Pressing (ECAP) ... 8

2.3.1 Introduction ... 8

2.3.2 Microstructural Characteristic of ECAPed Material... 11

2.4 Fatigue and Fatigue Test ... 13

2.4.1 Introduction of Material Fatigue Property ... 13

2.4.2 Fatigue Mechanisms ... 16

2.4.3 Fatigue Test Method ... 19

CHAPTER 3 EXPERIMENTAL PROCEDURE ... 23

3.1 Test Materials... 23

3.2 Sample Pretreatment ... 24

3.3 Sample Characterization Analysis ... 24

3.3.1 X-ray Diffraction (XRD) ... 24

3.3.2 Scanning Electron Microscopy (SEM) ... 25

3.3.3 Transmission Electron Microscopy (TEM) ... 26

3.3.4 Precession Electron Microscopy (PED) ... 27

3.3.5 Four Point Fatigue Test ... 27

CHAPTER 4 RESULTS AND DISCUSSION ... 30

4.1 Characterization of As-received Sample ... 30

4.2 Four Point Bend Fatigue Test and Fracture Mechanism ... 34

4.2.1 Four Point Bend Fatigue Results ... 34

4.2.2 Fractography ... 39

4.3 Characterization of Fatigued Sample ... 40

CHAPTER 5 CONCLUSION... 43

LIST OF FIGURES

Page

Figure 2-1 Mg-Al phase diagram ... 4

Figure 2-2 Deformation slip system in magnesium ... 5

Figure 2-3 Heat-treatable Al (Sc) dark-field transmission electron micrographs (TEM) ... 6

Figure 2-4 Schematic illustration of facility used for ECAP ... 9

Figure 2-5 Different processing routes in ECAP ... 10

Figure 2-6 Sample of AZ31 after single pass ECAP at (a): 150℃, (b): 200℃, (c): 250℃ ... 11

Figure 2-7 Typical microstructures of (a) unECAPed, (b) 1-pass ECAPed, (c) 2-pass ECAPed, (d) 3-pass ECAPed (e) 4-pass ECAPed magnesium alloy... 12

Figure 2-8 (0002) pole figure of magnesium alloy after extrusion (first row) and ECAP (second row)... 13

Figure 2-9 Damage on Aloha Airlines flight 243 ... 14

Figure 2-10 Schematic view of cyclic loading ... 14

Figure 2-11 S-N diagram of SAE 4130 steel, black dots represent individual fatigue tests ... 16

Figure 2-12 Phases of the fatigue life and relevant factors ... 17

Figure 2-13 Typical fatigue crack growth rates (da dN) for as function of stress intensity factor range ΔK ... 18

Figure 2-14 Four point bend specimen geometry and the loading state ... 19

Figure 2-15 Surface stress distribution with t/h ... 21

Figure 2-16 Surface distribution with L/t ... 21

Figure 3-1 Schematic view of as received sample rods ... 23

Figure 3-2 The size of the samples for the four-point bend test ... 24

Figure 3-4 Physical picture of four point bend fatigue test ... 28

Figure 4-1 XRD pattern of as-received AZ31 sample ... 30

Figure 4-2 (0002) Pole figure of as-received AZ31 sample ... 31

Figure 4-3 Backscatter electron SEM of AZ31 sample ... 32

Figure 4-4 EDS mapping of Mg, Al and Mn ... 33

Figure 4-5 Al-Mn phase diagram ... 33

Figure 4-6 Schematic of sample loading relation ... 34

Figure 4-7 S-N curve of the #33 ED, #33 TD, #36 ED and #36 TD fatigue test, plotted as maximum stress (MPa) verses number of cycles (N) ... 38

Figure 4-8 SEM images of #36 ED fracture surface that fatigued at 90% of yield stress (180 MPa) (a) bottom right, (b) middle ... 39

Figure 4-9 SEM high magnification Flake like fracture surface topography ... 40

Figure 4-10 Diffraction pattern of #36 TD before and after fatigue test ... 40

Figure 4-11 (0002) pole figure after fatigue test ... 41

LIST OF TABLES

Page

Table 2-1 Specific strength of AZ31B versus some commonly used Al alloys ... 2

Table 2-2 Some physical properties of Mg ... 3

Table 2-3 Grain size and mechanical properties of AZ61 materials ... 8

Table 2-4 Effective strain intensity and equivalent reductions ... 10

Table 2-5 Types of fatigue loading ... 14

Table 3-1 Chemical composition of AZ31 magnesium alloy (wt.%) ... 23

Table 4-1 Chemical composition of precipitates and matrix ... 34

NOMENCLATURE

Acronym Definition

hcp Hexagonal closed packed

SEM Scanning electron microscope

TEM Transmission electron microscopy

PED Precession Electron Microscopy

EDS Energy dispersive X-ray spectroscopy

XRD X-ray diffraction

FPB Four point bending

UFG Ultra-fine grained

ECAP Equal channel angular pressing

ED Extrusion direction

TD Transverse direction

ACKNOWLEDGMENTS

I would like to thank my committee chair, Professor Peter Collins, and my committee members, Professor Scott Chumbley, professor Ashraf Bastawros and Professor Ralph Napolitano, for their guidance and support throughout the course of this research.

I would also like to give my thanks to Doctor Peyman Samimi, for giving me many help for the whole project. I wish he had a good life in Oregon. In addition, I would like to thank other colleagues in our group: Alden Watts, Brain Martin, David Brice, Iman Ghamarian, Matthew Kenney, Michael Mendoza, and Thomas Alex. I felt so appreciated that they give me a happy memory in the United States. In addition, I would also like to thank my friends, other colleagues, the department faculty and staff for making my time at Iowa State University a wonderful experience.

What is more, I would like give my special thanks to my parents Yubao Li and Xiaoqing Lu, for their selfless, unconditional supporting me, giving me the chance to get higher target. They are my power when I feel depressed.

ABSTRACT

Magnesium alloys are of more and more interest in engineering applications because they have low density, good machinability and great damping capacity. However, the application of Mg alloys at their present stage has been restricted by their poor formability and limited room temperature ductility because of their hcp crystal structure. Therefore, many attempts have been made to improve the mechanical properties of Mg. Grain refining is considered an effective way of increasing strength, though there is less well-established understanding of how microstructural refinement influences other properties, including fatigue.

In this research, the Equal Channel Angular Pressing (ECAP) process was used to develop an ultra-fine grained AZ31 Mg alloy. Materials with ECAPed process have very small grain size, typically less than 1 μm, thus improve their tensile strength and hardness with fairly large ductility. Fatigue properties of AZ31 Mg alloy are measured using a four point bending fatigue testing configuration. The effect of texture on fatigue properties is studied. The

CHAPTER 1

INRODUCTION

1.1Motivation

Lighter weight of automobiles and aircraft is always the goal for various industries. Aluminum is a traditional structural material and has been widely used. However, extensive research has been conducted on aluminum, and it is more and more difficult to reduce the structural weight by using aluminum. Recently, magnesium has attracted lots of attention owing to its comparatively low density. Efforts have been made to use magnesium for a new generation of structural materials. Although low density has made magnesium an ideal material for industry, it suffers from poor plasticity due to its hexagonal close packed (hcp) crystal structure. Grain size refinement is one method to strengthen magnesium, but the influence of microstructure and defect populations on design properties such as fatigue are not well explored, so the objective of this study is to determine, via fractography and detailed characterization of defect populations, how damage accumulates in ultra-fine grained (UFG) Mg AZ31 alloy.

1.2Framework of the Thesis

Following this introduction and motivation, a literature review presents some basic concepts of magnesium alloys, including some approaches to strengthen alloys. An experimental methods section follows, in which the Equal Channel Angular Pressing (ECAP) process and the way fatigue tests methods are described. In part three some results of microstructure and

CHAPTER 2

LITERATURE REVIEW

2.1 Introduction of Magnesium and Alloys

Magnesium was first discovered by Joseph Black in 1755 as an element[1]. Later on, magnesium was found as a silver white metal, with an abundant storage volume in the earth’s crust. In nature, magnesium is most often observed in seawater in its ionic state, MgCl2. Sizable pure magnesium was made by reacting magnesium chloride with potassium in 1831. This made it possible for human society to use magnesium in industry.

Magnesium and its alloys drew more and more attention when they were found to be good candidates as structural materials in the automotive or aircraft industry[2]. Magnesium has a density of 1.74 g/cm3_{, which is the lowest among those mostly seen in structural materials} systems such as aluminum alloys, titanium alloys, and steel[3]. Table 2-1 shows the specific strength of AZ31B Mg alloy versus some widely used Al alloys. Based on various production method, the highest specific strength of AZ31B Mg alloy is double that of 319 Al alloy and it is slightly higher than 6061 Al alloy.

Table 2-1 Specific strength of AZ31B versus some commonly used Al alloys[4–6] AZ31B Pure Al Al-319 Al-6061 Al-7075 Yield strength (MPa) 130-219 7-11 124-179 290 414-469 Density (g/cm3) 1.77 2.70 2.796 2.7 2.81 Specific strength (kN·m/kg) 73.44-123.73 2.59-4.07 44.44-64.16 107.41 147.86-166.90

This lightweight material makes it possible for manufacturers to produce vehicles more environmentally friendly. According to a report from the United States Automotive Material Partnership: “Magnesium use in the automotive industry has grown by 10-15 percent per annum over the past 15 years. The goal is to substitute 340 lbs. of Mg components for 630 lbs. of current ferrous and Al parts.”[7] Some other physical properties of magnesium are shown in Table 2-2.

Table 2-2 Some physical properties of Mg[1] Melting point 923 K (650 °C, 1202 °F) Boiling point 1362 K(1091 °C, 1994 °F)

Atomic distance a 0.321 nm

c/a ratio 1.624

Elastic modulus 45 GPa

Shear modulus 17 GPa

Poisson’s ratio 0.290

Magnesium is typically alloyed with some other elements for specific applications and design properties, for example, enhancing creep resistance. Considering atomic size and

solubility, there are about ten elements that usually are alloyed with magnesium[8]. Aluminum is the most commonly used element to form an alloy, which can significantly improve material strength. The maximum solubility of Al in Mg can be 12.7 wt.%[1]. Figure 2-1 shows the Mg-Al binary phase diagram. Other alloying elements include zinc, manganese, calcium, lithium, zirconium and various rare earth elements.

Fig. 2-1 Mg-Al phase diagram[9]

However, its widespread application is affected by its poor ductility and strength, due to its hexagonal close-packed (hcp) crystal structure[10]. The ratio of the two lattice parameters for pure magnesium, (c/a), equals 1.62, which is close to the ideal value 1.633, so it is often

considered to be perfectly close-packed[11]. At least five independent slip systems are required for homogeneous deformation without cracks based on von Mises criterion[12]. However, there are only three basal slip system in magnesium, as shown in Figure 2-2, resulting in poorer ductility and requiring the activation of higher energy deformation modes.

Fig.2-2 Deformation slip system in magnesium[13]

2.2 Strengthening of Magnesium Alloys 2.2.1 Solid Solution Hardening

In order to optimize the strength of magnesium, a series of approaches have been conducted. One of the approaches, which is also used for most other metal strengthening, is alloying with other elemental species. Introducing different atoms into the magnesium matrix will cause a change of lattice parameter and there will be an interaction between the dislocation and the solute atom. The equation expressing the relation of yield stress and solute atoms is described by Fleischer[14]:

𝜎𝑦𝑆 = 𝜎𝑦0+ 𝑍𝐹𝐺(|𝛿| + 𝛽|𝜂|)3/2𝑐1/2

where 𝜎𝑦0 and 𝜎𝑦𝑆 is the yield stress before and after adding solute atoms, G is the shear

modulus of magnesium, c is the atomic solute atom concentration, ZF and 𝛽 are constants which can be estimated from experiment, 𝛿=(da/dc)/a, also known as the relative change of lattice

parameter, and 𝜂 is relative change of modulus. We can observe from the equation that the strengthening effect has a positive correlation to the atomic solute concentration.

2.2.2 Precipitation Strengthening

Besides solid solution hardening, precipitation strengthening is regarded as one method to improve the mechanical properties of magnesium. Precipitation strengthening is a kind of heat treatment that produce well-dispersed second phase particles in the matrix in order to strengthen the metal. Figure 2-3 is a typical precipitation strengthening microscopic image.

Fig. 2-3 Heat-treatable Al (Sc) dark-field transmission electron micrographs (TEM)[15]

The main factors that the precipitates will affect the strength of polycrystalline

magnesium include the size of the particles, their distribution and volume fraction in the matrix and the interface between particles and the matrix. Those tiny particles can interfere with

dislocation movement, thus strengthening the metal. The excess stress caused by precipitates can be expressed by the equation below[16]:

Δ𝜎_𝑦𝑃 ∝ 𝐺𝑏/𝐿

where G is the shear modulus, b is the magnitude of burgers vector, L is the average particle spacing.

Usually the strength of a metal is determined by the contributions of solute hardening, precipitation strengthening, and dislocation hardening. So in practice the yield stress can be described as[1]:

𝜎𝑦𝑎 = 𝜎𝑦𝑆+ [(𝜎𝑦𝐷)2+ (𝜎𝑦𝑃)2]1/2

where 𝜎_𝑦𝑆 represents solute hardening, 𝜎_𝑦𝑃 is precipitation strengthening and 𝜎_𝑦𝐷 is dislocation hardening.

2.2.3 Grain Refinement Strengthening

It has been shown that grain refining can significantly enhance the strength of a material. As we know, in most cases and for most length scales, the yield strength has a negative

correlation with grain size and is easily expressed by the Hell-Petch relationship[17,18]:

where d is average grain size, 𝜎₀ is a constant, and Kyis the stress intensity factor for plastic

yielding. Ky depends greatly on the experimental condition but in practice its value is often given

by 210 MPa·um1/2 for pure magnesium[1]. When the grain size shrinks from 400 um to 17 um, the elongation to failure increases by 15%, as reported by Yamashita et al.[19]. This result is further supported by W.J. Kim et al.. When the grain size of the alloy AZ61 decreases gradually from 24.4 um to 8.4 um, the elongation increases from 33% to 55%, as shown in Table 2-3:

Table 2-3 Grain size and mechanical properties of AZ61 materials[20] Grain size, d (um) Ultimate tensile strength (MPa) Elongation (%)

24.4 322 33 15.8 302 34 12.5 317 39 11.2 329 35 10.6 317 42 8.4 310 55

It is also mentioned by W. J. Kim et al. that while the yield strength of a material may not be strongly influenced by a reduction in grain size, the ductility does see a marked improvement. Over decades, people have devoted themselves to finding better ways to develop ultra-fined grain bulk materials, and one of the approaches, Equal Channel Angular Pressing (ECAP), will be introduced in the next section.

2.3 Equal Channel Angular Pressing (ECAP) 2.3.1 Introduction

There exist many methods to take advantage of large plastic strains in order to produce small grain materials. Rolling, forging and extrusion are examples of techniques that can result in

grain refining, but they all have obvious limitations. Many of them are restricted by the geometric size of the material so it is impossible for those approaches to be used in industry. Rolling, for example, can produce small grains only when the raw material is thin foil.

Therefore, a new technic was developed in the former Soviet Union[21]: Equal Channel Angular Pressing (ECAP), also called Equal Channel Angular Extrusion (ECAE). ECAP is a process where the bulk metal is pressed through a special die in order to obtain severe plastic strain, and thus produce fine-grained material. Figure 2-4 is a schematic cutaway view of an ECAP device. ECAP can be conducted at either room temperature or high temperature and sometimes the ECAP sample will be coated by graphite or moly based powder as lubricant. Generally speaking, using a higher ECAP processing temperature will improve recrystallization and help to avoid crack formation and increasing[22]. However it has also been reported that an ECAPed AZ31 alloy sample at 300℃ does not show any significant improvement in the mechanical properties nor finer grain size[23]. In order to introduce a different slip system in the sample, the sample is often rotated by 90° or 180° under different circumstance (see Fig. 2-5).

Fig. 2-4 Schematic illustration of facility used for ECAP[19]

Fig. 2-5 Different processing routes in ECAP[24]

Different from other conventional extrusion methods, the cross section does not change before and after pressing, so it gives an advantage for those applications that need special geometric requirements. However, ECAP is not a continuous process so it is not easy to be applied in commercial forming operations[25]. Meanwhile, material require high ductility to operate ECAP, some raw material like CP-Ti is hard to process at room temperature[26]. Usually the material will experience multiple passes through the die to achieve a more

homogeneous microstructure. It has been shown that the microstructure will be stable after four passes[27], as Table 2-4 shows.

Table 2-4 Effective strain intensity and equivalent reductions[28] Number of passes Total strain intensity Equivalent area reduction (%) Equivalent reduction ratio (x:1) 1 1.15 69 3 2 2.31 90 10 3 3.46 97 33 4 4.62 99 105 5 5.77 99.7 335

Table 2-4 continued

6 6.93 99.90 1073

7 8.09 99.97 3436

8 9.24 99.99 10100

It is also worth noting that the speed of pressing and the initial size of a sample may influence the final status of the sample[29]. Figure 2-6 shows the macrographs when samples are under various pressing speeds and temperatures.

Fig. 2-6 Sample of AZ31 after single pass ECAP at (a): 150℃, (b): 200℃, (c): 250℃

2.3.2 Microstructural Characteristic of ECAPed Material

Optical photographs of both unECAPed and ECAPed magnesium alloys are shown in Figure 2-7, according to Kim et. al.. The as-received samples in their experiment have an average grain size of 48.3 um. After just 1-pass of ECAP, the grain size decreases to an average of 8.1 um, but there still exist some coarse grains that have 15-20 um in diameter. After 4-passes the grain size further decreases to 2.5 um, and grain size distribution becomes homogenous by breaking those coarse grains.

It is worth noticing that the ECAP process will not only have an effect on the grain size, but this process will also change the texture of the samples as well. It can be seen that the (0002) plane is basically parallel to the extrusion direction (upper row in Fig. 2-8) in the extruded samples, while in the ECAPed samples, the (0002) plane incline about 45°.

Fig. 2-7 Typical microstructures of (a) unECAPed, (b) 1-pass ECAPed, (c) 2-pass ECAPed, (d) 3-pass ECAPed, (e) 4-pass ECAPed magnesium alloy[30]

Fig. 2-8 (0002) pole figure of magnesium alloy after extrusion (first row) and ECAP (second row)[31]

2.4 Fatigue and Fatigue Test

2.4.1 Introduction of Material Fatigue Property

Mechanical failure causes much financial loss every year. There exist many kinds of failure and one of them is fatigue. Material fatigue refers to a progressive degradation of a material due to some periodical change in physical parameter on its surface such as stress or heat. Common fatigue loading is listed in Table 2-5. One of the most well-known accidents caused by fatigue was “The Aloha Incident” (see Fig 2-9). The final investigation pointed out that crevice corrosion triggered the metal fatigue, and finally caused the accident[32].

Table 2-5 Types of fatigue loading[33]

Type Description

Mechanical fatigue Pure fluctuation

Fretting fatigue Cyclic stress with oscillatory relative motion and frictional sliding

Contact fatigue Rolling and sliding contact Corrosion fatigue Chemical aggressive or embrittlement

Creep fatigue Cyclic loading at constant elevated temperature Thermal fatigue Cyclic temperature at constant or zero load Thermo-mechanical

fatigue

Combined cyclic variation of both lading and temperature

Fig. 2-9 Damage on Aloha Airlines flight 243

Figure 2-10 shows the definition of cyclic loading that causes fatigue crack. When 𝜎𝑚𝑖𝑛>0, the cyclic loading is called a tension-tension cycle. If 𝜎𝑚𝑖𝑛=0, then it is called fully

reversed loading and if 𝜎𝑚𝑖𝑛<0 then it corresponds to tension-compression loading. The number

of 𝜎_𝑚𝑖𝑛/𝜎_𝑚𝑎𝑥 is defined as load ratio, R.

Considering the mechanisms resulting in fatigue, all fatigue can be divided into two parts: low-cycle fatigue and high-cycle fatigue. The former one is related to macro-plastic deformation in every cycle, and its loading is typically over the yield stress; the second one is associated with the elastic behavior of the material, and it has an applied stress that is below yield stress while above the fatigue limit.

Before there was a microscopic understanding of fatigue, some empirical and macroscopic means of quantifying the fatigue process have been developed to have a better knowledge of fatigue properties. S-N curve is one of the most important concepts in fatigue property[34], in which a specimen is loaded with a constant cyclic load S and the number of loading cycles until failure N is determined, such as shown in Figure 2-11. With the decrease of the stress, the S-N curve presents a plateau. Below this plateau, the sample may have cycled loading infinitely without failure. This lower limit of the loading is known as the fatigue limit, σe. The values of σe vary depending on the nature of the material, and usually lie between 35% to 50% of the tensile yield stress.

Fig. 2-11 S-N diagram of SAE 4130 steel, black dots represent individual fatigue tests[35]

2.4.2 Fatigue Mechanisms

When loading is applied on the sample, slip occurs within individual grains by

dislocations moving along crystallographic planes. This makes one or more planes within a grain slide relative to each other. Slip is considered as an accumulation of infinitely small

movements[36]. Fatigue cracks initiate in local slip bands and then begin to grow in a plane that has maximum shear stress. As cycling continues, the initial crack will grow. The whole fatigue process is named after Stage I and Stage II[37]. Figure 2-12 shows the fatigue life flow chart. The crack initiation period may sustain a large period of fatigue life under high-cycle

Fig. 2-12 Phases of the fatigue life and relevant factors[38]

Fatigue crack propagation has been studied for several decades. Fatigue life prediction was very much an empirical art rather than a science until Paris et. al.[39] in 1961 postulated that just as fast fracture, range of stress intensity factor ΔK might also describe crack growth under fatigue loading in the same way. The basic formula of Paris’ law reads as:

da

dN= 𝐶∆𝐾

𝑚

where a is the crack length and da

dN is the crack growth rate per loading cycle due to fatigue, C and

m are constants related to material environment and stress ratio, ΔK is the range of the stress intensity factor during the fatigue cycle. In this equation, K = ∆σY√𝜋𝑎, σ represents the loading and Y is a factor depended on geometry of the sample. Figure 2-13 shows how Paris’ law fits in practice.

Fig. 2-13 Typical fatigue crack growth rates (da

dN) for as function of stress intensity factor range

ΔK[40]

However, Pairs’ law is only quantitative for a long crack, while a short crack cannot be simply defined by liner elastic fracture mechanics (LEFM) because when considering the microscopic scale, the material is not uniform and isotropic any more[41]. From a microscopic point of view, a long fatigue crack starts at the surface of a sample and grows across several grains controlled by shear stress, then grows in a winding manner along the direction of maximum stress. Some fatigue cracks grow in between the grain but most of the cracks grow across grain boundaries. The behavior of short cracks is strongly correlated to

microstructure[42]. Taira et. al. proved that the presence of grain boundaries prevent small cracks from growing due to blocking of slip bands ahead of the crack tip by the grain boundaries[43]. A study on Al-Li alloys confirms that crack branching appears at the grain boundaries[44]. This

result may indicate that the density of grain boundaries is positive related to the resistance of the crack propagation. Further study on grain boundaries was done by Sarioglu et al.[45]. The slip length was determined by the grain size, crack propagation was stopped by the formation of large plastically deformed zone until new crack was formed at the neighboring grain.

To explore how the grain sizes affect the fatigue life, three sets of samples were prepared. The average grain sizes were 4.7 um, 15 um and 23 um. Tsushida et al. found that the specimen that had the smallest grain size owned the highest fatigue limit, while the other two had similar fatigue limits[46].

2.4.3 Fatigue test method

The four-point bend test is a convenient tool for fatigue studies. It has several benefits for characterizing the mechanical properties over other regularly used mechanical testing

procedures, such as tension, torsion or plane bending. First, four point bend testing does not need special sample gripping; therefore, it is friendly to those brittle materials and sample preparation is much easier. Secondly, four point bend testing is applicable to test samples by asymmetrically loading the specimen[47,48].

Previously there is no standard sample geometry for the four point bending test, it is difficult for different investigators to compare and use these results. In 1992, Grabowski et al. carried out a series of four point bend fatigue tests on Ni-based superalloys with different geometries[49]. They found that the inner roller spacing t, and the sample thickness h had a significant effect on fatigue properties. Although a standard four point bend measurement is set up, it is not always suitable for either a too small specimen or a high temperature. Therefore, there is an urgent need to determine the optimum testing geometry for the four point bend test. The inner roller distance/sample thickness ratio (t/h) and the support span/sample thickness ratio (L/t) are used to analyze geometric effects on the fatigue test. Figures 2-15 and 2-16 exhibit the stress distribution with t/h and L/t. Grabowski et al. proved that t/h=1.2-1.5 and L/t=4-5 is a proper testing geometry for FPB fatigue test, which can give the sample a uniform stress distribution. The nominal maximum stress σnom on the surface of the sample can be determined using the following equation:

𝜎_𝑛𝑜𝑚 =3𝐹(𝐿 − 𝑡) 𝑤ℎ2

where F is the force loaded on the roller, h and w are the thickness and width of the sample, and L and t are the outer span and inner span, respectively.

Fig. 2-15 Surface stress distribution with t/h[47]

Fig. 2-16 Surface distribution with L/t[47]

2.5 Summary

Magnesium is a promising structural material for spacecraft and automobiles due to its low density among commonly-seen metals. However, the application of magnesium is restricted by poor ductility and strength, attributed to its hcp crystal structure. Many approaches have been developed to improve the mechanical properties of magnesium, including grain refining.

ECAP is a novel method to produce a large scale of ultra-fine grained materials and it is more effective than other technics such as rolling. Other than this, ECAP is suitable for those materials that have a special geometry requirement.

The investigation into fatigue properties of materials has attracted attention for hundreds of years. Many mechanisms of fatigue behavior were raised to predict or to explain the fatigue and fracture. Paris’ law is one of the most successful models from the macroscopic view; meanwhile, microstructure also plays an important role in fatigue properties. Four-point bend fatigue testing was developed as an easy, self-aligning way to test the fatigue property of one specimen.

Few researches have been worked on fatigue behavior of ECAPed ultra-fine grained AZ31 Mg alloys. Therefore, it is necessary to verify the fatigue property, especially high cycle fatigue property, fracture mechanism of this strong textured material, and how texture will lead to anisotropy of fatigue behavior.

CHAPTER 3

EXPERIMENTAL PROCEDURE

3.1 Test Materials

The material used in this study is AZ31 magnesium alloy. The chemical composition of AZ31 magnesium alloy is listed below:

Table 3-1 Chemical composition of AZ31 magnesium alloy (wt.%)

Element Al Zn Mn Others Mg

percentage 2.5 1.2 0.3 0.60 Balance

Four sets of samples were prepared from two AZ31 magnesium plates, #33 and #36. The ECAP process of each plate was: #33: 2 Route A+ 1 Route C+ 1 Route A, #36: 2 Route A+ 2 Route C. Samples were cut along both the extrusion direction (ED) and the transverse direction (TD) from the plate, as shown in Fig 3-1. Three directions (ED, TD and normal direction (ND) were known and labeled on each of the sample rods for the following four-point bend fatigue test. The size of each sample rod was approximately 4 mm × 5 mm × 140 mm.

3.2 Sample Pretreatment

Specimen were cut 30 mm long for the four-point bend test, as shown in Fig 3-2. The edge of the samples were grinded round by abrasive paper in order to prevent stress

concentration. The top surface of the sample rod was first coarse polished by 15 μm diamond suspension polish agent, then further polished by 6 μm and 1 μm diamond polish agent. 0.25 μm diamond polish agent was used for the final polish. Ethanol based lubricant was used in case of corrosion and rapid oxidization.

Fig. 3-2 The size of the samples for the four-point bend test

3.3 Sample Characterization Analysis 3.3.1 X-ray Diffraction (XRD)

X-ray diffraction is a common and powerful technique for researchers to determine the crystal structures and atomic spacing. X-ray diffraction is based on interference of X-ray photons and repetition structure of crystal. When the lattice parameter of the crystal and the wavelength of the X-ray satisfy Braggs’ Law (nλ = 2d sin 𝜃), the diffraction X-ray pattern will be detected on the detector.

Pole figure measurements are performed by collecting the diffracted beam intensity of a Bragg reflection as a function of the sample rotation (β=0-360°) angle at various tilt angles of

the sample surface normal (α=15–90°, due to the restriction of the instrument, α cannot be smaller than 15°). The intensity I (α,β) plotted as a 2-dimensional map is known as pole-figure and is a representation of the distribution of a crystallographic orientation in space.

In this project, a Rigaku SmartLab diffractometer was used to study diffraction patterns and textures by pole figure. The sample was loaded on an alpha-beta stage. The incident

direction of the X-ray beam is perpendicular to the extrusion direction. Figure 3-3 shows the relative position between X-ray beam and sample.

Fig. 3-3 Sample position on alpha-beta stage

3.3.2 Scanning Electron Microscopy (SEM)

Scanning Electron Microscopy (SEM) is a powerful technique by hitting electrons on the sample surface to obtain the information of materials. Compared to an optical microscope, SEM is extremely useful for its microanalysis function and depth of field. The sample is hit by

high-Incident beam

Reflection beam

energy primary electrons. Secondary electrons (SE), backscattered electrons (BSE) and X-rays are collected for further analysis. Secondary electrons are sensitive to the surface topography and backscattered electrons are more governed by the atomic number of the sample. Characteristic x-rays generated in the sample allow investigators to determine the chemical composition of particles as small as a few hundred nanometers. The proper choice of the electron collection mode is important in order to more effectively study the microstructure.

In this project, a FEI Quanta 250 FE-SEM was used to study the sample topography and fracture surface of fatigued samples. By applying an accessory on SEM called X-ray Energy Dispersive Spectroscopy (EDS), the chemical composition distribution was analyzed

quantitatively. Before being analyzed, as-received samples were polished using a diamond suspension agent. After polishing, samples were cleaned by ethanol in ultrasound for 30 seconds and then dried with compressed air.

3.3.3 Transmission Electron Microscopy (TEM)

After being invented in 1936[50], TEM has gradually become the most powerful method to explore the microstructure of materials. Although both SEM and TEM use electron beams as probes, there is a significant difference between TEM and SEM. In TEM, the accelerated electrons penetrate thin samples, and the diffraction pattern will appear on the back focal plane. This pattern contains information from a large area of the specimen that interact with the beam, and this is the basis of defect characterization and how TEM works.

In this project, the sample was cut thin by using an FEI Helio NanoLab DualBeam Focused Ion Beam Microscope (FIB/SEM). The incident ion beam was coming in at a low angle

in the range of 5-8°. A thin large area would be achieved on the edge of the center hole. An FEI Tecnai G2-F20 Scanning TEM was used to image the deformed structure.

3.3.4 Precession Electron Diffraction (PED)

Precession electron diffraction (PED) is a newly developed method to collect electron diffraction patterns by utilizing the standard instrument configuration of a TEM. A tilted incident electron beam rotate around the middle axis of the microscope, diffraction conditions are collected and integrated to form as a PED pattern. PED is more advantageous over

conventional TEM through these two aspects[51] including: PED is able to acquire quasi-kinematical diffraction patterns, and PED has a broader range of measured reflections.

The PED datasets were acquired by using an FEI Tecnai G2-F20 TEM with an attached PED/ASTAR system, which controls two sets of coils in the TEM.

3.3.5 Four Point Bend Fatigue Test

As mentioned before, there exists an optimum geometry criterion for four point fatigue tests. The geometry of the fatigue specimens was a rectilinear solid whose dimensions were 29 mm × 5 mm × 4 mm. The tensile surface was polished before fatigue testing. The normal direction of the sample was positioned upward and in contact with the support rollers where maximum tension is experienced. The designed and machined upper and lower fixtures for four point bend testing are shown in Figure 3-4.

Fig. 3-4 Physical picture of four point bend fatigue test

The support and load span were t=5.74 mm and L=23 mm. According to Pilchak[36], before clamping the fixtures into the hydraulic grips, the crosshead and load cell must be

carefully aligned using a strain-gauged standard tensile specimen. The standard specimen, which has 6 strain gauges that measured bending moments induced by merely gripping the specimen with no load, was applied. With the load train properly aligned, the upper fixture was connected to a hemispherical bearing adapter which allowed final alignments to be made to distribute the load equally between the two loading rollers. A series of fatigue tests were conducted to determine the yield strength of the samples which would then be used as the benchmark for stress values employed in the four-point bending fatigue tests. For every group of samples (#33 ED, #33 TD, #36 ED and #36 TD) the fatigue life experiments were carried out under multiple stress levels ranging from 40% to 90% of the macroscopic yield strength (0.2% offset). The specimens were tested under the load ratio of R=σmax/σmin=0.1 and a frequency of 50Hz. Although high frequency fatigue tests can potentially suffer from specimen heating due to internal hysteresis damping which would, in turn, disguise the true fatigue behavior at room

temperature, it is known that for bending fatigue tests frequencies below 200 Hz have only a small effect on S-N behavior for most structural metals[52].

Interestingly, we determined the yield strength in the transverse direction was ~290MPa. This value is much greater than the values reported in Table 2-1, and the calculated specific strength is then ~164 kN·m/kg, which means that ultrafine grained AZ31B is competitive with Al-7075, a high-strength wrought Al alloy used for structural aerospace applications.

CHAPTER 4

RESULTS AND DISCUSSION

4.1 Characterization of As-received Sample

Figure 4-1 shows an X-ray diffraction pattern of the as-received AZ31 sample. A very strong (0002) peak can be found on the pattern. However, (10-10) diffraction peak can hardly be seen in this figure. This abnormal missing peak is caused by the strong texture of the as-received sample, which can be further confirmed by the (0002) pole figure. Figure 4-2 is the (0002) pole figure projection of the as-received sample. It can be found that only one hotspot appears in the middle of the pole figure. Because the (0002) plane is normal to the (10-10) plane in hcp crystal, and taking into consideration this strong texture, it can be concluded that the (10-10) plane has a very weak reflection because the projection of d spacing of the (10-10) plane on the extrusion-transverse surface is so small that not enough X-rays can be diffracted.

Fig. 4-1 XRD pattern of as-received AZ31 sample

0 2 4 6 8 10 12 14 30 35 40 45 50 x 1000 0 2-theta (10-11) (10-12) (10-10) (0002)

Fig. 4-2 (0002) Pole figure of as-received AZ31 sample

Figure 4-3 is the backscattered electron SEM image of the as-received sample. Some precipitates can be found in the matrix. The precipitates are approximately 20 um in diameter and distributed quite uniformly in the matrix. EDS based on SEM is used to find out the

chemical composition of the precipitates, and Figure 4-4 shows the EDS results. The bright area indicates that there is more signal of that element that can be detected. From the EDS mapping results, the precipitates found are compounds made of Mn and Al, while almost no Mg can be found in the precipitates. Table 4-1 is the chemical composition of both precipitates and matrix. We can calculate from the table that the atomic percent of Al in the precipitates is about 37% and Mn is 57%. Cao P. et al. discovered the presence of both γ2-Al8Mn5 intermetallic and β-Mn phase in the precipitates. Figure 4-5 shows the Al-Mn binary phase diagram. Based on the phase

diagram we may conclude that the mole ratio of Mn11Al15/β-Mn is approximate 0.2 in the precipitates. It is also shown that Mn as an additive in Mg alloys can not only get rid of Fe, but

also can give rise to obvious grain refinement[53]. However, the matrix shows the presence of only Mg itself, no signal of Al or Mn is found.

Fig. 4-4 EDS mapping of Mg, Al and Mn

Table 4-1 Chemical composition of precipitates and matrix wt.% Mg Al Mn Fe Zn Precipitate 1 0.23 22.66 74.14 2.22 0.11 Precipitate 2 0.44 23.18 70.78 3.47 0.10 Precipitate 3 0.24 23.62 72.20 2.26 0.09 Matrix 94.96 2.58 0.29 n/a 1.12

4.2 Four Point Bend Fatigue Test and Fracture Mechanism 4.2.1 Four Point Bend Fatigue Test Results

As illustrated in Figure 4-6, fatigue tests of the specimens were measured with the loading perpendicular to the ED-TD surface.

Table 4-2 Fatigue test results Stress Level

(% of Yield) σnom (MPa) Max Load (N) Min Load (N) Fatigue life #33 Extrusion direction 90 180 556 56 67500 80 160.00 494 49 97908 75 150 463 46 108143 70 140 441.2514 44.12514 1000000 60 120 384.1483 38.41483 1000000 #33 Transverse direction 90 261 806.489 80.6489 22927 70 203 627.2692 62.72692 46954 67.5 194 600.3862 60.03862 55857 65 188 582.4643 58.24643 1000000 60 174 537.6593 53.76593 1000000 #36 Extrusion direction 90 180 565.0985 56.50985 36654 80 160 505.2762 50.52762 55950 75 150 473.6964 47.36964 243850 70 140 443.847 44.3847 252000 65 130 412.1437 41.21437 1000000 60 120 391.5643 39.15643 1000000 #36 Transverse direction 90 261 838.7486 83.87486 20150 70 203 627.2692 62.72692 69606 60 174 537.6593 53.76593 78800 50 145 448.0494 44.80494 103450 45 130 403.2445 40.32445 137080 40 116 358.4396 35.84396 1000000 40 116 358.4396 35.84396 1000000

Table 4-2 shows the fatigue test results for all four sets of samples. According to what has been introduced before, the stress given to the sample is determined by this equation:

𝜎_𝑛𝑜𝑚 =3𝐹(𝐿 − 𝑡) 𝑤ℎ2

One million cycles is set as the longest fatigue life, and stress that does not make samples brake after one million cycles will be considered as lower than its fatigue limit. Figure 4-7 shows the S-N curves based on the fatigue results, with unbroken samples labeled with red spots, and others are blue. From the S-N curve obtained for the #33 ED and #36 ED (Fig. 4-6 (a)(c)), it can be seen that the fatigue life increases slightly as the maximum stress reduces from 90% to 75% (70% for #36 ED) of the yield strength. However, a further 5% reduction of stress applied on the samples leads to a significant increase in fatigue life, which surpasses the arbitrary fatigue limit, 106.

The same trend can also be observed for the #33 and #36 TD material. Nevertheless, a remarkable difference occurs between the stresses of any given fatigue life when ED and TD sample are compared. Obviously, the samples show strong fatigue strength anisotropy. It has been reported that AZ31 Mg alloy presents obvious differences in compression or tension

condition[55,56]. In the condition of compression, the initial preferred orientation of the grains is more likely to have {101̅2}〈101̅1〉 twinning, while in the condition of tension, the deformation is predominated by harder non-basal slip system[57]. Consequently, the measured tensile strength is larger than that of compression.

The fatigue anisotropy of the extruded AZ31B alloy is caused by the strong texture and the angular spread of basal poles towards the extrusion direction. The tilt of basal poles towards the extrusion direction causes yielding by activating the basal slip in the extrusion direction, which result in plastic deformation. Yielding in the transverse direction on the other hand

at room temperature. According to different models, critical resolved shear stress (CRSS) value of basal slip in polycrystal AZ31B alloy range between 10-50 MPa, while it is 55-110 MPa for prismatic slip[58]. The ratio of prismatic/basal vary from 2-5.5. As a result, mechanical

properties anisotropy can be found in textured hcp materials. Furthermore, twinning in the transverse direction requires higher stress levels[59].

Fig. 4-7 S-N curve of the (a) #33 ED, (b) #33 TD, (c) #36 ED and (d) #36 TD fatigue test, plotted as maximum stress (MPa) verses number of cycles (N)

(a) (b)

(c) (d)

4.2.2 Fractography

The fracture surfaces of the fatigue samples were examined by SEM. Figure 4-8 shows some fracture surface at large field of view. It can be seen that the fracture surface contains three regions: fracture initial, fatigue and fast fracture. At the bottom right of figure 4-8(a) is the place where the initial crack appears, while (b) is the border of the fatigue and fast fracture. Some flake like surface topography can be found in the fatigue crack propagation area (Fig. 4-9). The crack branching has happened extensively in the fatigue region and the flake boundaries could

potentially represent the grain boundaries of UFG Mg which would imply intergranular crack propagation.

Fig. 4-8 SEM images of #36 ED fracture surface that fatigued at 90% of yield stress (180 MPa) (a) bottom right, (b) middle

Initial crack region

Fast fracture

Fatigue fracture

Fig. 4-9 SEM high magnification Flake like fracture surface topography

4.3 Characterization of Fatigued Sample

Figure 4-10 is the XRD results comparison before and after fatigue testing. A significant decrease of the (0002) peak intensity can be found in the pattern. This can be attributed to the change of the basal plane direction. It can be seen that the (0002) pole tilt toward the extrusion direction by about 30°-40° from figure 4-11.

Fig. 4-10 Diffraction pattern of #36 TD before and after fatigue test

0 2 4 6 8 10 12 14 30 32 34 36 38 40 42 44 46 48 50 x 1000 0 2-theta as received fatigued (10-12) (0002) (10-11)

Fig. 4-11 (0002) Pole figure after fatigue test

The ASTAR™/PED scanned area is indexed and the extracted maps are presented in Figure 4-12. The grain boundary in the quaternion color map shows that the axis and angle of grain boundaries misorientation for all the detected grain boundaries remain more or less the same along their length. This observation implies that the grain boundaries in this map are not subject to severe twist, which is expected in highly deformed materials.

CHAPTER 5

CONCLUSION

This study is conducted with a view to examine the high-cycle fatigue properties of AZ31 magnesium alloy. The effect of texture and microstructure on fatigue life and fracture

mechanisms is discussed. This UFG AZ31 Mg alloy has an average grain size of less than 1 um and a fairly strong basal texture with the c-axis of most grains tilted toward the ND. A second phase of Al-Mn rich precipitates can be found by SEM and EDS. Samples reveal significant anisotropy in the fatigue behavior due to texture. Both #33 TD and #36 TD samples maintain a longer fatigue life than those of ED at the same stress level.

Twinning and detwinning likely to play a key role in the tension-compression loading cycle. Comparison of XRD pattern and pole figures before and after fatigue test confirms that the fatigue anisotropy of extruded AZ31B alloy is caused by the strong texture. Yielding in ED is caused by activating basal slip, which rotates towards the easy slip direction (~45°) during the fatigue process, while the activation of prismatic slip is required for yielding in TD. This rotation is especially important, as it implies that the grains themselves rotate as a type of “grain

aggregate” structure. This collective rotation of fine grain structure may be a way to overcome the traditional con Mises rule of 5 independent slip system, and thus occur at lower stresses. We would expect this stress would be lower than the stress required for prismatic slip. Topography of fracture surface has been examined. Initial crack starts from the corner and the crack

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