Uday Chakkingal

This paper describes the tensile properties, flow and work-hardening behavior of four metastable β-titanium alloys Ti-5Al-5Mo-5V-3Cr (A1), Ti-5Al-3.5Mo-7.2V-3Cr (A2), Ti-5Al-5Mo-8.6V-1.5Cr (A3), and Ti-5Al-3.5Mo-5V-3.94Cr (A4) in α+β hot-rolled condition. The decreasing order of average strength parameters (σYS and σUTS) is A4, A2, A1, and A3. The maximum strength observed in alloy A4 is due to the presence of highest wt. fraction of Cr. The elongation is the maximum and minimum in alloys A3 and A4, respectively. These alloys display moderate to high percent in-plane anisotropy (AIP) and reasonably low anisotropic index (δ) values. Both the AIP and δ values are maximum and minimum in alloys A1 and A3, respectively. The yield locus plots also exhibit the presence of anisotropy due to relatively large differences in yield strength values along tension and compression directions. The flow behavior of alloys A1, A2, and A4 follows Swift equation, while the alloy A3 displays best fit with Holloman equation. The presence of prestrain (ε0) in hot-rolled materials before tensile testing has an important bearing on the flow curves of A1, A2, and A4 alloys. The instantaneous work-hardening rate curves of the alloys A1, A2, and A3 exhibit all the three typical stages (stage I, stage II, and stage III) in RD samples, while the alloy A4 shows the presence of only stage I and stage III. The 45 deg to RD and TD samples of alloys A1, A2, and A4 display only stage I. The stages I and III as well as I and II are present in alloy A3 in 45 deg to RD and TD samples, respectively. Dislocation-controlled strain hardening occurs in all the three stages of these alloys in the absence of stress-induced martensitic transformation (α″) and twinning. Slip is the predominant deformation mechanism during tensile testing. Three types of slip lines, i.e., planar, wavy, and intersecting have been observed close to fracture surfaces of post tensile-tested specimens.

Nano-hydroxyapatite (nHA) reinforced magnesium composite (Mg-nHA) was fabricated by friction stir processing (FSP). The effect of smaller grain size and the presence of nHA particles on controlling the degradation of magnesium were investigated. Grain refinement from 1500 μm to 3.5 μm was observed after FSP. In vitro bioactivity studies by immersing the samples in supersaturated simulated body fluid (SBF 5 ×) indicate that the increased hydrophilicity and pronounced biomineralization are due to grain refinement and the presence of nHA in the composite respectively. Electrochemical test to assess the corrosion behavior also clearly showed the improved corrosion resistance due to grain refinement and enhanced biomineralization. Using MTT colorimetric assay, cytotoxicity study of the samples with rat skeletal muscle (L6) cells indicate marginal increase in cell viability of the FSP-Mg-nHA sample. The composite also showed good cell adhesion. © 2014 Elsevier B.V.

Titanium has been the material of choice for hard tissue replacements due to its excellent biocompatibility and high strength to weight ratio. Since, cells live in a nano-featured environment of extracellular matrix; there is great interest in the formation of submicron to nano size grain materials over conventional biomaterials. Equal channel angular pressing, groove pressing and mechanical milling of commercially pure titanium (cpTi) was carried out to obtain submicron/nano grain size materials. The processed samples were characterized using optical microscope, scanning electron microscope (SEM), X-ray diffraction (XRD), transmission electron microscope (TEM), hardness, tensile properties, atomic force microscope (AFM), and contact angle measurements. Microstructural and mechanical characterization of the processed samples exhibited grain refinement and improved mechanical properties when compared to as received condition. The bioactivity study of the fine grained samples in SBF exhibited dense and homogenous apatite layer on the surface. All samples were found to be non-toxic by MTT [3-(4, 5-Dimethylthiazole-2-yl)-2, 5-diphenyl tetrazolium bromide] assay to fibroblast cells and culture study using osteoblast cells show more cell adhesion and spreading on ultra fine grained samples compared to as received cpTi. The enhanced bioactivity in the fine grained samples is due to submicron/nano surface features with high wettability and surface energy.

Equal Channel Angular Pressing (ECAP) is currently being widely investigated because of its potential to produce ultrafine grained microstructures in metals and alloys. A sound knowledge of the plastic deformation and strain distribution is necessary for understanding the relationships between strain inhomogeneity and die geometry. Considerable research has been reported on finite element analysis of this process, assuming threedimensional plane strain condition. However, the two-dimensional models are not suitable due to the geometry of the dies, especially in cylindrical ones. In the present work, three-dimensional simulation of ECAP process was carried out for six outer corner radii (sharp to 10 mm in steps of 2 mm), with channel angle 105°, for strain hardening aluminium alloy (AA 6101) using ABAQUS/Standard software. Strain inhomogeneity is presented and discussed for all cases. Pattern of strain variation along selected radial lines in the body of the workpiece is presented. It is found from the results that the outer corner has a significant influence on the strain distribution in the body of work-piece. Based on inhomogeneity and average strain criteria, there is an optimum outer corner radius.

Equal channel angular pressing (ECAP) is an important severe plastic deformation process to produce ultrafine grained microstructures in metals and alloys. Magnesium and its alloys generally possess poor workability at temperatures below 250 °C. This investigation examines the influence of different passes and processing routes of ECAP on improving the workability of Mg alloy AZ31B. ECAP was carried out for three passes using a die of angle 120° using processing routes Bc and C. The operating temperature was 523 K for the first pass and 423 K for the subsequent two passes. The resultant microstructure and mechanical properties were determined. Workability of the alloy at 423 K (150 °C) was determined using upsetting experiments on cylindrical specimens machined from the annealed and ECAPed samples. Workability limit diagrams have been constructed for the various processed conditions. The workability data generated were also analyzed using five different workability criteria (also referred to as ductile fracture models) and the material constants for these five models were evaluated. Specimens processed by two passes through route C (pass 2C) exhibits better workability compared to other passes since the workability limit line after this pass shows maximum safe working area and lies above the other workability lines. Among the five different workability criteria investigated, the Freudenthal workability criterion is more suitable for prediction of failure in this alloy.

Formability of aluminum alloys poses a major challenge for their wider application in automotive sheet metal components as the deep drawability of aluminum is low when compared to steel. This is indicated by the low limiting drawability ratio (LDR) of aluminum sheet blanks which is characterized by the poor r value or the plastic strain ratio. Recently, a number of techniques have been attempted to improve the r value of an FCC metal like aluminum by altering the texture. In the present study, a groove pressing process was carried out on commercial purity aluminum sheets under three different orientations to its rolling direction. The r, rm and Δr values of the groove pressed specimens were experimentally determined. Improvements in these values were obtained. X-ray diffraction scans were carried out on the specimens to measure the relative intensities of the (1 1 1) and (2 0 0) peaks in the pattern. The LDR, determined by the Swift cup forming test shows an improvement for the aluminum sheet specimen groove pressed at 0° and 45° to the RD. This can be attributed to the improved r value due to the development of (1 1 1)//ND shear texture imparted to the specimen by groove pressing. © 2010 Elsevier B.V.

Present work describes the development of the α phase in β WQ specimens of Ti-5Al-5Mo-5V-3Cr (A1), Ti-5Al-3.5Mo-7.2V-3Cr (A2), Ti-5Al-5Mo-8.6V-1.5Cr (A3) and Ti-5Al-3.5Mo-5V-3.94Cr (A4) alloys as function of different ageing temperatures and time intervals. The α phase formed during ageing display different morphologies namely, thin film on prior β grain boundaries, sub boundary and islands of the α phase within the β matrix. Mechanism of the formation of these morphologies has been explained. The fine and even distribution of the α phase in β matrix exhibit two distinct features i.e. uniform distribution of α laths with two variants and clusters of parallel α laths. The α phase presents in all morphologies is transformed to Widmanstatten type morphology on further increase in ageing temperature and time. The development of the α phase in these alloys at 300 and 400 °C results in significant increase in hardness which is attributed to typical precipitation hardening.

In this study, the Influence of various passes and processing routes of equal channel angular pressing (ECAP) on the workability of aluminium alloy AA 6063 has been Investigated. Aluminium alloy specimens were subjected to ECAP using a 105° angle ECAP die using three processing routes for up to three passes at room temperature. Microstructure characterization and mechanical property measurements were carried out. Workability was determined by means of upsetting tests on cylindrical collar specimens machined from specimens processed by ECAP and workability limit diagrams for various processing conditions have been generated. A Cockcroft fracture criterion was used to analyze the workability data. It is observed that processing to first pass results in enhanced mechanical properties with only a slight decrease In workability. Taking Into account both mechanical properties and workability, ECAP to pass one can be considered as an optimum processing route for aluminium alloy AA 6063. © 2011 Wiley-VCH Verlag GmbH & Co. KGaA. Weinheim.

Commercial purity Ti was subjected to equal channel angular pressing (ECAP) for up to three passes at 400°C using a die with die angle of 120°. Compression testing of the ECAP specimens was carried out to determine the subsequent flow behavior. Two types of compression test specimen orientations, one parallel to the axis of ECAP and the other at 45° to the axis of the ECAP, were prepared from the specimens subjected to ECAP. Anisotropy in flow behaviour (as indicated by values of strength co-efficient, K and strain hardening exponent, n) was observed. The strain hardening rates were also calculated from the experimentally determined flow curves for the specimens tested in compression in the two orientations. The results have been interpreted in terms of the strain path change parameter between the two deformation steps (ECAP and compression). Strain hardening behaviour and microstructure evolution is discussed in terms of strain path change parameter. Specimens compressed in the direction parallel to the ECAP direction had lower strain hardening exponents while exhibiting higher initial flow stresses. The strain hardening rates were lower for specimens compressed at 45° to the ECAP direction compared to specimens compressed parallel to the ECAP direction.

Metallic materials are widely studied for load-bearing applications such as orthopaedic implants. Titanium and its alloys find applications for load-bearing medical implants due to their biocompatibility, good corrosion resistance, high specific strength and good bioadhesion. However, the bioactivity of titanium which can be defined as the ability to form a hydroxyapatite (HA) layer, which is similar to the mineral phase of the bone, on its surface when in contact with the biological environment is poor. On the other hand, magnesium and its alloys are becoming the prime choice for degradable biomaterials targeted for temporary applications in cardiac and orthopaedic fields. However, controlling the degradation rate is the essential issue in developing magnesium-based biomaterials. Synthesis of nano/ultra fine grain materials to enhance the biofunctionalisation of orthopaedic implants is of considerable interest as cells live in a nano-featured environment consisting of a complex mixture of pores and fibres of the extracellular matrix. Recently severe plastic deformation (SPD) processes which can achieve considerable grain refinement, typically to the submicrometre or nanometre level, have gained significant attention in materials research. Therefore, using SPD processes to develop grain-refined titanium and magnesium-based materials for implant applications has become a promising strategy in developing new-generation medical materials. Particularly for titanium, nanostructuring results in improved mechanical properties and increased bioactivity. Whereas for magnesium, grain refinement results in controlled degradation due to higher biomineralisation with enhanced tissue response. The present review aims to provide a comprehensive summary of the progress achieved using SPD processes in developing nano/ultra fine grain structured titanium and magnesium for implant applications. Role of smaller grain size on enhancing bioproperties is also discussed including the challenges involved in processing to achieve the grain refinement up to nano/ultra fine grain level.