Search

Research was conducted on the microstructural evolution and ensuing mechanical response from high-pressure torsion (HPT) processing of Ti-6Al-4V alloy in the as-cast and β-forged conditions with and without 0.1 wt.% boron addition. The boron addition produces refinement of the prior β grains and the (α+β) colonies and introduces an additional TiB phase but this affects the deformation response and the microstructural evolution only at low strains of 0.5 to 5 rotations. In the initial condition the orientation of the (α+β) colonies significantly affects the deformation response and leads to differences in substructure formation in both the as-cast and β-forged conditions. This orientation dependence counts on the initial microstructural differences between the unmodified and the boron modified alloys. At higher strains, there is a similar deformation response and microstructure evolution all the alloys. The hardness variation with equivalent strain is similar for the unmodified and boron modified alloys in as-cast and β-forged conditions and represents various deformation regimes in HPT-processing. Strength modelling confirms a simultaneous contribution from microstructural refinement and increased dislocation density towards the hardness increment during HPT processing. Overall, the as-cast and β-forged Ti-6Al-4V-0.1B alloys possess identical deformation response to the β-forged unmodified Ti-6Al-4V alloy in the initial and intermediate stages. At high levels of straining, all alloys respond in an equivalent manner, thus ruling out any possible effects from additional TiB phase or microstructural refinement for the boron-modified alloys.

Surface severe plastic deformation (S2PD) of additively manufactured/three-dimensional (3D) printed metallic parts is gaining increased attention as a post-manufacturing operation to enhance the material performance in a wide variety of applications. Surface mechanical attrition treatment (SMAT) is an S2PD technique that can yield a nanostructured surface layer induced by compressive stresses and work hardening. In the present study, SMAT was performed on 316L (austenitic) stainless steel (SS) processed by selective laser melting (SLM), and the consequent effects on mechanical response were investigated. The underlying mechanisms of microstructural evolution leading to the formation of nanocrystalline grains resulting from SMAT in SLM 316L SS are elucidated. The interactions between twins and deformation bands act as potential sites for impeding the movement of dislocations, which in turn leads to the formation of stacking faults, twinning, and occasionally transform to a different crystal structure. Twin-twin and/or twin-deformation band intersections sub-divide the matrix grains into smaller cells or low-angle disoriented blocks, which result in the formation of low-angle grain boundaries and finally in nanocrystallization at the surface. The size of nanocrystalline grains increases progressively with depth from the surface to micrometer size grains in bulk. The gradient nanostructure in the additively manufactured alloy after SMAT imparts an unusual combination of strength and ductility that markedly exceeds that of conventional, bulk nanostructured, or even high-performance 316L SS (containing nanoscale deformation twins embedded in submicron-sized austenitic grains obtained by dynamic plastic deformation processes). Analytical models revealed that strengthening results from a combination of grain boundaries and dislocations. The results of the present investigation pave the way for engineering high-performance SS for a variety of engineering applications.

In this study, high energy synchrotron radiation was used to perform an in-situ diffraction experiment in medium Mn Fe-6Mn-0.5C-1Al alloy to study the elemental partitioning and consequent austenite phase evolution at different stages, namely, during heating, holding and cooling. It has been observed that the austenite phase fraction significantly increases on annealing at the inter-critical annealing temperature and remains stable at room temperature. The austenite phase stability at room temperature is due to the rapid partitioning of manganese (Mn) and carbon (C) during annealing. The relative change in d-spacing (Δd/d) during heating-cooling confirms the partitioning of alloying elements during inter-critical annealing.

High strain rate deformation and shock-wave-assisted processes find applicability in a variety of applications including material processing. Design of a processing unit utilizing shock phenomenon and design of a material for protection from explosion shock mandate thorough understanding of material behavior under shock and high strain rate conditions. The material response to high strain rate and shock is very different from conventional deformation. It is marked by generation of abundant defects in material exposed to shock for a comparatively low amount of strain generated in it. The material response can vary widely and depends on various material parameters along with the parameters of shock wave. It is well known that the materials with different crystal systems deform by means of different active slip and twin systems. Moreover, for a class of crystal systems the response can further depend on a system’s specific material parameter such as stacking-fault energy in a face centered cubic (FCC) material. With an extensive amount of research dedicated to understanding of the phenomenon of defect evolution in a material under shock, the phenomenon is nonetheless not completely comprehended. The present chapter aims to give the readers a brief overview of the response of materials to shock. Various experimental methods used for research are discussed briefly. The material behavior under shock with a focus on materials with FCC, hexagonal close packed, and body-centered cubic crystal systems is reviewed. The effect of peak pressure and pulse width is explored thereafter. Recent studies have shown that residual strain in the material can have a strong effect on its microstructure under shock. The role of residual strain, which has largely evaded the discussion till now, on defects in shocked microstructure is discussed. Theories of defect generation mechanism under shock compression are reviewed with a focus on recent developments suggesting an atomic shuffling–based defect formation mechanism.

Toward engineering a new generation of low modulus titanium alloys for orthopedics, we present new insight into the control of nanoscale precipitation in a metastable β Ti-32Nb-2Sn alloy. Nanoscale α precipitates from β phase were obtained by one-step heat treatment at 500°C. The nanoscale precipitates markedly improve the tensile strength (≈1070MPa) while affording lower modulus (≈82GPa) than conventional metallic biomaterials. Besides age-hardening at 500°C, an unexpected phenomenon of age-softening is observed even in the presence of nanoscale α precipitates when aged at 600°C. This effect is attributed to significant softening of the β phase due to compositional changes, as revealed by the elemental mapping in transmission electron microscopy (TEM). TEM elemental mapping reveals that Sn partitions preferentially in the β phase on aging at 500°C and does not show any preferential partition on aging at 600°C. The passive layer at the surface enriches in Sn content after aging at 500°C and consequently affects the electrochemical behavior of the alloy. The alloy supports the proliferation, and osteogenesis of human mesenchymal stem cells. This study provides new understanding for processing Ti-Nb-Sn alloys in biomedical applications. Keywords: Titanium alloys, Ti-Nb-Sn, Aging, Corrosion, Biocompatibility, Biomedical alloys

A low modulus beta Ti-Nb-Ta-O alloy was subjected to heat treatment to investigate its phase stability upon aging. The resultant effect on the mechanical and functional properties was systematically evaluated. The aging of the beta-only microstructure, obtained by solutionizing and quenching, resulted in the formation of ultrafine alpha-precipitates with increasing order of size as the aging temperature increased from 400 degrees C to 600 degrees C. The variation in the size of alpha-precipitates effected the mechanical properties at the three different aging temperature. The highest hardening observed at 400 degrees C was associated with macroscopic embrittlement, whereas age softening was observed in samples aged at 600 degrees C due to coarsening of precipitates and softening of the beta-matrix. In contrast, aging at 500 degrees C resulted in about 32% increase in tensile strength from the beta-solutionized condition. As the samples aged at 500 degrees C showed optimum combination of mechanical properties among the aged samples, these were further characterized for their electrochemical, tribological and biological responses. The fretting wear studies showed that the wear rate of the solution-treated samples increased after aging due to the higher corrosion rate leading to a higher rate of tribocorrosive dissolution and formation of a transfer layer harder than that of solution treated sample. The Ti-Nb-Ta-O alloy supported the attachment and proliferation of osteoblasts similar to that on commercially pure Ti. Taken together, this work provides new insights into the preparation of next-generation Ti alloys for biomedical applications with high strength and low modulus through microstructural control induced by heat treatment.

We report a rare experimental evidence of an exceptional increase in grain-boundary diffusion-controlled growth kinetics of product phase by grain-refinement due to minor alloying. Ti-addition during growth of V3Ga superconductor by coupling V and Cu(Ga)-solid solution by bronze process increases the critical current density (Jc) significantly. In this manuscript, the exact role of Ti-addition is studied based on systematic and quantitative diffusion-couple experiments that mimic the bronze-method. Metallurgical aspects like microstructural features and growth mechanism are assessed critically. Results are compared when Ti is added either to V or Cu(Ga) and to both for a fixed Ga content. Ti-addition to Cu(Ga) compared to V increases growth kinetics of V3Ga more significantly. Quantitative EPMA-analysis indicates that a mixture of Ti-free and Ti-containing V3Ga phase grows when Ti is added to V. Small precipitates grow along with Ti-containing V3Ga when Ti is added only to Cu(Ga). TEM-study indicates that these precipitates are rich in V and Ti. Much finer grains found in the precipitate-containing region lead to higher growth kinetics due to grain-boundary diffusion. EBSD-analysis shows equiaxed grains and relatively random in orientation because of alloying. This study indicates that Ti-addition to Cu(Ga) is most effective considering metallurgical aspects that influence Jc. Keywords: Superconductor material, Microstructure, Grain size distribution, Grain boundary diffusion, Microtexture

The hot deformation behavior of Nb–1 wt.%Zr alloy was studied using uniaxial compression tests carried out in vacuum to a true strain of 0.6 in the temperature range of 900 to 1700 °C and the strain rate range of 3 × 10−3 to 10 s−1. The optimum regime of hot workability of Nb–1Zr alloy was determined from the strain rate sensitivity (m) contour plots. A high m of about 0.2 was obtained in the temperature and strain rate range of 1200–1500 °C and 10−3 to 10−1 s−1 and 1600–1700 °C and 10−1 to 1 s−1. Microstructure of the deformed samples showed features of dynamic recrystallization within the high strain rate sensitivity domain. Compared to the study on Nb–1Zr–0.1C alloy, Nb–1Zr showed a lower flow stress and an optimum hot working domain at lower temperatures. In the 1500 to 1700 °C range the apparent activation energy of deformation for Nb–1Zr was 259 kJ mol−1, the stress exponent 5, and the activation volume about 200 to 700 b3. Keywords: Nb–1Zr alloy, Hot deformation, Strain-rate sensitivity, Dynamic recrystallization