Materials Science Forum Vols. 584-586

Abstract: Investigations of mechanical properties of nanocrystalline (nc) materials are still in interest of materials science, because they offer wide application as structural materials thanks to their outstanding mechanical properties. NC materials demonstrate superior hardness and strength as compared with their coarse grained counterparts, but very often they possess a limited ductility or show low uniform elongation due to poor strain hardening ability. Here, we present the results of investigation of the microstructure and mechanical properties of nc Pd and Pd-x%Ag (x=20, 60) alloys. The initially coarse grained Pd-x% Ag samples were processed by high pressure torsion, which resulted in formation of homogenous ultrafine grain structure. The increase of Ag contents led to the decrease of the resulted grain size and change in deformation behavior, because of decreasing of stacking fault energy (SFE). The samples with larger Ag contents demonstrated the higher values of hardness, yield stress and ultimate stress. Remarkably the uniform elongation had also increased with increase of strength.

Abstract: This study introduces a process of high-pressure torsion (HPT) using ring samples and compares with the results of conventional disk HPT. Both types of HPT were conducted at room temperature on pure Al and pure Cu. The microhardness was measured along the diameters of the disks and rings. Microstructures were examined using transmission electron microscopy. When hardness values were plotted against equivalent strain, all data points fell on a single line for each material. There was a hardness maximum for pure Al but no such a maximum was present in pure Cu. In pure Al, many dislocations were visible within grains up to the equivalent strain corresponding to the hardness maximum but beyond this strain, grains with low dislocation density appear. All materials exhibited steady state where the hardness remains constant with respect to imposed equivalent strain. This study concludes that use of ring samples is effective as an alternative to the disk samples.

Abstract: Possibilities of X-ray methods, using diffractometric measurements of X-ray line profiles and crystallographic texture, are compared as applied to study of SPD metal materials. The texture analysis gives the most direct data on the degree of grain fragmentation under conditions of deformation. The consideration is illustrated by results, obtained by X-ray studies of ECAP rods from Cu and Ti.

Abstract: We suggest a simple method to study the mechanical behaviour of nanocrystalline (nc) samples in compression-torsion mode. High applied pressure prevents the fracture of sample, and quantitative parameters of sample response during torsion test can be compared with developed microstructure. Here we present and discuss the results of systematic investigation of mechanical behaviour of nc Pd with a mean grain size of 12 nm in a wide range of shear strains (0<γ<200) and at strain rates γ& = 3 10-1 s-1 and 3 10-2 s-1. We show that in the studied shear strain range the notable changes in the microstructure, namely a strain induced grain growth occurs, and that controls the relevant deformation mechanisms. For lower strains when the grain size is still small enough, the plastic flow governs by twinning and probably grain boundary sliding. The flow stresses are lower as compared with the later stages of deformation, when the grain size becomes larger and deformation is controlled exceptionally by dislocation glide. Finally, a steady stage is achieved, when the grain size, dislocation density and flow stress are saturated.

Abstract: A positron annihilation study of free volumes was performed on Cu-Zr and Al-Sm alloys in the course of repeated cold rolling (RCR) in order to contribute to the microscopic understanding of the complex processes of solid-state amorphization and nanocrystallization. In addition to positron lifetime spectroscopy, which yields information on the size of free volumes, twodimensional Doppler broadening technique was applied in order to study the local chemical environment of free volumes on an atomistic scale. Both in Cu60Zr40 and Al92Sm8 a characteristic variation of the chemical environment of free volumes with the number of folding and rolling (F&R) cycles could be observed. In the first (Al-Sm) and intermediate state (Cu-Zr) of F&R, free volumes with an enhanced amount of Sm or Zr content in the local environment occur, indicating interfacial segregation or the formation of solute-vacancy complexes. Upon further F&R cycling, a complete (Cu-Zr) or partially amorphous structure (Al-Sm) is obtained with the free volumes exhibiting a chemical environment characteristic of the average chemical composition. In contrast to melt-spun or ball-milled amorphous alloys, free volumes of the size of a few missing atoms are found in amorphous alloys prepared by RCR presumably due to strong athermal conditions of F&R cycling.

Abstract: A commercially pure niobium has been subjected to SPD at room temperature ( ~0.11 TM) via ECAP (90º, route BC) up to 16 passes and via HPT up to shear strains γ =1000. ECAP-ed samples show an equiaxed structure after 8 and 16 passes with a decreasing average grain size. The results show that both the microstructure and mechanical properties of ECAP-ed samples do not reach a steady state up to at least 16 passes. HPT samples show at outer region a finer structural size but similar hardness values at similar equivalent strains. The nanoindentation results show an evident indentation size-effect even for the most deformed samples. The hardness values at the nano level converge for the recrystallized, the ECAP-ed and the HPT samples. This implies that, at the nano level, when the geometrically necessary dislocation density overcomes significantly the (initial) statistically stored dislocation density, hardness depends mainly on the physical intrinsic properties of the material (Burgers modulus, bulk modulus...) and the contribution of bulk mechanical properties (i.e., bulk yield strength) to hardness is smoothed down. Strain-rate sensitivity (SRS) of plastic strength has been also measured by means of rate-jump nanoindentation tests. The SRS is proportional to the inverse of hardness.

Abstract: Al 5083 alloy powder was mechanically milled in liquid nitrogen to achieve a nanocrystalline (NC) structure having an average grain size of 50 nm with high thermal stability, and then consolidated by quasi-isostatic (QI) forging. The consolidation resulted in ultrafine grains (UFG) of about 250 nm, and the bulk material exhibited enhanced strength compared to conventionally processed Al 5083. The hardness of as-cryomilled powder and the UFG material was measured by nanoindentation using loading rates in the range of 50−50,000 /N/s, and results were compared with the conventional grain size alloy. Negative strain rate sensitivity was observed in the cryomilled NC powder and the forged UFG plate, while the conventional alloy was relatively strain rate insensitive.

Abstract: The structure and mechanical properties of amorphous alloy Ni44Fe29Co15Si2B10 after severe plastic deformation (SPD) in Bridgman chamber at the different temperatures (77 and 298 K) have been studied. It is shown that the early stages of the SPD of amorphous alloy cause a noticeable decrease in microhardness HV and significant changes in the physical properties. With increasing the value of SPD the transition from inhomogeneous to homogeneous (or to qualitatively different) mode of plastic flow is observed, which is accompanied by the effects of homogeneous nanocrystallization. The nanoparticle size does not exceed 10 nm. It is established that the thermally activated nanocrystallization processes can occur at very low temperatures (77 K).

Abstract: The temperature and strain rate fields in severe plastic deformation (SPD) are measured using infra-red thermography and Particle Image Velocimetry (PIV), respectively. Plane strain machining is used as the method of SPD to impose controlled strains and strain rates. For metals such as titanium, the temperature rise is small at small strain rates and SPD occurs at near-ambient temperature. The possibility of exploring dynamic recovery/recrystallization phenomena using the Zener-Hollomon parameter in this SPD framework is briefly discussed.

Abstract: Progresses in developments of high resolution digital cameras and processors power as well enable the use of optical systems for axial and biaxial strain measurements. The main benefit of these systems is a contact free measurement on the sample surface without any influence to the sample. The commonly used optical systems based on following markings attached to sample surface which have to be applied before testing. This paper presents an alternative optical measuring system using the speckle-correlation-technique. The laser based method does not need any special sample markings before testing. The speckle correlation in the presented extensometer measures up to two-dimensional surface strains in tensile as well as in compression tests; original gauge lengths down to 1.5 mm are supported.