Materials Science Forum Vols. 706-709

Abstract: This article presents the results of numeric simulation obtained with commercial software for thermo-mechanical analysis of plastic forging processes, Forge 2009, of the process of alternate forced pressing and multiaxial compression. The new method of alternate forced pressing and multiaxial compression suggested by the authors is characterized by the presence in the plastically forged material of the similar states of deformations to those present in the processes of the equal channel angular pressing and cyclic extrusion compressing. From the performed preliminary tests it can be stated that as a result of combining and repeating two alternate operations: pressing and multiaxial compression, strain accumulation and development of deformation state favorable to grain crushing take place.

Abstract: In the present study, in situ measurements of applied torque and compressive load were conducted during high-pressure torsion (HPT) on Ti-23%Nb-0.7%Ta-2.0%Zr-1.2%O (in at %) , Gum Metal, by using four active strain-gage method. The shear stress was then calculated from the measured torque. The in situ measurements revealed that the maximum shear stress reaches ~2 GPa during HPT. This value is comparable to the ideal shear strength of Gum Metal, which was reported as ~1.8 GPa from experiments using single crystals. The deformation mechanism strongly depends on body-centered cubic (bcc) phase stability at an early stage of HPT straining, where the shear stress is well below the ideal shear strength. On the other hand, the deformation mechanism may be insensitive to the bcc phase stability at a later stage of HPT straining, where plastic deformation occurs at a strength close to the ideal shear strength.

Abstract: Grain refinement and superplastic deformation behavior of Zn-Al alloys were investigated in this study. To obtain fine grain size in Zn-0.3Al alloys, rolling and equal channel angular pressing (ECAP) were conducted at temperatures from 40 to 160°C after casting and homogenization heat treatment. Material processing maps for Zn-0.3Al alloy were constructed from a series of compression tests conducted at temperatures from RT to 200°C and strain rates from 3×10^-2 to 10¹ s^-1. Superplasticity of ECAPed specimens were evaluated at the temperature of 100°C under the strain rate of 2×10^-4 s^-1. After ECAP of the Zn-0.3Al alloy, elongation was dramatically increased up to 500%. The effects of ECAP on the texture and anisotropy in the superplastic deformation bebavior were found to be negligible.

Abstract: Effective grain refinement through equal channel angular pressing (ECAP) for magnesium (Mg) alloys has been demonstrated by many researchers. Although with the capability to achieve superplasticity, the batch mode nature of this method and the required repetitive processing to attain ultrafine grained structure have prohibited it from being widely used in large-scale industrial production. In this study, a well-established metal forming method – the continuous extrusion forming (CONFORM) process – was employed as a severe plastic deformation route to refine the microstructure of Mg alloys. Cast Mg-3%Al-1%Zn (AZ31) rods were used as the feedstock and the cast structure (grain size of ~150 microns) was refined to ~1 micron after one pass CONFORM extrusion. Uniaxial tensile tests of the as-extruded samples were conducted at a temperature of 473K and an elongation of ~200% was achieved under a strain rate of 1×10^-4 s^-1. The significant grain refinement effect was attributed to the severe shear deformation occurred during the CONFORM process, which is very similar to ECAP but with even higher effective strains. The most important advantage of CONFORM over ECAP is that the former is a continuous route, so it is able to produce long products. It was also shown that CONFORM could be an additional forming method for Mg alloys to conventional rolling, forging and extrusion.

Abstract: The competitive precipitation behavior observed in microstructures with high dislocation density and ultra-fine grains has been studied experimentally and computationally for cold-rolled and severe plastic deformed Al-Mg-Si alloy. The age-hardenability at 443K was reduced by the two deformation processes due to the accelerated formation of larger precipitates on dislocations and grain boundaries, in place of the transgranular precipitation of refined β” in the matrix. The developed numerical model based on the classical heterogeneous nucleation theory clarified the dislocation density and grain size dependences of the volume fraction of precipitates nucleated at different sites, in good agreement with experimental results. It could be therefore possible that three strengthening mechanisms of strain hardening, hardening by grain refinement and precipitation hardening are optimally exploited according to the computationally estimated dependences.

Abstract: A new strategy for increasing the low-temperature toughness of structural materials is in urgent need for overcoming the general rheotropic brittleness in coarse-grain state. Here, a unique phenomenon was observed that ultra-fine-grained (UFG) SiCp/ZL108 composites after severe plastic deformation (SPD) exhibit higher impact toughness at temperatures slightly lower than room temperature. The enhanced impact toughness is attributed to the simultaneous increase of strength and ductility of UFG materials at lower temperatures, related to grains or grain fragment boundary modification. This result demonstrates the advantage of fabricating UFG materials by SPD method and spurs the interest to use UFG materials in low-temperature conditions.

Abstract: Effects of the elastic anisotropy on deformation behavior are examined in a Ti-23%Nb-0.7%Ta-2%Zr-1.2%O (in at %) alloy, Gum Metal, and in an Fe-19%Ni-34%Co-8%Ti alloy with body centered cubic (bcc) crystal structure, and microstructural development in the iron based alloy during severe plastic deformation (SPD) process is discussed. Strong elastic anisotropy with reduced shear modulus, C₁₁- C₁₂, results in low ideal shear strength, which implies dislocation mediated plasticity easily occurs at lower stress. On the other hand, high pressure torsion (HPT), a typical SPD method, realizes very high shear stress during processing, which seems to reach the ideal shear strength in these alloys. Significant refinement of the grain size to 20 - 50 nm in the Fe-Ni-Co-Ti alloy is discussed in relation to the unique deformation mechanism which might be activated at ideal shear strength.

Abstract: High-pressure torsion (HPT) is a processing technique in which samples, in the form of thin disks, are subjected to a high applied pressure and concurrent torsional straining. In principle, the strain introduced into the disk during the straining varies across the disk and there is a direct proportionality between the estimated strain and the radial position on the disk. This means that the strain is zero at the center of each HPT disk and it reaches a maximum value at the outer periphery. Contrary to these expectations, recent experiments show there is a gradual evolution with increasing numbers of revolutions such that the hardness of the disk gradually becomes reasonably homogeneous. This report examines the development of hardness and microstructural homogeneity with special emphasis on the evolution in hardness homogeneity along vertical sections of disks of high-purity aluminum processed by HPT. The results demonstrate that, at least for pure aluminum, the distributions in the hardness values are independent of the plane of sectioning.

Abstract: Coextrusion and corolling are the major processes to produce bimetallic rods, tubes and wires, the objective being to perform clad metals, bimetallic joints or seals. The aim of the present work is to produce bimetallic rods showing an ultrafine grained microstructure with enhanced properties. Bimetallic Cu-Al rods were deformed by equal channel angular pressing (ECAP) in order to study their microstructure. ECAP is an interesting process for producing bulk materials with refined microstructure and, consequently, changes in physical, chemical and mechanical properties can be observed. Higher shear strength and dimensional stability are among the advantages of this process. A comparative experimental study of pure commercial copper with cylindrical inner aluminium rods of different diameters processed by one-pass equal channel angular pressing has been carried out. The ECAP die used in this research was a 90º 2-channels intersecting angle. Electron backscattered (EBSD) and X-ray diffraction techniques were used for microstructure characterization (deformation, grain fragmentation and microstrain evaluation) at the interfaces and away from them. It was found that the microstructure in the ECAP deformed Cu-Al bimetallic rods was influenced by the dimensions of the aluminium inner rod. In fact, the microstructure appeared to be much more elongated and refined in the samples containing smaller diameter aluminium rods.

Abstract: Ti-stabilised interstitial free (IF) steel initially subjected to 8 passes, route B_C equal channel angular pressing (ECAP) was further cold rolled (CR) at room temperature to 95% thickness reduction. Both samples were isothermally annealed at 710 °C following which their microstructures and micro-textures were compared via electron back-scattering diffraction (EBSD). The mechanical properties first obtained by shear punch testing (SPT) were later corroborated by uniaxial tensile tests. In the case of the ECAP material, continuous recrystallisation is followed by abnormal growth at prolonged annealing times with minor increases in high angle boundary (HAGB) fraction. On the other hand, the additionally CR material shows continuous recrystallisation accompanied by a reduction in the HAGB fraction. After 15 s annealing, the ECAP and CR samples exhibit a good strength-ductility balance; which corresponds to ~52% and ~67% softening, respectively.