Showing papers by "Bhagwati Prasad Kashyap published in 2014"

Microstructure and properties of hot extruded Cu–1wt% Al2O3 nano-composites synthesized by various techniques

Abstract: Cu–1 wt% Al2O3 nano-composites were prepared by three different methods, namely mechanical alloying, combustion synthesis and electrical explosion of wire, in order to investigate the effect of power processing technique on microstructure and ambient properties. The powders produced by these techniques were hydrogen reduced at 500 °C and subsequently vacuum encapsulated in copper container followed by consolidation using hot extrusion at 800 °C and 600 MPa to nearly full density. As-synthesized composite powders as well as the extruded samples were characterized by scanning electron microscope, X-ray diffraction, transmission electron microscope, thermal conductivity and electrical conductivity. Microhardness and room temperature deformation behavior in compression at a strain rate of 10−4 s−1 were measured. The mechanically milled powders, upon consolidation, exhibited a finer grain size of 146 nm and a hardness of 164 HV1.0 compared to that obtained by two other techniques, which ranged between 179–390 nm and 135–156 HV1.0, respectively. The physical and mechanical properties were found to vary with grain size according to the Hall-Petch type relationship, irrespective of the processing technique employed for getting these grain sizes.

Correlation between microstructural features and creep strain in a near-α titanium alloy processed in the α+β regime

Abstract: High temperature creep is an important property of titanium alloys used in aeroengines. Creep resistance of titanium alloys generally varies with heat treatment, temperature and cooling rate. Both the parameters affect the morphology and topology of the α (HCP) and β (BCC) phase present in the material. Various theories have been proposed in the literature to explain (i) the increase in creep strain with decreasing solution treatment temperature and (ii) the U-shaped variation of creep strain with cooling rate. Some of these theories are quite contradictory. An attempt is made here to systematically (a) evaluate and establish a direct microstructure–mechanical property correlation and (b) to explain the observed variation in the creep behaviour of a near-α titanium alloy IMI 834. The results obtained indicate that the observed U-shaped variation of creep curve is due to the counter acting nature of various microstructural features present in the material.

Hot working and geometric dynamic recrystallisation behaviour of a near-α titanium alloy with acicular microstructure

Abstract: The hot working behaviour of near-α titanium alloy – TITAN 29A (equivalent to IMI 834) with an acicular starting microstructure was evaluated by carrying out hot compression tests over a range of temperatures (850–1060 °C) and strain rates (3×10−4–100/s). Using the flow curves, processing maps were generated to identify the safe processing window for the material. The material exhibits a deterministic domain between 920 and 1030 °C at low strain rates of 3×10−4–10−3/s where it undergoes geometric dynamic recrystallisation (GDRX) or globularisation of α lamellae. The initiation and evolution of globularisation was investigated using the flow curve analysis method. The work hardening rate (θ)–flow stress (σ) curve was used to estimate the critical strain (ec) required for initiation of globularisation and the saturation stress (σsat) for dynamic recovery (DRV). The recrystallised or globularised volume fraction (X) was estimated from the difference between the calculated DRV and experimental DRX curves. The estimated globularised volume fraction modelled using Avrami equation was found to match with the microstructural observations.

Hot Workability and Flow Characteristics of Aluminum-5 wt.% B 4 C Composite

Abstract: Flow behavior of aluminum-5 wt.% boron carbide (Al-B4C) composite was investigated by carrying out compression tests over a range of strain rates (10−4-100 s−1) and temperatures (200-500 °C). The flow stress data obtained from these tests at true strain 0.5 were used to develop processing map. The stable and instable flow regimes in the map were characterized by the microstructural examination using Scanning Electron Microscopy and Electron Backscattered Diffraction. The optimum condition for processing of Al-5%B4C composite was found to lie between 425 and 475 °C at the strain rate of around 10−4 s−1. A strain-compensated Sellars-McG Tegart constitutive equation was established to model high-temperature deformation behavior of the material.

Grain Growth and β-Mg17Al12 Intermetallic Phase Dissolution During Heat Treatment and Its Impact on Deformation Behavior of AZ80 Mg-Alloy

Abstract: Microstructure evolution after solutionizing and ageing treatment of cast AZ80 Mg alloy were investigated using optical and scanning electron microscopy. Effect of these treatments on grain size, β-Mg17Al12 intermetallic phase, mechanical behavior, and flow asymmetry were investigated. The initial continuous network of β-phase found to be reduced after solutionizing. The dissolution of β-phase and simultaneous grain growth are found to be interrelated. Mechanical properties including yield strength, maximum strength (ultimate compressive strength), and maximum strain attainable in compressive found almost twice than the corresponding values obtained in tension. The asymmetry in compressive and tensile properties is found to decrease with grain size at certain solutionizing duration. Particular heat treatment found to offer best combination of tensile compressive flow properties in AZ80 Mg alloy. Aging under certain conditions found to minimize the strength asymmetry.

Structure and annealing behavior of an Al-Mg-TM alloy processed by equal channel angular pressing

Abstract: 1 Institute for Metals Superplasticity Problems RAS, Khalturin St. 39, 450001, Ufa, Russia 2Vikram Sarabhai Space Center, Trivandrum-695022, Kerala, India 3Indian Institute of Technology Bombay, Powai, Mumbai-400076, India Effects of equal-channel angular pressing (ECAP) at 325 oC to a strain, e~10, and post-ECAP annealing for 1 hr in the temperature range of 350–520 oC on structure of the cast and homogenized Al-Mg-Sc-Zr alloy 01570C were analyzed. It is found that annealing of ECAP processed material containing near homogeneous ultrafine-grained structure with ~1 μm grain size led to normal grain growth, accompanied by coarsening of Al3(Sc,Zr) dispersoids. As compared to the cast alloy, the coarsening of dispersoids in the deformed alloy started at lower temperature and occurred more intensely, and resulted in more rapid loss of their coherency with surrounding matrix.

Effect of Rolling on Microstructure and Room Temperature Tensile Properties of Newly Developed Mg-4Li-1Ca Alloy

Abstract: Mg-30Ca and Mg-14Li (wt %) master alloys were melted successively in the induction furnace to obtain a Mg-Li-Ca ternary alloy containing 3.99 % Li and 1 % Ca. The as-cast material of thickness 4 mm was homogenised at 350° C for 120 mins and subsequently rolled to 62.5 % reduction in thickness at 300 °C to get 1.5 mm thick sheet. The microstructures of hot rolled samples were examined in as-rolled condition as well as after annealing at 350° C for various lengths of time. The presence of deformation twins was clearly seen in the as-rolled structure, whereas equiaxed twin-free grains were observed in the annealed condition. The average grain size was found to increase from 10 μm to 18 μm by annealing, according to the kinetics that follows a parabolic law. Tensile samples taken from rolled plate were deformed to failure at room temperature and a strain rate of 10-4 s-1. Ultimate tensile strength of as-rolled material increased to 213 MPa, while tensile elongation dropped to 6.5 % from the initial values of 134 MPa and 8.5 %, respectively. Annealing after rolling offered a good compromise between the enhanced tensile strength (160 MPa) and tensile ductility (9 %) suggesting viability of the proposed thermomechanical treatment as a means for enhancing both strength and ductility of Mg-4Li-1Ca alloy.

Effects of temperature and strain rate on compressive flow behavior of aluminum-boron carbide composites

Abstract: Flow properties of aluminum and aluminum-boron carbide (Al-B4C) composites, containing 5, 10 and 15 wt% B4C, were investigated by compression tests at strain rates of 10−4, 10−3 and 10−2 s−1 over the temperature range 25 to 500℃. The nature of stress–strain curves as a function of reinforcement, temperature and strain rate revealed that (1) flow stress initially increases as the reinforcement increases, but it decreases for Al-15% B4C composite, (2) flow stress increases with the increase in strain rate, with the strain rate sensitivity index varying from 0.01 for aluminum at 200℃ to 0.30 for Al-5% B4C composite. The activation energy for deformation is found to vary from 124 to 187 kJ/mol for Al-15% B4C and Al-5% B4C composites, respectively.