Papers by Author: Günter Gottstein

Abstract: In the present study, the substructure within grains of different orientations was investigated by means of EBSD technique. A 70% cold rolled commercial AA8079L alloy was annealed at two temperatures, 280°C and 320°C, for short times from 2 to 60 seconds. The subgrain size within the deformed grains of different orientations was measured. The results revealed no pronounced difference in subgrain size right after cold deformation. During further annealing at both investigated temperatures, Cube grains showed the highest recovery rate. It is concluded that the Cube oriented nuclei require an incubation time which is, obviously, much smaller than for grains of other orientations.

Abstract: An aged Al-5Zn-1.6Mg alloy with fine η' precipitates was grain refined to ~100 nm grain size by severe plastic deformation (SPD). Microstructure evolution during SPD and mechanical behaviour after SPD of the alloy were characterized by electron microscopy and tensile, compression as well as nanoindentation tests. The influence of η' precipitates on microstructure and mechanical properties of ultrafine grained Al-Zn-Mg alloy is discussed with respect to their effect on dislocation configurations and deformation mechanisms during processing of the alloy.

Abstract: An aluminum alloy with two different precipitation states was processed by Equal Channel Angular Pressing up to 16 passes. The thermal stability of the ultra-fine grain structure was examined during annealing at 300°C up to 1 h. As the present precipitates are coarse they don’t contribute to strengthening where as soluted elements improved the strength of the material. Additionally the soluted elements formed dispersoids upon annealing and thereby retarded softening.

Abstract: The interlayers from different materials were used in order to modify the interface structure/property and to improve the mechanical properties of NiAl composites reinforced by continuous single crystal Al2O3 fibers. It was found that the interface without interlayer had a good chemical stability during hot pressing (sample fabricating), resulting in high interface shear strength (about 250 MPa) at RT. But for the composites with interlayers, chemical reactions occurred in the interfacial area during diffusion bonding. The measured interface shear strength for the complete debonding was 70±30 MPa, 110±36 MPa and 32±5 MPa for the composites with hBN, Cr2AlC and V2AlC interlayers respectively. The possible influences by introducing the interlayers on mechanical performance of NiAl composites are discussed.

Abstract: In the present study, nucleation at Cube bands during annealing of a cold rolled commercial aluminium alloy AA8079 was investigated. The main goal of this work was to provide experimental data for subsequent modelling. By studying the deformed material the geometrical parameters of Cube bands were determined. The Cube band spacing in the deformed state was measured experimentally and accordingly calculated with a simple geometric model, taking the initial Cube fraction in the pre-deformed state and the deformation degree into account. The nucleation frequency during subsequent annealing of the cold deformed material was estimated from the geometry and size of deformed Cube bands prior to deformation. From investigations of the orientation relationship between the Cube oriented nuclei and the adjacent deformed matrix with S orientation, the fraction of 40°<111> grain boundaries having optimum growth conditions, was established.

Abstract: In present investigation the influence of second phase particles on microstructure evolution during confined channel-die pressing (CCDP) and mechanical behaviour after CCDP were studied using two-phase Al alloys of Al-1.5Mn and Al-5Zn-1.6Mg. The investigation results revealed different grain refinement kinetics during CCDP and distinct mechanical behaviours after CCDP for both alloys. The results are discussed based on the influence of the second phase particles on dynamic recovery and the interaction with mobile dislocations during plastic deformation.

Abstract: In the present study the microstructure evolution of the aluminum alloy 3103 subjected to ECAP up to eight passes applying route Bc was investigated after deformation and subsequent isothermal annealing. The deformed and annealed states were analyzed by SEM, EBSD, optical microscopy and microhardness tests. It will be demonstrated that this ECAP deformed material shows an increased stability against discontinuous recrystallization with growing number of passes.

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 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: A refined view of particle stimulated nucleation of recrystallization is presented, which utilizes a combination of advanced modeling tools. FEM simulations were carried out in order to model the evolution of the deformation zone around particles for various particle sizes and shapes. The results of these simulations were complemented by EBSD measurements to determine the number and orientation of nuclei. Finally, this information on particle stimulated nucleation was incorporated into a 3D cellular automaton recrystallization model CORe to model microstructure evolution. From these simulations the dependence of grain size and texture on particles size and shape was derived.