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 an important process for producing ultra fine grains in bulk metallic materials by means of severe plastic deformation. Workability of metals and alloys is an important parameter as it influences the fracture resistance of the material and the ease of subsequent forming by conventional techniques. In this study, the effect of various passes and processing routes of ECAP on the workability of commercially pure aluminum has been investigated. Aluminum specimens were subjected to ECAP using 90° angle ECAP die. ECAP was carried out using two processing routes for up to three passes. Microstructure characterization and mechanical property measurements were carried out. Workability was determined by means of upsetting tests on hexagonal collar specimens machined from specimens processed by ECAP. A Cockcroft fracture criterion was used to evaluate experimental results. It is observed that processing to two passes through Route C results in enhanced mechanical properties with only a slight decrease in workability. © 2007 Elsevier B.V. All rights reserved.

Commercial purity copper sheets were subjected to a severe plastic deformation technique called groove pressing (GP) to a strain of 3.48 at both room and cryogenic temperatures. The mechanical properties and microstructure were studied as a function of the number of passes. A deformed microstructure with cell sizes approximately 0.5 μm in size were obtained from a starting annealed grain size of 78 μm. © 2005 Elsevier B.V. All rights reserved.

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.

The feasibility of using the equal channel angular extrusion (ECAE) process for the breakdown of cast structures in commercial purity aluminium has been investigated. Billets of an as-cast coarse grained Al (99.7% pure) were subjected to ECAE at 500°C using a die that imparts an equivalent plastic strain of 0.67 per pass. Multiple extrusion passes were performed using two different processing routes: Route I in which the billet orientation was kept constant from pass to pass, and Route II in which the billet was rotated by 180° about its longitudinal axis from one pass to the next resulting alternately in shear in planes oriented at 120° to each other. Processing by Route I through five passes produced elongated grains showing evidence of dynamic recovery. Processing by Route II through five passes however produced largely equiaxed grains with a dynamically recovered substructure due to the intervention of static recrystallization during the reheating to extrusion temperature between passes. Both processing routes produced a dynamically recovered structure with equiaxed subgrains approximately 6μm in size. Sharp deformation textures were not obtained. The relevance of this technique to industrial processing is discussed. © 2001 Elsevier Science B.V. All rights reserved.

The binary Mg-Ca alloys are drawing increasing attention as temporary implant materials because of their excellent biocompatibility, biodegradability, and good mechanical properties. However, their applications are limited due to their high degradation rates in the human physiological environment, the consequent release of hydrogen gas, and rapid loss in mechanical properties. Furthermore, biocompatibility depends upon the degradability of the material. Various researchers have demonstrated that these issues can be addressed by control of Ca content, thermo-mechanical processing to obtain suitable microstructures, deposition of surface coatings, etc. In this manuscript, a detailed review of published literature on Mg-Ca alloys is presented. The challenges and future directions of research in this area are also described.

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.