Investigation of shape memory characteristics and production of HfZrTiFeMnSi high entropy alloy by mechanical alloying method

High entropy alloy (HEA) with shape memory effect (SME) has been the subject of great interest for the past few decades. However, with the increased demands for new materials for high thermal applications, the research activities on the multi elemental high entropy shape memory alloys (HESMA) have been increased by many folds recently. The nano crystalline HEA powder with shape memory effect developed in this study, HfZrTiFeMnSi, was produced by mechanical alloying (MA) for the first time. In this method equiatomic ratio of Hf, Zr, Ti, Fe, Mn, and Si were mixed together and milled by MA process for 100 h. The powder formed was of amorphous in nature. Elemental mapping of the powder from SEM-EDS revealed homogeneity of the alloying elements confirming successful fabrication of HfZrTiFeMnSi HEA powder. The DSC studies from ambient to 500 °C of the annealed alloy powders showed reversible austenitic to martensitic (A↔M) transformations. The A↔M transformation hysteresis seemed to vary with the milling time and annealing temperature. The enthalpy values, ΔH, for the transformation were calculated from the DSC plots using tangent method for peak area calculation. Regardless of the annealing temperature, the thermal analysis revealed that the ΔH, equilibrium temperature (T₀), and crystallization temperature values decreased with the increasing milling time.     

Alloying Effect Study on Thermodynamic Stability of MgH2 by First-principles Calculation

First-principles calculations based on density functional theory were performed to study the effect of alloying on the thermodynamic stability of MgH₂ hydride (rutile and fluorite structures) with transitional metals (TM=Sc, Ti, Y) and group IIA elements (M=Ca, Sr, Ba). The results indicate that fluorite structure of these hydrides are more stable than its relative rutile structure at low alloying content (less 20%), structural destabilization of MgH₂ appears in the alloying cases of Ti, Sr and Ba respectively. The structure-transition point from rutile structure to fluorite structure is at around 20% for MgH₂-TM, and about 40% for MgH₂-M. The formation enthalpy of fluorite Mg_0.5Ba_0.52 is about 0.3 eV and higher than that of fluorite MgH₂, indicating that its hydrogen-desorption temperature at atmospheric pressure will be much lower than that of pure MgH₂. Good consistency between experimental and calculated data suggests that above-adopted method is useful to predict structural transition and properties of MgH₂ based hydrides for hydrogen storage.     

钾离子电池负极的高性能潜在材料:锑

近年来随着对储能器件的需求增加,钾离子电池受到越来越多的关注。钾的物理化学性质与锂相似,且在地壳中的储量丰富,在储能领域中具有广阔的发展前景。但由于电极材料实际容量远小于理论容量等问题的存在导致应用在钾离子电池的性能仍有不足。金属锑(Sb)具有较高的理论容量被广泛应用在电极材料上,然而在充放电过程体积变化过大导致稳定性较差,需要通过形貌控制、合金化、与碳材料复合等形式来提高结构稳定性。本文主要介绍了Sb材料在钾离子电池负极材料的研究进展,并展望了Sb电极材料的应用前景。     

Impact and Static Consolidation of Mechanically-Alloyed Mixture of Copper and Graphite Powders

A mixture of copper and carbon powders was mechanically alloyed in a planetary ball mill. The supersaturated solid solubility of carbon in mechanically-alloyed copper is 25 v at% C, as determined from the lattice parameter change. The supersaturated powder consolidated by static compression at 1.4 v GPa, has the relative density of 95%, a Vickers hardness of 121, and a compressive strength of 1.4 v GPa. In contrast, powder consolidated dynamically by a 90.1 g projectile traveling at a velocity of 38.1 m/s and at an estimated impact compression of 2.3 GPa after 0.4 GPa static precompression has a relative density of 96.3% and a Vickers hardness of 200. X-ray diffraction patterns shift to higher angles after impact compression. The supersaturated solid solubility of 18 at% C in Cu+20 at% C mechanically-alloyed powders was reduced to 12.5 at% C by impact compression. Mechanically-alloyed powders can be consolidated by impact and static pressures while retaining the effects of mechanical alloying; for example, supersaturated solid solubility.     

980 nm 垂直腔面发射激光器低欧姆接触电阻制备

研究了980 nm的垂直腔面发射激光器(VCSEL)欧姆接触技术.降低VCSEL的欧姆接触电阻,可有效地提高VCSEL的输出功率和延长其可靠性.P面采用高掺杂的P-GaAs/Ti/Pt/Au系统,N面采用N-GaAs/Ge/Au/Ni/Au系统,通过优化合金温度,得到了最佳优化合金温度为440 ℃,最低欧姆接触电阻值为0.04 Ω,同时对比了440 ℃和450 ℃器件的输出功率和转换效率之间的对比关系.测试结果表明,440 ℃器件的欧姆接触电阻0.04 Ω,峰值波长980.1 nm,光谱的半高宽0.8 nm,平行发散角θ_‖ 15.2°,垂直发散角θ_⊥ 13.5°,输出功率1.4 W,转换效率最大值为14.4%,而450℃的器件欧姆接触电阻为0.049 Ω,输出功率为1.3 W,转换效率为12.8%.通过优化合金温度能有效地降低980 nm的VCSEL欧姆接触电阻.     

Plasma transferred arc boriding of a low carbon steel: microstructure and wear properties

Borided coatings on AISI 1018 steel with different boron contents were produced using plasma transferred arc (PTA) melting. The thickness of the coatings ranged from 1 to 1.5 mm and their hardness from 400 to 1600 HV. Hypoeutectic or hypereutectic compositions consisting of primary ferrite or primary Fe₂B borides, respectively, and a eutectic constituent of -Fe+Fe₂B were obtained. The presence of FeB attested in coatings with the highest boron contents seems to be responsible for the intergranular cracks extending from the surface of the coatings to the substrate. Crack free coatings corresponding to the minimum quantity of eutectic and with a minor quantity of FeB were subjected to pin on disk wear testing and compared to the steel of the substrate. It was found that the wear rate of the borided coatings was about four orders of magnitude lower than the wear rate of the steel substrate. A transition from mild to severe wear was observed for the steel substrate material, but it was absent in the case of the borided coatings for the entire range of the applied loads examined. It is shown that the transition in the case of steel occurs when grooving and plastic deformation is replaced by intense cracking of the material above a critical load. In the case of the borided layer the dominant wear mechanism is delamination of the eutectic, however, the platelike borides are able to support the load and remain in the mild wear range for all the loads tested. Both borided and plain steel surfaces have the same friction coefficient after a short transition period, because both develop an oxide layer leading roughly to the same tribosystem with the alumina counterbody.     

The measurement of total mass attenuation coefficients of CoCuNi alloys

The total mass attenuation coefficients for Co, Cu, Ni elements and CoCu, CoCuNi alloys were measured at different energies with 11.88, 13.93, 17.59, 21.09 and emitted an point source using transmission arrangement. The gamma rays were counted by a Si(Li) detector with a resolution of at . Also the mass attenuation coefficients of each alloy (CoCu, CoCuNi) were estimated using mixture rule. The measured values were compared with estimated values for alloys.     

Pd-Fe/C催化剂中Fe的状态对氧还原电催化活性的影响

直接甲醇燃料电池;Pd-Fe/C催化剂;氧还原;合金化     

基于第一性原理的合金化掺杂MgH2的热力学稳定性研究

基于密度泛函的第一性原理,系统研究了合金化掺杂过渡金属（TM=Sc,Ti,Y）和IIA族元素（M=Ca,Sr,Ba）对MgH₂（金红石和萤石结构）的热力学稳定性的影响。结果表明,在低掺杂量（<20%） 时,MgH₂的萤石结构比金红石结构相对更稳定。掺杂Ti,Sr,Ba时,MgH₂的结构发生了失稳现象。MgH₂由金红石结构转变到萤石结构的掺杂TM和M的比例分别大约在20%和40%左右。Mg_0.5Ba_0.52萤石结构的形成焓比MgH₂萤石结构高约0.3 eV,表明其放氢温度在标准大气压下将远低于纯MgH₂。理论计算数据与实验数据有很好的一致性.     

Assessment of the correlation between mechanical testing and positron annihilation outcomes for RPV model alloys

The correlation between recent PAS results and the outcomes from mechanical testing of RPV model alloys are presented, here significant changes due to different chemical composition and different irradiation levels are observed. The influence of alloying elements to the microstructure degradation process following irradiation was identified by analysis of the mean-lifetime parameter, since an interesting interdependency of this parameter with hardness was observed.