Materials Science Forum Vols. 584-586

Abstract: Strain rate sensitivity of the strength of TWIP (Twinning Induced Plasticity) steel with the mixture of recrystallized fine grains and rolling-deformation microstructures was studied. The 31mass%Mn-3%Al-3%Si TWIP steel sheet was severely cold-rolled to a reduction of 92% and subsequently annealed at various temperatures ranging from 600oC to 700oC in order to obtain the partial recrystallized microstructure with various fraction of recrystallized microstructure. The 600oC annealed specimen keeps similar morphologies as observed in the as-rolled structure consisting of both the fine lamellar dislocation cell structure and the twin/matrix lamellar structure; whereas, in the specimen annealed at 625oC or 675oC , the partially recrystallized fine grains (d~1µm) with a few dislocations evolve. The volume fraction of recrystallized fine grains increases with increasing of the annealing temperature while the mean diameter of the recrystallized grains is not changed largely. The tensile deformation behaviors were measured at various strain rates ranging from 10-3sec-1 to 102sec-1. The strength and elongation become smaller and larger, respectively, with increasing the fraction of the recrystallized microstructure. The activation volume of dislocations becomes larger with increasing the fraction of recrystallized microstructure.

Abstract: The evolution of texture and deformation in the grains during one pass of equal-channel angular pressing (ECAP) was examined for fine grained high strength and low strength Al alloys and a coarse grained low strength Al alloy. The materials were analysed using electron back-scatter diffraction (EBSD). The results are consistent with the materials responding to the intense macroscopic shear stress by deformation of individual grains through movement of dislocations on one or more of the slip crystallographic slip planes {hkl} that are favourably oriented, combined with the rotation of grains to directions that bring main crystallographic slip planes parallel to the macroscopic shear direction and crystallographic slip directions parallel to two main shear directions. Contrary to reports claiming up to 4 slip systems are activated, it was observed that only the {111}<110> and {001}<110> shear systems are activated. Macroscopic shear deformation occurs on two shear planes: the main shear plane (MSP), equivalent to the simple shear plane, and a secondary shear plane which is perpendicular to the MSP.

Abstract: The effect of equal-channel angular extrusion (ECAE) on mechanical properties of an AA2017 produced by powder metallurgy is investigated. Special attention is given to the influence of heat treatment, processing temperature and backpressure on the workability for achieving high strength and moderate ductility. This is of special interest, since it is often reported that Al-Cu alloys have low ductility and therefore are prone to cracking during severe plastic deformation. It is shown that ECAE at high temperatures (>220°C) does not necessitate backpressure to ensure homogeneous deformation but leads to a significant sacrifice in strength due to in-situ precipitation. Thus, most of the extrusions are done at considerably low temperatures. Performing room temperature-extrusion is most effective in achieving high strengths but also requires high backpressures. Due to severe strain hardening during processing, the strength increase is combined with a reduction in ductility. Recently it was reported that a post-ECAE aging of pre-ECAE solution treated material is effective in enhancing the ductility of aluminium alloys. This approach was successfully transferred to the current alloy. A high-temperature, short-time aging after only one extrusion, for example, doubles the failure strain to a value of ~13%. Compared to the naturally aged condition with coarse grains that serves as reference (T4), an increase of 15 % in yield stress (YS) was obtained while retaining the ultimate tensile stress (UTS). Another effective approach is the combination of a pre-ECAE solution treatment with subsequent under-aging prior to ECAE. It is shown that performing ECAE at medium temperatures (160-180°C) enables a better workability and additionally gives higher strengths and better ductility compared to the processing in the water quenched condition. A remarkable YS of 530 MPa and an UTS of 580 MPa combined with a moderate failure strain of 11.6 % were achieved.

Abstract: Grain refinement taking place in a commercial 7055 aluminum alloy under equal channel angular extrusion at a temperature of 250°C, was examined. The material was deformed up to a total strain, ε, of ~12. At ε≈1, the development of subgrain bands was found. Upon further straining the average misorientation of deformation-induced boundaries increases; low-angle grain boundaries (LAGBs) gradually convert into true high-angle grain boundaries (≥15°) (HAGBs). At ε≈4, a structure consisting of boundaries with low and high angle misorientations was observed. At ε≈12, a structure with an average grain size of ∼0.7 µm was formed. This size is roughly similar to that for subgrains developed at preceding strains. It was shown that the formation of submicrocrystalline grains occurs through continuous dynamic recrystallization both along initial boundaries and within interiors of original grains as well.

Abstract: An ultra-fine grained Al-1.5Mn alloy was fabricated in an oversaturated and a precipitated (Al6Mn particle) state by confined channel-die pressing (CCDP) at room temperature (RT). Specimens for microcharacterization and mechanical tests were taken from the centre area of the CCDP samples, where an equivalent strain of ε ≈1-1.4 was reached for one pass pressing. The samples were subjected to CCDP for 1, 4, 8 and 16 passes, respectively. The microstructure evolution during CCDP was investigated by transmission electron microscopy (TEM). The deformation behaviour of the ultra-fine grained Al-Mn alloy after the CCDP process was determined by compression tests at RT with different strain rates of ε& = 1× 10-4s-1, ε& = 5× 10-4s-1 and ε& = 1× 10-3s-1. The results demonstrated that the alloy with Al6Mn precipitates had a larger grain size, higher yield strength, a larger Hall-Petch coefficient, a stronger strain softening, a larger strain rate sensitivity and a smaller activation volume after CCDP compared to the oversaturated alloy. These phenomena indicate that second phase particles and solute element affect grain refinement and mechanical behaviour during and after CCDP differently.

Abstract: This work focuses on the effect of strain rate on the deformation behaviour of an ultrafine grained Al alloy 6082 produced by equal channel angular pressing. The uniform tensile elongation was found to increase with decreasing strain rate very substantially. This effect is discussed in terms of the mechanisms that control plastic deformation of the alloy.

Abstract: The present study is an assessment of the effects of precipitation heat treatments on tensile behaviour, work hardening (WH) characteristics and microstructural evolution of an Al-4%Cu alloy deformed by equal channel angular pressing (ECAP). Two ageing temperatures were employed (170 and 100oC) and their effect on strength and WH behaviour was compared with that exerted on the same alloy, but in two different initial conditions: quenched from solution temperature and slowly cooled before anneal. Grain and precipitate sizes of samples deformed by one and four ECAP passes and heat treated as described were measured employing transmission electron microscopy (TEM). It was concluded that the lower ageing temperature gives the best combination of strength and ductility, a high WH rate and, possibly, the smaller grain and precipitate sizes. The relative participation of the various hardening mechanisms to total strength was estimated from tensile tests and hardness measurements.

Abstract: Grain boundary structures in the commercial purity aluminum (1100Al) highly deformed by the accumulative roll bonding (ARB) process was observed by using conventional transmission electron microscopy (CTEM) and high resolution transmission electron microscopy (HRTEM). In the low angle grain boundary with a tilt angle (2θ) of 2.1o consisted of the periodic dislocations array, the interval of those dislocations could be explained by the dislocation model for grain boundary. However, the dense dislocation region locally existed at the vicinity of the low angle boundary. On the other hand, we also observed the high angle grain boundary of which the common axis and 2θ was <110> and 125.9o, respectively. In this grain boundary, we could describe the boundary configuration in terms of the combination of the kite-shaped structure unit characterized by Σ11 coincidence boundary with the 2θ of 129.52o around <110> and the additional dislocations to compensate the difference of the actual and geometrically coincided one.

Abstract: The microstructure evolution of an as-cast commercial Al-Mg-Sc-Zr alloy during Equal- Channel Angular Pressing (ECAP) at 325°C was investigated. In the early stages of deformation strain induced boundaries were created within the initial coarse grains and constitute the deformation bands. Repeated ECAP led to an increase of the number and misorientation of deformation bands. Further straining up to e~8 resulted in the formation of a new fine-grained structure with an average crystallite size of 1.2 /m. It is concluded that the progressive increase of the misorientation of deformation induced boundaries is the main mechanism of structure formation under high temperature ECAP.

Abstract: An Al-0.2wt%Zr alloy was severely deformed up to a strain of 8.0 by accumulative roll bonding (ARB) process, started from the solution-treated state. The microstructural evolution during ARB and its aging behavior were investigated. With increasing the number of ARB cycles, Vickers hardness of the specimens increased and reached to a constant value. The microstructural evolution during the ARB could be understood in terms of grain subdivision. The ultrafine grained (UFG) materials whose mean grain size was 0.4 -m were obtained by 10-cycle ARB process. In aging of the ARB processed specimens at high temperatures above 673K, the UFG microstructures quickly coarsened. On the other hand, it was suggested that the precipitation behaviors of the ARB specimen at 623K were quite unique and completely different from those of the conventionally solution-treated material with coarse grain size.