Transformation behavior of nearly stoichiometric Ni-Mn alloys
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I.
INTRODUCTION
II.
EXPERIMENTAL PROCEDURE
NI-Mn alloys in the vicinity of the stoichiometric composition are reported to transform martensitically from a high temperature beta phase to a low temperature theta phase around 700 °C. 1 The structures of the beta and theta phases have been confirmed as the CsC1 type (B2) ordered b c c , 1'2 and the CuAuI type (L10), ordered fct, 1'2"3 structures, respectively. Krisevec et al. made electron microscopic observations on theta Ni-Mn obtained by slow cooling4 and by quenching 5 from the beta phase region and characterized the defect structure in slowly cooled specimens as {011} twinning but observed a different twinning plane, {111}, in quenched specimens. Using the graphical version of the phenomenological crystallography theory, Lieberman suggested the product with {111} twins to be truly martensitic, while that with {011} twins not to be a product of martensitic transformation, though very close to it. 6 The question arises as to what kind of mechanism produces different twinning planes, depending only upon the prior cooling rate, as observed. The reported structural change between B2 and Llo is the same as that seen in shape memory alloys such as Ni-A17'8 and Ni-Zn-Cu, 9 which exhibit a thermoelastic martensitic transformation. Krdn et al. ~reported a small hysteresis loop in the Ni-Mn electrical resistivity vs temperature curves, and Kr~isevec t° observed a morphology resembling that of the "spear-type" coupling of martensite variants characteristically found in thermoelastic alloys. 11 Thus at first sight the Ni-Mn alloy appears a favorable candidate for shape memory behavior with a possible application temperature extended to the high temperature range because of its high transformation temperature. However. the shape memory effect has never been reported in this system. This paper reports the results of an examination to clarify the general transformation behavior of Ni-Mn alloys in the vicinity of the stoichiometric composition, with primary emphasis on the effect of prior heat treatment and alloy composition. It will also be shown that the shape memory effect cannot be expected in this system because of a tempering reaction at intermediate temperatures.
Binary Ni-Mn alloys containing 45, 47, 49, 50, and 51 at. pct Ni were prepared by conventional vacuum arc melting using 99.99 pct electrolytic manganese and 99.99 pct Mond nickel. Ternary alloys including various amounts of A1, Cu, Ti, and B were also melted in attempts to control the transformation temperature and/or improve the alloy embrittlement. The as-melted ingots were homogenized at 950 °C in an evacuated quartz tube for at least 24 hours. For electrical resistivity measurements flat rectangular slabs of size about 1 x 2 x 40 mm were carefully cut from the homogenized rod-shaped ingots with a low-speed diamond saw, annealed in vacuum at 850 °C for 24 hours, and then furnace cooled to room temperature. Heating and cooling the specimens was carried out in air at a rate of about 10 °C per minute in a hori
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