The Influence of Martensitic Intercalations in Magnetic Shape Memory NiCoMnAl Multilayered Films

Hysteresis and transformation behavior were studied in epitaxial NiCoMnAl magnetic shape memory alloy thin films with varying number martensitic intercalations (MIs) placed in between. MIs consists of a different NiCoMnAl composition with a martensitic transformation occurring at much higher temperature than the host composition. With increasing number of intercalations, we find a decrease in hysteresis width from 17 K to 10 K. For a large difference in the layers thicknesses this is accompanied by a larger amount of residual austenite. If the thicknesses become comparable, strain coupling between them dominates the transformation process, which manifests in a shift of the hysteresis to higher temperatures, splitting of the hysteresis in sub hysteresis and a decrease in residual austenite to almost 0%. A long-range ordering of martensite and austenite regions in the shape of a 3D checker board pattern is formed at almost equal thicknesses.     

Ultralong PtPd Alloyed Nanowires Anchored on Graphene for Efficient Methanol Oxidation Reaction

The rational synthesis of Pt-based alloyed nanowires still remains a great challenge because of the different reduction potentials between Pt and another metal and the intrinsic feature of isotropic growth in face-centered cubic (fcc) structured Pt. In this work, PtPd alloyed nanowires with ultrahigh aspect ratio anchored on graphene (PtPd NWs/graphene) were synthesized by a facile solvothermal method without the use of any templates or surfactants. Due to the integration of ultralong PtPd nanowires and stable graphene support, PtPd NWs/graphene exhibited outstanding electrochemical activity toward methanol oxidation reaction (MOR) in comparison with pure Pt NWs/graphene and commercial Pt/C catalysts. Meanwhile, PtPd NWs/graphene had a much higher current density than Pt NWs/graphene and commercial Pt/C catalysts at a constant potential for 7200s in alkaline methanol solution. Moreover, after 1000 cycles of durability testing, PtPd NWs/graphene retained 89.2% of its initial mass activity, much superior to the 63.7% retained for commercial Pt/C.     

Cu induced coercivity enhancement in the low viscosity GdCo5−xCux system

We investigate the influence of Cu substitution, on the coercivity and magnetic viscosity, in the ternary system GdCo_5−xCu_x (x=0, 0.5, 1, 1.5, 2 and 2.5) with different field sweep rates. All samples have been studied in the as cast state and crystallize in a single phase CaCu₅ structure. With Cu addition, the coercivity was 10 times enhanced for x =1.5. The behavior of the coercivity H_c against field sweep rate, dH/dt, shows that the GdCo_5−xCu_x system exhibits only a small magnetic viscosity effect, a desirable property for magnetic dynamic applications under high magnetic field.     

Effect of different alloyed layers on the high temperature oxidation behavior of newly developed Ti2AlNb-based alloys

The application of titanium aluminide orthorhombic alloys (O-phase alloys) as potential materials in aircraft and jet engines was limited by their poor oxidation resistance at high temperature. The Ti₂AlNb-based alloys were chromised (Cr), chromium-tungstened (Cr-W) and nickel-chromised (Ni-Cr) by the double glow plasma surface alloying process to improve their high temperature oxidation resistance. The discontinuous oxidative behavior of Cr, Cr-W and Ni-Cr alloyed layers on Ti₂AlNb-based alloy at 1093 K was explored in this study. After exposing at 1093 K, the TiO₂ layer was formed on the bare alloy and accompanied by the occurrence of crack, which promoted oxidation rate. The oxidation behavior of Ti₂AlNb-based alloys was improved by surface alloying due to the formation of protective Al₂O₃ scale or continuous and dense NiCr₂O₄ film. The Ni-Cr alloyed layer presented the best high-temperature oxidation resistance among three alloyed layers.     

A 3-D phenomenological constitutive model for shape memory alloys under multiaxial loadings

This paper presents a new phenomenological constitutive model for shape memory alloys, developed within the framework of irreversible thermodynamics and based on a scalar and a tensorial internal variable. In particular, the model uses a measure of the amount of stress-induced martensite as scalar internal variable and the preferred direction of variants as independent tensorial internal variable. Using this approach, it is possible to account for variant reorientation and for the effects of multiaxial non-proportional loadings in a more accurate form than previously done. In particular, we propose a model that has the property of completely decoupling the pure reorientation mechanism from the pure transformation mechanism. Numerical tests show the ability to reproduce main features of shape memory alloys in proportional loadings and also to improve prediction capabilities under non-proportional loadings, as proven by the comparison with several experimental results available in the literature.     

Statistical model for the formation of the alloy

The electronic structures of most semiconductor alloys are smooth functions of their composition. Binary alloys of group IV semiconductors are usually easy to prepare at any concentration, but this is not the case for the Ge_1-xSn_x alloy. Homogeneous alloys as required for nano- and optoelectronics device applications have proved difficult to form for x above a temperature-dependent critical concentration, above which Sn exhibits the tendency to segregate in the metallic cubic β phase, spoiling the semiconducting properties. The underlying mechanism for this segregation and critical concentration was not known.Through previous accurate ab initio local defect calculations we estimated the scale of energies involved in the immediate environment around a large number of Sn defects in Ge, the relaxed configurations of the defects, and the pressure directly related to the elastic field caused by the defects. This detailed information allowed us to build a simple statistical model including the defects most relevant at low x, namely substitutional α-Sn and non-substitutional β-Sn (in which a single atom occupies the centre of a Ge divacancy). Our model enables us to determine at which concentration β defects, which exhibit a tendency to segregate, can be formed in thermal equilibrium. These results coincide remarkably well with experimental findings, concerning the critical concentration above which the homogeneous alloys cannot be formed at room temperature. Our model also predicts the observed fact that at lower temperature the critical concentration increases.     

Al、Zn、Mn、Zr、Ca对Mg合金电子结构影响的第一性原理研究

应用密度泛函理论研究了合金元素Al、Zn、Mn、Zr、Ca对α-Mg合金电子结构的影响。对合金元素添加后的结构进行了优化。在稳定结构的基础上,通过对不同合金元素的形成能、态密度、布居分布、差分电荷密度的分析,认为引起合金性能变化的原因是各合金元素的电负性和原子半径的大小不同所致,对比了合金元素对材料电子结构的影响,从理论上解释了Zr、Ca强烈的合金强化、细化作用。