Elsevier

Acta Materialia

Volume 69, May 2014, Pages 68-77
Acta Materialia

Influence of dislocation–solute atom interactions and stacking fault energy on grain size of single-phase alloys after severe plastic deformation using high-pressure torsion

https://doi.org/10.1016/j.actamat.2014.01.036Get rights and content

Abstract

Several pure metals (magnesium, aluminum, iron, cobalt, nickel, copper, zinc, palladium and silver) and single-phase Al–Mg, Al–Ag, Al–Cu, Cu–Al, Cu–Zn, Pd–Ag, Ni–Fe and Ni–Co alloys were processed by severe plastic deformation using high-pressure torsion (HPT). The steady-state grain size was decreased and hardness increased by alloying in all the systems. It was shown that the dominant factor for extra grain refinement by alloying was due to the effect of solute–matrix atomic-size mismatch and modulus interaction on the mobility of edge dislocations. For the selected alloys, unlike pure metals, the grain size was almost insensitive to the melting temperature, and like pure metals, no systematic correlation was established between the grain size and stacking fault energy (chemical interaction) or between the grain size and valence electrons (electrical interaction). The presence of a power-law relation, with n  0.56, between the hardness normalized by the shear modulus and grain size normalized by the Burgers vector signified the large contribution of grain boundaries to the hardening. The contribution of the solid-solution effect to the total hardening appeared to be <15%.

Introduction

Severe plastic deformation (SPD) processes such as high-pressure torsion (HPT) and equal-channel angular pressing (ECAP) are effective for producing ultrafine-grained (UFG) materials [1], [2], [3], [4]. For all SPD methods, the grain size decreases with increasing strain at an early stage of straining but enters into a steady state at large strains, where the grain size remains unchanged with further straining. The steady-state grain sizes are characteristics of single-phase materials and they remain the same irrespective of the strain, pressure and initial microstructure [5]. Several theoretical models (e.g. [6], [7], [8], [9], [10], [11], [12], [13]) and experimental works (e.g. [14], [15]) have been developed to investigate the steady-state grain sizes, dS. Most of these reports deal with pure metals, whereas little is understood to date regarding the correlations between dS and the physical parameters of alloys after processing by SPD.

This study is thus initiated with two main objectives: one is to investigate variations of dS with respect to physical parameters such as melting temperature, diffusivity, valence electrons and stacking fault energy, as attempted earlier in Refs. [14], [15] for pure metals; and the other is to examine the effect of dislocation–solute atom interactions on grain size and hardness.

Section snippets

Theoretical background

Among the models developed to predict dS, a dislocation model developed by Mohamed has received much attention [10], [11]. The model can be represented as:dSb=Aexp-βQ4RTDP0Gb2ν0kT1/4γSFEGb1/2GHVS5/4,where b is the Burgers vector, A is a constant, β  0.04, Q is the activation energy for self-diffusion, R is the gas constant, T is the absolute temperature, DPO is the frequency factor for pipe diffusion, G is the shear modulus, ν0 is the initial dislocation velocity, k is Boltzmann’s constant, γSFE

Experimental materials and procedures

Samples of several pure metals (Mg, Al, Fe, Co, Ni, Cu, Zn, Pd, Ag) and several single-phase fcc alloys (Al–Mg, Al–Ag, Al–Cu, Cu–Al, Cu–Zn, Pd–Ag, Ni–Fe, Ni–Co) were used in this study. Table 2 gives a list of materials and their physical parameters taken from Refs. [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57]

Results

Fig. 1 plots the microhardness vs. shear strain for pure Al and for four selected Al–Mg alloys. Fig. 1 demonstrates that all hardness data for each material follow a unique function of shear strain and reach steady-state levels at large shear strains. The steady-state hardness appears to increase with an increase in the fraction of solute Mg atoms. It should be noted that the hardness–strain behaviors for all selected alloys in this study were similar to the ones represented in Fig. 1, as shown

Discussion

The current findings show that solute atoms influence SPD-processed materials not only through enhanced solid-solution hardening but also through a more enhanced grain refinement when compared to pure metals. The relation between dS and the stress required for dislocation motion in the presence of solute atoms having larger mismatch with matrix atoms provides a practical means to reduce the grain size to the nanometer scale (dS < 100 nm) through manipulation of the chemical composition. It should

Conclusions

Several pure metals and single-phase Al–Mg, Al–Ag, Al–Cu, Cu–Al, Cu–Zn, Pd–Ag and Ni–Fe alloys were processed by HPT and the following conclusions were obtained:

  • 1.

    The dominant factor for extra grain refinement by alloying is the effect of atomic-size and modulus mismatch on the mobility of edge dislocations. The solute atoms increase the localized stress needed for dislocation motion (Δτ in Eq. (3), Eq. (5) and Fig. 4), and thus diminish the dislocation recovery, recrystallization and grain

Acknowledgements

One of the authors (K.E.) thanks the Japan Society for Promotion of Science (JSPS) for a Grant-in-Aid for Research Activity (No. 25889043). We would like to thank Furukawa-Sky Aluminum Corporation (now UACJ Corporation) for providing the Al–8.8% Mg alloy. This work was supported in part by the Light Metals Educational Foundation of Japan and in part by a Grant-in-Aid for Scientific Research from the MEXT, Japan, in Innovative Areas “Bulk Nanostructured Metals” (Grant No. 22102004).

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