Papers by Author: Nong Gao

Abstract: Severe plastic deformation (SPD) processes, particularly high-pressure torsion (HPT) have been increasingly applied to metallic specimens fabricated by laser powder bed fusion (L-PBF) additive manufacturing (AM) for enhancing their mechanical and functional properties through nanoscale grain refinement (≤ 100 nm). In this study. L-PBF AM-fabricated Inconel 718 (IN 718) specimens are initially subjected to 10 HPT revolutions to produce nanosized grains. Subsequently, microstructural characterisation, as well as hardness and electrochemical tests are conducted to evaluate the evolution of microstructures, hardness, and corrosion performance of the as-received and HPT-processed specimens by using various microscopy, Vickers microhardness (HV) measurements, and corrosion performance, respectively. The results reveal an average grain size of ~ 46 nm, dense dislocation networks, and nanotwins after 10 HPT processing, which contribute to the two-fold hardness increase compared to the as-received condition. Such microstructures also contributed to the overall improved corrosion performance after 10 HPT processing, as quantified by the 83% and 73% reduction in corrosion rate and pitting potential, respectively.

Abstract: An Al-1050 alloy was processed by ECAP and HPT, respectively. Dry sliding wear tests were conducted on the as-received and SPD-processed samples under various sliding conditions. A comparison of wear rate indicated that SPD processing decreased the wear resistance. Two main wear mechanisms were observed. The initial wear stage was dominated by severe platelet wear mechanism and later wear was dominated by an oxidation wear mechanism. The results show the severe wear stage of SPD-processed Al-1050 is much longer than that of the as-received Al-1050, which is attributed to the loss of work hardening capacity after SPD processing.

Abstract: Various different severe plastic deformation processes (SPD) have been developed to produce ultra-fine grained (UFG) materials during the last two decades. One very important material property that the UFG materials should have for structural materials application is good wear resistance. This review paper presents some recent work related to the wear resistance of materials processed by SPD, in particular for alloys processed by using equal-channel angular pressing (ECAP) and accumulative roll-bonding (ARB).

Abstract: The strengthening of AlCuMg(Li) alloys subjected to high pressure torsion (HPT) deformation with strain reversals was studied by microhardness (Hv) tests and differential scanning calorimetry (DSC). It was found that the strengthening is lower for both cyclic HPT (c-HPT) and single reversal HPT (sr-HPT) as compared to monotonic HPT (m-HPT). The DSC results demonstrate that |HPT influences S phase precipitation. With increasing strain, the maximum heat flow (height of the S peak) and the heat content of S formation peaks increases. There is a larger S heat content reaction in the periphery of HPT processed disks compared with those in the centre. Strain reversal also has a significant influence on the S precipitation. The strengthening during HPT deformation is discussed in terms of the density of statistically stored and geometrically necessary dislocations.

Abstract: Room temperature equal-channel angular pressing (ECAP) was employed on commercial purity titanium in the present work. Mechanical twining was observed in ~90% grains while the grains were not significantly refined (~10 μm) after ECAP. TEM observations showed that the twins observed under OM are usually composed of a serial of parallel twin bands with a width of ~1 μm. Microhardness and tensile tests showed that such a coarse-grained microstructure combined with a high fraction of mechanical twins has a microhardness of ~ 240 Hv, yield strength of ~ 730 MPa, tensile strength of ~ 740 MPa and elongation to failure of ~ 16%. This yield strength is much higher than 620-640 MPa, the yield strength of ultrafine-grained titanium by 8~12 passes of ECAP at 450 oC with a grain size of 200-300 nm, and is close to ~ 790 MPa, the yield strength of commercial Ti-6Al-4V alloys.

Abstract: The phase decomposition was investigated in Cu-Al alloys processed to a nanostructure condition by High Pressure Torsion (HPT). The microstructures are characterized by optical microscopy (OM), X-ray diffraction (XRD) and Atom Probe Tomography (APT). The results show that the’ → (1 + decomposition reaction begins in the early stage of annealing and it is much faster than in the coarse-grained state although there are similar phases after annealing.

Abstract: Disks of a commercial purity aluminium Al-1050 alloy and Al-1%Mg alloy were processed by high-pressure torsion (HPT) at room temperature for up to a maximum of 5 turns under a pressure of 6 GPa. Following processing, hardness measurements were recorded across the surfaces of the disks. These measurements showed low values of hardness at the center and high values near the edges of the disks and the hardness increased in both alloys with increasing numbers of turns. The evolution of homogeneity in hardness was rapid in Al-1050 compared to the Al-1%Mg alloy. After 5 turns of HPT under a pressure of 6 GPa, the hardness was fully homogeneous across the total surface of the Al-1050 disk whereas there was a region of lower hardness around the center of the Al-1%Mg disk. The results reveal the significant difference between both alloys where the higher rate of recovery in the Al-1050 alloy leads to a rapid evolution of the hardness homogeneity.

Abstract: The evolution of texture and deformation in the grains during one pass of equal-channel angular pressing (ECAP) was examined for fine grained high strength and low strength Al alloys and a coarse grained low strength Al alloy. The materials were analysed using electron back-scatter diffraction (EBSD). The results are consistent with the materials responding to the intense macroscopic shear stress by deformation of individual grains through movement of dislocations on one or more of the slip crystallographic slip planes {hkl} that are favourably oriented, combined with the rotation of grains to directions that bring main crystallographic slip planes parallel to the macroscopic shear direction and crystallographic slip directions parallel to two main shear directions. Contrary to reports claiming up to 4 slip systems are activated, it was observed that only the {111}<110> and {001}<110> shear systems are activated. Macroscopic shear deformation occurs on two shear planes: the main shear plane (MSP), equivalent to the simple shear plane, and a secondary shear plane which is perpendicular to the MSP.

Abstract: Billets of a commercial purity aluminium Al-1050 alloy were processed by equal-channel angular pressing (ECAP) for up to a maximum of 6 passes. Following processing, the billets were sectioned and hardness measurements were recorded on both longitudinal and transverse sections. These measurements showed the hardness increases significantly in the first pass and continues to increase by small amounts in subsequent passes. Initially, there are regions of lower hardness running in bands near the top and bottom surface of each billet. The region of lower hardness near the upper surface disappears with increasing numbers of passes but near the bottom surface the lower hardness remains even after 6 passes. The results show that, neglecting the small region near the bottom of the billet, there is an excellent potential for achieving microstructural homogeneity within the Al-1050 alloy after pressing through a sufficient number of passes in ECAP.

Abstract: Differential Scanning Calorimetry (DSC) is a thermal analysis technique that measures the energy absorbed or released by a sample as a function of temperature or time. DSC has wide application for analysis of solid state reactions and solid-liquid reactions in many different materials. In recent years, DSC has been applied to analyze materials and alloys processed through Severe Plastic Deformation (SPD). The basic principle of SPD processing is that a very high strain is introduced into materials which achieve significant grain refinement and improve properties of materials. This review paper presents some recent examples of the applications of DSC for materials subjected to SPD, especially by Equal-Channel Angular Pressing and High-Pressure Torsion.