Spark plasma sintering microscopic mechanisms of metallic systems: Experiments and simulations

Abstract

The microscopic densification mechanisms of metallic systems (TiAl, Ag-Zn) by spark plasma sintering (SPS) have been studied by simulations and experiments. Finite element simulations showed that, despite very high current densities at the necks between metallic powder particles (≈5 × 10 4 A/cm 2 ), only very limited Joule overheating can be expected at these locations (<1°C), because of very fast heat diffusion. The microscopic plasticity mechanisms under these high electric currents have been studied by transmission electron microscopy. For this purpose, thin foils have been extracted by focused ion beam at the necks between TiAl powder particles. This is the first time, to the best of our knowledge, that microscopic plasticity mechanisms at the necks between powder particles are investigated by TEM during densification of a metallic powder. Dislocation glide and climb mechanisms were identified, followed by recovery and recrystallization.The elementary mechanism kinetically controlling these phenomena is proposed to be bulk diffusion of Al, which activation energy (360 kJ/mol) is close to the activation energy measured for densification (308 ± 20 kJ/mol). Comparisons of densification kinetics by SPS (≈60-110 A/cm 2 ) and by hot pressing (0 A/cm 2 ) showed no influence of current on these mechanisms. Finally, reaction experiments in the Ag-Zn system did not show any influence of very high currents (>1000 A/cm 2 ) on diffusion kinetics. Consequently, densification by SPS occurs by classical mechanisms not affected by the current. K E Y W O R D Sdiffusion/diffusivity, electron microscopy, intermetallic systems, plasticity, spark plasma sintering

Fundamental principles of spark plasma sintering of metals: part I – Joule heating controlled by the evolution of powder resistivity and local current densities

“…The breakdown of surface oxide films is more likely happening due to plastic deformation and reduction at higher temperatures [33], possibly in conjunction with the presence of carbon [34]. A multitude of experimental observations [35,36], analytical estimates, and finite element simulations [37][38][39][40] even come to the conclusion that with less than 1 K no relevant temperature difference between particle-particle contacts and the centre of the particles exists.…”

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

“…In the early works, very elevated local current densities, leading to local high overheating at the contacts (up to 10,000 °C), were computationally calculated [8,11]. The main limitation of these simulations is that they did not consider heat conduction within the particles, which was later shown to play a major role [12][13][14][15][16]. Even though more recent simulations predict local current densities as high as ≈ 5 × 10 4 A/cm 2 at the contacts between particles [15][16][17], it has been shown that, for regularsize powder particles (≈ 100 µm), the temperature homogenization by heat conduction takes only milliseconds, which was not compatible with stabilization of "hot spots" at the particle contacts [15,16].…”

“…The main limitation of these simulations is that they did not consider heat conduction within the particles, which was later shown to play a major role [12][13][14][15][16]. Even though more recent simulations predict local current densities as high as ≈ 5 × 10 4 A/cm 2 at the contacts between particles [15][16][17], it has been shown that, for regularsize powder particles (≈ 100 µm), the temperature homogenization by heat conduction takes only milliseconds, which was not compatible with stabilization of "hot spots" at the particle contacts [15,16]. Few experimental works with FeAl [18] and metallic glasses [19] concluded local overheating, but clear and direct evidence could not be provided.…”

Preparation and Properties of Titanium Obtained by Spark Plasma Sintering of a Ti Powder–Fiber Mixture

“…This technology integrates plasma activation, hot pressing and resistance heating, and directly connects the high-frequency pulse current between the pressurized powder particles. This technology depends upon heat from the plasma produced by spark discharge, which has the advantages of fast heating and cooling speeds, a low sintering temperature, short sintering time, fine and uniform grain size, controllable microstructure, and so on [9,10,11]. Therefore, advantages of SPS technology can be used to try to solve the defects of traditional porous titanium.…”