Twenty-five years of ultrafine-grained materials: achieving exceptional properties through grain refinement

High-pressure torsion (HPT) is an attractive processing method in severe plastic deformation techniques involving the application of high compressive pressure with concurrent torsional straining. Excellent grain refinement is anticipated when using this technique to average grain sizes of the submicrometer or even nanometer ranges. Because of the significant microstructural changes during processing, there are numerous reports showing evolution in local hardness toward homogeneity throughout a disk diameter with increasing numbers of revolutions. The achieved hardness after HPT is mostly much higher than that in the as-received condition because of exceptional grain refinement although there are a limited number of metals and alloys showing softening or weakening after HPT processing. This paper was initiated to review recent discoveries in the experimental results on hardness evolution toward homogeneity during HPT processing and discuss the different models of hardness developments with respect to imposed equivalent strain by HPT processing for a range of metals and alloys. Moreover, recent results of hardness homogeneity and heterogeneity through thicknesses of the processed disks are discussed toward a complete understanding of hardness evolution in the UFG metals processed by HPT.

Different models of hardness evolution in ultrafine-grained materials processed by high-pressure torsion

Advances in ultrafine-grained materials

The mechanisms of superplasticity occurring in conventional materials, having grains sizes of the order of a few microns, are now understood reasonably well. However, very recent advances in the processing of ultrafine-grained (UFG) metals have provided an opportunity to extend the understanding of flow behavior to include UFG materials with submicrometer grain sizes. In practice, processing through the application of severe plastic deformation (SPD), as in equal-channel angular pressing (ECAP) and high-pressure torsion (HPT), has permitted the fabrication of relatively large samples having UFG microstructures. Since the occurrence of superplastic flow generally requires a grain size smaller than ~10 μm, it is reasonable to anticipate that materials processed by SPD will exhibit superplastic ductilities when pulled in tension at elevated temperatures. This review examines recent results that demonstrate the occurrence of exceptional superplastic flow in a series of UFG aluminum and magnesium alloys after ECAP and HPT. The results are analyzed to evaluate the superplastic flow mechanism and to compare with materials processed using different techniques. The critical issue of microstructural inhomogeneity is examined in two-phase UFG materials after SPD processing and the influence of microstructural homogeneity on the superplastic properties is also demonstrated.

Review: achieving superplastic properties in ultrafine-grained materials at high temperatures

A Mg–Li alloy with 8 wt% Li was processed by severe plastic deformation (SPD) through the process of high-pressure torsion (HPT) to achieve ultrafine grains with an average grain size of ~500 nm. Tensile testing with an initial strain rate of 10 −3  s −1 showed that the alloy exhibited superplasticity at a temperature of 323 K or higher. Tensile testing in boiling water confirmed that the specimens were elongated to 350–480% at 373 K under the initial strain rates of 10 −3  s −1 to 1 0 −2  s −1 with a strain rate sensitivity of ~0.3. The current study suggests that not only superplastic forming but also superplastic hydroforming should be feasible after the grain refinement using the HPT method.

Ultrafine-grained magnesium–lithium alloy processed by high-pressure torsion: Low-temperature superplasticity and potential for hydroforming

An Al–33% Cu eutectic alloy was processed by high-pressure torsion (HPT) at a pressure of 6.0 GPa for 1/4 to 10 turns. Examination after processing showed a gradual evolution to a reasonable level of hardness and microstructural homogeneity after 5 or more turns. Tensile tests were conducted at a temperature of 723 K on specimens processed through 5 and 10 turns of HPT. These specimens exhibited excellent superplastic properties with a maximum elongation of ∼1250% at strain rates lower than 10−3 s−1. The results also showed that the maximum elongation is displaced to a faster strain rate when the HPT processing is conducted to a higher number of turns. An analysis demonstrates that superplastic flow in the Al–Cu alloy processed by HPT is well described by a theoretical relationship that was developed for conventional superplastic materials.

Julien Foissey

Papers

Development of hardness homogeneity and superplastic behavior in an aluminum–copper eutectic alloy processed by high-pressure torsion