The crystal strength is determined by a critical shear stress to actuate the glide of crystallographic line-defects called dislocations, which is in proportion to the shear elastic modulus, μ, i.e., approximately reaching 1/30 of μ, the Young's modulus, E, is in linear relationship with the shear modulus via a simple relation E = 2(1 ν)μ.
This physical metallurgical principle was broken by a recent discovery on a series of multicomponent body-centered-cubic (BCC) solid solutions based on Ti-Nb binary alloys termed as gum metals (GMs) that have extraordinary multifunctional properties such as ultralow elastic modulus, ultrahigh strength, and super-elasticity and excellent plasticity.
The "invalidity" of the second rule of the thermodynamics is observed in partial cycles of the transformation with invariant plane: at the interrupt of the reverse transformation at a temperature in the interval the released heat of the forward transformation measured in the same partial cycle is larger as the absorbed heat of the reverse one.
The stability of the two-phase system and connection between the observed energy effects and the dissipation of the elastic en-ergy is analyzed.
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Therefore, the underlying deformation mechanism responsible for the peculiar properties of this new class of metallic materials still remains elusive.
The martensitic microstructure of shape memory alloy (SMA) is essentially an aggregate of martensite plate groups.
It has been shown that the microstructure of Ni-Ti SMA consists of martensite plates clustered in a triangular morphology about the (100)[beta] pole and that of SMA's which transform to 9R, 3R and 2H martensites consist of diamond shaped 4-plate groups clustered about the (110)[beta] pole.
The primary shear, on a system with degenerate stress sense, periodically alternates its direction to minimize long-range stress fields, while the habit plane shear minimizes long-range stress fields by “self-accomodation” between variants.
The habit planes and shape strains, computed by this theory, are the same as those for the phenomenological theory; this is discussed.