Papers by Keyword: Grain Refinement

Abstract: Semi-solid processing over a cooling slope has emerged as an efficient technique for producing near-spherical grain structures in metallic composites. In this study, an Al-15Mg₂Si-4.5Si composite processed using this method exhibited globular Mg₂Si particles embedded within an α-Al matrix. The addition of 0.01% Sr further refined the microstructure by reducing the size of the Mg₂Si particles, acting as a grain-refining agent. To gain deeper insight into the effect of Sr addition on microstructure evolution during semi-solid processing, a two-dimensional phase field model is developed. Wherein, rather than modelling the nucleation of Sr-containing phases explicitly, the influence of Sr is realised by modifying the relevant phase field parameters. The simulations predicted key microstructural characteristics—including grain size, sphericity, area fraction and grain density of P-Mg₂Si —along with interfacial energy and mobility coefficients for both the base and Sr-modified composites. The phase field results show good agreement with experimental observations, validating the modelling approach. Keywords: Semi-solid, Magnesium silicide, Phase field, grain refinement

Abstract: This study investigates the mechanical properties and microstructure of sintered tungsten under varying sintering conditions. Bending strength tests revealed that sintering at 1400 °C resulted in low flexural strength due to inadequate temperature, whereas sintered tungsten at 1500 °C exhibited improved strength attributed to grain growth. However, temperatures exceeding 1600 °C led to excessive grain growth and a subsequent decline in strength, indicative of grain coarsening and potential localized bonding. Additionally, analysis of holding times at 1500 °C demonstrated that extended durations promoted neck bonding between grains, contributing to the formation of interconnected grains and enhanced mechanical properties. This study underscores the importance of optimizing sintering parameters to control grain growth and achieve desired mechanical properties in sintered tungsten materials.

Abstract: The Ti-13Nb-1.5Mo-3Ta alloy is a recently developed biocompatible metastable β-Ti alloy designed for biomedical application. In this present work, the influence of cold rolling and subsequent annealing heat treatment on grain refinement of Ti-13Nb-1.5Mo-3Ta alloy was investigated. The alloy was cold rolled (CR) to 60% and 90% thickness reductions at room temperature followed by recrystallization annealing at different temperature (800°C-900°C) and time (1.5mins-10mins) before ice-water quenching. X-ray diffraction (XRD) and optical microscopy (OM) were used to characterize the alloy, and microhardness tests were carried out using the Vickers microhardness tester. The results revealed that the annealed alloys exhibited a fully β-phase, while those subjected to cold rolling displayed introduction of stress induced martensite (SIM) α′′-phase along with β-phase. The microhardness of the 60% and 90%CR samples increased significantly to 253 and 283 Vickers hardness (HV), respectively, from an initial value of 198 HV. Annealed samples exhibited a recrystallized microstructure containing fine equiaxed grains with average size of 10-50μm for 60%CR and 8-34μm for 90%CR. The grain refinement mechanisms are probably attributed to the reversal of the SIM α′′-phase back to the more stable β-phase and the recrystallization of the deformed β-phase.

Abstract: Biodegradable magnesium alloy WE43 (Mg-4Y-3RE) has received great attention in orthopaedic applications as it can dissolve completely after bone tissue repair, eliminating the need for a second surgery to remove the WE43 implant. However, the rapid degradation of WE43 implants during bone healing remains a concern. Rapid degradation can deteriorate the mechanical strength and generate a significant amount of hydrogen gas via corrosion in physiological environments, negatively affecting bone healing and the surrounding tissues. To overcome the rapid degradation of medical implants, one commonly used method is surface modification via laser surface melting (LSM) to alter the surface microstructures and improve the corrosion resistance. This paper investigates the possibility of applying LSM technique to refine the surface microstructures of WE43 alloy and compares the microstructures induced by LSM with the extruded alloy without laser treatment. Results show significant grain refinement after LSM with average grain size decreased to 3μm as compared to 5μm before LSM, approximately 40% reduction in grain size. Different types of grain morphology are also identified at different locations in the melt pool due to different temperature gradients and cooling rates. It is observed that the depth of the melt pool increases with increasing laser power and decreasing laser scanning speed due to the higher heat input. It is also observed that grain size decreases with decreasing laser power and increasing laser scanning speed due to increased cooling rate. Results from this study show that LSM, a form of rapid solidification processing, can form a predominantly basal crystallographic texture, homogenise and refine the surface microstructures of WE43, which are beneficial for corrosion resistance.

Abstract: In this study friction stir processing (FSP) was utilized to refine the microstructure of thick AZ31D magnesium (Mg) alloy, followed by evaluation of its microstructure and corrosion behavior in a NaCl environment. The application of a tapered threaded pin profile resulted in enhanced material mixing, significant grain refinement and reduced defect formation due to minimal heat input and minimized thermal gradients across the stir zone (SZ). The SZ, dominated by the pin profile, exhibited a fine, equiaxed, and uniform grain structure throughout its thickness the average grain size reduced from 13.8 µm to 5.19 µm after second pass of FSP confirmed through field-emission scanning election microscopy (FE-SEM) analysis. This structural refinement significantly enhanced the corrosion performance of the FSPed alloys, as compared to the base material (BM), demonstrated by electrochemical testing in 3.5% NaCl solution. The FSPed alloy surface showed uniform corrosion behavior, instead of intergranular corrosion with deep mud cracking patterns observed in the BM. This improved corrosion resistance was due to the uniformity of the produced microstructure via FSP, which reduced localised corrosion sites. These findings suggest that FSP is a promising technique for improving the durability of Mg alloys in corrosive environments, potentially benefiting applications in the automotive and aerospace industries.

Abstract: Doping high-zinc aluminium alloys with Ti builds in-situ composites reinforced with ternary aluminides Ti (Al,Zn)₃ with a significantly grain-refined matrix. In a series of studies, Ti was introduced with Al-6 wt% Ti (AlTi6) and Zn-4 wt% Ti (ZnTi4) master alloys in amount to contribute about 3 wt% Ti in the examined alloys. The alloy microstructure has been studied using light microscopy, SEM / EDS and XRD measurements. The observed significant refinement of the alloys matrix should lead to improvement in ductility, while the in-situ reinforcement should improve tensile strength and wear properties.

Abstract: Al-Mg alloys processed by Severe Plastic Deformation (SPD) have higher strength due to grain refinement by dynamic recrystallization. Generally, the relationship between grain size and strength is expressed by the Hall-Petch relationship, but the strength of Al-Mg alloys processed by SPD is higher than expected from the Hall-Petch extrapolation. This phenomenon is called ``Extra-Hardening. In this study, Al-3mass%Mg alloys processed by SPD were annealed, and the fraction of grain boundaries was measured by EBSD to determine the effect of grain boundaries on the strength. The results suggest that Extra-Hardening may be an effect of strengthening by SGB, which changes significantly with the number of passes and annealing, and constant LAGB strengthening. Strength predictions using the fraction of grain boundaries were in good agreement.

Abstract: Austenite restoration during thermomechanical (TM) rolling of typical vanadium-microalloyed structural steels was studied to optimize strength in the as-rolled and air-cooled condition. Multi-pass plate rolling simulations were performed on V-N microalloyed and CMn steels to compare recrystallisation behaviour in various temperature regions. Included were a conventional schedule ending at high temperature and two TM schedules with mill exit temperatures in the intermediate and low austenite regions. Increasing delay periods after roughing enhance the suppression of recrystallisation after the start of finishing thereby increasing both nucleation site density and nucleation rate for ferrite formation and refinement in grain size. Good agreement was found between microstructures after industrial TM rolling and those obtained from laboratory simulations. Although precipitation of vanadium carbonitrides is an effective strengthening mechanism, appreciable gains in yield strength due to grain refinement can be achieved by rolling in the lower austenite region. Low nitrogen contents in V steels produce coarser final ferrite grain sizes and lower strengths probably due to a larger precipitate size. V-N steels display similar flow behaviour to CMn grades down to approximately 825°C at low to intermediate strain rates but may experience alternate regions of work hardening and dynamic softening at lower temperatures in austenite.

Abstract: The effect of Ti/Al ratio (mass%) on the evolution of the microstructures after casting and hot rolling of Ca treated 441 dual stabilized ferritic stainless steel (FSS) was investigated in order to understand its effects on grain refinement mechanism. Industrially cast and lab simulated hot rolled samples, were subjected to similar processing conditions but with different Ti/Al ratios of 2.4 and 7.8. The microstructures and inclusions were analysed by the OM, SEM-EDS, SEM-EBSD and AzTecFeature software. The results showed that the steel with higher Ti/Al ratio exhibited finer grains after continuous casting and hot rolling, i.e., the initial finer as-cast structure resulted in finer grains and less substructure after hot rolling. The steel with higher Ti/Al ratio contained more Ti-rich complex inclusions and precipitates (especially TiN), which led to more heterogeneous nucleation of the 𝜹-ferrite and grain refinement during solidification. On the contrary, the steel with low Ti/Al ratio exhibited coarser as-cast grain structure, less recrystallization and higher volume fraction of substructure after hot rolling. Therefore, it was deduced that the Ti/Al ratio is one of the essential parameters to achieving grain refinement in Ca treated 441 FSS during continuous casting.

Abstract: In this study, AZ91 alloy was used as the base material and calcium and cerium were added as alloying elements. Microstructural analysis through optical microscope (OM) and field emission scanning electron microscope (FESEM) revealed that AZ91 base alloy contains α-Mg matrix and β-Mg₁₇Al₁₂ interdendritic network. The inclusion of individual calcium and cerium resulted in a more homogeneous distribution of the interdendritic network in the AZ91-1wt.% Ca and AZ91-1wt.% Ce alloy. The secondary phase (Mg₁₇Al₁₂) was refined in the microstructure as a result of Ca and Ce addition where Ce addition forms a new rod-like phase that is recognized as Al₁₁Ce₃ and Ca addition forms a skeleton like structure of Mg₁₇Al₁₂ and Al₂Ca. Due to the formation of new Al₂Ca and Al₁₁Ce₃ intermetallics, the volume fraction of β-Mg₁₇Al₁₂ was more suppressed with Ca and Ce alloy additions. The grain size determined from Electron Backscatter Diffraction (EBSD) maps indicate the reduction in average grain size with individual Ca and Ce additions. The addition of these elements was found to improve the hardness of AZ91 alloy. Overall, the results of this study demonstrate the potential for using Calcium and Cerium as alloying elements in AZ91 alloy to improve its mechanical properties by modifying its microstructure.