Whenever the forward and/or reverse B2 cubic to B19' monoclinic martensitic transformation in NiTi shape memory alloy wire proceeds under external stress above certain threshold, it generates incremental plastic strains which accumulate during thermomechanical cyclic loading and lead to functional fatigue preventing many promising engineering applications from realization. In this work, unique thermomechanical loading experiments were performed on NiTi shape memory wire with the aim to reveal the mechanism by which the forward martensitic transformation upon cooling under external stress generates plastic strain. Recoverable transformation strains and plastic strains generated by the forward transformation on cooling under various tensile stresses were evaluated, martensite variant microstructures in grains were reconstructed by nanoscale orientation mapping in TEM, martensite textures after cooling at room temperature were evaluated by in-situ synchrotron x-ray diffraction and permanent dislocation defects in martensite were analyzed by TEM. Based on the obtained experimental evidence, it is proposed that the forward martensitic transformation on cooling under stress proceeds via habit plane interfaces between austenite and second order laminate of (001) compound twins in martensite. Depending on the magnitude of the applied stress, the induced martensite reorients, partially detwins and deforms plastically via [100](001) dislocation slip in martensite in extent permitted by the requirement for compatible deformation of grains in nanocrystalline NiTi wire via a single deformation system. During the forward MT upon cooling under highest stresses 600 MPa, the martensite deforms via kwinking deformation enabling generation of large plastic strains 8 %.

Although superelastic NiTi shape memory alloy wire displays very high resistance to plastic deformation in austenite and martensite phases, incremental plastic strains are recorded whenever the cubic to monoclinic martensitic transformation (MT) proceeds under external stress leading to functional fatigue degradation. Therefore, special closed-loop thermomechanical loading tests were performed to shed light on the mechanism by which the incremental plastic strain are generated. These tests revealed that both forward and reverse MTs occurring above certain stress thresholds generate plastic strains specific for the [temperature, stress] conditions under which the MTs occurred. While the forward MT upon cooling does not produce plastic strain or permanent lattice defects up to 500 MPa stress, the reverse MT upon heating starts to generate them from 100 MPa. While plastic strain generated by the forward MT merely elongates the wire, plastic strain generated by the reverse MT also reduces the recoverable strain. Since the reverse MT upon heating generates plastic strains at lower external stresses than the forward MT upon cooling, it is largely responsible for cyclic instability of NiTi actuators. The characteristic thresholds and magnitudes of plastic strains generated by the forward and reverse MTs define the functional fatigue limits for specific NiTi wires.

Deformation mechanisms activated during tensile deformation of nanocrystalline NiTi wire in martensite state were investigated by combination of two experimental methods: (i) analysis of the evolution of martensite-variant microstructures in grains of deformed wire by TEM and (ii) analysis of the evolution of martensite texture by in situ synchrotron X-ray diffraction. The obtained results are linked to the activity of various twinning processes in martensite. It is concluded that martensite reorientation proceeds via motion of interdomain interfaces, gives rise to reoriented martensite with microstructure consisting of single (001) compound-twinned domain in each grain and results in sharp two-fiber texture of the martensite. The reorientation process leaves behind only very small unrecovered strains and very few dislocation defects in the austenitic microstructure of the deformed wire after unloading and heating. Plastic deformation of B19′ martensite proceeds via peculiar deformation mechanism which combines (100) deformation twinning with [100]/(011) dislocation slip based kinking. It gives rise to very special martensite variant microstructures consisting of deformation twin bands and kink bands containing martensite lattice aligned with [010] direction and characteristic two-fiber martensite texture. Reverse martensitic transformation of plastically deformed martensite upon unloading and heating leaves behind large unrecovered strains and high density of lattice defects in austenite. But there are also significant recoverable strains up to 10%. While the martensite matrix in grains of plastically deformed wire transforms into parent austenite matrix, (20-1) deformation twins transform into {114} austenite twins.

According to the state-of-the-art view, superelastic deformation of NiTi wires at room temperature proceeds via stress induced martensitic transformation from B2 cubic austenite to B19'monoclinic martensite. With increasing test temperature, the stress induced martensitic transformation is substituted by plastic deformation of austenite at martensite desist temperature M-D. However, there are many unsolved problems with this widely accepted view. What are the texture and martensite variant microstructure in stress induced martensite and do they depend on test temperature? Does the austenite transform to martensite completely within the transformation plateau range? How the superelasticity changes into plastic deformation of austenite with increasing temperature - is it stepwise or gradual change? How the wire deforms plastically at various temperatures? Does plastic deformation occur in austenite or in martensite, via dislocation slip or deformation twinning? Are the deformation/transformation processes in nanocrystalline NiTi wires same as in large grain polycrystals? We have addressed these long standing but unsolved questions by performing series of in-situ synchrotron x-ray diffraction experiments on superelastic nanocrystalline NiTi wire subjected to tensile tests at 20, 90 and 150 degrees C until fracture supplemented by post mortem TEM analysis of lattice defects created by the tensile deformation. It was found that, in case of conventional superelasticity at 20 degrees C, austenite transformed almost completely to stress induced martensite within the transformation plateau range. The stress induced martensite displayed a sharp two fibre texture reflecting its (001) compound twinned microstructure. Stress induced martensite transformed back to the parent austenite without leaving any significant unrecovered strains and lattice defects in the austenitic microstructure. When loaded further into the plastic deformation range, this martensite deformed via combination of (20-1) and (100) deformation twinning and kinking assisted by [1001(001) dislocation slip in martensite. Recoverability of tensile strains on unloading and heating remained surprisingly large (similar to 10%) up to wire fracture at 62% strain. The superelasticity at elevated temperatures 90 degrees C (150 degrees C) was found to be very different. The austenite transformed into a mixture of phases containing 40% (10%) volume fraction of stress induced martensite within the transformation plateau range. The stress induced martensite displayed four fibre texture, which further evolved with increasing strain. The original < 111 > fibre texture of austenite evolved with increasing strain towards random orientation distribution. The recoverability of tensile strains on unloading and heating sharply decreased with increasing temperature. Two alternative deformation mechanisms are proposed to explain these changes. The first mechanism assumes that martensite stress induced at elevated temperatures appears in a form of thin internally twinned CVP martensite plates surrounded by austenite deforming via dislocation slip. Requirement for strain compatibility at habit plane interfaces affects selection of martensite variants under stress and texture. The second mechanism is based on the idea that martensite stress induced at elevated temperatures immediately deforms plastically and undergoes reverse martensitic transformation to austenite leaving behind unrecovered plastic strain, slip dislocations and {114} austenite twins in the austenitic microstructure of the wire. Since the second mechanism explains the experimental observations better, it is considered to be more realistic.

High-resolution in situ synchrotron X-ray diffraction was applied to study a cold-drawn and solution-treated 56Ni–44Ti wt% alloy subjected to uniaxial cyclic loading–unloading with incremental strains. The micro-mechanical behaviour associated with the partial and repeated B2↔B19′ phase transformation at the centre of the sample gauge length was studied with respect to the macroscopic stress–strain response. The lattice strains of the (110)B2 and different B19′ grain families are affected by (i) the transformation strain, the load-bearing capacity of both phases and the strain continuity maintained at/near the B2–B19′ interfaces at the centre of the gauge length, and (ii) the extent of transformation along the gauge length. With cycling and incremental strains (i) the elastic lattice strain and plastic strain in the remnant (110)B2 grain family gradually saturate at early cycles, whereas the plastic strain in the B19′ phase continues to increase. This contributes to accumulation of residual strains (degradation in superelasticity), greater non-linearity and change in the shape of the macroscopic stress–strain curve from plateau type to curvilinear elastic. (ii) The initial 〈111〉B2 fibre texture transforms to [120]B19′, [130]B19′, [150]B19′ and [010]B19′ orientations. Further increase in the applied strain with cycling results in the development of [130]B19′, [102]B19′, [102]B19′, [100]B19′ and [100]B19′ orientations.

The present digital image correlation study characterised the local axial and shear strain fields of a 56Ni-44Ti wt.% shape memory alloy with an average grain size of 100 mu m, under uniaxial monotonic and cyclic loading-unloading in tension. To elucidate the grain size effect, the results were compared with a previous investigation of the same alloy with an average grain size of 10 mu m. The maximum local axial strain rate signified the direction and extent of the localised transformation. The widened single inclined transformation band and multiple criss-crossing patterns assist in straightening the sample edge by releasing an in-plane moment instigated by local shear strains. Electron back-scattering diffraction analyses showed that the plastic strain within the B2 grains and the remnant B19' variants account for the residual strains after unloading. Smaller grain sizes correspond to greater constraint from grain boundaries, higher interfacial energy and higher elastic strain energy barrier for transformation, and smaller intragranular heterogeneity of plastic deformation. This is reflected in the increases to the transformation start stress, stress level and stress-strain slope within the macroscopic stress plateau region and smaller complete transformation strain, super-elastic and residual strains upon unloading. (C) 2018 Elsevier B.V. All rights reserved.

A cold-drawn and annealed 56Ni-44Ti wt% alloy was subjected to in-situ uniaxial monotonic tension in a synchrotron. Spatially resolved diffraction data was acquired along the gauge length by pausing the loading at five select macroscopic strains within the stress plateau region. This enables tracking localised transformation phenomena by sub-dividing the gauge length into transformation band, untransformed and apparently transformed regions. Within the macroscopic stress plateau region: (i) the highly strained B2 phase within the propagating transformation band and apparently transformed regions produces a relaxation of the B2 phase within the untransformed region. (ii) The newly formed B19' grain families exhibit a transition in relative lattice strain values from the transformation band through to the apparently transformed region. (iii) The < 111 >(B2) fibre texture transforms to the [(1) over bar 20](B19'), [(1) over bar 30](B19') and [010](B19') such that the latter fibres continue to record increases in maximum intensity up to maximum load. Within the slowly rising macroscopic stress region and beyond a critical stress value of similar to 426 MPa: (i) the relative lattice strains of the (1 (2) over bar0)(B19') and (020)(B19'), grain families deviate from linearity along the axial and transverse directions, respectively and, (ii) the anisotropy in crystallite size and micro-strains in all B19' grain families reduces markedly.

This digital image correlation study details the mechanical behaviour and pattern evolution of the transformation band of a 56Ni-44Ti wt% shape memory alloy subjected to monotonic uniaxial and loading-unloading cycles in tension. The broadened single inclined band front and multiple criss-crossing patterns are found to relieve the in-plane moment caused by local shear strains and straighten the sample edges during testing. The magnitude of the maximum local strain rate suggests its feasibility to understand the direction and extent of the localised transformation. Specifically, the changes to the maximum local strain rate during monotonic uniaxial tension are generally analogous to the stages in the macroscopic stress-strain curve. The microstructure before and after mechanical testing was characterised via electron back-scattering diffraction. Estimates of the kernel average misorientation show that residual strains upon unloading are linked to high intragranular misorientation within the original B2 grains and the remnant B19′ variants.

Small quantities of copper were added to non-oriented electrical steels to produce a metastable Cu-rich phase with a different structure from a twinned 9R or 3R and an average size on the order of 20 nm. These precipitates were observed in hot-rolled bands coiled at 650 degrees C for between 1 and 3 h. The coiling time and copper content have little effect on the average size of the precipitates, but the amount of the precipitates increases significantly with the Cu content, and the particles appear to coalesce when the copper level rises to 0.35%. The influence of the precipitates on the recrystallization textures and magnetic properties of non-oriented electrical steels was investigated, and it can be concluded that the Cu-rich precipitates can improve the recrystallization textures of soft magnetic materials by inhibiting the {111} texture component and promoting the Goss component. However, this beneficial effect is weakened when the Cu-rich particles coalesce or the coiling time is extended. Furthermore, the Cu-rich precipitates can significantly reduce the core loss of non-oriented electrical steels by improving the recrystallization textures without obviously decreasing the magnetic induction. (C) 2013 Elsevier B. V. All rights reserved.