Y. V. R. K. Prasad

Bio: Y. V. R. K. Prasad is an academic researcher from Indian Institute of Science. The author has contributed to research in topics: Strain rate & Hot working. The author has an hindex of 29, co-authored 103 publications receiving 3288 citations.

Processing maps for hot working of titanium alloys

Abstract: In recent years, processing maps are being used to design hot working schedules for making near-net shapes in a wide variety of materials. In this paper, the results obtained on the characterization of hot working behavior of titanium and its alloys using the approach of processing maps are described. In commercial purity α titanium, dynamic recrystallization (DRX) domain occurs at 775°C and 0.001 s−1 with an efficiency of power dissipation [2m/(m+1) where m is the strain rate sensitivity of flow stress] of 43%. The DRX domain shifts to higher strain rates when the interstitial impurity content is lowered. In the near-α and α-β alloys like IMI 685, Ti–6Al–4V, the preform microstructure has a significant influence on the processing maps. For example, in the transformed β (Widmanstatten) preform microstructures, these alloys exhibit a domain of spheroidization at lower temperature and a domain of β superplasticity at higher temperatures, both occurring at slow strain rates. These domains merge at the β transus because of the occurrence of damage processes which lower the tensile ductility. On the other hand, processing maps on alloys with equiaxed preform microstructure exhibit a clear superplasticity domain in the α-β range and the β phase undergoes DRX with a power dissipation efficiency of ≈45–55%. Titanium materials in general, exhibit wide flow instability regimes due to adiabatic shear bands formation at higher strain rates and hence careful process design has to be adopted for successful forging and microstructural control.

Hot deformation behaviour of Mg–3Al alloy—A study using processing map

Abstract: The hot deformation behaviour of Mg–3Al alloy has been studied using the processing-map technique. Compression tests were conducted in the temperature range 250–550 °C and strain rate range 3 × 10 −4 to 10 2 s −1 and the flow stress data obtained from the tests were used to develop the processing map. The various domains in the map corresponding to different dissipative characteristics have been identified as follows: (i) grain boundary sliding (GBS) domain accommodated by slip controlled by grain boundary diffusion at slow strain-rates ( −3 s −1 ) in the temperature range from 350 to 450 °C, (ii) two different dynamic recrystallization (DRX) domains with a peak efficiency of 42% at 550 °C/10 −1 s −1 and 425 °C/10 2 s −1 governed by stress-assisted cross-slip and thermally activated climb as the respective rate controlling mechanisms and (iii) dynamic recovery (DRV) domain below 300 °C in the intermediate strain rate range from 3 × 10 −2 to 3 × 10 −1 s −1 . The regimes of flow instability have also been delineated in the processing map using an instability criterion. Adiabatic shear banding at higher strain rates (>10 1 s −1 ) and solute drag by substitutional Al atoms at intermediate strain rates (3 × 10 −2 to 3 × 10 −1 s −1 ) in the temperature range (350–450 °C) are responsible for flow instability. The relevance of these mechanisms with reference to hot working practice of the material has been indicated. The processing maps of Mg–3Al alloy and as-cast Mg have been compared qualitatively to elucidate the effect of alloying with aluminum on the deformation behaviour of magnesium.

Deformation behaviour of beta titanium alloy Ti–10V–4.5Fe–1.5Al in hot upset forging

Abstract: The characteristics of hot deformation of a beta titanium alloy Ti–10V–4.5Fe–1.5Al have been studied by upset forging in the temperature range 650–900 °C and strain rate range $0.001–100 s^{-1}$. The true stress–true strain curves at 650 °C show continuous flow softening at strain rates above $0.1 s^{-1}$ whereas at lower strain rates, the flow stress attains a steady-state. At temperatures higher than about 750 °C, there is a distinct peak in the flow stress in the early stages of deformation followed by a steady-state at higher strains. The variation of flow stress with temperature and strain rate follows the standard kinetic rate equation at strain rates lower than about $0.1 s^{-1}$ and the apparent activation energy is estimated to be about $180\hspace{mm} kJ \hspace{2mm} mol^{-1}$. The processing map exhibited a domain in the temperature range 750–900 °C with a peak efficiency of about 48% occurring at 850 °C and $0.01 \hspace{2mm}s^{-1}$. On the basis of the microstructural features, the variation of grain size with temperature and the tensile ductility variations, the domain is interpreted to represent a process of dynamic recrystallisation (DRX). The workability is optimum under peak DRX conditions and the grain size in the DRX domain is linearly dependent on the Zener–Hollomon parameter. At strain rates higher than $10 \hspace{2mm} s^{-1}$ and in a wide temperature range, the material exhibits flow instabilities, which are manifested as flow localisation.

Hot deformation behaviour of as-cast Mg–2Zn–1Mn alloy in compression: a study with processing map

Abstract: The deformation behaviour of as-cast Mg-2Zn-1Mn alloy in the temperature range 300-500 degreesC and in the strain rate range 0.001-100 s(-1) has been studied using processing maps. For obtaining the processing map, the variation of the efficiency of power dissipation given by [2ml(m+1)] where 'm' is the strain rate sensitivity, is plotted as a function of temperature and strain rate. The map exhibited a domain of dynamic recrystallization (DRX) occurring at 450 degreesC and 0.1 s(-1) which are the optimum parameters for hot working of the alloy. The material exhibits grain boundary cracking domain with a higher efficiency at temperatures higher than 450 degreesC and at lower strain rates. At strain rates higher than 3 s-1, the material undergoes flow localization which has to be avoided in mechanical processing for obtaining consistent properties.

Recent developments in stainless steels

Abstract: This article presents an overview of the developments in stainless steels made since the 1990s. Some of the new applications that involve the use of stainless steel are also introduced. A brief introduction to the various classes of stainless steels, their precipitate phases and the status quo of their production around the globe is given first. The advances in a variety of subject areas that have been made recently will then be presented. These recent advances include (1) new findings on the various precipitate phases (the new J phase, new orientation relationships, new phase diagram for the Fe–Cr system, etc.); (2) new suggestions for the prevention/mitigation of the different problems and new methods for their detection/measurement and (3) new techniques for surface/bulk property enhancement (such as laser shot peening, grain boundary engineering and grain refinement). Recent developments in topics like phase prediction, stacking fault energy, superplasticity, metadynamic recrystallisation and the calculation of mechanical properties are introduced, too. In the end of this article, several new applications that involve the use of stainless steels are presented. Some of these are the use of austenitic stainless steels for signature authentication (magnetic recording), the utilisation of the cryogenic magnetic transition of the sigma phase for hot spot detection (the Sigmaplugs), the new Pt-enhanced radiopaque stainless steel (PERSS) coronary stents and stainless steel stents that may be used for magnetic drug targeting. Besides recent developments in conventional stainless steels, those in the high-nitrogen, low-Ni (or Ni-free) varieties are also introduced. These recent developments include new methods for attaining very high nitrogen contents, new guidelines for alloy design, the merits/demerits associated with high nitrogen contents, etc.

Modeling of dynamic material behavior in hot deformation: Forging of Ti-6242

Abstract: A new method of modeling material behavior which accounts for the dynamic metallurgical processes occurring during hot deformation is presented. The approach in this method is to consider the workpiece as a dissipator of power in the total processing system and to evaluate the dissipated power co-contentJ = ∫o σ e ⋅dσ from the constitutive equation relating the strain rate (e) to the flow stress (σ). The optimum processing conditions of temperature and strain rate are those corresponding to the maximum or peak inJ. It is shown thatJ is related to the strain-rate sensitivity (m) of the material and reaches a maximum value(J max) whenm = 1. The efficiency of the power dissipation(J/J max) through metallurgical processes is shown to be an index of the dynamic behavior of the material and is useful in obtaining a unique combination of temperature and strain rate for processing and also in delineating the regions of internal fracture. In this method of modeling, noa priori knowledge or evaluation of the atomistic mechanisms is required, and the method is effective even when more than one dissipation process occurs, which is particularly advantageous in the hot processing of commercial alloys having complex microstructures. This method has been applied to modeling of the behavior of Ti-6242 during hot forging. The behavior of α+ β andβ preform microstructures has been exam-ined, and the results show that the optimum condition for hot forging of these preforms is obtained at 927 °C (1200 K) and a strain rate of 1CT•3 s•1. Variations in the efficiency of dissipation with temperature and strain rate are correlated with the dynamic microstructural changes occurring in the material.

Constitutive analysis in hot working

Abstract: Constitutive equations including an Arrhenius term have been commonly applied to steels with the objective of calculating hot rolling and forging forces. The function relating stress and strain rate is generally the hyperbolic-sine since the power and exponential laws lose linearity at high and low stresses, respectively. In austenitic steels, the equations have been used primarily for the peak stress (strain) associated with dynamic recrystallization (DRX) but also for the critical and steady state stresses (strains) for nucleation and first wave completion of DRX. Since the peak strain is raised by the presence of solutes and fine particles, the stress is raised more than by simple strain hardening increase, thus causing a marked rise in activation energy in alloy steels. In contrast, large carbides, inclusions or segregates, if hard, may lower the peak strain as a result of particle stimulated nucleation. Due to the linear relation between stress and strain at the peak, flow curves can be calculated from the constitutive data with only one additional constant. Maximum pass stresses can also be calculated from a sinh constitutive equation determined in multistage torsion simulations of rolling schedules. Comparison is made between carbon, micro-alloyed, tool and stainless steels and to ferritic steels which usually do not exhibit DRX. Parallels to the effects of impurities and dispersoids on the constitutive equations for Al alloys are briefly discussed.

Metallic anodes for next generation secondary batteries.

Abstract: Li–air(O2) and Li–S batteries have gained much attention recently and most relevant research has aimed to improve the electrochemical performance of air(O2) or sulfur cathode materials. However, many technical problems associated with the Li metal anode have yet to be overcome. This review mainly focuses on the electrochemical behaviors and technical issues related to metallic Li anode materials as well as other metallic anode materials such as alkali (Na) and alkaline earth (Mg) metals, including Zn and Al when these metal anodes were employed for various types of secondary batteries.