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The influence of the application of severe plastic deformation on microstructure changes and creep deformation behaviour was investigated in P92 creep-resistant pipe steel. P92 steel in its as-received state, with a microstructure typical of tempered martensite, was subjected to high-pressure torsion at room temperature. An investigation of the microstructure using an electron backscatter diffraction technique revealed that severe plastic deformation led to the subsequent formation of an ultrafine-grained microstructure, containing {110} fibre texture and randomly misoriented boundaries. The microstructure characteristics and hardness measurements exhibited saturated values at an equivalent strain of higher than about 20. The creep behaviour of the ultrafine-grained specimen produced in the saturated region exhibited a minimum creep rate that was about two orders of magnitude faster, and a larger creep ductility, in comparison with the as-received state. Creep exposure at 873 K with a time to fracture of about 3 h led to a slight grain coarsening and to the formation of Laves phase.

The effect of an application of an external high hydrostatic pressure on creep behaviour, the microstructure, and nanoscale porosity in an ultrafine-grained cladding Zr-2.5 wt%Nb alloy processed by equal channel angular pressing (ECAP) was investigated by comparative creep testing. The results show that, under identical creep loading conditions, ECAP processed specimens subjected to subsequent high hydrostatic pressurization treatment prior to creep exposure exhibited no substantial differences in creep behaviour compared to specimens processed by ECAP only. The selected level of high hydrostatic pressurization treatment used in this study is not indicative of an effective removal of nanoscale porosity resulting from ECAP processing. By comparison to the as-received state, an increasing number of ECAP passes leads to a significant weakening of creep resistence of the alloy. It is suggested that the reasons for the occurrence of creep weakening after ECAP processing are an accumulation of high-angle grain boundaries, multiplication of dislocations, coarsening and/or dissolution of the Nd-rich second phase precipitates, and dislocation/particle interactions.

Ultrafine-grained (UFG) Al matrix composites reinforced with 15 and 30vol% in-situ Al3Ti filaments were fabricated by extrusion of Al-Ti powder mixtures followed by solid-state reactive diffusion. Fine Al powder particles (1.3μm) heavily deformed the coarser Ti particles (24.5μm) into filaments during extrusion. Upon a subsequent operation of hot isostatic pressing (HIP), the micrometric Al3Ti filaments elongated along the extrusion direction and formed in situ in the UFG Al matrix. Fabricated composites are free of pores and voids with perfect bonding created at the Al-Al3Ti interfaces. In parallel, a small portion (2.4vol%) of nanoscale γ-Al2O3 particles, which originate from native amorphous films on fine Al powders, formed in situ and were homogenously dispersed in the Al matrix. The microstructures of as-extruded and after HIP composites were analyzed by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive spectrometry (EDS) and electron back-scattered diffraction (EBSD). Owing to the presence of nanometric γ-Al2O3 particles with Al high angle grain boundaries (HAGBs), the UFG Al matrix remained stable even after HIP at 600°C for 9h. The mechanical properties and creep performance of composites at testing temperatures of up to 600°C were systematically studied. The Al-Al3Ti composites exhibited a combination of increased strength and Young's modulus in addition to excellent creep performance and structural stability, which indicates that the studied composites are potential structural materials capable of service at elevated temperatures. Fil: Krizik, Peter. Slovak Academy of Sciences; Eslovaquia Fil: Balog, Martin. Slovak Academy of Sciences; Eslovaquia Fil: Nosko, Martin. Slovak Academy of Sciences; Eslovaquia Fil: Castro Riglos, Maria Victoria. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina Fil: Dvorak, Jiri. Institute of Physics of Materials; República Checa Fil: Bajana, Oto. Slovak Academy of Sciences; Eslovaquia

The austenitic stainless steel 304 L was subjected to severe plastic deformation using high-pressure torsion technique at room temperature. The severe plastic deformation led not only to the grain size reduction but also to the transformation of austenite into deformation-induced martensite. At high imposed strains the martensitic boundaries disappeared and misorientation distribution exhibited nearly random misorientation distribution. The thermal exposure and tensile creep tests under constant load performed at 923 K and stresses ranging from 50 to 150 MPa revealed that austenitic microstructure contains σ phase. The ultrafine-grained microstructure coarsens during creep testing but the mean grain size is significantly finer than the predicted stationary subgrain. The results demonstrate that during creep testing new grains at precipitates of σ phase are formed and texture changes. The formation of new grains during creep keeps the microstructure of being ultrafine-grained and thus grain boundary mediated processes are enhanced in this steel.

The deformation behavior of an AlTiVNbZr0.25 high entropy alloy at 1073 K was investigated in creep in protective argon atmosphere at stresses of 100–560 MPa with and without changes. In the initial condition after annealing at 1473 K for 24 h the alloys was composed of B2 matrix phase and coarse Zr5Al3-type particles. During creep a limited amount of new Nb2Al-type sigma phase precipitates at the Zr5Al3/B2 hetero-phase interface and a quasi-stationary dislocation density of the expected order of magnitude are developed. The quasi-stationary creep rate obeys a three-power law and the transients after stress changes are inverted. This is interpreted in terms of the class A creep behavior with viscous glide due to cloud drag.

The creep behaviour of high purity aluminium and copper and their Al–0.2 wt%Sc and Cu–0.2 wt%Zr alloys was examined after processing by equal-channel angular pressing (ECAP) with an emphasis on creep ductility and the ECAP microstructural homogeneity. It was found that, under the same loading conditions, the creep ductility of the ultrafine-grained materials processed by ECAP continually increases with increasing numbers of ECAP passes. A detailed quantitative microstructural study was conducted using the electron backscatter diffraction (EBSD) methods. This analysis revealed that, with increasing numbers of ECAP passes, the mutual misorientation of neighbouring subgrains grows and the subgrains continuously transform to grains having high-angle grain boundaries.

The rapidly solidified AlCr4.7Fe1.1Si0.3 (at.%) alloy produced at large scale by direct extrusion of gas atomised powders was investigated. Hot-extrusion yielded sound compacts and did not create any major structural changes compared to the as-atomised state. Microstructural characterisation of the as-extruded material revealed submicron scale α-Al grains with an embedded Al7Cr crystalline phase and traces of a nano-scale quasicrystalline Al84.6Cr15.4 phase. The long-term thermal stability up to 300 °C of compacts was proven. The thermal stability of compacts was accompanied by high mechanical properties, especially high strength and Young's modulus, and creep performance up to 300 °C. The same material was prepared by laboratory scale extrusion of cryo-chopped melt-spun ribbons as a reference material. No major differences in the structure or properties of the industrially prepared material were observed compared to the reference material.

It is suggested that creep of pure aluminium processed by equal-channel angular pressing (ECAP) occurs by diffusion-controlled movement of intragranular dislocations and by grain boundary sliding. The contribution of grain boundary sliding is increasingly higher with increasing number of ECAP passes. The coexistence of a dislocation process and grain boundary sliding may explain the observed decrease of the creep resistance with increasing number of ECAP passes. This is because high-angle boundaries are necessary in order to achieve grain boundary sliding. Therefore, an increase in the fraction of high-angle boundaries with increasing number of ECAP passes will essentially lead to increasing contribution of grain boundary sliding to the total creep strain, as it was observed experimentally.