Materials Science Forum Vols. 667-669

Abstract: Two Al alloys (AA1570 and AA6061) in the solutionized state have been processed by HPT at room temperature to achieve a homogeneous UFG structure. After HPT, the grain size was found to have a mean value about 100 nm for both alloys. Measured yield stress values of HPT-produced UFG alloys being plotted in terms of the Hall-Petch relationship were found to exceed the plot predictions for the range of ultrafine grain size. For both alloys, Atom Probe Tomography measurements allowed to reveal segregation of solute elements along grain boundaries. The origin of the extremely high strength of the alloys nanostructured by HPT is discussed with a special attention to the influence of such segregations on the emission and the mobility of dislocations.

Abstract: Motivated by the large variety of enhanced properties of ultrafine and nanocrystalline materials such materials are under extensive investigation. Besides focusing on classical material parameters, like strength and ductility, the fracture toughness of these materials is also of great importance, especially when the damage tolerance is required. In this contribution an overview of the fracture behavior of different metals covering ultrafine-grained iron and nickel as well as a nanocrystalline steel processed via high pressure torsion (HPT) will be given. It will be shown that the specimen orientation can have a tremendous influence on the fracture behavior and toughness. Due to this toughness anisotropy an unexpectedly good combination of high strength and high fracture toughness can be achieved very often in these materials.

Abstract: Most ultrafine-grained (UFG) materials produced by severe plastic deformation (SPD) exibit only limited ductility which is correlated with the low strain rate sensitivity (SRS) of these materials. Recently, it was demonstrated that SPD is capable of increasing the room temperature ductility of aluminum-based alloys attaining elongations up to 150%, together with relatively high strain rate sensitivity. In the present work, additional results and discussions are presented on the effect of grain boundary sliding (GBS) and SRS on the ductility of some UFG metals and alloys. The characteristics of constitutive equations describing the steady-state deformation process are quantitatively analyzed for a better understanding of the effects of grain boundaries and strain rate sensitivity.

Abstract: Pure metals of 30 elements with various crystal structures (bcc, fcc, hcp, diamond cubic, complex cubic, primitive hexagonal and tetragonal) are processed by high-pressure torsion (HPT) and their mechanical properties are subsequently evaluated by Vickers microhardness measurements. For all metals, the hardness reaches steady states at large strains where the hardness remains unchanged with further straining. It is shown that the hardness values at the steady state are characteristics of each metal and are successfully expressed as a unique function of the homologous temperature, shear modulus and physical parameters of metals such as melting temperature, specific heat capacity and diffusion coefficient except for a few elements. The findings are well applicable to predict the ultimate steady-state hardness of metals attained by HPT processing through the correlation established in this study.

Abstract: It is now well established that the application of high-pressure torsion (HPT) leads to exceptionally small grain sizes in bulk solids, usually in the nanometer scale. The HPT method thus attracts considerable attention and has been utilized in processing many materials such as pure metals, alloys and even ceramics. This paper describes experiments conducted on samples of an Al-6061 alloy and an Al-6061 metal matrix composite, reinforced with 20 vol.% Al2O3 particulates, in order to evaluate the mechanical properties of the alloy and its composite processed by HPT. The tensile properties of the materials were obtained both at room temperature and at 773 K and representative microstructures were observed using transmission electron microscopy.

Abstract: A processing route enhancing the strength of a metallic material usually leads to a decrease in the ductility. Therefore, a mechanical processing technique improving the ductility as well as the strength of a structural material is of significant importance. The present study employs equal-channel angular pressing to impart high strains up to ~24 and ~8, via route BC, in Zn-22% Al and Al-3% Mg alloys, respectively. Tensile specimens, machined from the central region of the processed materials, were tested at room temperature between strain rates of 10-3 and 1 s-1. The Zn-22% Al did not show the presence of a critical strain above which high ductility-high strength could be simultaneously achieved at any strain rate. However, Al-3% Mg processed through 8 passes of ECAP showed a transition to a high strength-high ductility region at high strain rates. The occurrence of high strength and high ductility in Al-3% Mg after processing to very high strains is probably due to the transition from the heavily deformed and heterogeneous microstructure present at the lower strains when processing by ECAP to the more homogeneous structures that develop after larger numbers of passes in ECAP. The absence of a transition strain in Zn-22% Al is attributed to its low melting temperature so that even at room temperature (~0.54Tm where Tm is the melting temperature) it shows diffusion-controlled high temperature deformation behavior.

Abstract: This paper presents the research results of the grain refining effect on the ductile-to- brittle transition temperature (DBTT) of commercial purity tungsten. The as-received tungsten was subjected to eight passes of equal-channel angular pressing (ECAP) at decreasing temperatures from 1300 to 1150 °C. According to optical and TEM microscopy the average grain size was refined considerably from ~80 μm to ~1 μm. The mechanical tensile tests, carried out at various temperatures for the tungsten samples, showed that DBTT decreased approximately 80 °C as a result of microstructure refinement by ECAP, at the same time the strength also increased 50-100 % by grain refinement. SEM observation of the fractures confirmed the mechanical testing results.

Abstract: Ultrafine-grained (UFG) commercially pure (CP) Ti with a grain size of about 200 nm was produced by ECAP up to 8 passes using route BC at room temperature. For ECAP processing a proper die set was designed and constructed with an internal channel angle Φ of 120° and an outer arc of curvature Ψ of 20°. Strain rate sensitivity of UFG CP-Ti and CG CP-Ti were investigated by compression tests in the temperature range of 298~673K and strain rate range of 10-4~100s-1 using Gleeble simulator machine. Evolution of the microstructure during compression testing was observed using optical microscopy (OM) and transmission electron microscopy (TEM). Strain rate sensitivity value m of the UFG CP-Ti has been measured and is found to increase with increasing temperature and decreasing strain rate, and is enhanced compared to that of CG CP-Ti. Result of the deformation activation energy determination of UFG CP-Ti indicates that the deformation mechanism in UFG CP-Ti is correlated to the grain boundaries.

Abstract: Mechanical properties of two Cu alloys (electrolytic and fire refined) severely deformed by equal channel angular pressing (ECAP) process were investigated. They were treated with a annealing heat treatment to 600°C during 30 minutes and then they were extruded in a Φ=90º ECAP die at room temperature following route Bc. Heavy deformation was introduced in the samples after a considerable number of ECAP passes from 1, 2, 3, 4, 5, 6, 7, 8, to 16. The principal changes were introduced in the first pass by ECAP but a gradual increment in the mechanical properties was observed for the consecutive ECAP passes. Also, the electrical conductivity decreased with increasing numbers of ECAP passes.

Abstract: Room temperature equal-channel angular pressing (ECAP) was employed on commercial purity titanium in the present work. Mechanical twining was observed in ~90% grains while the grains were not significantly refined (~10 μm) after ECAP. TEM observations showed that the twins observed under OM are usually composed of a serial of parallel twin bands with a width of ~1 μm. Microhardness and tensile tests showed that such a coarse-grained microstructure combined with a high fraction of mechanical twins has a microhardness of ~ 240 Hv, yield strength of ~ 730 MPa, tensile strength of ~ 740 MPa and elongation to failure of ~ 16%. This yield strength is much higher than 620-640 MPa, the yield strength of ultrafine-grained titanium by 8~12 passes of ECAP at 450 oC with a grain size of 200-300 nm, and is close to ~ 790 MPa, the yield strength of commercial Ti-6Al-4V alloys.