Sample Contamination

The final important practical consideration in a milling experiment is the presence of impurities, either from the atmosphere within the containers, or resulting from wear o f the surface o f the milling tools. Atmospheric impurities can be controlled easily by both preparing the samples and seahng the milling containers within the confines of a high purity glovebox (usually containing argon), and then transferring them to the ball mill where they are clamped securely in place. By contrast, contamination from the milling tools is generally unavoidable and depends not just on the material from which they are manufactured but also on the milling time and the mass and hardness o f the sample.

U n m ille d Iro n 0.0 2.5

a

I

5.0

I

M illed Iro n Ô 0 . 0 4.0 8.0 6 4 0 2 - 2 - 6 - 4 Velocity (mm/s)

Figure 4.7 Mossbauer spectra of pure iron powder, and the same powder following ball milling for 70 hours in stainless steel containers under an argon atmosphere.

Sa m p l e Pr e p a r a t io n 7 3

As a starting point for the investigation into sample contamination, iron powder (purity 99.9 + %, particle diameter < 10 pm) was milled under an (inert) argon atmosphere. The Mossbauer spectrum o f this sample is used as a basis against which subsequent results can be compared. This spectrum, shown in figure 4.7, is quite different fi'om that of unmilled powder. The latter is a single sextet with hyperfine field Bhf = 32.9 T and narrow lines, corresponding to a small root mean square deviation in Bhf relative to the mean, ABhf = 0.5 T. The spectrum of the milled sample, on the other hand, was modelled as two sextets, one with Bhf = 32.8 T and ABhf = 0.4 T, and a second with Bhf = 31.1 T and ABhf = 4.4 T. These two subspectra may be regarded as approximating a broad range o f disturbed local environments for the iron atoms, ranging jfrom relatively unchanged (the Bhf = 32.8 T subspectrum) to significantly distorted (the Bhf =31.1 T subspectrum), with the large linewidths indicative o f the degree o f local disorder.

The physical origin o f this second sextet is not immediately obvious. It may be a sample dependent factor, such as the presence o f either a large number of lattice distortions, or atoms at grain boundaries and surfaces (of which there would be a significant proportion in a nanocrystalline material such as this). These atoms would be in a non-standard structural and magnetic environment and would therefore produce a Mossbauer signal with a degree o f variation in hyperfine parameters. Alternatively, the cause of the observed distortion in the spectrum could be impurity related, being the result of absorption of gases fi’om the milling atmosphere, or o f material worn fi’om the milling tools themselves. At different times, all o f the above arguments have been used in the literature to explain similar observed effects [64]. For instance, in a pure iron sample milled in a conventional cylinder ball mill for 2000 hours, the extension in the hyperjfine field distribution to lower values was explained by the presence o f impurity contamination, with it noted that there was approximately 5 at.% carbon, 15 at.% oxygen and 0.4 at.% nitrogen in the product [65].

Given the range o f possible effects that impurities may have on the properties of mechanically alloyed materials, and the way in which these are all dependent on the particular milling conditions employed, a systematic study was conducted looking at each factor in turn. A series o f samples were produced, using various milling atmospheres (argon nitrogen and air) and different milling tools (steel and Syalon). All

7 4 Ch a p t e r 4

external milling parameters were fixed at the values summarised in table 4.1, and pure iron powder was used as the sample material

Milling time 70 hours

Milling disc velocity 600 rpm (‘8’ on control dial)

Sample mass 5 g

Grinding balls per container 6

Table 4.1 Fixed milling parameters for mechanical alloying experiments.

(a) Milling Tool Contamination

In general, contamination levels due to wear o f the milling tools increase with time but also depend upon the material fi'om which the container is made. The most wear resistant o f the commonly available materials is timgsten carbide, which also has the highest density (14.75 x 10^ kg/m^), leading to reduced milling times and large impact energies. However, perhaps not surprisingly it is also expensive, at around three times the price of stainless steel, the most widely used material.

Much o f the work done in this thesis employed stainless steel containers (made fi'om type 4301 stainless steel, with a composition o f 68 at.% Fe, 19 at.% Cr, 8 at.% Ni, 2 at.% Mn, 2 at.% Si plus traces o f S and C), although later experiments used Syalon (a silicon nitride con^osite - SisN4 with small amounts of aluminium and yttrium

oxides). Stainless steel has the higher density (7.9 x 10^ kg/m^ compared with 3.2 x 10^ kg/m^ for Syalon) with its associated advantages o f greater abrasion resistance and higher impact energy. However, the fact that steel contains iron is a significant drawback, given that most of the samples to be milled are also iron based. Additionally, its minor constituents of chromium and nickel are miscible in iron. Therefore any of these impurities would be expected to ahoy with the bulk o f the sample, modifying its composition [66]. Syalon on the other hand, whilst more prone to wear, is made o f an inert, stable material which is not expected to decompose in the mill or alloy with the iron-based materials studied.

In dealing with impurities there are therefore two fundamental questions that must be answered. Firstly and most obviously, it is necessary to determine the absolute impurity level. This was accomplished by means o f EDAX measurements, as described in chapter 3. Secondly, however, it is important to consider what form the impurity may

Sa m p l e Pr e p a r a t i o n 7 5 take, i.e. whether it is miscible or immiscible with the sample material. The effects of impurities, firstly from stainless steel and then from Syalon are considered below.

ED AX measurements of iron milled in steel for 70 hours gave the final composition of the material as 96.5 at.% Fe, 2.5 at.% Cr, 0.4 at.% Ni and 0.3 at.% Mn (errors ± 0.4 at.%). This imphes a total impurity level (included added iron) o f approximately 10 at.%. The iron worn from the container is absorbed directly into the sample, but it is necessary to confirm whether the chromium is also incorporated into the powder and, if so, how this modifies the overall results. Evidence for alloying between the iron in the sample and the chromium from the steel cannot be obtained by XRD since the two elements both have bcc lattices and similar cell parameters (a = 2.866 Â and a = 2.883 Â respectively). Thus any sohd solution formation would result in minimal peak shift, particularly given the small proportion o f chromium in the material, and furthermore any shift would be difficult to measure accurately due to the large line broadening.

However, the effect o f the addition o f smaU quantities o f chromium to iron is clearly visible by Mossbauer spectroscopy. It has been shown, when producing iron-rich Fe-Cr from the melt using standard methods [67] that the resulting spectra consist o f a sharp a-Fe sextet attributed to iron atoms m an undisturbed environment together with a broader sextet with a lower mean Bhf due to iron with one or two chromium atoms occupying nearest neighbour lattice sites. With an increase in chromium concentration, the relative area of the second sextet grows at the expense of the first. There are obvious similarities between these published spectra and that described earher of iron milled in steel (figure 4.7). It is thus likely that the additional sextet shown in this figure is caused by the formation of an iron-chromium alloy.

To investigate this further, a set o f sanq)les were made containing iron doped with known quantities o f chromium. Milling was first carried out in stainless steel, so the resulting alloy contained both the added chromium plus that worn from the tools during processing. The experiments were then repeated using the (chromium free) Syalon containers, thus ensuring that the FeiCr ratio remained fixed at a known value. The spectra are shown in figure 4.8 and the data summarised in table 4.2. They were best fitted using three ferromagnetic components, one representing a-Fe in an undisturbed environment and the other two approximating a distribution resulting from iron with an increasing number o f chromium nearest and next nearest neighbours.

76 Ch a p t e r 4

I:

c/3

1

»

Ph 3.0 - Fe (s te e l) FCg-^Crg (S y a lo n ) FegyCrg ( s te e l) FeggCr^g (S y a lo n ) - 4 - 2 0 2 V e lo c ity (m m /s)

Figure 4.8 Mossbauer speclra of a scries of iron, and iron-clironiiuin samples, milled in slcel or Syalon containers. The solid lines are least-squares iits of the data, as discussed in the text.

Composition Area o f component 1 (a-Fe) % Area o f components 2+3 (Fe-Cr) % Fe (steel) 45.6 51.6 FegyCrg (Syalon) 36.7 60.4 Fe9?Cr3 (steel) 30.8 65.8 FeswCrio (Syalon) 18.1 80.4

Table 4.2 Relative areas of the Mossbauer components of the Fe-Cr alloys shown in figure 4.8. The components represent those atoms in (1) an undisturbed iron-like environment and (2+3) lattice sites

with at least one chromium nearest neighbour.

It can be seen that the relative area o f the additional Mossbauer components in the pure iron milled in steel is shghtly smaller than that in the FepyCra milled in Syalon. This is consistent with the formation, from the pine iron, o f an Fe-Cr sohd solution containing approximately 2.5 at.% chromium (or, in the case of FegjCrg in steel, an extra 2.5 at.% chromium), which is the composition that would be expected from the ED AX results. There is therefore conclusive evidence that even the relatively small amount o f

Sa m p l e Pr e p a r a t i o n ₇₇ Fe (S yalon) FOggSij FbqoS Ijo - 4 - 2 0 2

Velocity (mm/s)

Figure 4.9 Mossbauer spectra of a series of iron, and iron-silicon samples, milled in Syalon containers. The solid lines are least-squares fits of the data, as discussed in the text.

Composition Area o f component 1 (a-Fe) % Area of components 2+3 (Fe-Si) % Fe 78.0 22.0* Fe^Sh 68.0 32.0 FepsSis 56.8 43.1 FeçoSiio 15.0 85 0

Table 4.3 Relative areas of the Mossbauer components of the Fe-Si alloys shown in figure 4.9. The components represent those atoms in (1) an undisturbed iron-like environment and (2+3) lattice

sites with at least one sfiicon nearest neighbour. *Not necessarily fiom Fe-Si solid solution.

chromium worn fi*om the steel containers, when incorporated into iron during milling can produce a significant distortion in the resulting Mossbauer spectrum.

There is an assunqition in the above analysis that, although the spectra o f the steel milled samples were affected by chromium impurities, the Syalon milled samples were not similarly affected by SisN4, or indeed any o f the minor components of Syalon such

78 Ch a p t e r 4

is no alloying between Syalon and iron), a new set of samples were produced comprising iron doped with silicon. (If Syalon were to break up and alloy with iron, the most likely outcome would be the formation o f a disordered bcc Fe-Si sohd solution, given the miscibihty of iron and sihcon.) The resulting Mossbauer spectra are shown in figure 4.9. They are largely similar to those o f Fe-Cr, and like them were best fitted using three ferromagnetic components. It can be seen that there is significant distortion of the a-F e spectrum even with a sihcon impurity level o f I at.%. AdditionaUy, the second sextet due to those iron atoms with a sihcon nearest neighbour has a noticeable positive isomer shift with respect to a-Fe o f 0.06 mm/s, as is clear by con^aring the shapes o f lines one and six in the spectra.

In contrast, pure iron miUed in Syalon exhibits only a smaU second sextet in its Mossbauer spectrum, occupying 22.1 % o f the total spectral area and with no isomer shift relative to a-Fe. EDAX measurements indicate a sihcon impurity level of 7.8 at % in this sample, which if aUoyed with the iron would produce a much more dramatic change in the Mossbauer spectrum than that observed. It can therefore be said that Syalon, although worn fi*om the container surface in significant quantities during the course o f an experiment, is immiscible with the sample material and does not affect its structure. This immiscibihty is confirmed by x-ray scans of, for example, mihed FeizCo?» (figure 4.10) which show that the impurities are visible as a distinct phase. (The sample itself alloys to form a mixed phase Fe-Co sohd solution, as described in chapter 6.)

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Fe Co milled in Syalon

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