Deformation and Fracture Behaviors of Pd Cu Ni P Glassy Alloys

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(1)Materials Transactions, Vol. 43, No. 12 (2002) pp. 3266 to 3272 c 2002 The Japan Institute of Metals EXPRESS REGULAR ARTICLE. Deformation and Fracture Behaviors of Pd–Cu–Ni–P Glassy Alloys Chaoli Ma1 and Akihisa Inoue2 1 2. Inoue Superliquid Glass Project, ERATO, Japan, Science and Technology Corporation, Sendai 982-0807, Japan Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan. Thermal behavior and mechanical properties of Pd-based glassy alloys with compositions of Pd35+x Cu30 Ni15−x P20 (x = 0, 5, 7.5 at%, group-I) and Pd40+x Cu20 Ni20−x P20 (x = 0, 5, 10 at%, group-II) have been studied. Group-I alloys lie in an invariant eutectic reaction region of the quaternary system. The invariant eutectic reaction of L → Cu3 Pd + Ni2 Pd2 P + Cu3 Pd5 P2 + quaternary phosphide exists at a composition of approximately Pd42.5 Cu30 Ni7.5 P20 and at a temperature of about 800 K. Both group glasses possess a large supercooled liquid region of over 80 K before crystallization. The compressive strength, Young’s modulus and Vickers hardness in each group increase with increasing Ni content. The compressive fracture strength and Young’s modulus are in the range of 1610 to 1740 MPa and 100 to 110 GPa, respectively. Under a uniaxial compression mode, these glassy alloys deformed inhomogeneously and fractured adiabatically. No distinct plastic deformation was observed for group-I alloys, while group-II alloys exhibited serrated flow with a maximum plastic strain of about 1.3%. (Received August 14, 2002; Accepted October 24, 2002) Keywords: bulk metallic glass, mechanical property, adiabatic shear, serrated flow, fracture. 1. Introduction Metallic glasses possess superior physical, chemical and mechanical properties, e.g., higher strength, lower elastic modulus and higher corrosion resistance, as compared with their crystalline ones.1, 2) These properties are prerequisite for application of advanced structural materials. However, metallic glassy alloys in a bulk form had not been synthesized by conventional casting methods until the end of 1980’s, because of the necessary of high cooling rates of 105–7 K/s for the suppression of crystallization. In 1988, Inoue et al.3) found anomalous stability of supercooled liquid in Mg–Ln– TM (Ln = lanthanide metal, TM = transition metal) systems and subsequently synthesized La–Al–Ni bulk glassy alloys by a conventional copper mold casting method.4) The anomalous stability of supercooled liquid was pointed out in association with three empirical rules:5) 1) multicomponent systems consisting of more than three elements; 2) significant difference in atomic size rations above about 12% among the three main constituent elements; and 3) negative heats of mixing among the three main constituent elements. After that, the anomalous stability phenomenon of supercooled liquid was found in a number of multicomponent alloy systems with relatively low critical cooling rates (typically below 10 K/s).6–8) It is not difficult to produce bulk metallic glasses with large size even under conventional cooling conditions if appropriate alloy compositions are chosen. It is expected that this kind of novel glassy alloys are widely used as advanced structural and functional materials. Pd–Ni–Cu–P (PCNP) base alloys are the best glass formers. The critical cooling rate (Rc ) of PCNP alloys near Pd40 Cu30 Ni10 P20 has been reported to be less than 0.1 K/s, which is believed to be the lowest Rc among all metallic glassy alloys reported to date.9–11) (All compositions are given in mol percent through the paper, unless otherwise stated). A cylindrical glassy rod with a diameter of as large as 75 mm was made at the composition of Pd40 Cu30 Ni10 P20 . PCNP glassy alloys have been extensively studied on their glass-. forming ability,11) crystallization12, 13) and several physical properties.14, 15) Some compositions have also been tested as an electrode material.2) To extend their practical application fields, the knowledge of mechanical properties is necessary. The aim of the present paper is to describe the compressive deformation and fracture behavior of PCNP glassy alloys. Thermal properties of alloys used in the present study are also briefly described. 2. Experimental Procedure Two groups of PCNP alloys were examined in the present study, whose compositions are listed in Table 1 and schematically shown in a quasi-ternary Pd–Ni–Cu (20 at%P) diagram (Fig. 1). These compositions lie in an area surrounded by dot lines, in which cylindrical glassy rods with a diameter of at least 7 mm could be formed.16) The filled and open circles in the diagram denote the group-I and group-II alloys, respectively. Alloy ingots were first prepared by arc melting the mixtures of pure Pd, Ni and Cu metals and pre-alloyed. Fig. 1 Quasi-ternary diagram of Pd–Ni–Cu (20 at%P) system showing the compositions of studied alloys..

(2) Deformation and Fracture Behaviors of Pd–Cu–Ni–P Glassy Alloys. 3267. Table 1 Alloy compositions and thermal properties.. I. II. Composition (at%). Tg (K). Tx (K). ∆Tx. Te (K). Tl (K). Tg /Te. Tg /Tl. Pd42.5 Cu30 Ni7.3 P20 Pd40 Cu30 Ni10 P20 Pd37.5 Cu30 Ni12.5 P20 Pd50 Cu20 Ni10 P20 Pd45 Cu20 Ni15 P20 Pd40 Cu20 Ni20 P20. 576 574 578 605 602 613. 670 671 667 695 689 682. 94 97 89 90 87 69. 800 801 800 835 845 852. 835 861 939 963 961 960. 0.72 0.716 0.722 0.724 0.712 0.719. 0.689 0.666 0.615 0.628 0.626 0.638. Pd–P ingots in an argon atmosphere. The ingots were subsequently subjected to flux treatment with B2 O3 by the same arc melting method. Finally, from these flux treated ingots, cylindrical rods of 3 mm in diameter were prepared by injection casting of the molten alloys into copper mold with a cylindrical cavity. The cast structure was examined by X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Their thermal characteristics were examined by differential scanning calorimetry (DSC). The Vickers hardness was measured at RT with a load of 4.9 N for 15 s. The compressive strength was evaluated by compression tests using specimens which were cut from the cast cylindrical rods by 6 mm in length. The cut surfaces were mirror polished parallel. The specimens were compressed between hard BN platens in an Instron machine. Compression tests were carried out at an initial strain rate of 4 × 10−4 s−1 in laboratory atmosphere at room temperature. Fracture surface was examined using a SEM. 3. Results 3.1 Thermal properties The cast structures of the studied alloys were identified by XRD and TEM to be a fully glassy phase. The DSC curves of these glassy phases are shown in Fig. 2(a) for group I and (b) for group II glassy alloys. The thermal parameters of glass transition temperature (Tg ), crystallization temperature (Tx ), and melting temperature (Tl ) are measured from these curves and summarized in Table 1. The thermal properties of group-I have been reported previously.11) The present results are consistent well with them. It is clear from these DSC curves and Table 1 that both two group alloys possess a large supercooled liquid region (∆Tx = Tx − Tg ) before crystallization, indicating high thermal stability of these supercooled liquids. The reduced glass transition temperature (Tg /Tl ) is in the range of 0.615 to 0.689, indicating that these PCNP alloys have high glass-forming ability. These DSC curves also show that the two group alloys have different melting behaviors. Group-I alloys possess almost the same Te temperature of 800 K, indicating that they are located in an invariant eutectic reaction field. The Te is the invariant eutectic temperature of this quaternary system. The Pd42.5 Cu30 Ni7.5 P20 alloy possesses the narrowest melting region (Tl − Te ) among the group-I alloys, indicating that it is located at the nearest eutectic point. Combining the phase equilibria reported in other paper,13) the invariant eutectic reaction can be expressed as: L → Cu3 Pd + Ni2 Pd2 P + Cu3 Pd5 P2 + quaternary phosphide. It has been reported that the Pd42.5 Cu30 Ni7.5 P20 alloy possesses high glass-. Fig. 2 DSC curves of (a) group-I and (b) group-II Pd–Cu–Ni–P based glassy alloys.. forming ability (GFA) with the lowest Rc of 0.067 K/s and the GFA decreases with the deviation of composition from the eutectic point.11) The Te temperatures of group-II alloys are not constant and about 30 K higher than that of group-I, indicting that this group alloys lie outside the invariant reaction field. 3.2 Mechanical properties The compressive strength, Young’s modulus and Vickers hardness of the two group alloys are summarized in Table 2. These mechanical properties exhibit a similar tendency, i.e., an increase with Ni content for both two group alloys. Figures 3(a) and (b) show the nominal compressive stressstrain curves of the two group alloys. It is clearly seen that the deformation behavior is greatly different between the two group glassy alloys. Group-I alloys with higher Cu content of 30 at% in the invariant eutectic reaction region, deformed elastically and fractured immediacy after yielding. Their stress-strain curves exhibit a nearly linear relation. On.

(3) 3268. C. Ma and A. Inoue. Table 2 Mechanical properties of studied PCNP based glassy alloys.. I. II. Composition (at%). Hv. E (GPa). σf (MPa). ε (%). Pd42.5 Cu30 Ni7.3 P20 Pd40 Cu30 Ni10 P20 Pd37.5 Cu30 Ni12.5 P20 Pd50 Cu20 Ni10 P20 Pd45 Cu20 Ni15 P20 Pd40 Cu20 Ni20 P20. 496 504 524 484 507 520. 102 106 108 104 108 110. 1710 1715 1734 1610 1630 1736. 0 0 0 1.3 0.5 0.6. Fig. 4 SEM images showing the top and side appearance of the fractured Pd40 Cu30 Ni10 P20 glassy alloy.. Fig. 3 Compressive stress-strain curves of (a) group-I and (b) group-II Pd–Cu–Ni–P based glassy alloys.. the contrary, group-II alloys, which have a lower Cu content (20 at%) and lie outside the invariant eutectic reaction region, deformed elastically to yield point, followed by a distinct plastic deformation. The maximum plastic strain (which is caused by serration) of about 1.3% was observed for the Pd50 Cu20 Ni10 P20 alloy. Their stress-strain curves (Fig. 3(b)) exhibited serrated flow pattern. 3.3 Fractographic observation A detailed SEM fractographic analysis was carried out for these fractured samples. The purpose of the fractographic analysis was to examine the fracture mode in the two group alloys with different compressive deformation behaviors. Figures 4(a) and (b) illustrate the top and side looks of one fractured segment of Pd40 Cu30 Ni10 P20 , which is typical for the studied PCNP glassy alloys. It is clear that the fracture takes place along the maximum shear plane, which is declined by about 45-degree to the direction of the compressive load.. Several second cracks and fractured surfaces, which oriented random angle degrees to the load axis, were also observed. Figure 5(a) illustrates typical well-developed vein pattern on the major crack surface. This vein pattern is commonly observed in other metallic glasses.17–19) Theoretical20) and experimental21, 22) studies have indicated that the formation of vein pattern is due to the existence of a ‘soften’ or ‘viscous’ layer in shear slip bands, in spite of the mechanisms of forming the ‘soften layer’ being greatly different.20, 23, 24) The morphology of ‘vein pattern’, thus, depends on the fracture mode (tensile, shearing, tearing, or mixing) of the slip bands.22) Figures 5(b)–(d) illustrate variant ‘vein’ or ‘river’ morphologies observed on fracture surfaces, indicating that the fractured segments departed by various modes at final fracture stage. No apparent difference in the fracture morphology was recognized between the two group alloys. Surprisingly, a large number of ‘liquid droplets’ were observed on the fractured surface in all the samples, suggesting that ‘adiabatic heating’ occurred at fracture stage and high volumes of material in shear band were remelted. For instance, Fig. 6 shows some typical remelted morphologies. Note that these droplets shown in Figs. 6(a), (b) and (c) are not.

(4) Deformation and Fracture Behaviors of Pd–Cu–Ni–P Glassy Alloys. 3269. Fig. 5 (a) ‘Vein’ morphology of Pd40 Cu30 Ni10 P20 glassy alloy and (b)–(d) several river-like patterns formed on major shear-band cracks.. formed locally, but spurted from other parts of the fractured sample. Some of them flowed on the surface and covered the true fracture surface (Fig. 6(d)). The ‘remelted droplet’ phenomena have been reported in many other glassy alloys,25–27) but such a large number of ‘liquid droplets’ observed in PCNP glassy alloys are believed to be the first case. Note that no evidence of any crystalline phase even at the top edge was observed. This suggests that the remelted liquid solidified again into a glassy phase on the fractured surface. Multiple shear bands with many slip steps were seen on the lateral surfaces of group-II alloys. But it was rarely observed in the group-I alloys. Moreover, developed shear bands were also observed near the major fracture crack of some group II alloys. Figure 7 shows the morphology of multiple shear bands for group-II alloys. 4. Discussion 4.1 Strength and hardness The compressive strength of the studied PCNP glassy alloys lies in the range of 1610 to 1740 MPa (see Table 2). It is comparable to that of other Pd-based glassy alloys, e.g., 1570 MPa for Pd77 Cu6 Si17 , 1600 MPa for Pd64 Ni16 P20 , and 1800 MPa for Pd16 Ni64 P20 .28) It is evident from Table 2 that the compressive strength, Vickers hardness and Young’s modulus exhibit a composi-. tional dependence, i.e., these mechanical properties increase with substitution of Pd with Ni. A similar tendency has also been reported in (Pd1−x Nix )80 P20 glassy alloys.29) Unlike a common crystalline material, the glassy alloys do not include dislocation, grain boundary and crystalline phase. Consequently, composition and atomic configuration (atomic structure: structural short-range order and chemical short-range order) play a critical role in mechanical properties of glassy alloys. A detailed mechanism for the compositional effect on the strength of a glassy alloy is not clear. Experimental results have revealed that the yield strength of glassy alloys strongly depends on transition metal, which is a major constituent, and the yield strength tends to increase with increasing the number of both group and period in the periodic table. This phenomenon may be associated with the average outer-electron concentration of transition metals.30) Moreover, experimental results have also revealed that the tensile and compressive fracture strength of metallic glasses increases with increasing melting or glass transition temperature.31, 32) This implies the importance of bonding strength among component atoms. The present results show that the compressive strength, Young’s modulus, hardness and melting temperature increase with substitution of Pd with Ni. It is interpreted that the addition Ni enhances the bonding strength of the component atoms..

(5) 3270. C. Ma and A. Inoue. Fig. 6 ‘Remelting droplets’ morphologies observed on major shear-band cracks.. Fig. 7. (a) Developed shear band near major crack and (b) large shear step observed in group-II glassy alloy.. On the other hand, the short-range ordering, which arises from the strong interaction between metal and metalloid atoms, has been presumed to be responsible for the increase of Young’s modulus and strength.27) It has also been reported that the Pd40 Cu30 Ni10 P20 glassy alloy contains two kinds of atomic clusters, Pd–Ni–P and Pd–Cu–P.2) Any change in composition is expected to cause a change in the cluster structure and the interaction strength between metal and metalloid (P) atoms, affecting the strength.. 4.2 Inhomogeneous deformation Fractographic observation clearly reveals that the deformation of PCNP glassy alloys is extremely constrained in a few shear slip bands, which is declined by about 45-degree to the compressive load axis. This localized deformation behavior has commonly been observed in other metallic glasses as well. The basic physical process underlying the inhomogeneous flow phenomenon is a local softening of material.33) Several.

(6) Deformation and Fracture Behaviors of Pd–Cu–Ni–P Glassy Alloys. mechanisms have been proposed. Based on the concept of free volumes,34) Spaepen and Turnbull proposed that the viscosity of material in stress concentrated region decreased by the stress-induced dilatation.20) Leamy, Chen and Wang23) indicated that the viscosity of the material within a shear band was reduced by the adiabatic heating. Polk and Chen,24) on the other hand, angulated that the viscous flow is due to the creation of structural disorder. These modes have been supported by some experimental data. But the inhomogeneous deformation behavior could not be explained only by these modes. For example, Owen et al.35) measured that the shear bands traveled at a high speed of 1300 m/s, which could not be explained by the stress-induced dilatation mode or the structural disorder mode because the necessary increase in free volume could not be satisfied. On the other hand, the adiabatic heating mode could not be applied at the beginning stage of plastic deformation. It is likely that the two or more mechanisms operate simultaneously in the metallic glasses. It is of interest to note that the apparent plastic strain is ∼0% for group-I alloys and 0.5–1.2% for the group-II alloys. Moreover, the group-II alloys exhibit distinct jerky flow characterized by the serrated stress-strain curve, as shown in Fig. 3(b). Experiments have revealed that the load drops are associated with the formation of new shear slip bands.36, 37) When the shear bands are stopped by some reasons, the load increases again. This process repeats several times until the sample fractures. The experimental data also suggest that the serrated deformation easily occurs under compressive condition and in certain temperature and strain rate regions, indicating that the serrated flow is dependent on temperature, stress state and strain rate.22, 38, 39) This phenomenon was also observed in the glasses enhanced by nanoparticles.40) Thus, microstructural features greatly affect the deformation process. In the present study, all compression tests were conducted carefully under a certain uniaxial compressive condition as described in experimental section. Therefore, the difference in plastic flow behavior shown in Fig. 3 is a reflection of the compositional and structural effects. The serrated flow may result from the existence of some microstructural barriers that provide local crack-arrest point. To clarify this point, two additional experiments were conducted. Firstly, the plastic flow of cylindrical rod-samples with different diameters of 2 to 4 mm was examined for the Pd50 Cu20 Ni10 P20 (group-II) alloy. This results from the idea that the cooling rate in the cast rod is dependent on rod diameter, which would result in a different glassy structure and, consequently, affect the plastic flow pattern. Second, we further examined the glassy structure using the high resolution TEM technique to confirm the difference in structure. Unexpectedly, all the plastic patterns of the samples with different diameters are similar and show serrated flow curves, indicating the formation of a similar glassy structure of the cast rods. The high resolution TEM observation did not also recognize the existence of any crystalline phase. The shear band was not stopped by nano-particles. Detailed examination of the fracture surface revealed that crossed marks formed by shear slip bands were always observed on lateral and fractured surfaces of the jerky deformed samples. Figure 8 shows the SEM image of fracture surface of the Pd50 Cu20 Ni10 P20 alloy, revealing that the shear bands are stopped by other intersected shear bands.. 3271. Fig. 8 SEM image of fracture surface of Pd50 Cu20 Ni10 P20 glass alloy showing the shear bands stopped by other intersected shear bands.. This shows a possibility that the serrated flow is formed by the momentary interrupt of shear sliding.41) The reason why the serrated flow could not be observed in group-I alloys is unclear. Nevertheless, the present results clearly indicate that the adjustment of alloy composition can influence the propagation behavior of shear bands, which can give significantly enhanced ductility. 4.3 Adiabatic heating The ‘liquid droplets’ observed on fracture surface (see Fig. 6) suggests that adiabatic heating (fracture) occurs at the fracture stage. This adiabatic heating phenomenon has also been reported in many other metallic glassy alloys.25, 26, 42–44) The dissipation of plastic deformation energy in a localized shear region will result in local heating of this region. Bengus et al.25) measured the rise of temperature near the shear-bands when a Fe40 Ni40 P14 B6 ribbon sample was tested in liquid 3 He and estimated that the temperature rise in shear bands reached ∼1800 K. Liu et al.43) have also estimated the local increase in temperature by attributing the conversion of the stored elastic strain energy to the adiabatic heating in the localized region at the moment of tensile fracture for a Zr-based glassy alloy. They concluded that as thick as 62 µm material will be heated adiabatically to the temperature above the melting point. Although we did not measure the temperature increase in the shear-band crack region, the ‘liquid droplet’ morphology indicates conceivably that the temperature in the major shear band cracks reached the temperature above the melting point. The highest melting point of these PCNP alloys is about 960 K, indicating that the local temperature is above 960 K. It is obvious that the remelting occurs within the shear bands at the fracture stage of PCNP glassy alloys. The additional problems are summarized as follows: (1) when does the adiabatic heating occur in the whole deformation and fracture processes, and (2) what is the role of the adiabatic heating in the fracture mechanism. Bruck et al.44) have measured the temperature changes in a Zr-based glassy sample subjected to dynamic compression load. They found that the temperature increases only after the onset of inhomogeneous deformation. This means that the adiabatic heating is generated.

(7) 3272. C. Ma and A. Inoue. by the enormous plastic deformation, which reduces the viscosity of material within shear bands. The viscosity of the Pd40 Cu30 Ni10 P20 glassy alloy has been reported to strongly depend on temperature and show low values of 105 –108 Pa s in the supercooled liquid region.2) It is reasonable to believe that the adiabatic shear is the major mechanism for fracture of PCNP glassy alloys. Note that at very beginning of deformation, the increase in the free volumes due to extension20) may also give some contribution to diminishing of the viscosity in this layer. 5. Summary Thermal and melting behaviors of Pd-based glassy alloys with compositions of Pd35+x Cu30 Ni15−x P20 (x = 0, 5, 7.5 at%) and Pd40+x Cu20 Ni20−x P20 (x = 0, 5, 10 at%) were examined by differential scanning calorimetry. The Pd35+x Cu30 Ni15−x P20 (x = 0, 5, 7.5 at%) alloys lie in an invariant eutectic reaction region of the quaternary system. The invariant eutectic reaction of L → Cu3 Pd + Ni2 Pd2 P + Cu3 Pd5 P2 + quaternary phosphide exists at a composition of approximately Pd42.5 Cu30 Ni7.5 P20 and at a temperature of ∼800 K. The Pd40+x Cu20 Ni20−x P20 (x = 0, 5, 10 at%) alloys are located outside the invariant eutectic reaction field. Both group glassy alloys possess a large supercooled liquid region (>80 K) before crystallization. Mechanical properties of both group glassy alloys were examined by compression and hardness tests. The compressive stress, Vickers hardness and Young’s modulus increase with increasing Ni content. The compressive fracture strength and Young’s modulus are in the range of 1610 to 1740 MPa and 100 to 110 GPa, respectively. These glasses were deformed extremely inhomogeneously at room temperature. 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