Materials Science Forum Vol. 941

Abstract: Overall kinetics of lamellar overaging reactions in U-5.5Nb and U-7.5Nb were analyzed by Avrami-Arrhenius analyses of volume fractions measured from an extensive temperature-time (T-t) matrix of specimens. The cellular initiation site (grain boundaries, inclusions) and regimes of lamellar divergency-cum-slowing growth rate were explicitly accounted for. Avrami exponents n from T-t regimes of constant-growth rate were consistent with theory (1<n<3); those from divergent T-t regimes were smaller, n~0.7, which is not surprising given their different growth rate behavior. The apparent activation energies Q were similar for grain-boundary and inclusion-nucleated discontinuous precipitation, indicating that their nucleation site does not alter their overall kinetics. Avrami Analysis of Isothermal Aging Kinetics

Abstract: Effects of Sr levels on microstructure of the LPSO structure-containing Mg_98.5-xNi_0.5Y_1.0Sr_x(x=0, 0.05, 0.10, 0.20 at.%) alloy were studied by SEM/EDS and XRD. Without Sr addition, the Mg_98.5Ni_0.5Y_1.0 alloy consists of α-Mg and LPSO structure and the block LPSO structure is distributed along the grain boundary. After adding 0.05 at.% Sr element into Mg_98.5Ni_0.5Y_1.0 alloy, the amount of dendrites decreased. With the increase of Sr content, the size of α-Mg grains decreases firstly and then increases. Meanwhile, the LPSO structure is refined. The addition of Sr element also results in the obvious increment of the amount of lamellar structure, which stretched from the grain boundary to the matrix. The excessive Sr in high Sr-content alloys participates in the form of Mg-Ni-Y-Sr compound, which is distributed in the vicinity of LPSO structure. In addition, the Sr can also promote the formation of Ni-rich and Y-rich phases.

Abstract: Fe-Ga alloy is expected as energy harvest material having superior magnetostrictive properties. In this study, we compared angle dependence of amount of magnetostriction with ideal magnetostrictive model using cube-oriented single crystal Fe-18mol%Ga alloy. Magnetostriction in [100] of specimen was measured with changing the direction of statical magnetic field H by strain gauge. As a result, the measured magnetostriction could not fully explaind by traditional energy-based model of magnetostriction. We compared measurement result using strain gauge with calculation result using energy-based model. As a result, there was a large difference in maximum compressive strain value. We established modified energy-based model considering the deviation of magnetic domains so as to match the actual magnetostriction behavior.

Abstract: Fe-Ga alloys are used for practical magnetostrictive materials. The understanding of mechanical properties under magnetic field is very important for their application to vibration power generation devices. Especially, Poisson’s ratio is one of key parameters because it is strongly related with the volume change of materials. In the study, we investigated actual behavior of Poisson’s ratio due to magnetostriction, instead of just the mechanical one. The sample was cube-oriented Fe-18mol%Ga single crystal disc. Static magnetic fields were applied in various directions parallel to (001). Strain values in various directions were measured by strain gauge and X-ray diffraction. And then, tri-axial strain analysis for single crystal was carried out. Theoretically, Poisson’s ratio due to magnetostriction is known to be 0.5, (volume conservative), value is close to the mechanical one, 0.45. On the other hand, we found that Poisson’s ratio exhibits anisotropic behavior despite the elastic constants are equivalent in [010] and [001]. This phenomenon is attributable to the magnetic domain structure under zero magnetic field. In this presentation, we will discuss this apparent Poisson’s ratio considering the volume strain and magnetostriction comprehensively.

Abstract: A joint research project was accomplished with the aim to determine the effect of homogenization conditions (temperature, time, cooling rate) on the microstructure and hardness of 6063 alloy billets. Homogenization is crucial for the ability to extrude the piece in low cost, but mainly without defects. Thus, it is of importance to determine the most suitable homogenization conditions (temperature, time, cooling rate) and its effect on both microstructure and hardness of 6063 alloy billets. Furthermore, the size and morphology of the AlFeMnSi intermetallic particles (mean diameter, aspect ratio) and the precipitation behavior of Mg₂Si constituents are examined in detail via optical (OM) and scanning electron microscopy (SEM). The resulting mean diameter and aspect ratio data generated by such measurements using OM and image analysis of the intermetallics, that are relevant to the extent of beta-to-alpha transformation, are statistically processed with “ANOVA”. Differential Scanning Calorimetry (DSC) tests are used to determine the coherency level of the particles that were precipitated during the different cooling rates and to reveal the potential for resolutioning the precipitates during billet preheating. Samples received from the plant are compared to laboratory samples in order to facilitate the process of optimization the thermal treatment and improve extrudability.

Abstract: This study investigated the effects of carbon addition on the development of deformation and recrystallization textures in 3 mass% Si steels cold-rolled by 95% reduction in thickness. A 3% Si steel with 0.016 mass% carbon developed RD // <110> (α-fiber) cold-rolling texture to a much greater extent than a 3% Si steel containing carbon less than 0.0005 mass%, whereas it is well known that the development of α-fiber rolling texture is suppressed by the addition of carbon in steels without silicon. This unique effect of carbon addition in the 3% Si steel appeared to be originated from a change in active slip systems. Straight slip bands were observed in the specimen without carbon, and the active slip plane was identified as {110}. On the other hand, wavy slip bands were observed in the steel containing carbon resulted from the activation of {112} and {123} slip planes in addition to {110}. It was also observed that {411}<148> recrystallization texture developed in the carbon-bearing 3% Si steel. Since the recrystallized grains with {411}<148> orientation preferentially nucleated in the vicinity of highly-strained deformation bands formed in deformed grains having α-fiber orientations, it is concluded that the development of {411}<148> recrystallization texture was resulted from the formation of the α-fiber texture after heavy cold-rolling in the carbon-bearing 3% Si steel.

Abstract: 7475 aluminium alloy is widely used in aerospace industry and military field. There are many dispersoid former elements in 7475 aluminium alloy, such as Mn, Cr, Ti, Zr and Sc. And the dispersoid configuration has important effect on the performance of alloy wrought. The characteristic of dispersoids in the semi-continuous casting ingot of 7475 aluminum alloy and the evolution of their distribution during rolling have been investigated. The size, shape and distribution of dispersoids are controlled by pre-treatment before homogenization. The results show that there are many coarse triangular, irregular block and spindly rod-like dispersoids which contain some of Cr and Mg and a little of Cu and Zn in the 7475 aluminum alloy which subject to conventional one-step homogenization treatment. The size and the distribution of the Cr-containing dispersoids is not uniform. The width of the coarse dispersoids is close to 1 micron and the length of the spindly rod-like dispersoids is more than 4 microns. There is no dispersoids in the center of the dendritic grains, but the nonuniform distribution of dispersoids is eliminated when the hot rolling reduction exceed 80%. The dispersoids can be significantly refined and homogenously distributed by pre-treated before the conventional one-step homogenized. The pre-treatment can stimulate the nucleation of Cr-containing dispersoids, narrow down the dispersoids free zone. The size of the dispersoids is dramatically reduced by pre-treated before the conventional homogenization, and the width of the coarse dispersoids and length of the spindly rod-like ones are less than 300 nm and 1 micron.

Abstract: Electron beam melting (EBM) has been among the most widely applied additive manufacturing techniques providing a high production rate, low residual stress and good mechanical properties of as-fabricated parts. In order for a wider industrial application of EBM, knowledge on alloy suitability for the process is important. In this work, EBM of Co-29Cr-10Ni-7W alloy is studied. During EBM, the alloy solidifies in a typical columnar-dendritic manner with multiple carbide phases formed in interdendritic regions and grain boundaries. Under the commonly used EBM conditions, cracks are readily observed in the EBM-fabricated state. In the present work, the forms of cracking are described and microstructure analysis has been conducted in order to suggest how cracks propagate during EBM. We will also discuss the possibility of controlling EBM operation to alter grain growth orientations and thus to reduce hot cracking. Keywords: melt pools; columnar dendritic growth; hot tearing; liquation cracking

Abstract: The effects of peening conditions on the surface characteristics and fatigue life of titanium alloy was investigated using microshot peening, ultrasonic shot peening, and multiple shot peening. The use of microshot peening technology with minute media has become more widespread in consideration of the reduction of the notch effect in the material surface. The ultrasonic shot peening that uses media of several millimeters in size with ultrasonic vibration has attracted attention as a means to reduce the surface roughness. In the present study, an air-type and an ultrasonic type machine were used. In the microshot peening process, the media used was high-carbon cast steel and the hard powder, with an average diameter of 0.1 mm. The workpiece was commercial titanium alloy Ti-6Al-4V. In the microshot peening (MSP), work hardening was evident to the depth of approximately 0.3 mm from the surface. This depth was approximately three times the diameter of the media. However, the influence of the peening time on the hardness distribution was not great. In the ultrasonic shot peening (USP), work hardening was deeper in the material. This is because the diameter of the media used for ultrasonic machining was large. On the other hand, in the combined shot peening (CSP), a degree of hardness was higher at the top surface. However, the hardness patterns and values were pretty much identical to ultrasonic shot peening. The fatigue limit was thought to be greater in the microshot peening experiment because the work-hardened layer was formed near the workpiece surface.

Abstract: The saturation value of the magnetostriction curve in the [100] direction of a Fe-Ga alloy single crystal was decreased from 226 to 55 ppm by applying the tensile strain of 533 ppm to the measured direction. By magnetic domain observation using a magneto-optic Kerr effect microscope, a complex structure composed of various magnetic domains was observed under zero applied strain. On the other hand, a stripe structure composed of magnetic domains with the magnetization direction in two kinds of <100> magnetic easy directions parallel to the tensile direction, which were separated by straight 180° domain walls, was observed under the tensile strain of 533 ppm. The characteristic magnetic domain structure due to the tensile strain was successfully observed as a cause of the significant decrease of the saturation value of the magnetostriction curve.