Single-crystal elastic constants of Fe-15Ni-15Cr alloy
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I. INTRODUCTION
DETERMINATIONS of the absolute values of elastic constants have proven to be extremely valuable in increasing our understanding of a variety of materials’ properties including defects,[1] force-constant-related properties such as lattice vibrations,[1] properties of dislocations and their interactions,[2] interatomic potentials,[3] and thermodynamic properties such as the characteristic Debye temperature and the Grüneisen parameter.[4,5,6] Moreover, investigations of the relative temperature dependences of these elastic constants exhibit unambiguous discontinuities in the presence of phase transitions.[7] In the present study, we have carried out investigations of the elastic constants of Fe-Ni-Cr alloys (austenitic stainless steels) that lie in a composition range that provides the basis for many widely used commercial products. Unfortunately, the elastic constants of pure face-centeredcubic (fcc) iron cannot be measured directly since pure iron does not exist in the fcc phase in equilibrium at ambient temperatures. Nickel has been used to stabilize Fe alloys in the fcc phase with small effect arising from Cr, and this alloying has permitted indirect measurements of the elastic constants of “iron” to be made at ambient temperatures.[8] Extensive studies have been carried out in the design and processing of alloys in the Fe-Ni-Cr family to meet the increasing demands for nickel-based alloys to be used in aircraft gas turbine engines. These investigations are moti-
vated by the excellent high-temperature mechanical properties exhibited by alloys in this class of materials.[9–12] Detailed study has also been carried out on several aspects of solidification and welding phenomena in these crystals to understand the underlying principles that determine the microstructural development of these important processes.[13] In addition, having data on the complete set of elastic constants of an Fe-Ni-Cr alloy is essential to a number of extended applications and investigations, e.g., topics such as dislocation glide,[14] phase instability,[15] and anisotropic sound-wave propagation.[16] In the present investigations, we are primarily concerned with the low-temperature properties of Fe-Ni-Cr alloys and, in particular, in the useful temperature range of these materials. A number of Fe-Ni-Cr alloys undergo paramagneticantiferromagnetic phase transitions at relatively low temperatures, reflecting the antiferromagnetic properties of the ground state of fcc Fe.[17] However, unlike some Fe-Cr-Ni alloys, Fe-15Ni-15Cr undergoes an fcc to bcc structural phase transition near 190 K that results in a low-temperature ferromagnetic phase. Here, we report the results of RUS and PE measurements of the low-temperature dependence of elastic constants of the ternary alloy Fe-15Ni-15Cr. Discontinuities were observed in the temperature dependence of the elastic moduli for these alloys around 190 K corresponding to the structural phase transition of the alloy from fcc to bcc.
II. SAMPLE PREPARATION AND EXPERIMENT A. TEKLU, formerl
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