Analyses of Microstructures and Mechanical Properties of Nanocrystalline Materials Processed by High-pressure Torsion

Abstract

Nanocrystalline / ultra-fine grains materials have been a subject of extensive research over the past few decades due to their attractive mechanical, thermophysical, optical and magnetic properties. Methods of severe plastic deformation (SPD) are well-known procedures for obtaining dense ultrafine-grained or even nanocrystalline bulk materials. Among the SPD processes, high pressure torsion (HPT) is known as the most effective process in terms of grain refinement due to the continuous imposing strains larger than in any other SPD processes. To understand HPT process, analyses of grain refinement mechanism, evolution of dislocation densities and resultant mechanical properties are necessary. In this study, high strain by HPT was used to produce the UFG materials. Electron back scatter diffraction (EBSD) method was employed for elucidating the mechanisms of grain refinement in pure Cu as well as for quantitative analysis of the grain size, the density of geometrically necessary dislocations (GND), and their dependence on the strains imposed by HPT. The results revealed that after effective strain 2.7 hardness of pure copper reached to a steady-state value of 140 Hv even before saturation of the grain refinement. The hardness saturation occurring earlier than the grain size saturation was explained by calculation of GND densities. Before saturation of the grain refinement, that is, until 4/8 turns, average GND densities consistently increased, whereas after recrystallization started, GND densities decreased and finally established the steady state. Based on the average of the GND density and grain size, we analyses the grain refinement mechanism of pure Cu by HPT.HPT is a simple technique for applying very large strains in a material due to the applied hydrostatic pressure during deformation. The evolution of the microstructure of single-phase materials deformed by HPT demonstrated no further microstructural refinement even upon increasing strain. Recently, combining powder metallurgy processing of the “bottom-up”-type with and the “top-down”-type HPT process has presented lower saturated grain refinement and better microstructural stabilization due to dispersed oxide particles that are naturally produced on metallic powders. In this study, in order to achieve both grain refinement for high strength and microstructural stability after refinement, we adopted a powder consolidation approach by HPT called “cold sintering”. Atomized Cu powders were used as the starting material. The evolution of mechanical properties was investigated for bulk Cu specimens consolidated by HPT under two different processing temperatures and three different numbers of revolutions. In particular, thermal stability of the UFG microstructures was investigated after annealing treatments. After HPT consolidation, relative density over 98%, high tensile strengths of 642 and ductility of 3.5% with thermally stable ultrafine grained structures were achieved. The specimens HPT processed at RT showed higher tensile strength due to more dislocations and finer grain sizes than the specimen processed at 373 K. Higher ductility in the specimen processed at elevated temperature (373 K) compare to the RT processed specimen was attributed to good bonding between particles, decreased dislocation density, and increased grain size. Moreover, High tensile strength of 616 MPa and improved ductility of 7.6% were obtained in powder-consolidated pure Cu processed by high-pressure torsion (HPT) at room temperature followed by post-annealing at 673 K for 1 h. The powder-HPT consolidation process maintained nanocrystalline microstructures even after post-annealing due to the presence of well-dispersed nanosized oxide particles in the matrix. Higher ductility in the post-annealed specimen is attributed to higher fraction of stable Σ3 coincidence site lattice boundaries than that in the HPT-processed Cu.On the other hand, rapid solidified Mg95Zn4.3Y0.7 (at.%) alloy powders produced by an inert gas atomizer were consolidated using HPT at room temperature and 373 K. As the HPT processing temperature increases, the powders are more plastically deformed due to decreased deformation resistance, grain boundaries are more equilibrium, and powder bonding is enhanced due to increased inter-particle diffusion. As a result, tensile ductility and strength increases. At the same time, hardness decreases with the increased processing temperature, due to less dislocation density.Carbon nanotube (CNT) reinforced Cu powders were consolidated by HPT. Effects of CNT reinforcements on the microstructural evolutions of the Cu matrix showed that the Cu matrix grain size was reduced to ~114 nm and the CNTs were well dispersed in the matrix. Due to the effect of the UFG Cu and CNTs, the tensile strength (350 MPa) of the nanocomposite was higher than that (190 MPa) of the Cu processed by the powder HPT process without CNTs. However, the Cu-CNT 10 vol. % indicated a decreased tensile strength due to an increased interface area between the matrix and CNTs at high volume fractions of the CNTs. In addition, the high energy ball milled Cu-CNT and Cu powders were consolidated by HPT at RT. High energy ball milling process enhanced the CNTs dispersion on Cu powders. The HPT process could consolidate the powder more than 98% relative density. The homogeneous nanocrystalline structure of 57 nm and homogeneously distributed CNTs in Cu matrix were obtained. The dispersed CNTs in Cu-matrix provide considerably enhanced wear resistance by retarding the peeling of Cu grains during sliding wear process. It is concluded that the homogeneous distribution of CNTs in nanocrystalline Cu matrix is important to enhance the mechanical behavior and wear resistance of CNT/Cu nanocomposite.