Processing Model for Tungsten Powders and Extention To
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Processing model for tungsten powders and extension to nanoscale size range 2 2 Randall M. German*\ J. Ma , X. Wanr and E. Olevsky Nanoscale tungsten powders promise access to very hard, strong and wear resistant materials via the press-sinter route. A small particle size changes the response during sintering, requiring lower temperatures and shorter times to attain dense but small grain size structures. On the other hand, oxide reduction and impurity evaporation favour high sintering temperatures and long hold times. Accordingly, press-sinter processing encounters conflicting constraints when applied to small particles. Presented here is an analysis of press-sinter tungsten particle processing to isolate conditions that balance the temperature and size dependent effects. The calculations are pinned by existing data. Opportunities are identified for new consolidation approaches to deliver a small grain size in a full density structure. Keyword•: Tungsten, Naf'I06cale, Sintering, Compaction, Powder Metallurgy exists for particle sizes ranging from 20 nm to 18 j.llil Introduction 1 3 5 6 and sintering temperatures up to 2700°C. • • • ·20-44 This Tungsten, tungsten alloys and tungsten compounds body of knowledge provides a basis for predicting occupy unique positions in materials science. Most uses property levels, processing effects, and opportunities for derive from combinations of the high density, hardness, tungsten based materials at the nanoscale. These data melting temperature, elastic modulus, radiation opacity, are combined with processing models in a form that and thermal-electrical conductivity in conjunction with handles micrometre to nanometre particle sizes. From low thermal expansion. Tungsten was one of the first this base predictions are made for product properties as metallic nanoscale powders.1 Prior reports give data on 7 13 functions of the particle size and processing conditions. the processing of nanoscale W/-6 W-Cu, - W 16 When the earlier efforts employed nanoscale powders, Ni-Fe,14 W-Cu-Al-Ni/s W-Y and W-Y203. almost always traditional processing cycles were applied, Strength, hardness, and wear resistance will improve with little reported property gain. with a small grain size. For example, the Hall-Petch Hence, with the novel and new powders comes a effect (hardness increases in proportion to the inverse need to identify novel processing routes to produce square root of grain size) suggests significant gains if improved materials and properties. Although some of nanoscale powders could be consolidated to full density the processing attributes of tungsten powders scale with with a small grain size. In spite of nanoscale powder particle size, there are other aspects that encounter availability, success has been difficult because these temperature constraints. Nanoscale tungsten needs to be powders undergo rapid microstructure coarsening when 4 1 1 19 assessed in the context of this balance. Modelling, such sintered. · 0. 2- Consequently, novel but expensive as performed here, provides a means to identify consolidation routes are used to lower the densification processing options customised to nanoscale tungsten . wth 4 20 21 In temperature as a means to re duce gram gro . · · based materials. The model is constructed to predict traditional press-sinter processing, temperature depen performance parameters including hardness, ductile dent reactions are embedded in the high temperature brittle transition temperature, strength, and wear heating cycle, but low temperature consolidation tends resistance from the processing and initial powder to trap the contaminants. Thus, the conflicts between characteristics. Success here provides a basis for analys size dependent events, such as a low sintering tempera ing other materials and processing objectives. Based on ture, and temperature dependent events, such as oxide this model, future experimental studies with nanoscale reduction, create imbalances that often result in sintered powders based on high compaction pressures, low properties below expectations. sintering temperatures, and short sintering times seem Prior research has generated press-sinter data for most justified. tungsten powders over a broad particle size range. Data 'Mechanical Engineering Centre for Advanced Vehicular Systems, PO Modelling Box 5404, Mississippi Slate University, MS 39762-5405, USA "Mechanical Engineering Department, San Diego Slate University, 5500 Particle packing Campanile Drive, San Diego, CA 92182-1323, USA Small particles have inhibited packing owing to the high 4 •Corresponding author, email germanOcavs.msstate.edu surface area and interparticle friction. s The existing 0.6 fractional fractional 0 1 ~ apparent density " .------------- green density I - 500 MPa compation I • experiment I 0.2 I -model 0.01 L-----'--------'-----'---------' 0.0 '-----------'--------'--------' 0.01 0.1 10 100 0.01 0.1 10 particle size, 1-1m particle size, J..Lm Log-log plot of fractional apparent density versus par 2 Model predicted fractional green density versus tung ticle size, showing agreement between model and sten particle size for 500 MPa compaction pressure experimental results for deagglomerated tungsten value is 0·56. As a demonstration of equation (2), Fig. 2 tungsten packing data are fit as follows plots the green density versus particle size down to 10 nm for a constant compaction pressure of 500 MPa. log (p A)= log (p ) +a log D (1) 0 The relationship between the compaction pressure where PA is the fractional apparent density, p0 is the and fractional green density can be determined from fractional packing density at 1 f..UD, a is a constant, and plasticity theory for porous bodies.47 For isostatic D is the median particle size in f..UD. Large spheres pack compaction of porous materials to 0·6 apparent density, but colloidal particles pack to densities as low as 0·05. For deagglomerated tungsten (4) powder with a prismatic particle shape, p =0·143 and 0 where Ur is the radial stress, is the normalised bulk a= 115. These constants were derived using packing data 1/1 modulus, 8a is the green porosity of the powder taken from powders in the range of 20 nm-18 f..UD compact, and uy is the yield stress of the fully dense (Ref. 5). The fit is illustrated in Fig. 1. powder material. The first invariant of a stress tensor Compaction (the hydrostatic pressure) for rigid die compaction is Traditionally, powder is loaded at the apparent density 1 into hard tooling for compaction. A polymer might be P= 3(uz+2ur) (5) added to lubricate the tooling. Pressure is applied to the where ux is the axial stress. For isostatic pressing Uz=ur. powder to increase density via rearrangement and plastic Therefore deformation at the particle contacts. The density after compaction is termed the green density. Powders exhibit 1 a declining rate of densification with increasing pressure P= 3 (uz +2ur) = Ur (6) as the powder work hardens to resist further densifica from equations (4) and (6), we have tion. Hard particles are very resistant to compaction. 46 Typical compaction models rely on the apparent density P= 1/1(1- 8a) 112uy (7) as the zero pressure density, and include a means to account for particle rearrangement, deformation, and hence work hardening. For compaction pressures >50 MPa, p p these factors give the fractional green density Pa as a 1 2 1 2 (8) function of the compaction pressure P 1/1= (1-8a) 1 uy (Pa) 1 uy where Pa = 1 - 8a is the fractional green density of the Pa-PA =1-pAexp (B- f) (2) compacted specimen. From equation (2), one can derive Pas a function of C, B, pa, and PA as follows where PA is the fractional apparent density. The rearrangement term B for tungsten depends on the particle size and its packing PA B- [1-(pa-PA)] => B= 1·545 (3) ~=In Dl/7 C PA where the particle sizeD is in f..UD, compaction pressure (9) P is in MPa, and the constant C is 1650 MPa. The predicted density is limited to 100% for very high compaction pressures. Most of the experimental data for combining equations (9) and (7), it can be concluded that tungsten compaction were collected over the pressure range from 50 to 300 MPa (0·47-0·64 fractional 1/J= ~/2 {B-ln[1-(pa-PA)]} (10) density) and tend to have 1-2% scatter, so the precision (Pa) Uy PA of the constants in equations (2) and (3) is no better. For for the compaction of porous nanoscale materials in a example, at 207 MPa the fractional green density for a rigid die48 6·5 f..UD powder is 0·61 and equation (2) predicts 0·60, and for a 1·2 j.Lm powder at the same pressure the (11) measured fractional density is 0·55 and the predicted 140 120 100 fractional green 80 green density strength, MPa 60 0.2 40 20 00'-------------'---------'-------' 0.01 0.1 10 particle size, 11m 0.01 0.1 1 10 100 particle size, 1-1m 3 Comparison of modelling and experimental results for compaction of tungsten powders under 240 MPa stress 4 Predicted green strength from various tungsten parti cles compacted at 500 MPa in this case 6,.=0. Therefore, equation (11) can be rewritten as follows for the case shown in Fig. 2, where uz=500 MPa, C=1650 MPa, uy=680 MPa, and B= ';i,jf as the rear rangement term (Dis the particle size in J.Ufl), then using (12) equation (1) leads to Fig. 3, which compares the model with experimental results for tungsten powders com Uz= ~ez(~+ ~{0) pacted at 240 MPa (Ref. 5). The model results are in good agreement with the experimental data. Extrapo where the equivalent effective strain rate W lation to a 10 nm particle size results in a low rredicted green density, in agreement with early reports. 1 .2 ·2 1/2 (13) W= 172 (tJJr +~e-) Pa Green strength . (2)1/21.