Papers by Author: Terence G. Langdon

Abstract: The bulk ultrafine-grained (UFG) materials usually show superior mechanical properties. Since the occurrence of superplastic flow generally requires a grain size smaller than ~10 μm, it is anticipated that materials processed by severe plastic deformation (SPD) will exhibit superplastic ductilities when pulled in tension at elevated temperatures. Recent advances in the processing of UFG metals have provided an opportunity to extend the understanding of superplastic flow behavior to include UFG materials with submicrometer grain sizes. Recent studies showed the UFG materials demonstrated the development of plasticity in micro-mechanical response at room temperature by the significant changes in microstructure attributed to high-pressure torsion (HPT). Accordingly, this study summarizes recent results on excellent ductility and plasticity in a UFG Zn-22% Al alloy. Specifically, the alloy demonstrated the occurrence of exceptional superplastic flow at high temperature after equal-channel angular pressing and HPT and excellent room temperature plasticity of the alloy after HPT where the plasticity was evaluated by the nanoindentation technique. The significance of purity of the alloy is also considered for enhancing the ductility at room temperature.

Abstract: The occurrence of superplasticity may be traced to the classic work of Pearson conducted in the U.K. in 1934 when an elongation of 1950% was reported in a Pb-Sn eutectic alloy. Subsequently, much attention in Russia was devoted to this scientific curiosity and this led to the first book on superplasticity written by Prof. A.A. Presnyakov and published in 1964. Later, in 1985, Oscar Kaibyshev established in Ufa the Institute of Problems of Superplasticity of Metals of the Russian Academy of Sciences and this was, and remains to this day, the only institute in the world devoted exclusively to studies of the phenomenon of superplastic flow and the development through superplastic forming of complex-shaped parts. An important development occurred in 1988 with the publication of a classic report by Kaibyshev and co-workers describing the potential for achieving low temperature superplasticity in a metallic Al-Cu-Zr alloy that had been specially processed by severe plastic deformation (SPD) to produce a remarkably small grain size of only 300 nm. This report formed the basis for the later development of SPD processing as a major tool for the production of exceptional grain refinement and as a procedure for achieving large superplastic elongations that cannot be achieved using more conventional processing. This report describes this early work, the subsequent developments and the modern status of superplastic flow in ultrafine-grained metals.

Abstract: The lattice defect structure developed during plastic deformation in a High-Entropy Alloy (HEA) with the composition of Ti₃₅Zr_27.5Hf_27.5Nb₅Ta₅ was investigated. The crystallite size as well as the density and types of dislocations in a disk processed by High-Pressure Torsion (HPT) were determined by X-ray profile line analysis (XLPA). Additional transmission electron microscopy (TEM) investigations were carried out to monitor the grain size evolution during deformation. It was found that the dislocation density in the HPT-processed sample was very high compared to conventional materials. In addition, in Ti₃₅Zr_27.5Hf_27.5Nb₅Ta₅ HEA the initial body-centered cubic structure transformed into a martensitic phase during HPT. The hardness of this HEA was investigated along the HPT-processed disk radius and correlated to the microstructure.

Abstract: The effect of different plastic deformation methods on the phase composition, lattice defect structure and hardness in 316L stainless steel was studied. The initial coarse-grained γ-austenite was deformed by cold rolling (CR) or high-pressure torsion (HPT). It was found that the two methods yielded very different phase compositions and microstructures. Martensitic phase transformation was not observed during CR with a thickness reduction of 20%. In γ-austenite phase in addition to the high dislocation density (~10 × 10¹⁴ m^-2) a significant amount of twin-faults was detected due to the low stacking fault energy. On the other hand, γ-austenite was gradually transformed into ε and α’-martensites with transformation sequences γ→ε→α’ during HPT deformation. A large dislocation density (~133 × 10¹⁴ m^-2) was detected in the main phase (α’-martensite) at the periphery of the disk after 10 turns of HPT. The high defect density is accompanied by a very small grain size of ~45 nm in the HPT-processed sample, resulting in an very large hardness of 6130 MPa.

Abstract: An Al-3% Mg-0.2% Sc alloy was subjected to annealing or solution treatment and further processed by HPT at room temperature. Microhardness measurements were taken along the middle-sections of the discs and they demonstrated that a very substantial hardening is achieved during HPT processing regardless of the initial heat treatment. Hardness values of ~200 Hv were recorded at the edge of the samples although the microhardness distribution remained inhomogeneous along the diameters of the discs after 20 turns of high-pressure torsion. In addition, the microhardness of the solution treated Al-Mg-Sc samples continued to increase with the equivalent strain imposed by the anvils even after 30 turns of HPT processing whereas the hardness at the edges of the annealed discs saturated after 10 turns. These differences in the hardness evolution are attributed to the higher Mg content in solid solution in the case of the solution treated samples and its influence on delaying the recovery rate of this aluminium alloy.

Abstract: High-pressure torsion (HPT) is one of the major severe plastic deformation (SPD) procedures where disk metals generally achieve exceptional grain refinement at ambient temperatures. HPT has been applied for the consolidation of metallic powders and bonding of machining chips whereas very limited reports examined the application of HPT for the fabrication of nanocomposites. An investigation was initiated to evaluate the potential for the formation of a metal matrix nanocomposite (MMNC) by processing two commercial metal disks of Al-1050 and ZK60 magnesium alloy through HPT at room temperature. Evolutions in microstructure and mechanical properties including hardness and plasticity were examined in the processed disks with increasing numbers of HPT turns up to 5. This study demonstrates the promising possibility for using HPT to fabricate a wide range of hybrid MMNCs from simple metals.

Abstract: Ultrafine-grained (UFG) materials produced by severe plastic deformation (SPD) may show both enhanced ductility and strength and hence resolve the so-called strength-ductility paradox. To gain mechanistic insights into such resolution, the effect of high-pressure torsion (HPT) on the microstructure and mechanical behavior was studied using a cast Al-7 wt. % Si alloy. As expected, the grain size decreased while the fraction of high-angle grain boundaries and microhardness increased due to HPT processing. However, tensile testing at room temperature revealed a simultaneous increase in strength and ductility compared to the as-cast sample. The samples showing simultaneous increase in strength and ductility also showed an increased contribution from grain boundary sliding (GBS), even at room temperature, which is attributed to the existence of a high fraction of high-angle and high-energy grain boundaries. It is proposed that the occurrence of moderate GBS, providing ductility, in very small size grains provides Hall-Petch strengthening and this suggests a potential combination for simultaneously achieving high strength and high ductility in SPD-processed UFG materials.

Abstract: The Al-1% Mg and Al-0.1% Mg alloys were both processed by high-pressure torsion (HPT) at room temperature. In the Al-1% Mg alloy, the hardness values in the disc centre area are lower than in the disc edge area after 1/2 and 1 turn, and the area of lower hardness values in the disc centre decreases as the number of turns increases from 1/2 to 1 turn. Finally, the hardness values are reasonably homogenous along the disc diameter as the number of turns increases to 5 and 10 turns. The Al-0.1% Mg alloy displays a different hardness evolution behavior: the hardness values in the disc centre are higher than at the disc edge 1/2 and 1 turn, and the area of higher hardness values decreases as the numbers of turn increases from 1/2 to 1 turn. The hardness values evolve towards homogeneity along the disc diameter after 5 and 10 turns. EBSD microstructure investigations in the Al-0.1% Mg alloy reveal that a few low-angle boundaries exist at the disc edge after 1/2 turn. It is suggested that the higher hardness values in the disc centre in the Al-0.1% Mg alloy are related to rapid recovery at the disc edge where the material is subjected to heavy straining.

Abstract: High-Pressure Torsion (HPT) is one of the most effective severe plastic deformation techniques in grain refinement. The goal of this study was to investigate the influence of HPT on the microstructure and hardness of a Ti-rich High-Entropy Alloy (HEA). The evolution of the grain size due to 1 turn of HPT was studied by transmission electron microscopy. Besides the refinement of the microstructure, a phase transition also occurred during HPT, as revealed by X-ray diffraction. The initial bcc structure transformed into a martensitic phase throughout the material. The features of this phase transformation were studied on a sample compressed to low strain values. The hardness as a function of the distance from the center in the HPT-processed disk was measured and correlated to the microstructure.

Abstract: The evolution of phase composition, microstructure and hardness in 316L austenitic stainless steel processed by high-pressure torsion (HPT) was studied up to 20 turns. It was revealed that simultaneous grain refinement and phase transformation occur during HPT-processing. The γ-austenite in the initial material transformed gradually to ɛ-and α’-martensites due to deformation. After 20 turns of HPT the main phase was α’-martensite. The initial grain size of ~42 μm was refined to ~48 nm while the dislocation density increased to ~143 × 10¹⁴ m^-2 in the α’-martensite phase at the disk periphery processed by 20 turns. The microstructure and hardness along the disk radius became more homogeneous with increasing numbers of turns. An approximately homogeneous hardness distribution with a saturation value of ~6140 MPa was achieved in 20 turns.