Densification of Ni and TiAl by SPS: Kinetics and Microscopic Mechanisms

Abstract

Densification by spark plasma sintering (SPS) of ductile (Ni) and brittle (TiAl) metallic materials have been studied to elucidate the mechanism of densification in the two cases. Isothermal densification experiments were carried out to determine the activation parameters in Ni. Transmission electron microscopy (TEM) observations of thin foils extracted by focused ion beam (FIB) in the contact regions between particles of TiAl and Ni powders are presented. Macroscopically, the most striking feature observed here is that the densification of Ni takes place in the wide temperature range of 0.2-1.0 Tm, whereas that of TiAl varies in 0.7-0.9 Tm, which is significantly narrower (Tm being the melting temperature of Ni and the peritectic temperature of TiAl). In Ni, the low activation energy (164 ± 30 kJ/mol), the high dislocation density in the inter-particle contact region, and the formation of recovery cells involving dislocation climb, indicate that the rate-controlling mechanism is probably self-diffusion in dislocations. In TiAl, high dislocation densities leading to reorganization into sub-boundaries point to dislocation climb mechanisms, which are kinetically controlled by volume diffusion. The difference in densification kinetics between Ni and TiAl is then accounted for in terms of the difference in their respective rate-controlling mechanisms operative during densification.

Elaboration of Metallic Materials by SPS: Processing, Microstructures, Properties, and Shaping

“…Moreover, at the microscopic scale, with the noticeable exception of the study of Yanagisawa et al [8], no evident phenomena have been experimentally detected. The densification mechanisms are consequently similar to those classically involved in densification theories: deformation of the powder particles by power-law creep involving dislocations glide and climb, followed by recovery and recrystallization [23,24]. Therefore, it can reasonably be stated that the densification of metallic powders by SPS experiments is not governed by electrical effects.…”

“…Consequently, the local increase of the temperature can lead to acceleration of plastic deformation by creep. Other experimental investigations have been performed by focused ion beam and transmission electron microscopy (TEM) in the contact regions between powder particles in TiAl [23] and Ni [24]. It has been shown that the elementary deformation mechanisms were dislocation glide and climb, followed by dislocations recovery and recrystallization.…”

“…It has been shown that the elementary deformation mechanisms were dislocation glide and climb, followed by dislocations recovery and recrystallization. For example, Figure 5 shows TEM images of a thin foil extracted in the contact region between two Ni powder particles [24]. High dislocation densities, and recovery walls, can be observed.…”

Sintering Mechanisms of Metals Under Electric Currents

“…Fig. 7 gives the evolution of the relative density of powders of metallic systems exhibiting emblematic mechanical behavior: brittle (TiAl) and ductile (Ni) (Trzaska et al 2018). The striking feature is that in the two cases, densification occurs within minutes, that is, quite rapidly.…”

Deformation Behaviour of a FAST Diffusion Bond Processed from Dissimilar Titanium Alloy Powders

“…[8] It has been observed that the electric current improves metal powder densification by the electromigration phenomenon, yet temperature still has the most dominant effect on densification. [10] Trzaska et al [11] also observed enhanced densification during the FAST of metal powders due to high dislocation densities and diffusion rates at interparticle contact regions. Additionally, previous studies have demonstrated an enhancement in diffusion when pulsed current is used.…”