Materials Science Forum Vols. 783-786

Abstract: The creep behavior of high purity aluminum and copper, Al-0.2wt.%Sc and Cu-0.2wt.%Zr alloys was examined after processing by equal-channel angular pressing (ECAP) with an emphasis on the link between microstructure and creep. The microstructure was revealed by electron backscatter diffraction (EBSD) and analyzed by stereological methods. Representative microstructural parameters were obtained using orientation imaging microscopy and EBSD on the relationship between creep behavior and microstructure.

Abstract: While uniform elongation is a measure of ductility of the material, reduction in area in tensile tests is also an important measure of ductility. It was found that the reduction in area - tensile strength balance is far better than the conventional ferrite+pearlite steels and even superior to martensitic and bainitic steels. Formability of ultrafine-grained steel is examined by applying to form a M1.7 micro screw using these ultrafine-grained steels. Screws are formed through the process of cold heading and rolling. Relationship between cold heading, rolling, uniform elongation and reduction in area are investigated to clarify the formability of ultrafine-grained steels. Low-carbon ultrafine-grained steel has excellent cold headability and favorable rolling properties, i.e., excellent formability. Reduction in area is a measure to determine formability on cold heading. Ultrafine grained steel wire with length of several hundred meter were developed with the technology of warm continuous multi-directional rolling. This wire also have a good formability which can form microscrews. High strength microscrew with ultrafine grained structure was obtained.

Abstract: High-pressure torsion (HPT) was conducted on disks of commercial purity Ti under applied pressures of 3 and 6 GPa. Measurements of the Vickers microhardness showed improving hardness homogeneity with increasing numbers of HPT turns. Transmission electron microscopy demonstrated that a higher HPT pressure leads to a smaller grain size after straining and these grains contain a high dislocation density with arrays of twins. This is consistent with the higher hardness of the Ti samples processed by HPT under 6 GPa pressure.

Abstract: The microstructure and aging behavior of Cu-1.8wt%Be-0.2wt%Co alloy specimens processed by high-pressure torsion (HPT) at room temperature (RT) and 150°C after solution treatment have been studied. Application of HPT processing at RT and 150°C under an applied pressure of 5 GPa for 10 revolutions at 1 rpm to alloy specimens (RT-and 150°C-specimen) produces an ultra-fine grained structure with a grain size of 70 nm. The hardnesses of the RT-and 150°C-specimens increase with equivalent strain up to 7 and then saturate at constant values of 400 and 430 Hv, respectively. Annealing the RT-specimen at 150°C for 10 min increases the hardness from 400 to 430 Hv. Transmission electron microscopy observations of the 150°C-specimen and the RT-specimen annealed at 150°C reveal that there are no intragranular and intergranular precipitates. It is suggested that the higher hardness of the 150°C-specimen than the RT-specimen is ascribed to the segregation of Be atoms on dislocations during HPT processing at 150°C. The RT-and 150°C-specimens harden rapidly and exhibit maximum values of hardness at 3 min during aging at 320°C. The increase in the hardness is attributed to the precipitation of finely dispersed G.P. zones.

Abstract: A two-scale finite element analysis method based on a micro-macro decoupled scheme is applied to an equaled channeling angular extrusion. At first, the macro-scale finite element analysis for one process of an equaled channeling angular extrusion is carried out with a non-liner explicit method to handle the contact and friction between die and bullet. Using the deformation history at a macroscopic material point in this process, the micro-scale finite element analysis is conducted for the multiple processes with a single crystal plasticity and a nonlinear implicit method. As the results, the deformation process of the polycrystalline aggregate during the equaled channeling angular extrusion is numerically reproduced.

Abstract: Using a novel plastic deformation technique, termed linear plane-strain machining, large shear strains up to ~2.3 have been imparted to 316L stainless steel at rates of up to 1700/s. Combinations of hardness and magnetic measurements, X-ray diffraction (XRD) and transmission electron microscopy (TEM) experiments were used to monitor the microstructural and mechanical property changes for the room temperature plastic deformation processing. Grain refinements to the ultra-fine grained and even the nanocrystalline size regime have been achieved without formation of significant volume fractions of strain-induced martensite. The mechanical strength enhancements in the linear plane-strain machined 316L have been attributed to grain refinement and stored strain. The suppression of martensite formation has been correlated to significant adiabatic heating of the 316L during high strain rate plastic deformation processing.

Abstract: A magnesium AZ31 alloy was processed by ECAP at a temperature of 473 K using a die with a channel of 110^o. The results show that the grain size was reduced from 5.4 μm to ~1.0 μm. Micro-tensile tests of the AZ31 alloy were conducted using as-received and ECAP-processed AZ31 alloy at a temperature of 473 K. The ductility of the AZ31 alloy can be improved significantly after ECAP processing at high temperature, which demonstrates a potential application in micro-forming technology for the UFG AZ31 alloy.

Abstract: In the present work the influence of various parameters on formation of nanoor ultrafine-grained structure in commercial-purity titanium during large deformation was quantified using TEM and EBSD. The beneficial effect of twinning on the kinetics of microstructure refinement in titanium was revealed. It was shown that deformation twinning (and therefore nanostructure formation) can be intensified via decrease in temperature, increase in the initial grain size and decrease in the impurities content. The minimum grain size at which twinning can still operate in commercial-purity titanium was determined to be ~1μm. It was shown that rolling to a thickness strain of 93% at-196°C resulted in the formation of a microstructure with a grain/subgrain size ~80 nm.

Abstract: The enhanced demands made to safety and comfort for vehicle occupants have, in recent years, resulted in a steady increase of the vehicle weight. The achievement of a good vehicle performance has been accompanied by the development of more effective combustion motors. However, all this entailed the increased out-put of detrimental exhaust fumes. More restrictive environmental requirements demand the future reduction of the vehicle weight. In order to follow those requirements, different lightweight concepts are currently being followed. The application of higher-strength steel materials allows the reduction of the vehicle weight with, at the same time, increase of the part strength. A further possibility is composite construction which, depending on local demands on the material, means the application of the proper material at the proper position in the vehicle. The highest importance is attached to the combination of steel and aluminium. The development of modern, digitally controlled arc processes has rendered the process-reliable reproducible joining of steel to aluminium possible. Within the framework of a publicly sponsored project, the joining of vehicle-relevant materials with different short-arc processes using low-melting zinc-based brazing solders is currently investigated.

Abstract: Dissimilar joints between 1050 Al alloy and Cu were prepared by ultrasonic spot welding technique to understand the joint characteristics. The joint strength was evaluated by tensile shear strength test. The joint strength increased with increasing joining energy and the joint produced with sufficiently high energy was fractured at the base metal region of Al alloy. The interface microstructure in Al alloy consists of severely deformed region due to ultrasonic vibration. In addition, the fine and equiaxed grains were observed near the joint interface in the specimens fractured at the base metal. These characteristics were significantly different from the microstructure in the bulk region of Al alloy. In contrast, the microstructure in Cu was hardly changed around the interface after ultrasonic welding. Additionally, thin intermetallic compound layer with the thickness of 40 nm was found to be formed at the joint interface in the specimens fractured at the base metal. Peak temperature during ultrasonic welding was found to be approximately 480 K at the interface, which was measured using embedded thermocouple.