软磁Mn-Zn铁氧体纳米晶的低温自蔓延合成及表征

采用低温自蔓延方法合成了Mn-Zn铁氧体纳米晶,EDS测试结果表明该纳米晶的化学式为(MnZn)_(0.5)Fe_2O_4。由ESR谱图求得其g值为9.9560,远大于自由电子的g值(g_e=2.0023),这主要由Mn~(2+),Fe~(3+)的3d~5半充满价电子层结构和它们在尖晶石型晶胞(MnZn)_(0.5)Fe_2O_4中的磁矩偶合作用所致;XRD测试结果显示该Mn-Zn铁氧体纳米晶结晶良好。TEM测试结果表明该Mn-Zn铁氧体纳米晶粒径较均匀,晶粒粒径为10～20 nm,与由Scherrer公式算得的晶粒尺寸16.9 nm较吻合。     

Formation of local melting regions in a solid body near the melting temperature

The formation of local melting regions is shown to be ensured by the fluctuation of the vibrational (kinetic) energy in a crystal. This approach is based on the Frenkel’ idea about heterophase fluctuations. Equations that relate the additional volume (ΔV(T)), electrical resistivity (Δρ(T)), or enthalpy (ΔH(T)) increment in solids to the presence of local melting regions are obtained. An analysis of the experimental data demonstrates that 6% of the solid body atoms have the properties of a liquid at the melting temperature. This approach is used to explain some unexpected facts and to calculate the energy of formation (E _f) and concentration (C _m) of vacancies in some metals.     

Behavior of steels near the incipient melting temperature

A new method of incipient melting temperature (IMT) detection has been developed in which a constant-strain-rate tensile deformation is applied to a specimen whose temperature is simultaneously increasing. The IMT is determined in a single test, and any phase transformations before the IMT will also be detected by the effects on the stress vs strain behavior in the same experiment. By means of such tests, the incipient melting behavior of a series of steels with carbon levels from 0.031 to 0.45 wt pct was examined. For the steels containing 0.08 to 0.097 pct C and about 1.5 pct Mn, it was found that incipient melting occurs in the two-phase (γ + δ) region in the temperature range from 1470 °C to 1480 °C and is significantly influenced by microalloying elements. In the ultralow-carbon steel (0.031 pct C), the IMT is in the single-phase δ region, and for the medium-carbon steel containing 0.42 pct C (hyperperitectic) it is in the γ single phase.     

金属粉末特性对选区激光熔化工艺及其制件性能影响

高品质金属粉末是选区激光熔化（selective laser melting,SLM）制备高性能制件的重要基础。粉体特性对选区激光熔化技术的影响及其机理研究是理解选区激光熔化技术不可或缺的重要组成部分。本文从粉末物理和化学特性出发,论述了粉末特性对选区激光熔化工艺、制件微观组织与性能的影响。结果表明,粉末的物理特性,尤其是粉末形貌和粉末粒度分布能显著影响其流动性和粉末床堆密度等关键工艺特性;而粉末的化学成分,特别是杂质成分,是影响制件相组成和微观组织的重要因素。在此基础上,本文进一步介绍了选区激光熔化过程中高能量源与粉末颗粒的冶金作用机理研究进展。