喷雾干燥-氢还原制备超细/纳米晶W-10Cu粉末及其烧结行为

采用喷雾干燥-氢还原法制备超细/纳米晶W-10Cu(质量分数,%)复合粉末,并经过压制和烧结制备W-Cu复合材料,系统研究烧结温度和保温时间对该材料性能和组织的影响,以及在1 100～1 300℃温度范围内的烧结激活能。结果表明,W-10Cu还原粉末晶粒度仅为30～60 nm;在1 200℃烧结时开始发生明显的致密化行为;随烧结温度升高相对密度增大,当烧结温度升高到1 300℃时W-10Cu复合材料的相对密度为90%,但当温度达到1 460℃时有所降低。1 420℃保温90 min时材料相对密度高达99.1%,且此时晶粒度仅为1.8μm。W晶粒尺寸为30～60 nm的W-10Cu复合粉末在1 100～1 300℃烧结的平均激活能为129.14 kJ/mol。烧结温度为1 420℃时W-10Cu的电导率随保温时间延长先增大后减小,保温90 min时最大达到19 MS/m,超过国标有关规定。     

W-Ni-Fe高比重合金的SPS烧结行为

采用放电等离子烧结(SPS)设备制备了93W-5.6Ni-1.4Fe高比重合金,烧结温度范围为1100~1180℃,保温时间为5min.对不同烧结温度下的样品进行了密度、硬度、抗弯强度等性能测试,采用场发射SEM观察了样品表面形貌及断裂行为.结果表明:采用SPS烧结,可以在较低的温度下实现93W-5.6Ni-1.4Fe高比重合金的固相烧结,使合金致密化,并能有效控制钨晶粒长大,提高材料的硬度、抗弯强度等力学性能.     

纳米Fe包覆Si3N4及烧结组织分析

非均相沉淀-热还原法制备Fe/Si3N4颗粒复合粉末及常压烧结与热压。通过SEM、TEM、EDS、XRD等方法观察Fe/Si3N4复合粉末的结构形貌,常压烧结与热压后的微观组织。结果表明:复合粉末主要存在Fe相与Si3N4相,结构为纳米薄层Fe均匀包覆Si3N4颗粒;常压烧结与热压的样品微观组织有较大的不同:常压烧结样品存在的主要衍射峰为Fe/Si化合物、SiC和Si3N4,SEM图观察Fe/Si化合物晶粒粗大(2～3μm),分布在Si3N4(1μm左右)相之中;热压样品存在的主要衍射峰为Fe、Fe/Si化合物和Si3N4,SEM图观察Fe/Si化合物晶粒较细(1μm左右),镶嵌在类似玻璃态物质中。并通过热力学分析进行讨论与解释。     

Mechanism and sintering kinetics of moistened Y—Ba—Cu—O powders

The influence of preliminary moistening of YBa₂Cu₃O_7−x powder on the kinetics of sintering and recrystallization, and the evolution of element and phase compositions in the bulk and on grain boundaries, was studied by the methods of scanning electron microscopy, x-ray microprobe analysis, and x-ray diffractometry. It was established that moistening the powder leads to a change of the sintering mechanism from bulk diffusion (for unmoistened powder) to diffusion through solid layers in the grain boundary and subsurface regions of moistened powder, formed from the interaction products of YBa₂Cu₃O_7−x with the moist atmosphere. The volume diffusion coefficients in YBa₂Cu₃O_7−x, and the diffusion coefficients in the solid layers, were calculated from the sintering kinetics data. The diffusion coefficients in the solid layers were 2–3 orders of magnitude hither than the volume diffusion coefficients. This results in more rapid sintering of the moistened than the unmoistened powders. Kharkov University. Institute of Monocrystals, Ukrainian Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya. Nos. 1–2, pp. 50–54, January–February, 1997.     

Transient liquid phase sintering of high density Fe3Al using Fe and Fe2Al5/FeAl2 powders Part 1: Experimentation and results

Abstract

High density Fe₃Al was produced through transient liquid phase sintering, using rapid heating rates of greater than 150 K min^-1 and a mixture of prealloyed and elemental powders. Prealloyed Fe₂Al₅/FeAl₂ (50Fe/50Al, wt-%) powder was added to elemental iron powder in a ratio appropriate for producing an overall Fe₃Al (13·87 wt-%) ratio. The heating rate, sintering time, sintering temperature, green density and powder particle size were controlled during the study. Heating rate, sintering time and powder particle size had the most significant influence upon the sintered density of the compacts. The highest sintered density of 6·12 Mg m^-3 (92% of the theoretical density for Fe3Al) was achieved after 15 minutes of sintering at 1350°C, using a 250 K min^{- 1} heating rate, 1-6 μm Fe powders and 5·66 μm alloy powders.

SEM microscopy suggests that agglomerated Fe₂Al₅/ FeAl₂ particles, which form a liquid during sintering, are responsible for a significant portion of the remaining porosity in high sintered density compacts, creating stable pores, larger than 100 μm diameter, after melting. High density was achieved by minimising the Kirkendall porosity formed during heating by unbalanced diffusion and solubility between the iron and Fe₂Al₅/FeAl₂ components. The lower diffusion rate of aluminium in the prealloyed powder into the iron compared with elemental aluminium in iron, coupled with a fast heating rate, is expected to permit minimal iron-aluminium interdiffusion during heating so that when a liquid forms the aluminium dissolves in the iron to promote solidification at a lower aluminium content. This leads to a further reduction in porosity.     

Cyclic sintering of cermet powders

Summary It is shown that after eleven cycles of sintering under conditions of 1200–800 °C a stage of porosity stability of the sample sets in, and a stage of exhaustion of shrinkage.Cyclic sintering has no advantages over isothermal sintering: with equal temperatures and summary durations during sintering, the magnitude of porosity of the samples is the same.     

Alloy powders in the Fe—Cu, Fe—Cr, Fe—Mn, and Fe—Cr—Mn systems

Studies have been done on iron-copper, iron-chromium, iron-manganese, and iron-chromium-manganese powder alloys, which have been made by dispersing the melts with high-pressure nitrogen. The use of such alloys in the preparation of low-alloy construction materials eases the stringent specifications for the oxidation potential of the controlled gas medium, and it also produces a more uniform distribution of the alloying elements in the sintered material.     

Formation of Cu pockets in Fe grains during the sintering of Fe-Cu alloys

The mechanisms for forming liquid pockets in grains during the liquid phase sintering of Fe-Cu alloys are examined. The number of liquid pockets is observed to increase with a slower rate of heating to the sintering temperature and with a higher compacting pressure. These results are consistent with two processes which occur in the solid state: sweeping of Fe grain boundaries across solid Cu particles and formation of C-shaped Fe grains. The liquid pockets are therefore formed when an extensive grain growth occurs in the solid state during heating.     

Supersolidus liquid-phase sintering of prealloyed powders

A model is derived for the sintering densification of prealloyed particles that form internal liquids when heated over the solidus temperature. The model considers the powder size, composition, and microstructure, as well as the processing conditions of green density, heating rate, maximum temperature, hold time, and atmosphere. Internal liquid forms and spreads to create an interparticle capillary bond that induces densification during sintering. Densification is delayed until the particles achieve a mushy state due to grain boundary wetting by the internal liquid. This loss of rigidity and concomitant densification of the semisolid particles depends on the grain size and liquid quantity. Viscous flow is the assumed densification mechanism, where both viscosity and yield strength vary with the liquid content and particle microstructure. Densification predictions are compared to experimental data, giving agreement with previously reported rapid changes in sintered density over narrow temperature ranges. The model is tested using data from steels and tool steels of varying carbon contents, as well as boron-doped stainless steel, bronze, and two nickel-based alloys.     

Selective laser sintering of compacted powders

The selective laser sintering of preliminarily compacted powders was investigated. Depths of sintering lower than those with freely poured powders were attained. This has a negative effect on the production of multilayered articles. The main disadvantage of the process, however, was the difficulty in separating the article from the unsintered compacted powder. Institute of Technical Acoustics, Byelorussian Academy of Sciences, Vitebsk. Translated from Poroshkovaya Metallurgiya, Nos. 7–8(402), pp. 27–31, July–August, 1998.