W.H. Wang

Martensitic transformation and shape memory effect in ferromagnetic Heusler alloy Ni2FeGa

We have synthesized ferromagnetic Heusler alloy Ni2FeGa using the melt-spinning technique. The Ni2FeGa ribbon, having a high chemical ordering L21 structure, exhibits a thermoelastic martensitic transformation from cubic to orthorhombic structure at 142 K and a premartensitic transformation. The alloy has a relatively high Curie temperature of 430 K, a magnetization of 73 Am2/kg, and a low saturated field of 0.6 T. The textured samples with preferentially oriented grains show a completely recoverable two-way shape memory effect with a strain of 0.3% upon the thermoelastic martensitic transformation.

Vacancy-tuned paramagnetic/ferromagnetic martensitic transformation in Mn-poor Mn1-xCoGe alloys

It is shown that a temperature window between the Curie temperatures of martensite and austenite phases around the room temperature can be obtained by a vacancy-tuning strategy in Mn-poor Mn1-xCoGe alloys (0≤x≤0.050). Based on this, a martensitic transformation from paramagnetic austenite to ferromagnetic martensite with a large magnetization difference can be realized in this window. This gives rise to a magnetic-field–induced martensitic transformation and a giant magnetocaloric effect in the Mn1−xCoGe system. The decrease of the transformation temperature and of the thermal hysteresis of the transformation, as well as the stable Curie temperatures of martensite and austenite, are discussed on the basis of the Mn-poor Co-vacancy structure and the corresponding valence-electron concentration.

Magnetic properties and martensitic transformation in quaternary heusler alloy of NiMnFeGa

Quaternary Heusler alloy Ni2(Mn,Fe)Ga has been studied systematically for the structure, martensitic transformation, and magnetic properties in two systems of Ni50.5Mn25−xFexGa24.5 and Ni50.4Mn28−xFexGa21.6. Substituting Fe for Mn up to about 70%, the pure L21 phase and the thermoelastic martensitic transformation still can be observed in these quaternary systems. Iron doping dropped the martensitic transformation temperature from 220 to 140 K, increased the Curie temperature from 351 to 429 K, and broadened the thermal hysteresis from about 7 to 18 K. Magnetic analysis revealed that Fe atoms contribute to the net magnetization of the material with a moment lower than that of Mn. The temperature dependence of magnetic-field-induced strains has been improved by this doping method.

Stress-free two-way thermoelastic shape memory and field-enhanced strain in Ni52Mn24Ga24 single crystals

Stress-free and two-way thermoelastic shape memory, with 1.2% strain and 6 K temperature hysteresis, has been found in single crystalline Ni52Mn24Ga24. The deformation can be enhanced more than three times, up to 4.0% shrinkage with a bias field 1.2 T applied along the measurement direction, or changed to 1.5% expansion by the 1.2 T applied perpendicular to the measurement direction. For achieving a large deformation, the magnetic field exhibits a more evident contribution than an external stress on this material. These characteristics can be attributed to the low level of internal stress and the preferential orientation of the martensitic variants.

Intermartensitic transformation and magnetic-field-induced strain in Ni52Mn24.5Ga23.5 single crystals

We have found a complete thermoelastic intermartensitic transformation between modulated and unmodulated martensite in single-crystal Ni52Mn24.5Ga23.5. This intermartensitic transformation provides a much larger strain than that of the martensitic transformation. A giant switching-like strain of ±5.0% can be achieved by a small magnetic field of 0.2 T upon the intermartensitic transformation. In the modulated martensite, a large recoverable magnetic-field-induced strain of up to 1.2% has been obtained.

Hidden order in serrated flow of metallic glasses

We report results of statistical and dynamic analysis of the serrated stress-time curves obtained from compressive constant strain-rate tests on two metallic glass samples with different ductility levels in an effort to extract hidden information in the seemingly irregular serrations. Two distinct types of dynamics are detected in these two alloy samples. The stress-strain curve corresponding to the less ductile Zr65Cu15Ni10Al10 alloy is shown to exhibit a finite correlation dimension and a positive Lyapunov exponent, suggesting that the underlying dynamics is chaotic. In contrast, for the more ductile Cu47.5Zr47.5Al5 alloy, the distributions of stress drop magnitudes and their time durations obey a power-law scaling reminiscent of a self-organized critical state. The exponents also satisfy the scaling relation compatible with self-organized criticality. Possible physical mechanisms contributing to the two distinct dynamic regimes are discussed by drawing on the analogy with the serrated yielding of crystalline samples. The analysis, together with some physical reasoning, suggests that plasticity in the less ductile sample can be attributed to stick-slip of a single shear band, while that of the more ductile sample could be attributed to the simultaneous nucleation of a large number of shear bands and their mutual interactions

CaLi-based bulk metallic glasses with multiple superior properties

We describe a class of metallic glasses that consists of low-cost components and exhibiting multiple superior properties such as exceptionally low glass transition temperature (∼35–60 °C), ultralow elastic moduli (∼23 GPa) comparable to that of human bones, high elasticity and strength, ultralow density (<2 g/cm3), exceptional thermodynamic and kinetic stability in supercooled liquid state, strong liquid fragility, ultrahigh specific strength and lower electrical resistivity, and polymerlike thermoplastic formability near room temperature. Such metallic glasses could have potential applications and facilitate studies of the nature of glasses.

Soft ytterbium-based bulk metallic glasses with strong liquid characteristic by design

A family of Yb-based bulk metallic glasses with excellent glass-forming ability has been fabricated based on the elastic moduli correlations. The YbZnMg(Cu) glasses exhibit very strong liquid characteristic in fragility (m=26±5), while soft mechanical characteristics, such as low bulk elastic modulus (e.g., Young’s modulus is about 26.5 GPa), small Poisson’s ratio (0.276), low Vickers hardness (1.52 GPa) and Debye temperature, and exceptionally low glass transition temperature (Tg∼347 K). The soft bulk metallic glasses with exceptional values of Tg, fragility, Debye temperature, and elastic moduli confirm some found correlations in metallic glasses.

Correlations between elastic moduli and molar volume in metallic glasses

We report clear correlations between bulk modulus (K) and average molar volume Vm, and between Poisson’s ratio ν and Vm for various bulk metallic glasses. The origin for the correlations between elastic moduli and Vm are discussed. The established correlation, associated with Poisson’s ratio ν, and since the ν correlates with plasticity of metallic glasses, indicates that the average molar volume is important factor to be considered for plastic metallic glasses searching. The found correlations also suggest a close relation between the mechanical properties and the short-range atomic bonding, and assist in understanding deformation behavior in metallic glasses.

Structure and magneto-history behavior of DyNi2Mn

The compound DyNi2Mn with a MgCu2-type structure was synthesized, even though the atom ratio of Dy to transition metal was confirmed to be 1:3, by using the energy dispersive spectrum method. The X-ray diffraction pattern (XRD) could be well indexed with a cubic Laves cell with the lattice parameter a = 7.1406(2) Angstrom and space group Fd3m. The refinement result of the XRD pattern indicates that 73.8% of the 8a sites (Dy sites) are occupied by Dy atoms, 21.4% by Mn atoms, and 4.8% are empty. The compound shows ferrimagnetic coupling of the Dy and Mn moments with an ordering temperature T-C of 94 K. The spontaneous magnetization at 5 K is 6.7mu(B)/f.u. A large coercive field of 0.73 T at 5 K is found for the bulk sample, which can be understood in terms of narrow-domain-wall pinning