Non—Isothermal Shrinkage of Compacts

Schatt, W.; Vetter, B.; Friedrich, E.

doi:10.1179/pom.1991.34.3.179

Cited by 11 publications

(14 citation statements)

References 9 publications

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“…Until this critical value is reached, an expanding zone of dense dislocation tangles is observed in the neck area, and the dislocations play an active role in the sintering process. 38 At supercritical values of x/a, normal recovery processes take over and the sintering mechanisms involving high dislocation densities become less important.…”

Section: Resultsmentioning

confidence: 99%

Sintering of β Titanium: Role of Dislocation Pipe Diffusion

Sundaresan¹,

Raghuram²,

Mallya³

et al. 1996

Powder Metallurgy

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Coefficients are tabled on basis of diffusion equation consisting of intrinsic diffusion coefficient D! and extrinsic diffusion coefficient D2: D = Do.! exp(QdRT) + DO•2 exp(-Q2/RT).model for this 'anomalous' diffusion behaviour is the extrinsic vacancy model, in which it is proposed that interstitial impurities in the matrix give rise to an extrinsic vacancy concentration in addition to the intrinsic thermal equilibrium vacancy concentration. The excess diffusion in the fJ phase is attributed to this extrinsic vacancy concentration. 12 -I5 EXPERIMENTAL Sintering was followed essentially by isothermal dilatome try. Two types of titanium powder were studied:(i) irregular shaped sponge fines (ii) spherical powder obtained by vacuum centrifugal atomisation. The characteristics of the powders are given in Table 2 and shown in Fig. 1. Compacts were made from the sponge fines by isomechanical compaction to a green density of about 65% theoretical. The atomised powder was not amenable to cold compaction and compacts were obtained by presintering of tap filled moulds at 1023 K for 30 min. The density in this case was about 66% theoretical.A vacuum dilatometer with a platinum wound furnace capable of operating at temperatures up to 1600 K was employed. The dilatometer, which incorporated an impermeable alumina tube, was fabricated in house. Sintering was carried out at a vacuum of 10-3 Pa. The compact was pushed from the cold end of the evacuated furnace tube into the hot zone maintained at the isothermal sintering temperature, and anchored by means of an alumina push rod. At the other end of the tube the shrinkage was monitored as indicated on a sensitive dial gauge (0'001 mm sensitivity, ± 5 mm range) using an alumina transducer.A PtjPt-13Rh thermocouple passing through the transducer tube was used to measure the compact temperature directly.The sensitivity of 0·001 mm was adequate to record the thermal expansion of the system together with that of the specimen. Thermal expansion of a metal specimen 20 mm in length will be of the order of 0·020 mm at t'.' 1250 K, while the simultaneous expansion of the alumina tube would lead to a much lower reading on the dial gauge. To correct for thermal expansion effects, specimens of solid titanium were used in blank runs at the various isothermal hold temperatures. Reproducibility was confirmed over several blank runs to assure that the shrinkage values obtained from the sintering runs were valid. 9 10 '11 130'6 3·58 X 10-4 152·8 251'2 131 1·9 X 10-3 1·09 4·54 x 10-4Table 1 Diffusion coeflicients* for p titaniumSintering of titanium in its high temperature fJ phase was studied by isothermal dilatometry. The sintering shrinkage y did not follow the normal time exponent type of behaviour, instead being described by the equation y = Kt m /[ 1 -(A + Bt)2J, where m = 1·93 ± 0,07, with an activation energy of 62-90 kJ mol-I. A detailed analysis of these results, based on the 'anomalous' diffusion beha viour reported for fJ titanium, is carried out. It is shown that the generation of a h...

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Section: Resultsmentioning

confidence: 99%

Sintering of β Titanium: Role of Dislocation Pipe Diffusion

Sundaresan¹,

Raghuram²,

Mallya³

et al. 1996

Powder Metallurgy

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“…On a microstructural level, changes in the pore structure are observed in green compacts, with elongated pores present mainly perpendicular to the compaction direction as the contact areas between the powder particles become larger [9,10]. The interparticle contact regions also further change through sintering during which a self-activation effect of the material in regions of increased dislocation density can enhance the sintering process [11][12][13]. This structural activity and its role during sintering can, in turn, be further affected by several processing parameters such as compaction pressure and heating rate [12,14].…”

Section: Introductionmentioning

confidence: 99%

Sintering behaviour of compacted water-atomised iron powder: effect of initial state and processing conditions

et al. 2020

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Conventional ferrous powder metallurgy relies on uniaxial die-pressing of powder before sintering. The operation causes particle rearrangement and deformation and the initial state of the compact ultimately determines the sintering behaviour. In this study, sintering was investigated for water-atomised iron powder compacted at three different pressure levels, with and without admixed graphite. Electron back-scatter diffraction of the powder and compacts showed a large increase in low-angle grain boundaries after compaction. The sintering of the compacts was investigated by means of dilatometry in hydrogen. The initial compaction strongly influences sintering shrinkage, particularly in the low-temperature, high-diffusivity ferrite region which can account for up to 80% of the recorded shrinkage. Only a small fraction of the total shrinkage originates from the austenite phase, even at high sintering temperatures. However, the ratio of sintering shrinkage in ferrite and austenite changes with compaction pressure, carbon content and heating rate.

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“…Non-isothermal dilatometric measurements of SBN and SBNþLN-x (x ¼ 0.1 and 0. typical shrinkage rates between 10 À4 and 10 À1 % min À1 [35][36][37][38]. Furthermore, DSC measurements of SBNþLN-0.2 up to 1350 C did not show any indication of the formation of a liquid phase (Fig.…”

Section: Sintering Microstructure and Phase Composition Of The Ceramicsmentioning

confidence: 87%