Plastic deformation kinetics of electrodeposited Cu foil at low and intermediate homologous temperatures

Abstract

INTRODUCTIONIn recent years, there has developed a renewed interest in the effect of grain size on the mechanical properties of metals and ceramics with special attention to the submicron grain-size range. In a review 1 by the senior author (HC), it was suggested that the effect of grain size on the flow stress of Cu at low and intermediate homologous temperatures (i.e., T Ͻ 0.5 T M ) could be considered in terms of three grain-size regimes (Fig. 1). Regime I includes grain sizes in the range of approximately 1-100 m, Regime Ia in the range of ϳ0.1-1 m, and Regime II in the range of ϳ0.001-0.1 m. It was concluded that the plastic deformation kinetics in Regimes I and Ia were governed by intragranular dislocation motion. The rate-controlling mechanism in Regime I was deduced to be the intersection of dislocations. The rate-controlling mechanism in Regime Ia appeared to be grain boundary shear induced by dislocation pileups at T Ͻ ϳ350 K and the occurrence of cross slip or the intersection of dislocations at higher temperatures. Grain boundary shear produced by the applied stress alone was proposed to be rate controlling in Regime II.The behavior of polycrystalline Cu with grain size in Regime Ia is of interest in the present paper. The only data in sufficient detail for evaluating the plastic deformation kinetics in this regime known by the present authors are those by Embury and Lahaie (E-L) 2 on 30-m thick, vapor-deposited (VP) Cu foil with a grain size 0.5 Ϯ 0.3 m tested in tension at 77-473 K (Fig. 2). Their results show an irregular variation of the apparent activation volume v = kT ∂ ln ε ⋅ /∂σ with temperature. At T Ͻ ϳ350 K, v increases with decrease in temperature (i.e., with increase in flow stress ), while at T Ͼ 350 K the opposite occurs (i.e., v increases with decrease in ). The latter is the more usual behavior for thermally activated dislocation motion. In the analysis 1 of the E-L data, it was proposed that the rate-controlling mechanism at T Ͻ 350°C was grain boundary shear induced by the pileup of dislocations at the grain boundaries and that at T Ͼ 350 K cross slip or the intersection of dislocations appeared to be the likely mechanism. The objective of the present investiga-The plastic deformation kinetics of electrodeposited (EP) Cu foil (grain size d ϭ 0.6 m) were determined at 296-448 K and compared with those for vapor-deposited (VP) foil (d ϭ 0.5 m) tested at 77-473 K. The apparent activation volume v = kT ∂ ln ε ⋅ /∂σ for both materials exhibited a minimum at ϳ350 K, and at this temperature, there occurred a marked increase in the temperature dependence of the flow stress . The rate-controlling mechanism in both materials at T Ͻ 373 K appears to be grain boundary shear induced by dislocation pileups at the grain boundaries. The results at T ϭ 373-473 K suggest that the dislocation pileups are relieved or prevented and that either cross slip or intersection of dislocations is rate controlling with stronger support for the latter. The determination of the rate-controlling mechanism at the h...

Compressive behavior and deformation kinetics of ultrafine grained aluminum processed by equal channel angular pressing

“…This conclusion is in good agreement with the experimental data. Since the pre-exponential strain rate, _ ε 0 , is of the order of 10 6 -10 8 /s [56] which is several orders of magnitude higher than that in the present tests, values of TS is expected to be negative according to Equation (9). This indicates the reduction in flow stress with increasing temperature.…”

“…V* in the range of 70-150b 3 and quasi-static SRS 0.013-0.058 for current UFG-Al [3,15,54,62]. Similarly, the grainboundary shear could also be ruled out since it bears the same order of activation volume with Coble creep [56,63]. More importantly, recent analysis presented evidence that such diffusion-assisted GB processes are more likely operate at very low rates and high temperatures, especially in metals with grain size below tens of nanometers [3,14,64,65].…”

“…In case that the grain size is reduced to ultrafine level (below 1 μm but above 100 nm) or nanograin regime (grain size below 100 nm), GB-related processes, such as Coble creep [58][59][60][61], GB sliding/shear mediated by GB diffusion [11,56], and dislocation-boundary interactions [14,25,32], have been proposed to be the possible rate-controlling mechanisms. T for the present UFG-Al at quasi-static (blue circles) and dynamic loading rates (wine triangles) as well as UFG-Al by May et al [15] deformed under static loading condition (triangles).…”

“…Thus, during plastic deformation, moving dislocations may travel almost unimpeded through the lattice of the interior of the grains but slow down and spend most of time on fighting against the forest dislocations near/at grain boundaries and therefore enable the rate-controlling mechanism to be medicated by dislocations intersection near/at grain boundaries [3,10,56,77]. Therefore, one possibility for the larger value of estimated dislocation density in the presence UFG-Al compared to results reported in literatures might be that the locally high density of dislocations arranging near/at the submicron grain boundaries [56,78,79].…”

Investigation on mechanical behaviour of ECAPed 2A12 aluminium alloy

“…This conclusion is in good agreement with the experimental data. Since the pre-exponential strain rate,ε 0 , is of the order of 10 6 -10 8 /s [17] which is several orders of magnitude higher than aare expectedin rates in the present tests, values of temperature sensitivity is expected to be negative according to Eq. (4).…”