Superelasticity by reversible variants reorientation in a Ni–Mn–Ga microwire with bamboo grains

Abstract

a b s t r a c tThe link between microstructure and mechanical properties is investigated for a superelastic Ni-Mn-Ga microwire with 226 lm diameter, created by solidification via the Taylor method. The wire, which consists of bamboo grains with tetragonal martensite matrix and coarse c precipitates, exhibits fully reversible superelastic behavior up to 4% tensile strain. Upon multiple tensile load-unload cycles, reproducible stress fluctuations of $3 MPa are measured on the loading superelastic stress plateau of $50 MPa. During cycles at various temperatures spanning À70 to 55°C, the plateau stress decreases from 58 to 48 MPa near linearly with increasing temperature. Based on in situ synchrotron X-ray diffraction measurements, we conclude that this superelastic behavior is due to reversible martensite variants reorientation (i.e., reversible twinning) with lattice rotation of $13°. The reproducible stress plateau fluctuations are assigned to reversible twinning at well-defined locations along the wire. The strain recovery during unloading is attributed to reverse twinning, driven by the internal stress generated on loading between the elastic c precipitates and the twinning martensite matrix. The temperature dependence of the twining stress on loading is related to the change in tetragonality of the martensite, as measured by X-ray diffraction.

Strain hardening in Fe–16Mn–10Al–0.86C–5Ni high specific strength steel

“…Recently, the use of diffraction techniques has supplanted this macroscopic description of internal stresses by the measurements of lattice strains in individual phases [45,46]. The unload-reload tests [36,38,47] are also used for the study of internal stresses in thin films or composite wires where compression cannot be applied.…”

Microstructure and phase transformation of Ni-Mn-Ga wires produced by the in rotating water melt-spinning technique

“…This bamboo grains structure resembles single grains spanning the whole wire cross-section. Although similar bamboo grains structure had been previously reported in other rapidly solidified Ni 55 Mn 20.6 Ga 24 alloys ( crucible melt extraction [9,18], custom-built Taylor machine [12,14] ), there is little report about the transformation from dendrictic-like structure to bamboo grains structure by annealing for Ni 55 Mn 20.6 Ga 24 wires. The special bamboo grains structure has two advantages.…”

“…Moreover, bulk alloy material exhibits large current loss at high frequency [5]. To solve these problems, small size Ni-Mn-Ga alloys(e.g., wires, ribbons, films) and "consructs" from these structural elements (e.g., mats, laminates, textiles, foams and composites) are widely studied by various researchers [6][7][8][9][10][11][12][13][14][15]. These materials have presented superior properties by matching grain and sample sizes [15].…”

“…Ni-Mn-Ga wires or ribbons were usually fabricated by the Taylor-Ulitovsky method and melt spinning(and the related melt extraction method). The Taylor-Ulitovsky method often led to the trouble of removing glass sheath while the film-like shape wires fabricated by most of melt spun wires usually limit the application of Ni-Mn-Ga alloys, especially about Ni-Mn-Ga/polymer composites [6][7][8][9][10][11][12][13][14][15]. In rotating water melt-spinning (INROWASP) technique can achieve wires thin round shape [16].…”

Superlattice in austenitic Ni-Mn-Ga shape memory microwires

“…Meanwhile, large MFIS is closely related to the existence of a long period modulated structure. Here, in order to overcome the intrinsic brittleness of Ni-Mn-Ga SMAs, the Taylor-Ulitovsky method was used to fabricate oligocrystalline structured microwires [35][36][37]. The rapid solidification process that caused by quenching the melt alloys may lead to a rich variety of microstructures and properties, which has already been observed before [38,39]…”