蒸发冷凝法制备超细镁粉

采用电阻加热蒸发和感应加热蒸发法分别制备出超细镁粉,系统研究了充气压力对粉体产率和粒度的影响,采用XRD、TEM等分析手段对所制备的粉体结构进行表征.结果显示:感应加热蒸发法的产率远大于电阻加热蒸发法的产率;随充气压力提高,粉体产率降低,粉体平均粒径增大.所制备的超细镁粉为链球形结构、多晶态,纯度较高,粒度分布范围窄.     

真空蒸发-冷凝法制备片状CU粉

采用蒸发-冷凝法在真空条件下制备了片状Cu粉,并用激光粒度分析仪(SL)、扫描电镜(SEM)、辉光放电质谱仪(GDMS)和X射线衍射仪(XRD)对片状Cu粉进行表征.结果表明,制备的Cu粉为面心立方结构,表面平整,分散性好,粒径为10～30μm,厚1～2μm.试验考察了蒸发温度、真空度、保温时间和冷凝高度4个因素对片状...     

Observations of the formation and kinetics of surface steps during evaporation and condensation

Surface step patterns produced by crystal growth or evaporation can be observed, e.g., on NaCl, AgBr, Ag, and Si, by means of the electron microscopic method of decoration. These observations give insight into the mechanisms (random two-dimensional (2-D) nucleation, formation of hills or pits by spirals or repeated preferential 2-D nucleation, kinematic step interaction, orientation dependence of step motion, light-influenced evaporation, and stage of coalescence of thin films) and molecular processes (surface and edge diffusion) of crystal and thin film growth. Combining the decoration and platinum-carbon replica techniques enables an interesting insight into the step kinetics during the process of faceting. The pinning of moving steps at impurities and their piling up are decisive particulars. This paper is based on a presentation made in the symposium “The Role of Ledges in Phase Transformations” presented as part of the 1989 Fall Meeting of TMS-MSD, October 1–5, 1989, in Indianapolis, IN, under the auspices of the Phase Transformations Committee of the Materials Science Division, ASM INTERNATIONAL.     

Thermodynamics of strontium molybdate sublimation

High-temperature mass-spectrometric analysis is used to study the sublimation of SrMoO_4(s) at 1570–1800 K. SrMoO_4(g) molecules are found to be present in the vapor. The vapor pressure of the SrMoO_4(g) molecule is determined to be log p = ?19980/T + 7.99. The heat of sublimation ΔH _{s, 0} ^o (SrMoO_4(s)) = 480 ± 40 kJ/mol is determined using the third law of thermodynamics. The atomization energy of the SrMoO_4(g) molecules is calculated to be ΔH _{at, 0} ^o (SrMoO_4(g)) = 2860 ± 40 kJ/mol.     

非平衡态热力学分析

为了研究非平衡态热力学理论在具体的冶金过程中的应用,本文用该理论模拟计算了转炉的炼钢过程,并建立了该过程的数学模型。得到了转炉冶炼过程的脱碳速度表达式、钢液中硅锰含量变化表达式、氧含量变化表达式以及温度变化表达式。用C语言对整个冶炼过程进行模拟,结果表明非平衡态热力学模型能够描述整个冶炼过程主要元素的成分变化和熔池温度变化,对转炉炼钢生产实际的过程控制提供了新的理论和方法。     

Nonequilibrium synthesis of nbai3 and Nb-Ai-V

In this study, the isothermal oxidation behavior of laser-clad NbAl₃ has been investigated in the temperature range between 800 °C and 1400 °C in air. The effect on oxidation of vanadium microalloying, used to increase the ductility of the otherwise brittle NbAl₃ and discussed in Part I, [1] has also been considered. Bulk and surface oxide chemistry has been investigated using X-ray diffraction and X-ray photoelectron spectroscopy (XPS), respectively. Oxidation kinetics have been determined from weight gain data. The XPS and X-ray diffraction data show that NbAl₃ does not exclusively form a protective A1₂O₃ layer when oxidized in air. The oxidation products at 800 °C are a mixture of Nb₂O₅ and A1₂O₃, while at 1200 °C, a mixture of NbAlO₄, Nb₂O₅, and Al₂O₃ is formed. At 1400 °C, a mixture of NbAlO₄, A1₂O₃, NbO₂, NbO_2.432, and Nb₂O₅ forms. Upon addition of vanadium, the oxidation rate is found to dramatically increase and may be related to the formation of (Nb, V)₂O₅ and VO₂, which grows in favor of protective A1₂O₃. Consequently, although vanadium may be a good additive in terms of its potential for improving ductility in NbAl₃, it is not in terms of its deleterious effects on oxidation.     

Nonequilibrium synthesis of NbAl3 and Nb-Al-V

NbAl₃, like many ordered intermetallic compounds, appears to have great potential for application as high-temperature structural material because of the high melting point and stability in high- temperature oxidizing environments. However, the D0₂₂ (tI8) lattice structure shows limited ductility at room temperature. Addition of group IVB, VB, and VIB elements to A1₃X (D0₂₂ type of compounds) as partial replacement of X is a method being applied by various researchers to promote ductility in these materials by activation of numerous dislocations and ordered twin- ning. In this study, V was added in different amounts to improve the ductility of NbAl₃. How- ever, this has an adverse effect on the oxidation response of the system, as is explained in Part II of this article. This article deals with the microstructure evolution, phase identification, and characterization of laser-clad (rapidly solidified) nominal NbAl₃ and also studies the effect of V additions on the final microstructure in this system. Laser cladding of nominal NbAl3 produces two metastable unreported phases [body-centered tetragonal (bct) and base-centered orthorhombic phases]. Addition of V suppresses the primary phase (NbAl₃) in the Nb-Al-V system. on leave from the Department of Metallurgy on leave from the Department of Metallurgy     

Nonequilibrium solidification of Fe-P alloys

The specific features of solidification of Fe-P alloys in the concentration range 5?C25 at % P have been studied by differential thermal analysis and X-ray diffraction. The equilibrium ??-Fe + Fe₃P eutectic is shown to form immediately from melt only for compositions with 17?C19 at % P when a melt is superheated below temperatures of 1130?C1160°C, at which the type of composition short-range ordering changes (Fe₃P ?? Fe₂P) under insignificant (??50°C) supercooling conditions. A nonequilibrium ??-Fe + Fe₂P eutectic forms during cooling of the alloy at a large (??200°C) supercooling.     

Model free energy,mechanics, and thermodynamics of shape memory alloys

The shape memory effect (SME) in alloys is due to a first order martensitic phase transition. Based on Landau's theory a phenomenological model free energy function is presented which accounts for the SME. The mechanical and thermodynamical consequences of the model (such as stress-strain curves, elastic constants, and the latent heat of the phase transition) are discussed. Comparison with experiments yields the four involved parameters for the alloy Au₂₃ Cu₃₀ Zn₄₇.     

界面─晶界非平衡偏聚

简要介绍了偏聚特别是晶界非平衡偏聚研究的历史和现状．并扼要介绍了品界非平衡偏聚与材料冷脆之间的关系。