The Influence of Porous Structure on the Interdiffusion Kinetics of Cu-Ni System During Spark Plasma Sintering
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cation feature of spark plasma sintering (SPS) stems from its accelerated atomic diffusion by electric current.[1–3] However, the multi-field coupling of temperature, electric, and magnetic fields makes it difficult to understand the mechanisms of atomic diffusion during SPS. Thus, the mechanisms of current-enhanced atomic diffusion remain obscure. It has been suggested that the role of the current in enhancing diffusion may relate to changes in defect concentration or mobility.[4,5] That is, the imposition of a current increases the concentration of vacancies in metals, enhancing the growth of the diffusion layer. Although this view has been accepted by many researchers,[6,7] it still lacks definitive evidence. Additionally, electromigration has been considered another
RUIDI LI, PENGDA NIU, LINJUN TANG, SIYAO XIE, and TIECHUI YUAN are with the State Key Laboratory of Powder Metallurgy, Central South University, Changsha, 410083, P.R. China. SHENGHUA DENG is with the School of Material Science and Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, P.R. China. Contact e-mail: [email protected] QIGANG WENG is with the Zunyi Titanium Co., Ltd, Zunyi, 563004, P.R. China. Manuscript submitted February 25, 2019. Article published online January 21, 2020 METALLURGICAL AND MATERIALS TRANSACTIONS A
mechanism of current-accelerated diffusion and has been supported by experimental studies on the interdiffusion of Ni-Al,[8] Cu-Ni,[9] W-Ti,[10] Ag-Sn[11] and Ni-Sn[12] interdiffusion systems under direct current. However, Trzaska[13] and Rudinsky[14] demonstrated that due to low current density (£ 1000 A/cm2), no significant electromigration phenomenon was observed during SPS. In general, the electromigration effect is known to occur at a current density of 8 9 102 to 1 9 105 A/cm2 in dense metals.[9,14,15] Nevertheless, the current density distribution of powder material during sintering is significantly different from that of dense materials. Collard’s[16] numerical simulations of powder materials during SPS show that the local current density at the tips of necks between particles is nearly 5.87 9 108 A/m2 (5.87 9 104 A/cm2), which is several orders of magnitude higher than the apparent current density. Thus, the current-enhanced atomic diffusion kinetics of powder materials may well be different from those of dense materials. Unfortunately, few studies have compared the differences in atomic diffusion behaviors between dense materials and powder materials. In this study, to investigate the effect of porous structures on atomic interdiffusion behaviors, the Ni/ Cu/Ni diffusion couple was implemented in a special
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sandwich structure (foil/foil/powder) to compare the mechanisms of current-enhanced diffusion kinetics between the foil/foil and foil/powder interfaces.
II.
EXPERIMENTAL PROCEDURE
The diffusion couples were assembled into two types of sandwich ensembles: Ni foil/Cu foil/Ni foil and Ni foil/Cu foil/Ni powder, as shown in Figure 1. The Cu foils had a thickness of 25 lm and a purit
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