Materials Science Forum Vols. 584-586

Abstract: The role of the deformation pre-history in high-cycle fatigue properties of copper produced by severe plastic deformation is discussed. The focus is placed on comparison of the structures and mechanical behaviours of two types specimens produced either by rolling or by ECAP, i.e. by the pure shear or simple shear mode, respectively. It is shown that the deformation either by simple or pure shear mode to the same equivalent strain results in alike mechanical properties, both monotonic and cyclic. The significant influence of the initial stages of strain hardening on fatigue is highlighted.

Abstract: This paper reports on the microstructures and fatigue properties of ultrafine-grained (UFG) AM60 magnesium alloy processed by equal channel angular pressing (ECAP) at various temperatures. After ECAP processing, samples exhibited an increase in fatigue endurance limit, which correlates well with a decrease in grain size. In case of lowest ECAP temperature, the mean grain size is as small as 1 2m which leads to an increase in fatigue endurance limit by 70 % in comparison to coarse-grained alloy. The temperature of ECAP not only governs the grain size and misorientation angles of grain boundaries but also the volume fraction of precipitates, thus affecting the probability of twinning and grain growth after fatigue treatment.

Abstract: In order to study the effect of trace impurities on high-cycle fatigue damage of ultrafine grained (UFG) copper, fatigue tests were carried out for two samples: oxygen-free copper (OFC, 99.99 wt% Cu) and deoxidized low-phosphorous copper (DLP, 99.95 wt% Cu). After the processing by equal channel angular pressing (ECAP) using eight passes, equiaxed grains with an average size of 250 nm were formed for both the samples. Fatigue strength of UFG copper was enhanced by the impurities. The formation behavior of surface damage and the change in surface hardness was monitored. A close relationship was observed between the change in hardness and the formation behavior of surface damage. The physical background of the effect of trace impurities on the fatigue damage was discussed from the viewpoints of surface damage formation behavior.

Abstract: Crack growth in AA6060 after two and eight equal-channel angular extrusions (ECAE), showing a bimodal microstructure and a homogenous ultrafine-grained microstructure, respectively, are compared to the coarse grained counterpart. Furthermore, an optimized condition, obtained by combining one ECA-extrusion and a subsequent short aging treatment is included. Fatigue crack growth behaviour in the near-threshold regime and the region of stable crack growth is investigated and related to microstructural features such as grain size, grain size distribution, grain boundary characteristics and ductility. Micrographs of crack propagation surfaces reveal information on crack propagation features such as crack path deflection and give an insight to the underlying microstructure. Instrumented Charpy impact tests are performed to investigate crack initiation and propagation under impact conditions. Due to the recovery of ductility during the post-ECAE heat treatment, the optimized condition shows improved fatigue crack properties and higher energy consumption in Charpy impact tests, when compared to the as-processed conditions without heat treatment.

Abstract: Fatigue-crack-growth in an ultrafine-grained (UFG) Al-6%Mg-0.3%Sc alloy is investigated in conjunction with a precise analysis of the fracture surface. The comparison of the crack growth behavior of the UFG and ordinary polycrystalline materials has shown that the fatigue crack growth rate in the UFG alloy is higher than that in the coarse-grained material only in the near-threshold region. In the intermediate fatigue stage, propagation of the fatigue-crack in the UFG structure becomes insensitive to the grain size. At larger stress-intensity-factor-increments,
K, the crack resistance of the UFG material is better than that of un-ECAPed specimen. Analysis of the surface features indicates that such inhibition of the crack growth in the UFG structure upon increasing
K may be related to the gradual transition from intergranular- to transgranular mode of fatigue fracture.

Abstract: Ti-6Al-4V ELI (extra low interstitials) was processed by equal channel angular pressing in order to obtain an ultrafine-grained (UFG) microstructure which is known to enhance the fatigue behavior of metallic materials. Fatigue properties of UFG Ti-6Al-4V ELI were studied by strain and stress controlled fatigue tests. UFG Ti-6Al-4V ELI shows an improvement of the fatigue behavior compared to conventional grain (CG) size counterpart. Microstructural investigations prior to and after fatigue testing confirm a high structural stability of the UFG material. Hence, the UFG alloy has a high potential for prospective use in biomedical and engineering applications.

Abstract: Commercial purity aluminium AA1050 and aluminium alloy AA6016 were accumulative roll bonded and subsequently friction stir welded. The microstructure of the conventional and ultrafine-grained materials produced by accumulative roll bonding is strongly affected by friction stir welding. The elongated ultrafine-grained microstructure of roll bonded sheets becomes coarser and equiaxed in the nugget region. Hydraulic bulge tests showed that higher burst pressure can be achieved for samples without friction stir welding than for the ones with friction stir welding. Localised deformation, crack initiation and propagation, as well as the final fracture occurred within the nugget. Friction stir welded AA1050 sheet showed similar achievable burst pressures and von Mises equivalent strains compared to the as-received conventionally grained sheets. On the other hand, significantly higher burst pressures and at the same time higher von Mises equivalent strains were observed for the friction stir welded ultrafine-grained material than for the friction stir welded conventionally grained material.

Abstract: The fatigue behaviour of aluminium-magnesium alloys has been investigated in the recrystallized CG state and in an ultrafine-grained (UFG) state after equal channel angular pressing (ECAP). A strong improvement of the fatigue behaviour up to 12 ECAP passes has been found for an AlMg0.5 alloy. The results have been interpreted in consideration of the microstructure of the different states. Additionally, for an investigation of the influence of impurities on the cyclic stability of Aluminium, 3 different AlMg alloys with Magnesium contents of 0.5, 1, and 2 wt.-% have been compared. Total strain controlled fatigue tests have shown an improvement of the cyclic stability with increasing Mg content.

Abstract: Experiments were conducted on an Al-0.2wt.%Sc alloy to evaluate the effect of equalchannel angular pressing (ECAP) on its creep behaviour. ECAP was conducted at room temperature with a die that had an internal angle of 90° between the two parts of the channel. The subsequent extrusion passes were performed by route BC up to 8 ECAP passes. Creep tests in tension were performed on the as-pressed samples at 473 K under an applied stress range between 10 to 50 MPa. For comparison purposes, some creep tests were performed also on the unpressed alloy. Following ECAP and creep testing, samples were prepared for examination by means of transmission electron microscopy (TEM) and scanning electron microscopy (SEM) equipped with an electron back scattering diffraction (EBSD) unit. The observation of the surface of the ECAPed samples after creep exposure showed the occurrence of mesoscopic shear bands. The EBSD data reveal that these bands are separated by high angle grain boundaries. The creep resistance of an alloy is a little decreased after one ECAP pass. However, successive ECAP pressing lead to a noticeable decrease of the creep properties. Thus, the Al-0.2wt.%Sc alloy processed by 8 ECAP passes exhibited faster creep rate by about two and/or three orders of magnitude than the unpressed alloy when creep testing at 473 K and at the same applied stress. The detrimental effect of ECAP on the creep resistance is probably a consequence of a synergetic effect of mesoscopic sliding of groups of grains along shear bands, more intensive grain boundary sliding and creep cavitation in creep of the ultrafine-grained material.

Abstract: Martensitic transformations in NiTi shape memory alloys (SMAs) strongly depend on the microstructure. In the present work, we investigate how martensitic transformations are affected by various types of ultra-fine grained (UFG) microstructures resulting from various processing routes. NiTi SMAs with UFG microstructures were obtained by equal channel angular pressing, high pressure torsion, wire drawing and subsequent annealing treatments. The resulting material states were characterized by transmission electron microscopy and differential scanning calorimetry (DSC). The three thermomechanical processing routes yield microstructures which significantly differ in terms of grain size and related DSC chart features. While the initial coarse grained material shows a well defined one-step martensitic transformation on cooling, two-step transformations were found for all UFG material states. The functional stability of the various UFG microstructures was evaluated by thermal cycling. It was found that UFG NiTi alloys show a significantly higher stability. In the present work, we also provide preliminary results on the effect of grain size on the undercooling required to transform the material into B19’ and on the related heat of transformation.