Materials Science Forum Vols. 633-634

Abstract: The paper has viewed the manifestation of the paradox of severe plastic deformation (SPD), caused by the occurrence of preexisting deformation twins in ultrafine-grained Cu, which has been obtained by the combination of the SPD method, accomplished by an equal-channel angular pressing with the conventional methods of deformation-thermal treatment. The high strength of the obtained samples has proved to be conditioned by the occurrence of the high density of the coherent twin boundaries, serving as effective obstacles on the way of slipping dislocations. Moreover, the occurrence of the twins creates favorable conditions for the dislocation density increase both in the grains with the twins and in the grains without them. As a result the sample hardens, contributing additionally into its strength. Simultaneously it manifests high ductility. By doing so the deformation behavior of the sample is mainly conditioned by the grain boundaries of grains free from the twins. The results were obtained on the basis of the dislocation-based model which develops models of Y. Estrin and L. Tóth, M. Zehetbauer, and L. Remy.

Abstract: Recently, the necessity to grade grain size to ultrafine and nano scale for understanding the mechanical behavior of these materials has been recognized. However, the nature of such classification has remained unclear. As an example, ultrafine (100 nm -1 μm) and nano (<100 nm) grained FCC metals, compared to their coarse grained counterparts, exhibit a grain size strengthening that may deviate from the Hall-Petch relationship. To explain the mechanism of such deviation, previous dislocation theories seem insufficient. To solve this problem, a critical grain size criterion governing the shift of deformation mechanism is proposed in this work. This model employs an energetic approach; it relates the grain boundary energy density to certain critical energy values; and it permits, for the first time, a quantitative grading of grain sizes. Predictions based on this model were evaluated. The prediction on copper polycrystals of various grain sizes showed a very good agreement with experimental results. It is thus wished that the grain size theory on plastic deformation mechanism could be unified with the dislocation theory. In this study, such unification is attempted by using a parameter defined as the defect energy density. The possibility of such generalization is further reasoned upon the fact that the defect energy approach should be a unique but common form applicable for both dislocations and grain boundaries.

Abstract: Ultrafine grained materials prepared by methods of severe plastic deformation appear to show good ductility for their high strength. To a large extent this ductility enhancement, for the given strength, is shown to correspond to the fracture ductility and not the uniform ductility at maximum stress. The improved fracture ductility is often due to the refinement or removal of the coarse defects that act as sites for failure nucleation. The low work hardening rate inherent to the very fine microstructures produced by severe plastic deformation essentially condemns such materials to very low uniform ductility. Stress relaxation occurring during unloading after processing, and changes of internal stresses during reloading for mechanical testing, appear to play a significant role in determining deformation behaviour near the onset of plastic flow, and this can affect the measured uniform strain.

Abstract: Influence of SPD process realized by ECAP on structural formation and mechanical properties was searched. Samples after ECAP were heat treated at various temperature and time conditions. Investigation material bases were high purity aluminium and aluminium alloys EN AW 6082, EN AW 2014. The best material properties are describing in dependence on experimental conditions.

Abstract: An effect of ageing on mechanical properties of ultrafine grained Al 6061 alloys has been investigated in the present work. The solution treated bulk Al 6061 alloy was subjected to cryorolling to produce ultrafine grain structures and subsequently ageing treatment to improve its both strength and ductility. The hardness and tensile properties of solution treated, cryorolled, cryorolled and aged Al alloys were measured and explained by using their corresponding microstructural morphologies. The pre-cryorolled solid solution treatment combined with post-CR ageing treatment (1300C-30h) has been found to be the optimum processing condition to obtain the ultrafine grained microstructure with improved tensile strength (362MPa) and good tensile ductility (10.7%) in the Al 6061 alloy. The combined effect of precipitation hardening and recovery are responsible for the simultaneous improvement of both strength and ductility observed in the present work.

Abstract: The effect of equal channel angular pressing (ECAP) on the structure and mechanical properties of Al-4% Mg-1.5% Mn-0.4% Zr and Al-4% Mg-1.5% Mn-0.4% Zr-0.4% Sc alloys in the initial as-cast state was studied. The ECAP processing was shown to lead to the formation of predominantly submicrocrystalline structure with an average grain size of 850 nm in the Al-Mg-Mn-Zr-Sc alloy and 1060 nm in the Al-Mg-Mn-Zr alloy. It is remarkable that both strength and ductility of the two alloys were enhanced by ECAP. The highest strength was observed in the Al–Mg–Mn–Zr–Sc alloy (UTS = 425MPa), in combination with elongation to failure of EL=17 %.

Abstract: In this work ultrafine-grained (UFG) structure was successfully produced in the commercial Al 6061 and Al-30%Zn alloys using new modifications of two severe plastic deformation (SPD) techniques, namely equal channel angular pressing (ECAP) with parallel channels (PC) and high pressure torsion (HPT) with enhanced load. Variation of SPD processing regimes made it possible not only to perform strong grain refinement but also to modify the phase composition through the formation of grain boundary (GB) segregations and precipitations. This enabled to achieve a unique combination of high strength and ductility in the Al 6061 alloy and demonstrate super-ductility in the Al-30%Zn alloy, when elongation to failure exceeded 150% at room temperature.

Abstract: The influence of severe plastic deformation induced by ECAP on microstructure modification and aging effect was studied in two modified Al-Mg-Si aluminium alloys. The microstructure of both alloys in different heat treated and deformed state was characterised by X-Rays diffraction and polarised light microscopy. The effect of artificial aging was investigated after ECAP performed on samples in the as extruded condition. The aging effect was followed by hardness and electrical conductivity measurements. At higher aging temperature (170°C) the alloys showed an increasing softening with time due to recovery or/and grain coarsening effect. At the lower aging temperature, the hardness remains almost constant due to enhanced precipitation hardening effect.

Abstract: The processing of bulk metals through the application of severe plastic deformation provides an opportunity for achieving exceptional grain refinement to the submicrometer or even the nanometer range. This paper examines the characteristics of metals processed by equal-channel angular pressing with special emphasis on the levels of ductility that may be attained. It is shown that the amount of ductility is dependent not only upon the composition of the material but also, and to a major extent, upon the testing temperature. Specifically, the ductilities are often low at ambient temperatures where the strength of the as-processed material is relatively high but, by contrast, exceptionally high superplastic ductilities may be achieved over short ranges of strain rate when testing at elevated temperatures.

Abstract: Microstructure of ultra fine grained (UFG) Mg-Gd alloy prepared by high-pressure torsion (HPT) was investigated in the present work. Lattice defects introduced by HPT were characterized at first. Subsequently thermal stability of UFG structure and its development with annealing temperature were studied and correlated with changes of hardness and ductility. Precipitation effects in the alloy with UFG structure were compared with those in a conventional coarse-grained alloy. Defect studies were performed by positron annihilation spectroscopy (PAS), which represents well established non-destructive technique with a high sensitivity to open volume lattice defects like vacancies, dislocations, misfit defects etc. PAS investigations were combined with transmission electron microscopy (TEM) and X-ray diffraction (XRD). Changes of mechanical properties were monitored by Vicker’s microhardness (HV) and deformation tensile tests. It was found that HPT deformed Mg-Gd alloy exhibits UFG structure with mean grain size of 100 nm and a dense network of dislocations distributed uniformly throughout the whole sample. Although recovery of dislocations takes place at relatively low temperatures, it is not accompanied by grain growth and the mean grain size remains around 100 nm up to 300oC. Tensile tests performed at elevated temperatures to examine ductility showed that HPT-deformed alloy exhibits a superplastic behavior at 400oC. Moreover, it was found that the precipitation sequence in HPT-deformed alloy differs from that in conventional coarse-grained material.