Showing papers by "Bhagwati Prasad Kashyap published in 2008"

Trigger Stress for Stress-Induced Martensitic Transformation during Tensile Deformation in Ti-Al-Nb Alloys: Effect of Grain Size

Abstract: The effect of β grain size on trigger stress for stress-induced martensitic transformation during tensile deformation in Ti-Al-Nb alloys was investigated. The trigger stress for stress-induced martensitic transformation (SIMT) in Ti-Al-Nb alloys exhibited a U-shaped behavior with variation in grain size. The variation of trigger stress with grain size was explained qualitatively, in terms of the contrasting change in the internal elastic energy stored in matrix due to formation of martensite (ΔEel) and the irreversible work done in overcoming the internal frictional resistance to phase boundary movement (∂Eirr) with β grain size.

Effect of Al and Nb on the trigger stress for stress-induced martensitic transformation during tensile loading in Ti–Al–Nb alloys

Abstract: The effect of composition on trigger stress for stress-induced martensitic transformation (SIMT) during tensile deformation in Ti–Al–Nb alloys was investigated. The trigger stress in these alloys was found to decrease with an increase in Nb as well as Al content in the range of Nb (8–12 at.%) and Al (15–18 at.%) studied. The effect was explained on the basis of effect of these elements on the critical free energy required for the SIMT.

T-shaped equi-channel angular pressing of Pb-Sn eutectic and its tensile properties

Abstract: Equi-channel angular pressing (ECAP) of a Pb–Sn eutectic alloy up to six passes in a T-shaped die, rather than a conventional L-shaped die, was studied for grain refinement. The effect of ECAP on the hardness and tensile properties was studied. Microstructure predominately changed in the early part of the ECAP process and became equiaxed and uniformly distributed in both the longitudinal and the transverse sections after four passes. There occurred substantial softening over the first two passes—hardness of 10 Hv, yield strength of 14.2 MPa and tensile strength of 16.3 MPa in the as-cast condition decreased upon two passes to 6 Hv, 9.7 MPa and 13.0 MPa, respectively. The ductility (% elongation) increased drastically from

Fretting wear study on Ti–Ca–P biocomposite in dry and simulated body fluid

Abstract: In the present work, powder metallurgy processing was used to synthesize a titanium rich composite containing in situ formed bioactive calcium-phosphatic phases. The potential application of such a composite includes load-bearing implants. In view of the importance of friction and wear in biomedical applications, the present work was taken up to investigate the friction and wear properties of such Ti–Ca–P composite at fretting contact against bearing steel in simulated body fluid (SBF) environment. A comparison was also made with fretting behaviour in dry conditions. Tribological experiments were carried out on a biocomposite against bearing steel at different loads (2, 5 and 10 N) for 10,000 cycles with displacement stroke set to 80 μm and at 10 Hz frequency using a low amplitude reciprocatory fretting wear tester. In addition to reporting the measured tribological data, a major focus of the work was in understanding dominant wear mechanisms under dry ambient and physiological environment.

Effect of Volume Fraction of Primary α 2 on the Trigger Stress for Stress-Induced Martensitic Transformation in Two-Phase Ti-Al-Nb Alloys

Abstract: Ti-Al-Nb alloys in the composition range studied in the present investigation were found to undergo stress-induced martensitic transformation (SIMT) in α2 + β heat-treated (two-phase) condition under tensile loading. The effect of different volume fractions of primary α2 on trigger stress for SIM transformation in these alloys was investigated. The trigger stress required for SIM transformation decreased initially when the volume fraction of primary α2 was increased from 0 to 5 pct. However, subsequently, the trigger stress was found to increase with further increase in volume fraction of primary α2 from 5 to 40 pct in all three alloys. An increase in the volume fraction of primary α2 is achieved by changing the solution treatment temperatures, which also results in a change in the β composition and β grain size. The observed variation of trigger stress with the volume fraction of primary α2 from 5 to 40 pct was then explained on the basis of the effect of composition and grain size of the β phase on trigger stress required for stress-induced transformation of β to martensite.

Characterisation of dynamic recovery during hot deformation of spray formed Al-Li alloy (UL40) using processing map approach

Abstract: Processing maps, also known as power dissipation maps, for spray formed Al–Li alloy (UL40), encompassing a wide range of hot working temperature (375–575°C) and strain rate (3 × 10−4–101 s−1) have been developed. The constant true strain rate compression tests were carried out under isothermal conditions to generate the stress–strain data required for computing the efficiency of power dissipation η. The maps exhibit three distinct regimes. The two high efficiency (>55%) regimes occur at the low strain rates ( 1 s−1), and the other with moderate efficiency values (20–23%) occurs at the intermediate strain rate (10−1 s−1). The latter one has been interpreted to be associated with dynamic recovery (DRV) process while the other two regimes have been found to be associated with cracking processes. The DRV regime in the map has been characterised in terms of stress–strain response, microstructure and tensile ductility. The kinetic rate analysis carried out i...

The effect of grain size and composition on the fracture toughness of Ti–Al–Nb alloys undergoing stress-induced martensitic transformation

Abstract: The effect of grain size and composition on the fracture toughness of Ti–Al–Nb alloys in β solution-treated condition was investigated The fracture toughness of the alloys was found to increase with an increase in grain size initially, reach a maximum and subsequently decrease with further increase in grain size This trend was attributed primarily to the effect of grain size on the enhancement of fracture toughness due to stress-induced martensitic transformation (SIMT) at the crack tip, which in turn can be related to the effect of grain size on trigger stress for SIMT Alloys containing higher Al and Nb showed a higher toughness for the same grain size, which was also explained in terms of effect of composition on the trigger stress

Influence of volume fraction of primary α2 on the fracture toughness of Ti–Al–Nb alloy undergoing stress-induced martensitic transformation

Abstract: The effect of volume fraction of primary α2 on fracture toughness of Ti–18Al–8Nb alloy, which undergoes stress-induced martensitic transformation (SIMT), was investigated. The fracture toughness of the alloy was found to decrease with an increase in volume fraction of primary α2. The results were explained on the basis of the effect of change in volume fraction of α2 on fracture toughness contributions due to SIMT as well as that due to the subsequent fracture process involving ductile fracture of the β/martensite/α2 microstructure.

Tearing Crack Growth and Fracture Micro-Mechanisms Under Micro Segregation in Zr-2.5%Nb Pressure Tube Material

Abstract: In the case of Zr-2.5%Nb pressure tube material, certain tramp elements (Cl, P, and C) have a deleterious effect on the fracture properties. In order to dramatically reduce the amount of these impurities, vacuum arc re-melting is adopted. The effect of the melting practice (double or quadruple) on the fracture properties of this material has been previously studied in detail. However, in these studies, the micro-mechanisms of fracture and the role of the trace impurities on crack initiation and propagation in the sub-critical regime were not investigated in detail. In the present study, the mechanisms operating during the three stages of crack growth, i.e., initiation, propagation and fracture, in the case of low toughness double melted material, are proposed based on a detailed study. In our observation, the tramp elements segregate in the form of fine stringers. In the regions away from the segregation, the material is found to be very ductile with the appearance of ligaments with high local plastic deformation before fracture. The presence of such ligaments on the fracture surface is an indication of fracture occurring in a transition zone (ductile to brittle). In the case of the failure in this transition region, both cleavage and ductile mechanisms can occur in the same specimen. In the transition region, near the upper shelf region, the initiation of the crack occurs by cleavage at a local discontinuity, but the toughness increases rapidly and crack propagation occurs in ductile manner by formation of microvoids. The crack front propagates finding the local discontinuities, leaving behind the unbroken regions with high toughness (ligaments). As the crack propagation continues and the crack face opens, the ligaments left well behind the crack tip rupture after the crack front moves further. The electron micro-beam analysis and X-ray mapping show a build-up of Cl concentration at the stringer sites. In the present analysis, the fracture behavior of double and quadruple melted Zr-2.5%Nb material is compared using a “tearing instability” criterion. Using this approach, an attempt is made to assess the enhanced safety margins, in terms of critical crack length, achieved by modification in the melting practice. First, an estimate from the small specimen J-R data was obtained. However, to obtain a realistic estimate for fracture under burst condition an appropriate scaling factor was applied.