Materials Science Forum Vols. 667-669

Abstract: In this paper, martensitic transformation from ultrafine grained (UFG) austenite fabricated by accumulative roll bonding (ARB) process in a metastable austenite alloy was studied. Microstructural observations and crystallographic analysis were carried out by FE-SEM/EBSD. The results showed that elongated UFG austenite having 200-300 nm in thickness surrounded by high angle boundaries was obtained after 6 cycles of the ARB process. The martensite transformed from the UFG austenite showed characteristic morphology and texture. The martensite transformation starting (Ms) temperature increased after 1 cycle ARB, which is related to increasing amount of nucleation sites, such as low angle boundaries, introduced during early stage of ARB process. In contrast, by increasing the ARB cycles, Ms temperature decreased. Decreasing the Ms temperature could be correlated to strengthening of austenite by the ARB process.

Abstract: The ultra-fine grained (UFG) materials have been widely investigated due to their mechanical properties such as super high strength and super high plasticity. The finite element simulation schemes are planned for the deformation mechanism of ECAP pressing. It will greatly affect the extrusion process when processing parameter changed such as die geometrical parameters and friction condition. It is considerable to determinate the range of die channel angle and die corner angle during ECAP process, and a moderate die corner angle is usually chosen. The friction condition of the ECAP should be lubricated as good as possible in the pressing on the basis of optimal die geometrical conditions. The ECAP process is a non-uniform shear deformation process in the cross-section of the workpiece for first-pass pressing. There have low angle grain boundaries along the cross-section of the grain microstructures. For the multi-pass pressing, although the pressing pass numbers are same, the processing routes were of important significance on the grain sizes and grain distribution and grain boundaries (GBs) orientations of workpieces. The formation mechanism of nanostructure was given for the ECAP through studying the deformation behavior and dislocation’s evolution. The dislocation is one of the main defects in the processed materials, but the form of the dislocations and its density are difference for different processing route. The processed workpiece with high surface quality, refined grain microstructure and optimization grain boundaries were obtained with optimal processing parameters and routes.

Abstract: To understand and model grain refinement in severe plastic deformation, some analysis of Nb single crystals has been carried out in previous work. To bridge the gap with normal polycrystalline materials, supplementary experiments on large polycrystals, deformed at moderate strains appear to be necessary to explain the grain subdivision step by step. In the present work, successive uniaxial compression tests have been carried out on a large grained Niobium polycrystal up to height reductions of 30% with small strain increments. Electron backscatter diffraction (EBSD) analysis was done after each compression step to characterize the evolution of orientation and microstructures. It is observed that a “rotation front” forms inside the grain and moves with increasing strain from one side to the other side of the grain. In one grain, this process results in a grain boundary affected zone in the vicinity of the grain boundary. Both static orientation evolution inside the grain and historical evolution of the average orientation have been studied, which indicates that the grain orientation rotates around one of the (110) poles at low strain.

Abstract: The microstructural evolution and grain refinement of Cu-Al alloys with different stacking fault energies (SFEs) processed by equal-channel angular pressing (ECAP) were investigated. The grain refinement mechanism was gradually transformed from dislocation subdivision to twin fragmentation with tailoring the SFE of Cu-Al alloys. Concurrent with the transition of grain refinement mechanism, the grain size can be refined into from ultrafine region (1 m~100 nm) to the nanoscale (<100 nm) and then it is found that the minimum equilibrium grain size decreases in a roughly linear way with lowering the SFE. Moreover, in combination with the previous results, it is proposed that the formation of a uniform ultrafine microstructure can be formed more readily in the materials with high SFE due to their high recovery rate of dislocations and in the materials with low SFE due to the easy formation of a homogeneously-twinned microstructure.

Abstract: Multi-pass of equal channel angular pressing (ECAP) at a single temperature as low as room temperature from 200 °C were measured using electronic back scatter diffraction (EBSD). The effect of texture and grain size on mechanical properties was investigated to realize the strengthening and large plastic deformation mechanism. A room temperature ECAP with multi-pass procedure is effective to product high strength and large plastic Mg, as a result of submicron grain structure and texture strengthening.

Abstract: Disks of as-extruded Mg-4Nd alloy were processed by high-pressure torsion (HPT) through ¼ to 5 turns at room temperature. The first 1/4 turn of HPT induces large numbers of twins and some dislocation tangles in the center region of the disk. With increase of torsional straining, the twinning is inhibited gradually and the dislocation density increases relating to the formation of dislocation substructures and ultimately transforming to high fractions of equiaxed gains which have an average grain size of ~200 nm and high-angle boundaries. HPT significantly improves the values of microhardness of this alloy. The hardness values in both the central and edge regions show a sharp rise after HPT for 1/4 turn and exhibit nearly saturation after 1/2 turn although there is a trend of a slight increase with increasing numbers of turns. The experimental results suggest more homogeneous microstructures may be produced by larger numbers of turns in the HPT process.

Abstract: The processing of metals through the application of severe plastic deformation provides the potential for achieving exceptional grain refinement in bulk solids. Several SPD methods are now available but processing by high-pressure torsion (HPT) has attracted much attention over the last five years. Numerous reports are now available describing the application of HPT to a range of pure metals and simple alloys and excellent grain refinement were achieved using this process with the average grain size often reduced to the nanoscale range. However, in order to make this technique more practical, the nature of the sample characteristics immediately after conventional HPT must be considered in order to understand the fundamental principles of HPT processing. This report examines the procedure with special emphasis on the evolution in hardness homogeneity in both high-purity aluminum and a Zn-22% Al eutectoid alloy processed by HPT.

Abstract: An ultra-low carbon IF steel was heavily deformed up to an equivalent strain of 36 at various high temperatures of ferrite single-phase region and various strain rates. Effects of temperature and strain rate on the microstructures evolved in torsion deformation were clarified. On the other hand, it was found that homogeneous ultrafine grained structures were not obtained by the present torsion deformation though very high strain was applied. The coarser grain sizes than those obtained by conventional severe plastic deformation (like ARB) were due to the deformation at higher temperature and lower strain rate, but lower fraction of high-angle grain boundaries in the torsion specimen was suggested to be attributed to the characteristics of monotonic torsion (or simple shear) deformation including the way of strain evaluation.

Abstract: Evolution of structure of Nb subjected to high-pressure torsion (HPT) with various strain at cryogenic and room temperatures and further annealing in the temperature range of 100-7000C has been studied by transmission electron microscopy (TEM) and microhardness measurements. HPT in liquid nitrogen enables to obtain true nanocrystalline structure with crystallite sizes of about 75 nm and the record-breaking microhardness of 4800 MPa. The thermal stability of the structure obtained is analysed.

Abstract: Titanium microalloyed steel whose grain size is around 3μm has been developed by CSP process. Samples after different plastic deformation were obtained by suddenly stopping the six-stand finishing train during tandem rolling. Experimental results and analysis show that plastic deformation during tandem rolling plays a major role for grain refinement. With the successive severe strain, microstructure of the same stock during tandem rolling is further refined because of repeated recrystallization at higher temperature and strain accumulation at lower temperature. Technological features of CSP process, such as large reduction per pass, rapid cooling after rolling and high solidification and of thin slab, are important reasons for obtaining ultafine-grained structure. Besides these, chemical composition of titanium microalloyed steel has beneficial effects on grain refinement.