Materials Science Forum Vols. 706-709

Abstract: Influences of cracking of the coated DyBa₂Cu₃O_̇˽δ˰superconducting layer in composite superconductor under applied tensile strain on V(voltage)-I(current) curve, critical current and n-value were studied experimentally and analytically. The measured variations of V-I curve and critical current with increasing applied strain were described well by the modeling analysis. Also, the variations of shunting current with increasing imposed current and also with extension of cracks were revealed, from which the influences of shunting current on the variation of n-value with current were elucidated.

Abstract: In a recent work, the present authors developed a new formal kinetics methodology suitablefor the situation in which transformations take place simultaneously or sequentially. In this work themain results of such a methodology are briefly presented in a simplified form. It is shown here howthis new methodology may be used for obtaining expressions for simultaneous or sequential reactionsfrom models of the kinetics of each reaction in isolation. It is also shown how one may extract theo-retical information from the experimentally measured quantities. Moreover, the methodology is usedto analyse published data on the recrystallization kinetics of IF steels.

Abstract: The microstructural heterogeneities in steels may have different origins. The tendency in industry to decrease the total amount of reduction during hot working decreases the possibilities of applying approaches based on “thermo-mechanical homogenization”. In this situation, alternative approaches need to be evaluated to reduce the heterogeneities present in the initial microstructure and, simultaneously, to avoid the possibility of inducing additional heterogeneities during processing. This paper analyzes the relevance of several processing parameters in different cases.

Abstract: In this paper, shape memory epoxy foams, obtained by the new solid-state foaming process for thermosetting resin powder, are investigated. Foaming experiments in different configurations, compression tests, constrained stress recovery tests, and density measurements are discussed. The interesting results seem to be very promising for the aerospace application of shape memory epoxy foams as light actuators, structural parts with reduced size during shipping, and expandable/deployable structures. Finally, an attractive experiment is introduced. It is designed for the next Space Shuttle STS-134/ULF-6 in I-25/26, on April 2011, with the aim to study the behavior of this new class of materials in microgravity.

Abstract: A significant amout of stabilized austenite can be obtained in high carbon steel containing high amounts of manganese and silicon (1.5-2 %). At relatively low temperatures the bainite plates formed are extremely thin, making the material very strong. In this study, the influence of the thermal cycle of austempering on the mechanical behavior of a spring steel 0.56C-1.43Si-0.58Mn-0.47Cr (wt. %), with TRIP effect was investigated. The thermal cycle consisted of heating three groups of hot-rolled wire steel at austenite field of 900°C for 300 s, and quickly transferring those to a metallic bath maintained at 200, 220 or 270°C, respectively, for different heat treatment times. The samples were then tested in tension and their microstructures were examined by scanning and transmission electron microscopy. The samples treated at 220°C showed higher elongation, yield strength and tensile strength than those maintained at 200 or 270°C. The high level of strength and ductility is due to a mixture of martensite and very fine bainitic ferrite with interlath film of retained austenite. The temperature has shown a strong influence on bainite formation kinetics. The fracture behavior of the steel was also evaluated using SEM fractography.

Abstract: Yield strength of ferritic steel increases with grain refinement standing on the Hall-Petch relation. In low carbon ferritic steels, the following relation is established between yield strength σ_y and grain size d: σ_y [MPa]= 100+600/√d [μm]. The Hall-Petch coefficient of interstitial free steels is substantially small as 0.15MPa·√m but it can be greatly increased by the existence of small amount of solute carbon less than 60ppm. As for the effect of substitutional elements such as Cr and P, some papers reports fairly large influence to the Hall-Petch coefficient of ferritic iron. However, the effect of small amount of carbon is sometime neglected or not cleared on the evaluation of Hall-Petch coefficient in ferritic steels. In order to evaluate the effect of substitutional elements, the research should be performed using interstitial free steels to eliminate the influence of solute carbon and nitrogen. In this paper, Hall-Petch relation was examined in iron, Fe-Cr alloys and Fe-P alloys with 0.02-0.05mass% Ti and the following results were obtained: 1) The Hall-Petch coefficient of interstitial free iron is about 0.15MPa·√m. 2) Chromium does not give any influence to the Hall-Petch coefficient of ferritic iron, although the friction stress σ₀ is enhanced in proportional to chromium content (Δσ₀ [MPa]=7×(mass%Cr)). 3) Phosphorus does not affect the Hall-Petch coefficient of ferritic iron or reduce it somewhat but increases markedly the friction stress σ₀ (Δσ₀ [MPa]=250×(mass%P)^1/2). 4) Even under the co-existence of carbon with chromium and phosphorus, carbon dominantly works to increase the Hall-Petch coefficient of ferritic steels, but it is changeable due to the interaction between carbon and the other substitutional elements.

Abstract: Al-Si alloys are widely used as cast alloys for their excellent castability, low thermal expansion coefficient, good wear resistance and corrosion resistance properties. However, the poor ductility of these alloys, caused by the presence of coarse and non-deformable Si phase in the as-cast state, has inhibited their applications as wrought materials. Recently, a process based on traditional technologies, i.e., direct chill (DC) casting followed by hot deformation and heat treatment, has been developed for potential mass production of wrought Al-Si alloys with superior mechanical properties in view of their strength and ductility. In this work, the microstructural evolutions of DC cast Al-Si alloys involved in solidification, recrystallization and precipitation during the processing are highlighted, aiming at understanding the correlations between the microstructures and the mechanical properties.

Abstract: FSP is an allied technology of friction stir welding (FSW). These processes are reviewed and the additional considerations such as processing pattern and step over distance in FSP are introduced. The application of FSP to cast metals including AA5083, Al-7Si and NiAl bronze is summarized. As-cast microstructures may be converted to a wrought condition in the absence of external shape change and the extent of grain refinement and homogenization of microstructure is documented. The FSP-induced superplastic response of AA5083, enhanced ductility of Al-7Si and surface hardening of NiAl bronze are summarized as examples of properties developed by FSP.

Abstract: In titanium alloys it is known that in bulk sections the solidification conditions in ALM commonly lead to undesirable, coarse, columnar β grain structures. Here, we have investigated the effect of build geometry on the grain structure and associated texture in Ti-6Al-4V ALM components produced by Selective Electron Beam Melting (SEBM). Through reconstruction of the primary β-phase, it has been confirmed that in thick sections large columnar β grains grow with a strong <001>_β fibre texture, although there is a significant skin effect. In contrast, in thin walls nucleation off the surrounding powder and growth inwards dominates. Local heterogeneities are also observed within section transitions. It is shown that the weaker α transformation texture arises from a random distribution across the possible habit variants.

Abstract: IMI834 (Ti-5.8Al-4Sn-3.5Zr-0.7Nb-0.5Mo-0.35Si) is a high-tech near-α titanium alloy with improved creep resistance and mechanical property retention at temperatures up to 600°C [1]. It is used in the aerospace industry for compressor disks and blades due to its excellent balance between creep resistance and fatigue strength [2]. The linear friction welding (LFW) behaviour of IMI834 displaying an initial bimodal α+β microstructure was investigated using varying axial pressures during welding. Electron backscatter diffraction (EBSD) was used to characterize the texture and phase fraction of the welded IMI834 samples in the weld zone (WZ) and thermomechanically affected zones (TMAZ) in relation to the base material. Based on microhardness evaluation of the weldments, the WZ was determined to be slightly harder than the base material.