Materials Science Forum Vol. 941

Abstract: The typical textures developed in aluminium alloys for deep drawing applications are less favourable as those in competing steel sheet material. The {111} fibre texture in steel, associated to high r-values, is favourable to this purpose, but the typical textures of the aluminium materials, the {001}<100> "cube" texture component and the β-fibre component, are not. Asymmetric rolling (ASR) as part of the production process generates a shear component at the expense of the unfavourable components. Modelling was tried out as a possible tool to fine-tune the process parameters. A multiscale FEM model (with a built-in polycrystal deformation model to predict the texture) was used to this purpose. The effect of the shear component on the resulting texture is discussed in function of the values of the process variables, as well as its effect on the resulting plastic anisotropy parameters (r and q values).

Abstract: The subsurface fatigue crack generation processes in near α type titanium alloy were divided into four steps: (1) development of a saturated dislocation structure by cyclical micro-plastic strain accumulation, (2) generation of localized slip and/or microcracking to relax the stress concentration in the vicinity of a boundary, (3) microcrack growth and transition to main crack, and (4) crack propagation. The experimentals on transgranular facets formation in Ti-Fe-O alloy were reviewed and a subsurface fatigue crack generation model was discussed. The β platelets which were aligned between the recrystallized α grain and the recovered α grain were responsible for the microcrack generation to form (0001) tansgranular facet in the recrystallized α grains. A combination of the shear stress and tensile stress normal to the basal plane may give a trigger of the (0001) microcracking in the recrystallized α grain. The localized shear stress following slip off on the basal plane was activated at the microcrack tip in the recrystallizedαgrain, and the microcrack grew into the recrystallized α grain to form (0001) transgranular facet.

Abstract: In this work asymmetric accumulative roll bonding (AARB) was applied to a AA1050 aluminum up to ten cycles at 350 and 400°C. The texture was measured by x ray diffraction and EBSD. Hardness and tensile tests characterized the strain distribution and bonding efficiency. At 350 °C the microstructural refinement was stabilized after four cycles and mean grain sizes of one micron and a saturation yield strength of 160 MPa was achieved. At 400°C grain growth took place yielding a bimodal microstructure with mean grain size of 9 microns. During repeated bonding cycles recovery and dynamic recrystallization were observed and extra shear in the interfacial region yielded a fairly well homogeneous strain distribution and weak shear texture across the sheet for both temperatures. The strongest component in both cases was the rotated cube orientation. The last bonding surface was the weakest bond but adding an extra 50% reduction step to the process increased the interfacial strength considerably.

Abstract: Young’s modulus varies with crystallographic orientation, temperature and alloying, but also with cold working and heat treatment. In this work, the evolution of Young’s modulus in polycrystalline pure aluminium (99.5%) with different cold-working levels determined at room temperature is presented. The deformation process was carried out in a universal tension machine and measurements were performed by ultrasounds. The Young’s modulus diminished from 70 to 65 GPa for 0-5% of deformation (elongation) and then increased with successive cold-working (68 GPa for 8.5% of elongation). These values were obtained 8 hours after plastic deformation was applied. This behaviour is compared with the Young’s modulus determined by extensometry in the same material. In this case, the modulus decreased from 70 to 63 GPa (3.5% of elongation) and then increased until 68 GPa for 10% of elongation. Results obtained on pure iron (Armco) deformed in the same conditions are included for comparative purposes. Values of Young’s modulus measured during the springback process after plastic deformation at different level are also included. Values obtained are between 10-15% lower than those measured 8 hours after plastic deformation.

Abstract: Molten Mg-AZ31 cools and solidifies to a sheet in horizontal twin-roll casting. In the present investigation, this process was numerically analyzed in two dimensions under various conditions. Steady-state solutions were obtained including plastic deformation after solidification. Based on results of the analyses, an optimum process schedule was proposed for production of a sheet of 1 mm in thickness where the sheet thickness decreased from 3 mm through a couple of transitions during operation. However, the schedule was recommended up to 2 mm in thickness due to the restriction in strength of the sleeve material.

Abstract: In this study, to clarify the effects of Mo addition on deformation behavior of Ti-Mn alloys, the mechanical properties and the deformation structures of the alloys were investigated using Ti-Mn and Ti-Mn-Mo alloys polycrystals and single crystals. We found that the elongation of Ti-Mn alloys are improved from approximately 5% to 30% by Mo addition, with maintaining ultimate tensile strength of 900 MPa. The excellent strength-ductility balance of Ti-Mn-Mo alloys is caused by {332}<113> twinning, which is unique twinning for metastable β-type titanium alloys. Additionally, the deformation behavior of Ti-Mn and Ti-Mn-Mo alloys was investigated in detail by using single crystals focusing on a critical resolved shear stress (CRSS). As a result, we found for the first time that CRSS for {332}<113> twinning in Ti-Mn-Mo alloy was lower than that in Ti-Mn alloy. Moreover, in Ti-Mn-Mo alloy, CRSS for {332}<113> twinning was lower than that for {112}<111> slip. These results suggest that CRSS for {332}<113> twinning in Ti-Mn alloys is decreased by Mo addition.

Abstract: In the present study, effects of heat treatment on microstructures and tensile properties of the cylindrical bars of Ti-48Al-2Cr-2Nb (at.%) alloy with unique layered microstructure consisting of equiaxed γ grains region (γ band) and duplex-like region fabricated by electron beam melting (EBM) were investigated. We found that it is possible to control width of the γ bands (W_γ) by heat treatments at 1100°C and 1190°C. The W_γ increases with decreasing heat treatment temperature. The bars heat-treated at 1190°C exhibit high elongation of 2.9% at room temperature (RT) with maintaining high strength. The RT elongation increases with increasing the W_γ because of increasing deformable regions. In contrast, the RT elongation of the bars decreases with increasing the W_γ when W_γ is very large. This is because the large γ band leads intergranular fracture. These results indicate that there is appropriate width for the γ band to obtain excellent tensile properties at RT.

Abstract: Fe-20Al-5Ti (at.%) single crystals composed of the bcc Fe-Al matrix and the Fe₂AlTi precipitates with the L2₁ structure was examined. In the single crystals furnace-cooled (FC) from 1373 K to room temperature, coarse Fe₂AlTi phase about 300 nm in diameter were precipitated in the bcc matrix. A misfit strain and a dissolution temperature of the L2₁ precipitates are +0.59% and 1151 K, respectively. The single crystals exhibited high yield stress above 600 MPa up to 973 K while further increase in temperature resulted in a decrease in yield stress due to the dissolution of the precipitates. In the FC crystals, 1/2<111> dislocations in the bcc matrix bypassed the coarse L2₁ precipitates due to their large misfit strain, resulting in high strength. In contrast, the fine L2₁ precipitates about 30 nm in diameter were observed in the crystals after solutionization and annealing at 873 K. The crystals with the fine L2₁ precipitates demonstrated high yield stress above 1100 MPa at and below 773 K. Uncoupled or paired 1/2<111> dislocations cut the fine L2₁ precipitates, leaving an anti-phase boundary (APB) inside the precipitates. The APB inside the precipitates was considered to be responsible for strong precipitation hardening.

Abstract: We present transport and thermodynamic properties of CeRu₂Al₁₀ controlled by pressure in a vicinity of a critical pressure P_C ~ 4GPa, where antiferromagnetic ordering disappears. The resistivity under pressure was measured with DC four terminal method and the AC specific heat under pressure was measured by Joule heating type technique. The pressure was applied by cubic-anvil-apparatus and palm-cubic-anvil-apparatus. The results of AC specific heat indicate T_N holds at high temperature up to 3.9 GPa but suddenly disappears above this pressure. We confirmed T_N from thermodynamic properties. Although CeRu₂Al₁₀ is in a Kondo semiconducting ground state at 4 GPa, temperature dependences of electrical resistivity at 4.6 GPa and 5.9 GPa indicate metallic ground state in these pressures. CeRu₂Al₁₀ does not show superconductivity down to 0.7 K at 4.6 GPa and 5.9 GPa.

Abstract: High specific strength of Ti-based alloys and composites makes them highly requested materials in various structural applications, especially when lightweight is desired in high-strength constructions. When these alloys are used in layered structures, far advanced set of characteristics that combine different mechanical properties often non-compatible in a single layer uniform structure can be attained; for instance, high hardness or moduli systems are usually lacking of sufficient toughness. Mechanical properties of individual layer in multilayered materials can be controlled by changing chemical composition and microstructure within each layer specifically. In present study layered materials were formed by combination of the layer of Ti-6Al-4V alloy and metal matrix composites on its base reinforced with fine TiB and TiC particles. Structures were fabricated using blended elemental powder metallurgy (BEPM). The effect of different post-sintering thermo-mechanical treatments on structure of layered BEPM materials was studied. Processing parameters were assessed in terms of their influence on materials’ porosity, grain size and structure, distribution of reinforcement particles and layers integration. The effect of above mentioned structural characteristics on hardness of layered materials was evaluated.