P. Málek

Superplasticity in an AlZnMgCu alloy

Abstract The deformation behaviour of a fine-grained AlZnMgCu alloy was investigated in a broad strain rate interval at temperatures kbetween 672 K and 789 K. Superplastic characteristics were found at all temperatures used at strain rates between 10−4 s−1 and 10−3 s−1. The non-superplastic behaviour at low strain rates (in what is known as region I) was explained taking into account the existence of a threshold stress. Using a suitable value of the threshold stress, region I seems to be a continuation of the superplastic region II with nearly the same values of the strain rate sensitivity parameter m and activation energy. Possible sources of the threshold stress are discussed.

The deformation structure of the superplastic Zn–Al alloy

Abstract Fine grained ( d ≅1 μm) and coarse grained ( d >20 μm) samples of the Zn-1.1 wt.% Al alloy were strained in a broad range of temperatures and strain rates corresponding both to the optimum superplastic region II and to the non-superplastic region I. The evolution of the microstructure during straining was studied and compared with the assumptions of the co-operative grain boundary sliding model. No pronounced localization of grain boundary sliding was found during straining at optimum superplastic strain rates. A tendency toward sliding of groups of grains was observed during straining at low strain rates. The stress dependence of the size of these grain groups might be a possible reason for the origin of the non-superplastic region I at low strain rates.

The influence of processing route on the plastic deformation of Al-Zn-Mg-Cu alloys

Abstract A fine-grained microstructure of Al–Zn–Mg–Cu alloys was produced by several processing routes — by thermomechanical treatment of conventionally cast ingots, by modern powder metallurgy, and by severe plastic deformation in equal channel angular pressing. The plastic deformation of these materials was investigated in a temperature range between 523 and 773 K and the deformation characteristics were correlated with the results of microstructure investigation. The grain size and its stability at elevated temperatures were found to play the decisive role in the deformation behaviour of the alloys studied.

The optimization of ECAP conditions to achieve high strain-rate superplasticity in a Zr- and Sc-modified AA 7075 aluminum alloy

Abstract An AA 7075 aluminum alloy with Zr and Sc additions was subjected to 6 – 8 passes of equal-channel angular pressing at 120, 170, and 220 °C in order to study the effect of the pressing conditions on the microstructural characteristics and the consequences for high-temperature mechanical properties. Using the method of electron backscatter diffraction, a decrease in pressing temperature was found to lead to a more pronounced refinement of the microstructure and a higher fraction of high-angle grain boundaries, but at the same time compromised the ability of the Al3 (Zr, Sc) phase to stabilize the ultra-fine-grained microstructure at elevated temperatures. Optimum superplastic properties were therefore achieved after pressing at the intermediate temperature of 170 °C. The material exhibited high strain-rate superplasticity with a ductility of 650 % at an initial strain rate of 1 × 10 – 1 s – 1.

Spark Plasma Sintering of a Gas Atomized Al7075 Alloy: Microstructure and Properties

The powder of an Al7075 alloy was prepared by gas atomization. A combination of cellular, columnar, and equiaxed dendritic-like morphology was observed in individual powder particles with continuous layers of intermetallic phases along boundaries. The cells are separated predominantly by high-angle boundaries, the areas with dendritic-like morphology usually have a similar crystallographic orientation. Spark plasma sintering resulted in a fully dense material with a microstructure similar to that of the powder material. The continuous layers of intermetallic phases are replaced by individual particles located along internal boundaries, coarse particles are formed at the surface of original powder particles. Microhardness measurements revealed both artificial and natural ageing behavior similar to that observed in ingot metallurgy material. The minimum microhardness of 81 HV, observed in the sample annealed at 300 °C, reflects the presence of coarse particles. The peak microhardness of 160 HV was observed in the sample annealed at 500 °C and then aged at room temperature. Compression tests confirmed high strength combined with sufficient plasticity. Annealing even at 500 °C does not significantly influence the distribution of grain sizes—about 45% of the area is occupied by grains with the size below 10 µm.

Microstructure and high temperature deformation of an ultra-fine grained ECAP AA7075 aluminium alloy

Abstract An AA 7075 aluminium alloy with an ultra-fine grained structure was prepared through equal channel angular pressing (ECAP) at pressing temperatures TECAP of 120, 170, and 220 °C. A decrease in TECAP from 220 to 120 °C was found to lead to a more pronounced refinement of the microstructure and to worse stability of the microstructure – the onset of grain coarsening was displaced to lower temperatures. The material pressed with the highest TECAP exhibited superplastic behaviour at temperatures close to 400 °C and grain boundary sliding was identified as the dominant operating deformation mechanism. The materials prepared with both of the lower TECAP exhibited only enhanced ductility of about 200 %, however this behaviour was observed at temperatures as low as 200 °C. It was found that this “low temperature superplasticity” resulted from a combined operation of grain boundary sliding at selected grain boundaries and glide of lattice dislocations.

High-temperature mechanical properties of Fe-40 at.% Al based intermetallic alloys with C or Ti addition

Abstract Tensile and creep properties of vacuum-induction-melted and cast Fe-40Al-1C and Fe-40Al-1Ti (at.%) were studied. The ingots were hot-rolled at 1200 °C to plates of 12.5 mm thickness using a stainless steel sheath and cooled in air. The alloys showed microstructures with coarse elongated grains having diameters up to 500 and 300 m in the direction of rolling, respectively. Tensile tests were carried out at temperatures 20, 400, 600, 700, and 800 °C. The creep tests were performed under constant load of 100 MPa and 150 MPa at temperatures 550, 600, 650, and 670 °C. Fracture surfaces of tensile specimens tested up to 700 °C exhibited mainly intergranular decohesion. With increasing temperature, the proportion of ductile dimpled fracture increased, and at 800 °C the fracture surfaces of both alloys were practically completely covered by ductile dimples. In comparison to tensile test specimens, fracture surfaces of creep specimens showed an increased fraction of intergranular ductile fracture.

The dependence of the threshold stress for superplastic flow on structure

Abstract The model of a threshold stress for superplastic flow is tested in Al-Zn-Mg-Cu based alloys prepared both using ingot (IM) and powder metallurgy (PM) routes. The threshold stress is found to increase with increasing grain size in the IM alloy. The true value of the parameter p t characterizing the grain size dependence of the strain rate is close to two at low and medium strain rates and decreases to zero at high strain rates. A significantly higher threshold stress in the PM alloy may result from the prevailing small angle character of grain boundaries.

High Temperature Deformation of Twin-Roll Cast Al-Mn-Based Alloys after Equal Channel Angular Pressing

Twin roll cast Al-Mn- and Al-Mn-Zr-based alloys were subjected to four passes of equal channel angular pressing. The resulting grain size of 400 nm contributes to a significant strengthening at room temperature. This microstructure is not fully stable at elevated temperatures and recrystallization and vast grain growth occur at temperatures between 350 and 450 °C. The onset of these microstructure changes depends on chemical and phase composition. Better stability is observed in the Al-Mn-Zr-based alloy. High temperature tensile tests reveal that equal channel angular pressing results in a softening of all studied materials at high temperatures. This can be explained by an active role of grain boundaries in the deformation process. The maximum values of ductility and strain rate sensitivity parameter m found in the Al-Mn-Zr-based alloy are below the bottom limit of superplasticity (155%, m = 0.25). However, some features typical for superplastic behavior were observed—the strain rate dependence of the parameter m, the strengthening with increasing grain size, and the fracture by diffuse necking. Grain boundary sliding is believed to contribute partially to the overall strain in specimens where the grain size remained in the microcrystalline range.

Deformation behaviour of ultrafine-grained 7075 aluminium alloy

Abstract Samples of a 7075 aluminium alloy were prepared by the equal-channel angular pressing (ECAP) technique. The channel had a 10 mm × 10 mm cross section and an angle of 90°. A total of six passes was performed using route BC. The ECAP processing was carried out at 120, 170, and 220 °C. The ECAP-processed samples with grain sizes between 300 and 2000 nm were machined to tensile specimens with their tensile axis parallel to the pressing direction. Uniaxial tensile tests were carried out using an INSTRON testing machine at temperatures between room temperature and 300 °C at a constant crosshead speed giving an initial strain rate of 10 – 3 s – 1. The maximum stress decreases with increasing testing temperature. The elongation to failure increases with increasing testing temperature. The values of the yield stress, the maximum stress (ultimate tensile stress), and the elongation to failure depend also on the ECAP processing conditions.