Abstract
We used mechanical alloying with a Spex 8000 mixer/mill to synthesize a series of Fe100−xNix alloys from x=0 to x=49. The Spex mill was modified so that it could also operate at a reduced milling intensity, and we compared the alloys synthesized after long times with the normal and reduced milling intensities. X-ray diffractometry and Mössbauer spectrometry were used to measure the volume fractions of the bcc and fcc phases in the alloys, and to determine the chemical compositions of the individual phases. We found that the composition ranges of the bcc and fcc single phase regions were extended well beyond their equilibrium ranges. At the higher milling intensity, we found that the bcc phase was destabilized with respect to the fcc phase, and the two-phase region shifted to lower Ni concentrations. For those alloys with coexisting bcc and fcc phases, we present evidence that the chemical compositions of the two phases are nearly the same. We explain the destabilization of the bcc with milling intensity as originating with a higher defect density in the bcc alloys than in the fcc alloys. We argue that this defect density is not homogeneous throughout the alloy, however, and the distribution of defect enthalpies can explain the two- phase coexistence in the as-milled alloys.
Highlights
It is generally expected that the synthesis of new materials by ball milling depends on the thermochemistry of the alloy, plus the microstructural defects generated during ball milling
Our Spex mill was modified so that it could operate at a significantly reduced milling intensity, and we compared the materials milled for long milling times with the normal and reduced intensities
We found that the composition ranges of the bcc and fcc singlephase regions were greatly extended with respect to their equilibrium ranges
Summary
Ball milling has been used for many years for the synthesis of dispersion-strengthened superalloys for jet engine parts,[1] recently it has attracted much attention from the materials science community, in large part because of the discovery that ball milling can cause solid-state amorphization.[2,3,4,5,6,7,8,9,10,11,12,13,14,15,16] Nanocrystalline materials with grain sizes of typically 10 nm are another class of materials of recent interest than can be synthesized by ball milling.[13,14,15,16] Many types of phase transformations have been observed during ball milling, such as polymorphic transformations of compounds and disordering of ordered alloys.[17,18,19,20,21,22,23,24,25] It is wellestablished that high energy ball milling can be used to synthesize alloy phases with extended solid solubilities,[13,14,15,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40] for which the Hume–Rothery systematics for size and electronegativity may be relevant.[37]. Our interest was in understanding how milling intensity affected the region of two-phasebcc plus fcccoexistence that we found from about 15–34 at.% Ni. Model free-energy calculationswith Thermo-Calc softwarewere performed for Fe–Ni alloys, and the general preference of the alloys for fcc or bcc structures was predicted from the calculated polymorphic transformation compositionthe Ni composition at the intersection of the bcc and fcc free-energy curves, which was 28% at low temperature. Downloaded¬12¬Jan¬2006¬to¬131.215.240.9.¬Redistribution¬subject¬to¬AIP¬license¬or¬copyright,¬see¬http://jap.aip.org/jap/copyright.jsp we show that the region of two-phase coexistence can be predicted if ball milling creates heterogeneities of 2–3 kJ/ mole in the free energy of the bcc phase. We find that this defect enthalpy is consistent with the average heat release measured by differential scanning calorimetry. Our interpretation of the region of two-phase coexistence in ball-milled materials is essentially the same as that used for understanding Monte Carlo simulations of bcc alloys with thermal and ballistic atom movements.[58,59]
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