A good combination of strength and ductility of ultra-coarse-grained Cu-Al alloy with coarse-grained surface layer via pre-torsional treatment
Graphical abstract
Introduction
It has been a long standing obstacle to obtain metallic materials with a combination of high strength and good ductility (Huang et al., 2006; Lu et al., 2004; Ritchie, 2011; Valiev et al., 2016; Wang et al., 2002; Wu et al., 2015). In recent ten years significant progress in obtaining both high strength and good ductility have been achieved in metals and alloys via establishing gradient structures (Fang et al., 2011; Gu et al., 2018a; Lu, 2014; Wang et al., 2013; Wei et al., 2014; Wu et al., 2014; Zhang et al., 2003). Pre-torsional treatment (Fu et al., 2012; Gu et al., 2018a, 2018b, 2019a, 2019b; Wei et al., 2014), surface mechanical grinding treatment (SMGT) (Li et al., 2008) and surface mechanical attrition treatment (SMAT) (Lu and Lu, 2004; Wu et al., 2016) were widely used to obtain gradient structures for metals and alloys. By introducing gradient structure in pure copper, the strain hardening introduced by grain refinement and the strain softening introduced by grain growth occur simultaneously at various layers of the gradient structure, leading to the yield strength double and the uniform elongation remaining constant (Fang et al., 2011). Wei et al. (Wei et al., 2014) introduced a gradient nanotwin density in a twinning induced plasticity (TWIP) steel via pre-torsional treatment, which successfully overcomes the strength-ductility trade-off dilemma. The distinctive microstructures and hierarchical deformation mechanism of gradient structure lead to the combination of both high strength and good ductility (Cao et al., 2018; Liu et al., 2019). In our previous investigations (Gu et al., 2018a, 2018b), we have systematic studied the effects of stacking fault energy and grain size on the microstructures and mechanical properties of Cu alloys after pre-torsional treatment. A combination of high strength and good ductility of Cu-6 wt.%Al alloy with ultra-coarse-grained (UCG) structure was obtained via pre-torsional treatment (Gu et al., 2018a). It should be noted that as a convenient method to implement, pre-torsional treatment can be used to modified the microstructures and mechanical responses of metallic alloys without changing the original shape and size of the samples (Gu et al., 2018b, 2019a, 2019b).
Previous investigations (Fu et al., 2012; Roland et al., 2006) also indicated that the surface layer in the metals and alloys is beneficial to their mechanical properties. In this study, the ultra-coarse-grained (UCG) Cu-6 wt.%Al alloy with a coarse-grained (CG) surface layer was successfully fabricated by surface mechanical attrition treatment, and the effects of pre-torsional treatment on the microstructural evolution and mechanical properties of the UCG-CG alloy were systematic investigated.
Section snippets
Material preparation
Cu-6 wt.%Al ingot (with the dimension of 250 mm × 85 mm × 45 mm) with an SFE of ∼6 mJ/m2 (An et al., 2019) were fabricated by vacuum induction melting technique in a high-purity argon atmosphere, using high-purity copper and aluminum (higher than 99.9 %) as the raw materials. The ingot was homogenization annealing treated at 1173 K for 2.5 h, followed by hot rolling deformation at 773 K and with the thickness reduction of ∼67 %. The dimension of the final hot rolled plate is 700 mm × 90 mm × 15
Results and discussion
Fig. 1 shows the hardness evolutions of the UCG-CG specimens along the radial direction before and after pre-torsional treatment for 180°, and then tensile testing to failure. The hardness of the UCG core is homogenous of ∼105Hv before pre-torsional treatment and increases obviously after pre-torsional treatment along the radial direction, which is similar to the hardness evolution after HPT treatment (Cao et al., 2011, 2014). After tensile testing to failure the hardness becomes homogeneous
Conclusions
An ultra-coarse-grained (UCG) Cu-6 wt.%Al alloy with a coarse-grained (CG) layer was successfully fabricated. Initial torsional treatment lead to the obvious strength and ductility trade-off due to the dramatic strain hardening of the CG layer. With further torsional treatment, the strength and ductility was simultaneously enhanced due to the combination of gradient structure in the UCG and deformed CG layer.
Declaration of Competing Interest
The authors declared that they have no conflicts of interest to this work.
Acknowledgements
The financial supports from National Natural Science Foundation of China (51771229), Natural Science Foundation of Hunan Province (2018JJ3649) and Science and Technology Program of Shenzhen City (JSGG20170824162647398) are appreciated. The Advanced Research Center of Central South University is sincerely appreciated for TEM technical support.
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