Phase Transformations in Nearly Stoichiometric Nickel-Manganese Alloys
Adachi, Kenji
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https://hdl.handle.net/2142/71805
Description
Title
Phase Transformations in Nearly Stoichiometric Nickel-Manganese Alloys
Author(s)
Adachi, Kenji
Issue Date
1983
Department of Study
Metallurgy and Mining Engineering
Discipline
Metallurgical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Metallurgy
Abstract
The transformation behavior of Ni-Mn alloys in the vicinity of the equiatomic composition has been studied using transmission electron microscopy and diffraction, x-ray diffraction, optical microscopy, and electrical resistivity measurements. The transformation behavior was found to be markedly different comparing Mn-rich alloys and Ni-rich alloys. In Mn-rich alloys a martensitic transformation between the (beta) and (theta) phases takes place, while in Ni-rich alloys an order-disorder transformation between the (gamma) and (theta) phases takes place.
The martensitic transformation proceeds both on quenching and on slow cooling from the (beta) region. However the martensite crystallography is nearly destroyed on slow cooling through a "tempering" process which involves the shrinkage of internal twins and the nucleation and growth of low-density twinned crystals along the variant-variant interfaces and grain boundaries.
The martensitic structural change, from L2(,o) (B2) to L1(,o), is the same as that for Ni-Al shape memory alloys. The A-D (fork) type, A-C (spear) type, and A-B (wedge) type combinations of thin-plate martensite, characteristic of thermoelastic alloys, have been identified using transmission electron microscopy, and the crystallography of the (theta) martensite has been found to agree very well with the predictions of the phenomenological crystallographic theory.
However, the Ni-Mn alloys were found to be extremely brittle and showed no memory-type deformation. This has been attributed to weak grain boundaries and matrix inclusions due to manganese oxidation, but crystallographic factors limiting the memory-type deformation enter in as well, which include mixed variant combinations other than the basic three types, numerous lattice defects within the martensite plates, and the large twinning shear magnitude. Possibilities of various different combinations of variants were also examined using the crystallographic theory and a complementary plate grouping has been suggested to be feasible for the self-accommodation of certain martensite variants.
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