Materials Science Forum Vol. 1016

Abstract: Microstructural evolution during the strain-induced phase transformation of austenite in an Austempered ductile iron (ADI) under various thermomechanical processing conditions is studied in the present study. An alloyed ductile iron is taken as the base material, and thermomechanical treatment is carried out on a Gleeble 3800 thermomechanical simulator coupled with dilatometry. The effect of deformation on the austempering process has been studied by microstructure characterization using optical microscopy (OM), scanning electron microscopy (SEM), and X-ray diffraction (XRD) techniques. The variations in retained austenite volume fraction and its carbon content with respect to different austempering times are analyzed to study the effect of strain-induced transformation of austenite. It has been observed that the thermomechanical treatment significantly influences the phase transformation kinetics during the austempering process. The thermomechanical treatment produced a martensite free ausferritic microstructure for all austempering times with a high volume fraction of carbon enriched retained austenite as compared to the conventional heat treatment.

Abstract: The aim of this study is to analyze how the fatigue behavior of DMLS built AlSi10Mg may be affected by heat and surface treatments. A three-by-three factorial plan has been arranged for this purpose. Particularly, regarding the heat treatment, three levels were considered (as built, age hardening and stress relief), whereas, for the surface treatment, three levels were investigated (micro shot-peening, micro shot-peening plus fine blasting and DMLS production, machining and lapping). Ten specimens per type were built with vertical orientation and with a cylindrical smooth geometry. 90 samples were tested on a four-point rotating bending machine, aiming at the determination of S-N curves in the finite life domain. The experimental results were statistically processed and compared. The outcome was that heat treatment significantly affects the fatigue response and stress relief may have a detrimental impact. Fractographic studies were performed as well, to individuate the crack initiation points. Finally, density measurements were carried out to quantify the porosity level.

Abstract: Subtransus multiaxial hot forging of α+β Ti-6Al-4V (TA6V) titanium alloy with a β-transformed microstructure aims at obtaining an equiaxed microstructure through α phase globularization. The activation of mechanisms involved in microstructural evolution, such as globularization, depends on parameters such as time, temperature, strain and strain rate. It is also sensitive to the crystallographic orientation of α-lamellae. As a result, multiaxial processing of titanium alloys leads to significant microstructural gradients depending on thermomechanical conditions and initial microstructure. In this study, we focused on the effect of complex thermomechanical paths on microstructural evolutions. Thanks to the MaxStrain Gleeble device, we were able to reproduce such thermomechanical treatments to β-transformed TA6V samples. Stress strain fields obtained with finite element modelling of the MaxStrain test were compared to experimental microstructure gradients. This experimental method offers the opportunity to get closer to industrial open die forging conditions.

Abstract: Automobiles are equipped with EGR (Exhaust Gas Recirculation) coolers to improve fuel economy and exhaust gas suppression performance. Inside the EGR cooler, the moisture in the gas is condensed by cooling the hot exhaust gas. This condensed water is highly corrosive because sulfur oxides dissolve. Therefore, stainless steel and Ni-based brazing metal having excellent corrosion resistance are used for the EGR cooler.Until now, stainless steel has been brazed under a vacuum atmosphere. However, there are increasing opportunities to braze stainless steel in an inert atmosphere gas at atmosphere for cost reduction and mass production. In this case, a paste-type brazing filler metal consisted of a powder brazing filler metal and a binder is used. As is well known, a debinding process that volatilizes the binder is needed. From previous research in this laboratory, it is clarified that the binder causes voids. In addition, it is said that the size and location of voids generated at the brazed joint affect the product performance. On the other hand, the detailed investigation about the influence which the installation position of a paste type brazing filler metal on the void formation process has not yet been made. Therefore, in this study, the arrangement method and influence on heating rate and debinder temperature on void formation were investigated by X-ray CT.

Abstract: Dislocation densities of GLIDCOP®, dispersion-strengthened copper with ultra-fine particles of aluminum oxide, were evaluated by employing the X-ray line profile analysis using the modified Williamson-Hall and modified Warren-Averbach methods. X-ray diffraction profiles for GlidCop samples with compressive strains applied at 200oC were measured with synchrotron radiation. The dislocation densities of GlidCop with compressive strain ranging from 0.6 to 4.3% were in the order of 3.2 × 10¹⁴–5.8 × 10¹⁴ m^-2. The dislocation density increased with increasing the compressive strain within the measured strain range.

Abstract: Pure copper, pure silver, Cu-6.8at%Al, Cu-1at%Mn and Cu-5at%Ni (stacking fault energies (SFEs) are about 41, 22, 23, 43 and 105mJ/m², respectively) were processed by equal channel angular pressing (ECAP) at the same homologous temperatures to investigate the effect of SFE and solute atoms on microstructural evolutions. The final grain size of Cu-6.8at%Al after eight passes of ECAP was the smallest followed by that of Cu-1at%Mn with little difference. The former alloy reaches saturation for grain size and grain boundary misorientation in early passes of ECAP while the latter continues decreasing even after eight passes. The role of shear bands and deformation twins is predominant for grain fragmentation in the early stage for Cu-6.8at%Al and Ag with low SFE while evolution from cell walls to grain boundaries is main mechanism for Cu, Cu-1at%Mn and Cu-5at%Ni with medium or high SFE. Solute Mn atom of Cu-Mn with high atomic size misfit and may suppress the dynamic recovery which transforms cell walls to grain boundaries, and allow accumulation of higher dislocations and reduction of cell size to smaller scales.

Abstract: The properties of anodized aluminum, and wear resistance in particular, are of high interest for the scientific community. In this study, discs of AA6082 were subjected to a peculiar hard anodizing process leading to anodized samples having different thicknesses. In order to investigate the wear mechanism of samples, unidirectional tribological tests were performed against alumina balls (corundum) under different loading conditions. Surface and microstructure of all the samples were characterized before and after the tribological tests, using different characterization techniques. The tribological tests showed remarkable differences in the friction coefficient and wear behavior of the anodized AA6082 samples, related to the microstructure modifications and to the specific applied sliding conditions.

Abstract: Performing X-ray stress measurement by a new method called the cosα method is spreading in Japan. The cosα method is characterized in that the entire diffraction ring is used and analysed. As a result, the measurement speed is fast, and the device can be downsized. On the other hand, the conventional sin²ψ method requires precise measurement of diffracted X-rays at several X-ray incident angles, which increases the measurement time and the size of the device. In addition, since only a small part of the diffraction ring generated during the X-ray irradiation is used, some of the information on the diffraction ring cannot be used so it is a wasteful method. In 2012, cosα method equipment was released in Japan. Since then, cosα method X-ray stress measurements have been performed at more than 300 facilities. In response to this situation, the Japan Society for Nondestructive Inspection (JSNDI) together with public and private institutions established a study group on the cosα method in 2014, and examined the cosα method and its appropriate use. In this paper, we report the results of this study group.

Abstract: Recent advances in transmission electron microscopy (TEM) in respect of structural characterization down to atomic scale have enabled confirmation of stabilization of long ignored hexagonal omega (ω) phase in steel. The presence of ω phase is suggested to increase the strength of steel, and one of the factors concerning its stabilization is enrichment caused by the presence of certain solute atoms in the nanometer sized areas. Here, we report a density functional theory study conducted on a (3×3×2) ω –Fe supercell by introducing alloying elements in such a way that at a particular instant, either interstitial or substitutional C co-exist with any one of the elements Mn, Cr, Al, Si, and Co in substitutional position. From total energy calculations, we show that the cohesive energy of ω-Fe supercell increases in general, and the most stable combinations in the decreasing order of stability are C_sub-Cr > C_sub-Co > C_sub-Si. Even though the ferromagnetic state is more stable when compared to non-magnetic and antiferromagnetic configurations, the total magnetism of the supercell decreases as some of the atoms acquire negative magnetic moments. The density of states analysis shows that the d-band width of Fe decreases in presence of alloying elements, and this can lead to increased cohesive energy. Our results elucidate that the presence of minor alloying elements can be a factor in stabilizing the metastable ω-phase in steel.

Abstract: Effects of peening direction on the reverse transformation induced by the shot-peening for the Fe-33 mass%Ni alloy with large amount of martensite (α’) are investigated. When the angle between the peened surface and the peening direction (Hereafter, peening angle) is 90 ^o, the reverse transformation occurs and subsequently martensitic transformation is induced by the shot-peening. On the other hand, in case of the peening angle of 30 ^o, only reverse transformation occurs. Furthermore, the volume fraction of austenite (γ) in the specimen after the shot-peening increases as the peening angle decreases. This means that the reverse transformation induced by the shot-peening is enhanced by decreasing the peening angle. Moreover, residual compressive stress around the peened surface increases as the peening angle decreases. Since the hydrostatic compressive stress decreases phase transformation temperature, the phase transformation temperature around the peened surface would be decreased by the shot-peening. Therefore, the reverse transformation behavior depending on the peening angle can be explained by the residual compressive stress due to the shot-peening.