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.

Reversibility of the magnetocaloric effect in materials with first-order magnetostructural transformation is of vital significance for practical magnetic refrigeration applications. Here, we report a large reversible magnetocaloric effect in a Ni49.8Co1.2Mn33.5In15.5 magnetic shape memory alloy. A large reversible magnetic entropy change of 14.6 J/(kg K) and a broad operating temperature window of 18 K under 5 T were simultaneously achieved, correlated with the low thermal hysteresis (∼8 K) and large magnetic-field-induced shift of transformation temperatures (4.9 K/T) that lead to a narrow magnetic hysteresis (1.1 T) and small average magnetic hysteresis loss (48.4 J/kg under 5 T) as well. Furthermore, a large reversible effective refrigeration capacity (76.6 J/kg under 5 T) was obtained, as a result of the large reversible magnetic entropy change, broad operating temperature window, and small magnetic hysteresis loss. The large reversible magnetic entropy change and large reversible effective refrigeration capacity are important for improving the magnetocaloric performance, and the small magnetic hysteresis loss is beneficial to reducing energy dissipation during magnetic field cycle in potential applications.

Here we report a transition in superelastic hysteresis loop from sharp with plateau to smooth without plateau for a Ni55−xCoxFe18Ga27 (x = 0-12) alloy system with increasing Co substituting for Ni up to 10 at.%. With the Co content reaching 10 at.%, the alloy exhibits the obvious characteristic of the strain glass transition, i.e., the frequency-dependent shift in temperature of the internal friction peak and frequency-dependent dip temperature, Tg, of the storage modulus following the Vogel-Fulcher relationship. The high-energy X-ray diffraction provides the direct evidence that the smooth hysteresis loops stem from a finite avalanche martensite transformation mode, inducing a long-range inhomogeneous stress field in the remained parent phase during deformation.

No abstract