Mapping of Texture and Phase Fractions in Heterogeneous Stress States during Multiaxial Loading of Biomedical Superelastic NiTi

Abstract

Thermoelastic deformation mechanisms in polycrystalline biomedical‐grade superelastic NiTi are spatially mapped using in situ neutron diffraction during multiaxial loading and heating. The trigonal R‐phase is formed from the cubic phase during cooling to room temperature and subsequently deforms in compression, tension, and torsion. The resulting R‐phase variant microstructure from the variant reorientation and detwinning processes are equivalent for the corresponding strain in tension and compression, and the variant microstructure is reversible by isothermal loading. The R‐phase variant microstructure is consistent between uniaxial and torsional loading when the principal stress directions of the stress state are considered (for the crystallographic directions observed here). The variant microstructure evolution is tracked and the similarity in general behavior between uniaxial and torsional loading, in spite of the implicit heterogeneous stress state associated with torsional loading, pointed to the ability of the reversible thermoelastic transformation in NiTi to accommodate stress and strain mismatch with deformation. This ability of the R‐phase, despite its limited variants, to accommodate stress and strain and satisfy strain incompatibility in addition to the existing internal stresses has significance for reducing irrecoverable deformation mechanisms during loading and cycling through the phase transformation.

A Giant Magneto‐Superelasticity of 5% Enabled by Introducing Ordered Dislocations in Ni34Co8Cu8Mn36Ga14 Single Crystal

“…Recoverable elastic strain of 0.2% exists for most metals. [ 16 ] Large recoverable elastic strains of serval percent, as called superelasticity, have been observed in some metallic materials such as shape memory alloys, [ 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 ] high entropy alloys, [ 30 , 31 , 32 ] metallic glasses, [ 33 , 34 , 35 ] gum‐metal, [ 37 , 38 , 39 ] metallic nanocomposites, [ 40 , 41 ] etc. All these superelasticities are obtained under external stress, which can not be utilized for contactless work of materials in the devices.…”

A Unique Quenching and Deformation Dilatometer for Combined In Situ Neutron Diffraction Analysis of Engineering Materials

“…[17] Using this particular device, many studies related to phase transformations kinetic, texture evolution, and developing mechanical properties in alloys have been conducted successfully in recent years, highlighting the demands for this kind of in situ studies. [18][19][20][21] Compared to these thermomechanical devices, the dilatometer is used as a phase transition research equipment including loading and heating/quenching in one device with an exceptionally resolution of the sample length change. It offers highly stable and accurate temperature control and, as instrumentation of this type is also used in many material laboratories as standard equipment for kinetic studies of phase transitions, it is common to a wider user community.…”