Effect of Twin Boundary–Dislocation–Solute Interaction on Detwinning in a Mg–3Al–1Zn Alloy

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In the present study, the influence of solute atoms together with dislocations at {101¯2} twin boundary (TB) on mechanical behavior of a detwinning predominant deformation in a Mg alloy AZ31 plate was systematically studied. The results show that a large number of {101¯2} twins disappear during recompression along the normal direction. Both the TB–dislocation interaction and TB–solute–dislocation interaction can greatly enhance the yield stress of the recompression along the normal direction (ND). However, the solute segregation at {101¯2} TBs with an intensive interaction with <a> dislocations cannot further enhance the yield stress of ND recompression. The samples with TB–dislocation interaction show a similar working hardening performance with that subjected to a TB–solute–dislocation interaction. Both the TB–dislocation interaction and TB–solute–dislocation interaction greatly reduce the value of work hardening peaks during a detwinning predominant deformation.

Introduction

Magnesium (Mg) alloys with low density and high specific strength and stiffness have shown great potential for applications where weight is a primary concern[1], [2], [3]. A strong basal texture can be readily developed during plastic processing of Mg and its alloys with hexagonal close-packed (hcp) structure, which will lead to a poor working ability and strong mechanical anisotropy[4], [5], [6], [7]. Twinning-derived deformation constitutes one of the main deformation modes due to the limited active slip systems at room temperature. Due to the pole nature, twinning in Mg alloys is normally triggered by both the loading directions and the types of load. For example, {101¯2} twinning is generally initiated under a compression stress perpendicular to the c-axis or a tensile one parallel to the c-axis[8], [9], [10]. Twinning intimately affects the strength, formability, mechanical anisotropy of Mg alloys[11], [12], [13], [14]. Recently, twin boundary (TB) has also been extensively used to tailor the strength, damping capacity, and tension–compression yield asymmetry of Mg alloys[2], [15]. It is reported that the reorientation by {101¯2} twinning can effectively improve hot rolling ability and reduces tension–compression yield asymmetry[16], [17], [18]. Cui et al. found that the introduction of a large number of {101¯2} TBs could further enhance damping capacity[16], [17], [19].

TB migration (e.g. twin growth or detwinning) or dislocation–TB interaction is an important issue during reloading of metals containing pre-existing TBs[20], [21], [22]. For example, reverse reloading or reloading along specific strain paths of a Mg AZ31 plate with {101¯2} twins is often a detwinning predominant deformation[5]. Intensive TB–dislocation interaction often accompanies the loading of nano-twinned fcc metals[23], twinning in hcp metals[8], [24], [25], or cyclic loading of Mg alloys[26]. The effect of TB–dislocation interaction on a slip predominant deformation has been extensively studied. It is well established that the accumulation of dislocations at TBs significantly affects the strength, ductility and strain hardening of Mg alloys[23]. The possible dislocation reactions at TBs involve the dislocations slip into the twinning plane, formation of sessile dislocations at TBs, and dislocations transmit across TBs[23], [27].

However, studies to address the influence of TB-dislocation on a TB migration predominant deformation are rare. In a recent publication, it was found that interaction between {101¯2} TBs and a dislocation greatly enhances the activation stress for TB motion and retards TB migration during reloading[28]. Recently, Nie et al. reported that solute atoms tended to segregate at TBs of Mg alloys to minimize the elastic strains after annealing. In this case, solute atoms will pin the TBs when the TBs migrate[29], [30]. It is hypothesized that mechanical performances might be different when a dislocation–TB–solute atom exists. Nevertheless, no open publications have addressed this particular matter. In the present study, a pre-twinned Mg alloy AZ31 plate was annealed at 180 °C for 2 h to allow the solute to segregate at {101¯2} TBs, followed by are-tension along rolling direction (RD) to achieve an intensive interaction of a dislocation with TBs. Subsequently, detwinning was initiated by reloading, with the aim to understand the mechanical difference when dislocation and solute atoms work synergistically at TBs.

Section snippets

Sample preparation and mechanical tests

As seen in Fig. 1, the initial material was a hot-rolled Mg alloy AZ31 plate with fully recrystallized grains and a typical basal texture (basal poles largely parallel to normal direction of the plate). Fig. 2 schematically illustrates the preparation of pre-twinned materials and specimens for mechanical tests. To prepare the materials with {101¯2} twins, bulk AZ31 plates with a dimension of 90 mm (RD) × 10 mm (TD) × 20 mm (ND) were rolled at room temperature with a 3.0% thickness reduction

Mechanical behavior

True stress–strain curves of samples under compression along ND are presented in Fig. 3. A plateau, the typical feature of a detwinning predominant deformation in Mg alloys, exists in the curves of the pre-rolled or the pre-rolled and annealed samples, while is not obvious in the sample with pre-tension along RD. As seen in Table 1, annealing enhances the yield strength of pre-twinned sample by about 29 MPa. A further pre-tension along TD greatly increases both yield stress and peak stress.

Discussion

It has been extensively reported that, for a hot rolled plate with a strong basal texture, compression along TD or RD is a {101¯2} twinning predominant deformation[8], [28]. Therefore, the pre-rolling with thickness reduction along TD generally generates a large number of {101¯2} twins as observed in Fig. 6. As {101¯2} twinning reorients the twinned regions by approximately 86° toward the compression directions[31], basal poles around TD in R3.0% come from the {101¯2} twins. As seen in Fig. 9

Conclusions

In the present study, a pre-twinned Mg AZ31 plate was annealed at 180 °C for 2 h to allow the solute to segregate at {101¯2} TBs, followed by RD re-tension to achieve an intensive interaction of a dislocation with TBs. Subsequently, detwinning was initiated by reloading, with the aim to understand the mechanical difference when dislocation and solute atoms work together at TBs. Several key conclusions were drawn as follows:

  • (1)

    Both the TB–dislocation interaction and TB–solute–dislocation

Acknowledgments

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Nos. 51371203 and 51571041) and the National Key Basic Research Program of China (No. 2013CB632204).

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