Elucidating the Multi‐Scale Temperature Evolution and Densification Mechanisms of Amorphous Powders During Spark Plasma Sintering: Experiments and Modeling

Abstract

With unique atomic stacking structures and outstanding performances, amorphous alloys have been formulated during the past several decades via various solidification techniques. [1,2] The cooling rates required to produce amorphous alloys are generally as high as 10 4 -10 5 °C s À1 . [3,4] This imposes restrictions on manufacturing amorphous alloys into giant-sized and geometrically complex parts. Nevertheless, amorphous alloys exhibit viscous-like behavior in the supercooled liquid region (SLR). Micron-sized amorphous powders can be fabricated from most alloy systems, even those having low glass-forming ability. [4] These provide a practical approach to densifying amorphous powders via powder metallurgy (P/M) processes. [5,6] Among the various P/M techniques available, spark plasma sintering (SPS) is promising for consolidating amorphous powders due to its fast heating rate and short cycle time. [5] In the past decade, consolidation of amorphous powders using the SPS process has been reported for Zr-based, [7,8] Ti-based, [9][10][11] Fe-based, [12] and Cu-based [13] alloys.The amorphous powders are heated to the SLR and densified by viscous flow. [10] However, the crystallization tendency is more intense with the increased temperature. [14] Therefore, the research focuses on fast densification while maintaining the amorphous structure. Amorphous powders' densification and crystallization dynamics race each other. Temperature is the key factor affecting this competitive process. Local overheating can lead to partial crystallization, resulting in severe embrittlement. For example, Nowak et al. reported that excessive overheating is generated in the vicinity of particle necks during the initial stages and is responsible for partial devitrification. [15] Similar phenomena have been widely observed in Fe-based [16] and Al-based amorphous alloys. [17] Despite the significant advantages of SPS, its further popularization is hindered by severe temperature and stress gradients, [18] especially for complex-shaped and large-sized parts. Therefore, it is of great concern to figure out the temperature field evolution in the amorphous alloys prepared by SPS. However, to the authors' knowledge, the current understanding of the temperature distribution in amorphous alloys prepared by SPS is limited.The temperature gradients in the sintering molds and samples can be divided into inhomogeneity at the macro and particle scales. In terms of the macroscopic temperature field, Song et al. developed a coupled thermal-electric-mechanical model to simulate the SPS process of Fe powders, [19] which concerns

Multiphysics Simulation and Experimental Investigation of the Densification of Metals by Spark Plasma Sintering

“…Second, the finite difference solution of the surface and grain boundary diffusion problem ( 16), (18) was realized in an in-house Cþþ code tool by means of staggered scheme. [46] Finally, the solutions for volumetric diffusion (17), electromigration (20), and thermomigration (19) coupled with mechanical stress are obtained using ANSYS.…”

“…They focus mainly on the influence of the electric current on material transport that leads to contact growth and/or densification, [3,13,14] as well as on the local temperature distribution in the powder particles leading potentially to softening or melting of the contact zone. [6,[15][16][17][18] Additionally, as for other pressure-assisted densification methods, the influence of pressure, temperature, [19] and microstructure [16,20,21] has to be taken into account. The latter three parameters have been shown to be the most important factors in the majority of experiments.Still, the problem of separating different effects during densification in experimental setups led to the need for detailed simulation of SPS.…”

“…There exist different approaches to capture sintering processes; the simplest approach is a two-way coupled thermoelectric analysis. [17,[30][31][32][33][34][35][36][37][38] The three-way coupling of thermal, electric, and mechanical fields is taken into account by only a few researchers. [39][40][41][42][43][44] The more complex problem, taking into account both coupled diffusion and mechanics, in which the analysis of the sintering process is carried out at the microscopic level considering the inhomogeneity of the stress state, was investigated in refs.…”