Nanomaterials by severe plastic deformation: review of historical developments and recent advances

International audienceSevere plastic deformation (SPD) is effective in producing bulk ultrafine-grained and nanostructured materials with large densities of lattice defects. This field, also known as NanoSPD, experienced a significant progress within the past two decades. Beside classic SPD methods such as high-pressure torsion, equal-channel angular pressing, accumulative roll-bonding, twist extrusion, and multi-directional forging, various continuous techniques were introduced to produce upscaled samples. Moreover, numerous alloys, glasses, semiconductors, ceramics, polymers, and their composites were processed. The SPD methods were used to synthesize new materials or to stabilize metastable phases with advanced mechanical and functional properties. High strength combined with high ductility, low/room-temperature superplasticity, creep resistance, hydrogen storage, photocatalytic hydrogen production, photocatalytic CO2 conversion, superconductivity, thermoelectric performance, radiation resistance, corrosion resistance, and biocompatibility are some highlighted properties of SPD-processed materials. This article reviews recent advances in the NanoSPD field and provides a brief history regarding its progress from the ancient times to modernity

Microstructure Mechanisms Governing the Creep Life of Ultrafine-Grained Cu-0.2 wt%Zr Alloy

Equal-channel angular pressing was conducted at room temperature and extrusion was performed up to 12 passes using route where the billets were rotated by 90° in the same sense between consecutive passes. Tensile creep tests were performed at 473, 573 and 673 K at different constant applied stresses. It was observed that the original coarse grain size of unprocessed alloy was reduced to 0.3 μm after 8 equal-channel angular pressing passes and the grain growth during creep was restricted by precipitates with the mean diameter ≈ 4.0 nm. No significant effect on creep resistance was found after one equal-channel angular pressing pass at 473 and 573 K. However, the longest time to fracture was exhibited by alloy after 2 equal-channel angular pressing passes at 573 and 673 K but with further increasing number of equal-channel angular pressing passes a decrease in the time to fracture was observed. Nevertheless, the beneficial effect of equal-channel angular pressing on creep resistance was still documented after 8 passes for temperatures of 473 and 573 K. By contrast, creep tests performed at 673 K showed that the time to fracture of ultrafine-grained material is shorter as compared with that for as-received state. The 3D laser measurement of surface showed that the creep fracture process is accelerated by formation of vertical surface step relief and cavitation at the intersection of the shear bands during creep

Microstructural processes in creep of an AZ 91 magnesium-based composite and its matrix alloy

Constant stress tensile creep tests were conducted on an AZ 91–20 vol.% Al2O3 short fiber composite and on an unreinforced AZ 91 matrix alloy. The creep resistance of the reinforced material is shown to be considerably improved compared with the matrix alloy. The creep strengthening arises mainly from the effective load transfer between plastic flow in the matrix and the fibers. Microstructural investigations by TEM revealed good fiber–matrix interface bonding during creep exposure. The microstructures of the AZ 91 alloy and its composite were similar with regard to two types of β-phase precipitates; the enhanced precipitation of the Mg17 (Al, Zn)12 phase on the fibers is promoted by heterogeneous nucleation due to the Al enrichment of the matrix near to the alumina fibers

Flow mechanisms in creep of short fibre AZ91 alloy-based composite

A comparison between the creep characteristics of an AZ91 magnesium alloy reinforced with 20 vol.% Al2O3 short fibres and an unreinforced AZ91 matrix alloy shows that the creep resistance of the reinforced material is considerably improved compared to the matrix alloy. It is suggested that the creep strengthening in the composite arises mainly from the existence of a threshold stress and the load transfer effect. The values of the threshold stress in the creep of the composite at temperatures in the range from 373 to 673 K were estimated using standard methods. It is proposed that the threshold stress arises from an attractive interaction between mobile dislocations and Mg17(Al, Zn)12 precipitates

Microstructure Mechanisms Governing the Creep Life of Ultrafine-Grained Cu-0.2 wt%Zr Alloy

Equal-channel angular pressing was conducted at room temperature and extrusion was performed up to 12 passes using route where the billets were rotated by 90° in the same sense between consecutive passes. Tensile creep tests were performed at 473, 573 and 673 K at different constant applied stresses. It was observed that the original coarse grain size of unprocessed alloy was reduced to 0.3 μm after 8 equal-channel angular pressing passes and the grain growth during creep was restricted by precipitates with the mean diameter ≈ 4.0 nm. No significant effect on creep resistance was found after one equal-channel angular pressing pass at 473 and 573 K. However, the longest time to fracture was exhibited by alloy after 2 equal-channel angular pressing passes at 573 and 673 K but with further increasing number of equal-channel angular pressing passes a decrease in the time to fracture was observed. Nevertheless, the beneficial effect of equal-channel angular pressing on creep resistance was still documented after 8 passes for temperatures of 473 and 573 K. By contrast, creep tests performed at 673 K showed that the time to fracture of ultrafine-grained material is shorter as compared with that for as-received state. The 3D laser measurement of surface showed that the creep fracture process is accelerated by formation of vertical surface step relief and cavitation at the intersection of the shear bands during creep