高温原位测试材料相变的新方法：高温法拉第磁秤

高温原位磁化率测试法利用顺磁性材料在相变时磁化率变化的原理,直接测试出材料相变时的转变温区,是研究顺磁性材料相变的一种新的尝试.我们采用高温法拉第磁秤测试了Ag2O和CuO在升温和降温中磁化率的变化曲线,通过磁化率的转变演示了Ag2O和CuO相变的动态过程.以此介绍了原位观测顺磁性材料晶体学相变的高温法拉第磁秤方法.高温法拉第磁秤在相变研究上的应用为顺磁性材料制备工艺提供了真实可靠的实验参数.

金属熔体中的液−液结构转变现象及其对凝固的影响

液态合金结构和性质的研究一直是凝聚态物理和金属材料科学领域的重要研究方向之一。液态合金并非理想的均质液体,在温度和压力等作用下会发生不连续转变。液−液结构转变广泛存在于金属和合金中,并且对最终凝固组织和性能有着很大的影响。系统阐释金属熔体中液−液结构转变现象,总结液−液结构转变对最终凝固组织和宏观性能的影响等方面的主要研究进展。结果表明,利用熔体液−液结构转变可以有效调控金属及合金的组织,改善其宏观性能。

强磁场下Cu−Co难混溶合金凝固过程中晶体取向和性能的调控

对Cu50Co50合金在不同磁场强度下进行非平衡凝固实验,研究富钴相的取向、显微组织演化和磁性能。结果表明,凝固后相分离的棒状富钴相可以实现排列形态,在5 T强磁场下富钴相的晶体取向为111平行磁场方向。然而,发生二次液−液相分离的富铜相的取向没有受到HMF的显著影响。在0 T磁场下凝固的合金表现出近磁各向同性行为,而在5 T磁场下凝固的合金则表现出磁各向异性行为。合金的饱和磁化强度略有增加,矫顽力相应降低,这与富钴晶体的取向演化行为密切相关。

Magnetic field intensity dependent microstructure evolution and recrystallization behavior in a Co-B eutectic alloy

Systematic understanding on the magnetic field intensity dependent microstructure evolution and re-crystallization behavior in a Co-B eutectic alloy under a constant undercooling(ΔT≈100 K)were carried out.Absent of the magnetic field,the comparable size of divorced FCC-Co and Co_(3)B eutectic ellipsoidal grains coexist with a few regular lamellas.When the magnetic field is less than 15 T,the elongated primary FCC-Co dendrites parallel to the magnetic field with the dispersed FCC-Co nano-particles em-bedded within the Co_(3)B matrix occupy the inter-dendrite regions.Once the magnetic field increases to 20 T,the FCC-Co/Co_(2)B anomalous eutectic colonies dominate.The formation mechanism of Co_(2)B phase is discussed from several aspects of the competitive nucleation,the chemical redistribution induced by the thermomagnetic-induced convection and magnetic dipole interaction,and the strain-induced trans-formation.Furthermore,the application of magnetic field is found to promote recrystallization,proved by the lower density of misorientation,the appearance of FCC-Co annealed twins and more Co_(3)B sub-grains.This work could further enrich our knowledge about the magnetic-dependent microstructure evolution and recrystallization process in the undercooled Co-B system and provide guidance for controlling the microstructures and properties under extreme conditions.

Liquid state dependent solidification of a Co-B eutectic alloy under a high magnetic field

The structure transition inside the Co-81.5at.%B alloy liquid has been studied by an in-situ magnetization measurement.A crossover was observed on the 1/M-T curve during the overheating process,indicating that a liquid-liquid structure transition(LLST)took place in the melt.Based on this information,the effects of LLST on the solidification behavior,microstructure and tribology property were investigated experimentally.The sample solidified with the LLST exhibits significantly different solidification behaviors,i.e.,the nucleation undercooling and the recalescence extent are conspicuously enlarged,and the solidification time is shortened.As a result,the microstructure is effectively refined and homogenized,and the hardness and wear resistance are significantly enhanced.The present work might be helpful for not only theoretically understanding the influence of LLST on the solidification behavior but also providing an alternative approach to tailor the microstructure and properties.

Tailoring mechanical and magnetic properties of AlCoCrFeNi high-entropy alloy via phase transformation

Phase constitutions,either changed by alloying or by phase transformation,are the key factors to determine the magnetic and mechanical performances of high-entropy alloys(HEAs).Using the AlCoCrFeNi HEA as a candidate alloy,this paper demonstrates the effect of phase transformation on both the mechanical and magnetic properties in the multi-phase system.With increasing heat treatment temperature,the sigma(σ)and face-centered-cubic(FCC)phases disappeared at 1000℃and 1200℃,respectively.Such volume fraction changes ofσ,FCC and body-centered-cubic(BCC)phases have divergent effects on mechanical and magnetic properties.The excellent strength-ductility combination will be achieved as the disappearance ofσphase and formation of FCC phase.As for the magnetic properties,the volume fraction of BCC phase plays a major role in determining its saturation magnetization.When the volume fraction change of BCC phase is not evident,the higher volume fraction of FCC phase will influence its magnetization at 2 T.Our present work might provide insights into analyzing the evolution of both mechanical and magnetic properties of HEAs caused by complex phase transformation.

Effects of an ultra-high magnetic field up to 25 T on the phase transformations of undercooled Co-B eutectic alloy

While there have been multiple recent reports in the literature focusing on the effects of magnetic field on the phase transformation behaviors,the research conducted with an ultra-high magnetic field greater than 20 T is still preliminary.In the current study,the structure evolution of Co-B alloys are experimentally studied with undercooling.The effects of a 25 T magnetic field on the solidification behavior and the subsequent solid-state phase transformation behavior have been investigated.The 25 T magnetic field is confirmed to have little effect on the homogeneous nucleation,but have some influence on the heterogeneous nucleation of Co_(3) B and Co_(23)B6 phases by modifying the wetting angleθ.The decomposition of Co_(23)B6 phase in the subsequent cooling process can be effectively suppressed by applying the 25 T magnetic field.The present work might be helpful for not only theoretically understanding the influence of ultra-high magnetic field on the phase transformation behaviors but a potential technology of field-manipulation of magnetic materials.