The characteristics of Titanium-nickel alloys produced by powder technology

Igharo, Monday (1987). The characteristics of Titanium-nickel alloys produced by powder technology. PhD thesis The Open University.



A range of titanium-nickel alloys near the equiatomic composition have been processed by cold compaction and vacuum sintering. The effects of compaction pressure, sintering temperature and powder particle size on dimensional changes and densities of sintered compacts are presented. The influence of composition and heat treatment on micro hardness and transformation temperature (Ms) is described.

During sintering, anisotropy of dimensional change occurs, with expansion in the radial and contraction in the axial direction of cylindrical compacts. Greater porosity is found in the sintered samples compared to that in the as-pressed condition. It is proposed that these observations are connected with the dissimilar interdiffusion rates of Ti and Ni, the segregation of powder particles in the green compacts and the occurrence of a transient liquid phase during sintering above 955°C. Subsequent hot isostatic pressing of the sintered material allows densification to near full density.

The transformation temperatures and hardness of TiNi alloys containing excess amounts of nickel (> 51 at -% Ni) are sensitive to cooling rate after solid state heat treatment, which is in contrast with samples of the exact equiatomic composition. This phenomenon has been related to the decrease in the homogeneity range of TiNi compound with temperature, resulting in either the formation of second phase precipitates in the slow cooled samples or the production of a supersaturated structure in water quenched material.

The pressed and sintered specimens display a well defined shape memory behaviour. The extent of shape recovery observed, following deformation and heating through the reverse transformation range, is explained in terms of the volume of pores in the sintered compacts.

Ribbons of equiatomic TiNi alloy have been rapidly solidified by the chill block melt spinning technique under an argon atmosphere.The effects of rapid solidification processing and subsequent heat treatments on the transformation behaviour are presented. The crystal structures at room temperature have been analysed by X-ray powder diffraction and thin foil transmission electron microscopy. Some of the ribbons have been chopped and ball milled to produce prealloyed particulate from which compacts have been prepared by cold compaction followed by vacuum sintering. The consolidation response of the prealloyed powder is compared with that of elemental blends.

The grain size of the rapidly solidified material is found to beat least an order of magnitude smaller than those observed in wrought specimens. The s temperature of TiNi alloy is depressed by rapid solidification processing, thus allowing the R-phase to be observed in addition to the high temperature parent phase. This depression has been correlated with the fine grain structure of the spun ribbon.

Sintering temperatures in excess of those employed for elemental blends are required for the prealloyed particles. This is related to the dominant effect of the alloy formation energy in elemental powders sample. However, while the volume of porosity increases with sintering temperature in elemental mixture compacts, densification takes place in the case of RS prealloyed specimens. In spite of the need for a higher sintering temperature for RS prealloyed compacts,the resulting grain size is smaller.

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