Materials Science Forum Vols. 838-839

Abstract: The surface of body-centred cubic Fe-7Al (mass%) alloy plate was successfully modified by friction stir processing with SiC particles addition. The stir zone with SiC addition had an average grain size of 5.9 μm, smaller than that of 10.1 μm in the stir zone without SiC addition. SiC particles introduced by friction stir processing were converted to fine Fe₃AlC_x particles by reaction with the ferrite matrix. The hardness near the surface of the stir zone was significantly increased to 351 HV by introduction of particles, compared to the hardness of 200 HV in the stir zone without particles addition. The dispersed particles also contributed to suppression of grain growth of the matrix at elevated temperature.

Abstract: Friction stir processing (FSP) in a lap configuration of a metal sheet and an alloy plate has been examined to produce surface alloy layer with nanostructures. The 1-pass friction stir lap processing (FSLP) over 0.5 mm-thick Cu sheet on an AZ91 substrate produced multilayer structure with nanograins and/or nanoprecipitates in each layer, but the structure distributed only partially in the stir zone (SZ). Through the 3-pass FSLP along the same line, the multilayer structure has disappeared and the fine structure with precipitates in size ranging from several 100 nanometer to 3 micrometer has yielded among the entire SZ. 2-dimensional microhardness mapping have shown that the standard deviation of microhardness values in the SZ has decreased by half from 1-pass to 3 pass FSLP. Homogeneous microstructure involving nanostructures has been successfully produced via multi-pass FSLP.

Abstract: The commercial Al-5.4Mg-0.2Sc-0.1Zr alloy was subjected to equal-channel angular pressing at 300°C to a true strain ~12 followed by cold rolling to a total thickness reduction of 80%. The ultrafine-grained sheets were joined by friction stir welding (FSW). To evaluate superplastic properties of the weldments, the tensile samples including all of the characteristic FSW microstructural zones were machined perpendicular to the welding direction and pulled up to failure in the temperature range of 400 to 500°C and at strain rates of 2.8×10^-4 s^-1 to 5.6×10^-1 s^-1. The friction-stir welded material exhibited excellent superplastic properties. The highest elongation-to-failure of ~1370% was recorded at a temperature of ~450°C and an initial strain rate of 5.6×10^-2 s^-1, where the strain rate sensitivity coefficient was about 0.64. The relationship between superplastic ductility and microstructure is discussed.

Abstract: Hydrogenation effect on the development of superplastic deformation in the submicrocrystalline Ti–6Al–4V alloy at temperatures (0.4–0.5)Т_melt is investigated. Hydrogenation of the submicrocrystalline Ti–6Al–4V alloy to 0.26 mass% during superplastic deformation is found to result in solid solution strengthening, plastic deformation localization, and as a consequence, decrease of the deformation to failure. Possible reasons for the decrease of the flow stress and increase of the deformation to failure in the submicrocrystalline Ti–6Al–4V–0.26H alloy during deformation under conditions of superplasticity and simultaneous hydrogen degassing from the alloy are discussed.

Abstract: The paper presents the results of finite element analysis of plastic deformation in the contact zone of dissimilar superalloys subjected to pressure welding. The results are compared for the two cases: with and without relief on the surfaces of parts to be welded. It is shown that the relief made on the surface can either decrease or increase the area of deformation stagnant zones, depending on the relief shape. The results of simulations are compared with the experimental test results.

Abstract: Theoretical and practical aspects of fabrication and processing of bulk metallic nanomaterials are presented. The effect of different deformation modes on the structure formation is shown. Development of nanotechnology with respect to fabrication of gas turbine engines (GTE) parts made of nanostructured superalloys is exemplified.

Abstract: The molecular dynamics simulation method in two-dimensional case is presented for the simulation of grain refinement and can be applied to the investigation of grain boundary sliding and defects movement under severe plastic deformation. Nanopolycrystalline system is shown as the example of the application of the method proposed. Atomistic details of structure formation and grain growth (refinement) are shown by the example of change of loading scheme. It was shown that elongated grains which appear under plastic deformation can grow up even larger or be destroyed, depending on the direction of the applied maximal shear stresses.

Abstract: The structure transformations in the D16 (2024) aluminum alloy caused by isothermal rolling with effective strain up to e ~3.5 at a temperature of liquid nitrogen were investigated. It is shown that under straining to e ~2.0 the dislocation structure containing cells of the nanometric size is formed. At higher strains the dynamic recovery and continuous recrystallization result in the development of a mixed nano(sub) grain structure, which after e ~3.5 is characterized by the size and volume fraction of grains ~ 150 nm and 40-45%, respectively. Nature of the alloy structure transformations is discussed.

Abstract: An AA2139 alloy belonging to Al-Cu-Mg-Ag system was subjected to thermomechanical processing (TMP) included solution treatment at 525°C for 1 h, quenching in water, over-aging at 380°C for 3 h followed by equal-channel angular pressing (ECAP) at 250°C to a total strain of ~12 via route B_C. This TMP produced a partially recrystallized structure with fine crystallites having an average size of ~0.9 μm and a fraction of high-angle boundaries of ~56%. Tensile test was performed in the temperature interval 300-500°C at initial strain rates ranging from ~1.8×10^-4 to ~1.7×10^-1 s^-1. It was shown that the highest elongation-to-fracture of ~660% appears at a temperature of 400°C and an initial strain rate of 1.4×10^-3 corresponding with the coefficient of sensitivity coefficient, m, of 0.34.

Abstract: The Al-5.4Mg-0.2Sc-0.1Zr alloy with initial coarse grained structure and containing coherent nanoscale Al₃(Sc,Zr) particles with an average size of ~9 nm was subjected to equal channel angular pressing (ECAP) at temperatures ranging from 300 to 450°C up to a total strain of ~12 and friction stir processing (FSP) with the rotation speed ranging from 350 to 800 rpm. ECAP led to the formation of a uniform microstructure with an average grain size of ~ 0.9 μm. Increasing deformation temperature leads to a slight increase in the average grain size to 1.4 μm and coarsening of Al₃(Sc,Zr) precipitates to 13 nm. FSP with a tool rotation speed of 350, 500, 650, 800 rpm and traveling speed of 75 mm/min led to the formation of fully recrystallized uniform microstructures with an average grain size of ~1.6, 1.9, 2.7 and 2.9 μm, respectively. The coarsening of Al₃(Sc,Zr) dispersoids from 9 to 27 nm occurred under FSP but most of them retained coherency with the matrix.