Materials Science Forum Vols. 783-786

Abstract: A nanostructured Al-1%Si alloy containing dispersed Si particles was produced by heavily cold-rolling to study the effect of second phase particles on the tensile instability of nanostructured metals. Tensile tests were conducted on the as-deformed sample and the samples after recovery annealing treatments. The structural features of deformed and annealed samples were characterized by transmission electron microscopy. By comparing with the behavior of nanostructured commercial purity Al without dispersed particles, a remarked improvement in the tensile stability was found. This is related to a prevention of localized deformation by the presence of finely dispersed Si particles in the nanoscale matrix structure.

Abstract: A copper alloy, Cu-0.1% Zr, was processed at room temperature by high-pressure torsion (HPT) in order to evaluate the microstructural evolution and grain refinement mechanism. Transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) techniques were employed to measure the grain morphology, grain size distributions and the distribution of the misorientation angles. The results demonstrate that this processing procedure has a potential for producing an ultrafine-grain structure containing reasonably equiaxed grain with high-angle boundary misorientations. The grain refinement mechanism is primarily governed by dislocation activities.

Abstract: The mechanism of grain refinement in an Al-5.4Mg-0.4Mn-0.2Sc-0.09Zr alloy subjected to equal-channel angular pressing (ECAP) at 300°C through route B_C is considered. It was shown that the formation of geometrically necessary boundaries (GNB) aligned with a {111} plane at ε≤1 initiates the occurrence of continuous dynamic recrystallization (CDRX). Upon further strain the GNBs transform to low-to-moderate angle planar boundaries that produces lamellar structure. In the strain interval 2-4, 3D arrays of planar boundaries evolve due to inducing the formation of 2^nd order and higher orders families of GNBs in new {111} planes. GNBs gradually convert to high-angle boundaries (HAB) with strain. A uniform recrystallized structure is produced at a true strain of ∼8. The role of slip concentration and shearing patterns in the formation of GNBs is discussed.

Abstract: Severe plastic deformation (SPD) is an attractive processing method for refining microstructures to have ultrafine grain sizes within the submicrometer or even the nanometer levels. In SPD, the most promising techniques are equal-channel angular pressing (ECAP) and high-pressure torsion (HPT). A conventional superplastic Zn-22% Al eutectoid alloy was processed by ECAP and HPT. Experiments were conducted to demonstrate the evolution of hardness and microstructure and the enhancement of superplastic properties of the Zn-Al alloy after processing by the SPD techniques. In addition, flow mechanisms of the Zn-22% Al alloy are discussed by utilizing a deformation mechanism map.

Abstract: Ultra fine grained steels have been developed by many researchers. However, the study of influence on processes and product functions from different grain size are limited because the size of bulk material was small for these products. Authors have developed the production process of thin ultra fine grained stainless steel coil, and the effects are able to be clarified. This paper will firstly report the influence on micro hole piercing by comparing different grain size materials. Secondly, orifices are produced from these materials, and the liquid flow volume is measured as the functional effect of different grain size. The effects of grain size differences were discussed with observing the hole conditions and measuring flow volume. The effects of reduction of the grain size were summarized as follows: (1) Accurate small hole is produced when ultra fine grained stainless steel is employed. (2) Product functional improvement is possible, and the phenomena are useful for liquid control devices.

Abstract: The studies of diffusion bonded samples of Ti-6Al-4V and Nitinol alloys were carried out considering the titanium alloy in two states: ultra-fine grained and bi-modal microstructures, the last one consisted of small and large α-phase grains. Depending on microstructure and chemical composition of the alloys, the diffusion bonding had been made at temperatures from 600°C to 850°C. The microstructures of joints was studied by scanning electron microscope using detector of backscattering electron diffraction. The shear strengths of joints were measured. It was concluded that the ultra-fine grained Ti-6Al-4V alloy could be applied for joints manufactured at a temperature lower than 750°C. The bi-modal Ti-6Al-4V alloy is an effective material for producing the joints at the temperature larger that 750°C.

Abstract: Mechanochemical (MC) processes are often modified to a large extent with use of process control agents (PCAs). An appropriate PCA is, however, chosen semi-empirically for any MC process such as ball-milling. In this study, milling period and medium, PCA kinds and amount, and powder kinds, are altered as major parameters to systematically and mechanistically investigate the effect of PCAs on powder milling and the related phenomena. The model systems selected are TiO₂ and ZnO powders with various PCAs including alkanes, alcohols, water, and a few ionic liquids. According to X-ray line broadening analyses the viscosity and the structure of PCA molecules strongly affect the MC process. In addition, the MC process for harder TiO₂ is quite different from ZnO, and oxygen-derived polarity of PCA molecules would cause major difference on the milling behavior. Various parameters controlling the MC process will be discussed.

Abstract: Capabilities of the Mössbauer (nuclear gamma-resonance) spectroscopy for investigation of the state of grain boundaries in ultra-fine grained materials are analyzed, and the main problems of such studies are discussed. The emission and absorption NGR spectroscopy are compared, and it is demonstrated that the emission mode of the Mössbauer spectroscopy is preferential for GB studies. These studies enable to reveal differences in the state of GBs in ultra-fine grained materials and coarse-grained polycrystals with GBs of recrystallization origin.

Abstract: The effect of grain refinement on cold formability behavior on medium C-Boron steel has been studied. In particular a 30MnB4 steel long product has been considered to be used for screw production as final application. One of the issues during the cold forming of screw heads is given by the possible material failure in the most strained area, like the flange tip in the head. The probability of having a high scrap rate is strictly linked to the material cold formability limits. The study focused on the comparison between steel with conventional and ultrafine microstructure. Relationships between steel microstructure and plastic deformation behavior of the material were investigated by FEM modeling. Calibration of the model was carried out by analyzing the ultrafine grained steel, produced in pilot mill whose grain size was about 2-3 μm and the conventional steel industrially produced (grain size about 12 μm). Specific laboratory tests to characterize the material formability have been carried out. Finite element simulations have been performed to characterize the strain and stress state both in the forming process and in experimental tests to determine the material ductile fracture locus. Mechanical tests allowed to build the ductility curves related to the different microstructures to be used to simulate cold forming taking into account the material damage. Results put in evidence a significant improvement in the forming behavior of UFG steel, meaning that the refinement of grain size should produce a more evident effect in scrap reduction during the cold forming process.

Abstract: The microstructure evolution and the deformation behavior of a Cu-0.3%Cr-0.5%Zr alloy subjected to multidirectional forging at a temperature of 673 K under a strain rate of about 10^-3 s^-1 were studied. Following a rapid increase in the flow stress during straining to about 1, the strain hardening gradually decreases, leading to a steady-state flow behavior at total strain above 2. The multidirectional forging led to the development of ultrafine grained microstructures with mean grain sizes of 0.9 μm and 0.64 μm in the solution treated and aged samples, respectively. The presence of second phase precipitates promoted the grain refinement. After processing to a total strain of 4, the fractions of ultrafine grains (D < 2 μm) comprised 0.36 and 0.59 in the solution treated and aged samples, respectively.