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ark plasma sintering (SPS) has emerged as an advanced sintering technique in the last few decades, owing to its versatile ability to sinter a variety of materials to full density with ultrafine grains in a shorter duration compared to conventional sintering technology.[1,2] Its particularity is that a pulsed direct current is applied through the entire sintering process, which accelerates the densification of powders.[3–6] However, the micromechanisms involved in the densification have not been identified unambiguously due to the complex multifield of the coupling temperature, electricity, and pressure.[7–9] Atomic diffusion, as the smallest kinetics behavior in SPS, needs to be investigated to understand the SPS mechanisms. It has been proven that atomic diffusion can be significantly enhanced by direct current (DC).[5] For example, Garay et al.[7] studied the effect of DC on interfacial reactions in Ni-Ti system and compared the reactivity with the current-free diffusion couples, pointing out that the intrinsic growth rate constant of NiTi2 intermetallic under DC was 43 times

RUIDI LI, PENGDA NIU, TIECHUI YUAN, and GUANGHONG LIU 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] Manuscript submitted April 3, 2019. METALLURGICAL AND MATERIALS TRANSACTIONS B

higher than that without DC. Some researchers believe that the current enhanced atomic diffusion may relate to an increase in the defect concentration, which is induced by the current passing these phases or influencing their mobility.[7,10] However, while knowledge about the effect of the current on defect mobility is sparse,[11,12] on-going investigations are aimed at determining this possible effect. Moreover, electromigration has been considered as another mechanism of direct currentaccelerated diffusion, which has been supported by some investigations on Ni-Al,[13] Cu-Ni,[14] W-Ti,[15] Ag-Sn,[16] and Ni-Sn[17] interdiffusion systems under the influence of DC. The electromigration effect has been observed directly by many works under the DC density range of 3.16 9 102-105 A cm2.[13,18,19] However, the enhancing mechanism of atom diffusion under DC is significantly different from that under pulse current (PC) in SPS.[5] So far, only a few attempts have been undertaken to address the different kinetic behaviors between the DCactivated diffusion and PC-activated diffusion. To solve this problem, in this work, a Ti-Ni diffusion couple, which has been reported by Garay,[7] was selected as a representative diffusion couple to study the effect of PC on the diffusion kinetics by comparing the reaction behavior under PC and DC. In the present work, the diffusion couples were composed of three layers of foils and wafers arranged in a sequence of Ti/Ni/Ti. The thickness of Ni foils was 35 lm with 99.5 pct purity and the thickness of Ti wafers was 3 mm wit