SINTERING KINETICS IN IRON-COPPER ALLOYS WITH AND WITHOUT A LIQUID PHASE

Abstract

Sintering of iron-copper alloys has been studied in the temperature range 950–1250°C. The factors involved include compacting pressure, sintering temperature, sintering time, and atmosphere. The results are interpreted as a decrease in pore volume due to the filling of voids between particles by a diffusion mechanism. An empirical equation of the Arrhenius type, based upon volume change as a function of sintering time, has been derived in order to evaluate the rate constant of the sintering process.

Volume diffusion is considered to be the primary mechanism of material transport in alloys containing 0·5–2·0% copper, when sintered in the range 950–1250°C, and in alloys containing 5·0–10·0% copper, when sintered in the range 950–1050°C. The activation energy derived for the sintering process is 53·4 kcal/mole. Surface diffusion appears to be the operative mechanism of material transport in alloys containing 5·0–10·0% copper, when sintered above the melting point of copper. The activation energy for this sintering process is 32·6 kcal/mole.     

Characterisation of nickel alloy powders processed by spark plasma sintering

Abstract

In this study, nickel alloy powders were consolidated by spark plasma sintering. Experiments were performed between 700 and 750°C temperature range under 50 MPa pressure with holding times from 5 to 10 min. In addition to these main spark plasma sintering parameters three different heating rates ranging from 100 to 235°C min^?1 and two different particle size ranges (75–106 μm narrow size distribution and ?45 μm wide size distribution) were used for the experiments. After sintering, the sliding wear behaviour of the samples was investigated. The results revealed that the density of the material increased with raising the sintering temperature and holding time. However, heating rate and particle size also played an important role in the densification and these parameters were investigated in detail.     

Character of porosity of compact materials obtained from metalorganics

The possibility of obtaining highly porous (60–80% pores) materials by combining techniques of powder metallurgy with chemical-metallurgical processes of formation of nanodimensional activators of sintering during the thermal destruction of metalorganics is shown. It is noted that, irrespective of the composition of starting charges, the surface porosity of such materials is represented by pores <30 μm, the fraction of which does not exceed 1%. The basis (86–89%) is represented by pores <4 μm. The character of porosity inside the sintered articles is determined by the sizes and shape of the powderlike skeleton material introduced into the starting charge to prevent the outflow of metalorganics during thermal treatment. When using powders with highly developed particle surfaces as such, the character of the internal porosity is similar to the surface porosity. The combination of scale particles up to 100 μm in size and highly dispersed (<10 μm) particles promotes the formation of long porous channels (100–350 μm) 10–50 μm in width.     

微波烧结锇的工艺研究及动力学分析

采用微波烧结技术制备锇（Os）烧结体,研究了生坯压制压强和微波烧结主要工艺参数（升温速率、烧结温度和保温时间）对Os烧结体组织结构和相对密度的影响规律,分析了微波烧结致密化的机理。结果表明,1350 ℃微波烧结后Os平均晶粒尺寸约0.22 μm,与粉体颗粒尺寸差别不大;随着烧结温度增加到1500 ℃,晶粒尺寸长大到0.76 μm。1500 ℃烧结时,延长保温时间,Os烧结体的相对密度先快速增加,后缓慢增加。1500 ℃微波烧结60 min后,Os烧结体相对密度为94.3%,平均粒径小于1 μm。烧结动力学分析表明,Os的致密化过程是体积扩散和晶界扩散共同作用的结果,随着烧结温度的升高,扩散机制从晶界扩散逐渐向体积扩散转变。     

低温下氢气还原氧化铁的动力学研究

 用热重分析法研究了低温下不同粒度氧化铁的氢还原动力学,得出在同一温度下,铁矿粉粒度从107.5 μm降到2.0 μm后,由于粉体的表面积大幅度增加,提高了粉气接触面积,从而使得化学反应的速度提高了8倍左右,还原反应的表观活化能从78.3 kJ/mol降低到36.9 kJ/mol;当反应速度相同时, 粒度6.5 μm的粉体的反应温度比107.5 μm的降低了80 ℃左右。同时,通过理论推导和实验结果表明,当反应扩散层厚度相同时,铁矿粉粒度越小,反应扩散层厚度越薄,其还原率越高。     

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.     

Sintering and magnetocaloric properties of gas atomised LaFe11.0Si1.2Co0.8

ABSTRACT

The sintering behaviour of LaFe_11.0Si_1.2Co_0.8 powder produced by gas atomisation was examined to provide a basis for the application of powder metallurgical shaping technologies to magnetocaloric La(Fe,Si)₁₃-alloys. The aim was to establish sintering parameters for attaining both high densification and good magnetocaloric properties for the investigated particle sizes <10 µm and <25?µm. Dilatometry measurements and sintering trials were carried out and density, microstructure and entropy change ΔS of the sintered samples were analysed. For the fine particles <10?µm, the lowest investigated sintering temperature 1150°C results in a relative density of 97%, a low α-Fe content and a high ΔS?=??5?J?kg^?1?K^?1 (ΔH=2?T). For powder <25?µm, a two-stage process is required to achieve similar properties.     

Structure and properties of W-Ni-Fe-Co heavy alloys compacted from nanopowders

Heavy tungsten alloy (HTA) W-Ni-Fe-Co nanopowders synthesized by a chemical-metallurgical method are used to produce a compacted material with a theoretical density and a grain size of 2.9–4.6 μm. Upon solid-phase sintering (SPS) at 1350–1450°C, the binder composition of the produced material coincides with the binder composition of the alloy fabricated by liquid-phase sintering (LPS) according to a traditional technology. The hardness of the material is 4400–3400 MPa (as compared to 1750 ± 50 and 2950 ± 50 MPa after LPS followed by a hardening treatment for a standard HTA), and its ultimate tensile strength after SPS is 950–1050 MPa (as in the case of the standard alloy after LPS). The melting temperature of the binder is 25–30°C lower than that of the traditional alloy.     

金纳米颗粒烧结的分子动力学模拟

采用分子动力学模拟方法研究了不同尺寸Au纳米颗粒在烧结过程中晶型转变及烧结颈长大机制.研究发现纳米颗粒的烧结颈生长主要分为两个阶段:初始烧结颈的快速形成阶段和烧结颈的稳定长大阶段.不同尺寸纳米颗粒烧结过程中烧结颈长大的主要机制不同:当颗粒尺寸为4 nm时,原子迁移主要受晶界（或位错）滑移、表面扩散和黏性流动控制;当尺寸在6nm左右时,原子迁移主要受晶界扩散、表面扩散和黏性流动控制;当颗粒尺寸为9 nm时,原子迁移主要受晶界扩散和表面扩散控制.烧结过程中Au颗粒的fcc结构会向无定形结构转变.此外,小尺寸的纳米颗粒在烧结过程中由于位错或晶界滑移、原子的黏性流动等因素会形成hcp结构.      

铝粉粒度和烧结温度对硅太阳能电池铝背场结深及光电转化率的影响

分别采用4种不同粒度(平均粒度为1.37、3.95、6.08和9.73μm)的铝粉制备单晶硅太阳能电池铝浆,研究铝粉粒度和烧结温度(800、830、850和870℃)对铝背场结深和光电转化率的影响。结果表明:采用不同粒度的铝粉在相同条件下制备的铝背场,经800℃烧结后,结深均为4.05μm左右,其中粒度为6.08μm的铝粉,其光电转化率最高,达到17.2%;烧结温度从800℃升高到870℃时,铝背场结深从4.05μm增加到4.75μm,转化率略有提高。     

Investigation on compressibility of Al–SiC composite powders

Abstract

The consolidation behaviour of particulate reinforced metal matrix composite powders during cold uniaxial compaction in a rigid die was studied. Al–SiC powder mixtures with varying SiC particle size, ranging from nanoscale (50 nm) to microscale (40 µm), at different volume fractions up to 30% were used. Based on the experimental results, the effect of the reinforcement particles on the densification mechanisms, i.e. particle rearrangement and plastic deformation, was studied using modified Cooper–Eaton equation. It was found that by increasing the reinforcement volume fraction or decreasing its size, the contribution of particle rearrangement on the densification increases while the plastic deformation becomes restricted. In fact, when percolation network of the ultrafine reinforcement particles is formed, the rearrangement could be the dominant mechanism of consolidation. It was also shown that at tap condition and at the early stage of compaction where the particle rearrangement is dominant, the highest density is achieved when the reinforcement particle size is properly lower than the matrix (0˙3<the size ratio<0˙5) and the fraction of hard particles is relatively low (<10%). At high compaction pressures, the reinforcement particles significantly influence the yield pressure of composite powders, thereby retarding the densification.     

Mechanical alloying of Cu–SiC materials prepared with utilisation of copper waste chips

Abstract

The aim of this work was to study the structure and particle size of copper based composite materials reinforced with a high content (15–35 wt-%) of silicon carbide and prepared by mechanical alloying in the high energy planetary mill. Raw materials consisted of grinded copper chips with a size of <5000 μm and reinforcing particles with an initial size of 10 μm. Duration of milling was 20–80 min. It was shown that the formation of Cu–(15–25 wt-%)SiC composites occurred successfully. With an increase in the silicon carbide content of above 25 wt-% (48 vol.-%), the efficiency of mechanical alloying was decreased. The average size of composite particles was ～20 μm.     

广义热弹性扩散下非等径颗粒的烧结驱动力分析

在广义热弹性扩散理论框架下建立非等径两颗粒系统三维有限元模型，研究颗粒系统温度场和浓度场的分布规律，分析场分布对脉冲电流烧结初期迁移驱动力的影响。结果表明，颗粒颈部空位浓度梯度、温度梯度、由温度场和应力场产生的浓度梯度是颗粒颈部物质迁移的共同驱动力。烧结颈部的温度会产生两次突变，烧结过程中小颗粒一直保持高温状态；温度变化会引起浓度改变，使得颈部浓度高于边缘浓度；热扩散占总扩散通量的2/3，浓度扩散占1/3，因此烧结颈部的热扩散驱动力和浓度扩散驱动力是脉冲电流烧结过程的主导驱动力，提高热扩散能力和浓度扩散通量可显著提高烧结过程驱动力。非等径颗粒的烧结驱动力远远大于等径颗粒，为非等径颗粒的烧结比等径颗粒更为迅速提供了理论依据。     

Spark plasma sintering of cryomilled copper powder

Abstract

The densification and sintering behaviour of a cryomilled copper powder (grain size of 17±2 nm and dislocation density of 6·26±0·04×10¹⁶ m^?2) were investigated and compared to those of an atomised copper powder with the same mean particle size in order to highlight the effect of the nanostructure on spark plasma sintering (SPS). Oxygen and nitrogen contamination of the cryomilled powder gives rise to extensive degassing during SPS up to 400°C. The cryomilled powder is more resistant to plastic deformation than the atomised one, but the huge density of dislocations and grain boundary activates sintering at low temperature. Densification is therefore promoted by deformation in the atomised powder and by sintering shrinkage in the cryomilled one. As a consequence, in the SPS conditions investigated, the atomised specimen is densified but not sintered, while the cryomilled one is effectively sintered and consequently densified.     

Determination of fracture toughness and bridging tractions from crack-opening displacement measurements in particulate composites of diamond in zinc sulfide

The fracture toughness of zinc sulfide ceramic and a series of ZnS diamond particle composites was studied through measurements of crack opening displacement profiles. Five composites were fabricated using weight fractions of 10, 20 and 30% of 0–1 μm diamond particles, and 10 and 20% of 1–3 μm particles in a ZnS matrix. The cracks were grown using a novel specimen geometry and a loading technique that permitted stable crack growth even in small specimens. The fracture toughness of each material was calculated on the basis of the displacement profiles and the material properties, as opposed to the applied loads and the specimen geometry. The pure zinc sulfide material exhibited nearly ideal brittle behavior, and the toughness measurements agreed closely with other methods. The greatest toughening occurred in the 1–3 μm particle size composites, in which weak bridging tractions (⩽ 100 MPa) over a short distance along the crack (50–100 μm) could explain the reduced displacements near the crack tip. Even smaller size bridging zones (5–10 μm) may have been present in the 0–1 μm particle size composites, but elastic shielding alone can explain the observed toughening. The specimen geometry used here permitted toughness measurements using small specimens (< 5 mm) but is limited to materials having bridging zones that are less than about 250 μm.     

Effects of Particle Sizes on Sintering Behavior of 316L Stainless Steel Powder

In rapidly evolving powder injection molding technology, the wide prevalence of various microstructures demands the powders of smaller particle sizes. The effects of particle size on the sintering behavior are critical to not only shape retention of microstructure but also its mechanical properties. This study investigates the effects of three different particle sizes on the sintering behavior of the 316L stainless steel (STS316L) samples, prepared by powder injection molding, via the dilatometry experiments. For this purpose, the STS316L powders of three different mean particle sizes, i.e., 2.97, 4.16, and 8.04 μm, were produced for STS316L. The samples for the dilatometry test were prepared through powder-binder mixing, injection molding, and solvent and thermal debinding. Dilatometry experiments were carried out with the samples in a H₂ atmosphere at three different heating rates of 3, 6, and 10 K/min. The shrinkage data obtained by dilatometry experiments was collected and analyzed to help understand the densification and the sintering behaviors in terms of particles size and heating rate. The master sintering curve (MSC) model was used to quantify the effects of particle sizes. In addition, we investigated the microstructure evolutions in terms of particles sizes.     

A Study of Grain Growth Kinetics in Sintered NdFeB Magnets

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Abstract

This work proposes a hypothesis for the interpretation of shrinkage anisotropy during sintering of an Fe–Cu–C alloy based on the effect of the structural modifications of the powder, due to the prior compaction, on the mass transport phenomena. Dislocations are introduced by cold compaction in the contact regions between particles, with different densities along the compaction direction and the transversal one. Therefore, the mass transport by volume diffusion is strongly activated in both directions, and a prevailing effect in the compaction direction is shown. The volume diffusion coefficients derived from the kinetic model correspond to the dislocation pipe diffusion mechanism.     

The influence of gas atmospheres on the first-stage sintering of high-purity niobium powders

Niobium and tantalum surfaces easily absorb oxygen. With decreasing particle size the content of oxygen increases. The role of this surface oxygen and oxygen in the sintering atmospheres on the first-stage sintering is not well established. Therefore the sintering behavior of high-purity niobium powders was studied by annealing cylindrical powder compacts (particle size <63 μm) in the temperature range from 1000°C to 1600°C in ultra-high vacuum and under low oxygen partial pressures, as well as in inert gas atrnospheres with low oxygen contents. The specific surface of the samples was determined by metallographic methods, adsorption, and capacitance measurements. Low oxygen partial pressures (10^-3 Pa) lead to a slight enhancement of the surface diffusion which is controlling first-stage sintering. High heating rates (0T > 3000 min^-1) to temperatures above the melting point of Nb₂O₅ (Tm = 1495 °C) enhances the neck growth due to the formation of a liquid oxide phase on the surface of the powder particles. This paper is based on a presentation delivered at the symposium “Activated and Liquid Phase Sintering of Refractory Metals and Their Compounds” held at the annual meeting of the AIME in Atlanta, Georgia on March 9, 1983, under the sponsorship of the TMS Refractory Metals Committee of AIME     

Sintering study of silver particles by in situ electron microscopy

Micron size silver spherical particles were sintered in the hot stage of an electron microscope between 370 and 720°C. In the 370 to 520°C temperature range, sintering can be explained either by grain-boundary diffusion or dislocation slip. At higher temperatures, the controlling mechanism changes from surface or grain-boundary diffusion to volume diffusion from 570 to 720°C. These conclusions are based on observations of neck growth and shrinkage and on analyses of the shapes of the sintering curves.