Kuldeep Kumar Saxena

Role of activation energies of individual phases in two-phase range on constitutive equation of Zr–2.5Nb–0.5Cu alloy

Abstract Dominant phase during hot deformation in the two-phase region of Zr–2.5Nb–0.5Cu (ZNC) alloy was studied using activation energy calculation of individual phases. Thermo-mechanical compression tests were performed on a two-phase ZNC alloy in the temperature range of 700–925 °C and strain rate range of 10 −2 –10 s −1 . Flow stress data of the single phase were extrapolated in the two-phase range to calculate flow stress data of individual phases. Activation energies of individual phases were then calculated using calculated flow stress data in the two-phase range. Comparison of activation energies revealed that α phase is the dominant phase (deformation controlling phase) in the two-phase range. Constitutive equations were also developed on the basis of the deformation temperature range (or according to phases present) using a sine-hyperbolic type constitutive equation. The statistical analysis revealed that the constitutive equation developed for a particular phase showed good agreement with the experimental results in terms of correlation coefficient ( R ) and average absolute relative error (AARE).

Hot Deformation Behaviour and Microstructural Evaluation of Zr-1Nb Alloy

In nuclear water reactors, zirconium alloys are extensively used as fuel cladding material and in other structural applications. Uniaxial hot compression tests were performed to understand the deformation behavior of Zr-1Nb alloy. Therefore, hot compression tests were performed in the temperature range of 700-1050°C, which envelopes α-phase, (α+β) phase, and β-phase. True stress-strain curves, processing maps, microstructural observation and kinetic analysis were used to discuss the deformation behavior of Zr-1Nb alloy. Deformation at a strain rate of 10-2 s-1 reveals softening at lower temperatures and steady state behavior at higher temperatures. Processing map also reveals domain of high efficiency at 10-2 s-1 strain rate for a wide range of deformation temperatures. The flow softening and high power dissipation efficiency predicts dynamic recrystallization or dynamic recovery during the hot deformation of studied alloy.

Flow behaviour of TiHy 600 alloy under hot deformation using gleeble 3800

Abstract To understand deformation behaviour of TiHy 600 alloy at higher temperatures, hot compression tests are performed in α region (1173 K), α + β regions (1223, 1248, and 1273 K) and β region (1323 K) at strain rates (0.001, 0.01, 0.1, 1 and 10/s) for up to 50% deformation in Gleeble 3800® thermo-mechanical simulator. Flow curve plots are drawn at each strain rates and temperatures and it is observed that dominant deformation mechanism at higher temperature 1323 K (β region) and strain rates (1 and 10/s) is dynamic recovery (DRV) whereas dynamic recrystallization (DRX) is mostly observed at lower strain rates (0.001, 0.01/s) in medium temperature range of 1223 K (α region) to 1248 K (α + β region). Hyperbolic sine law equation is used to calculate the activation energy (Q) and other material sensitive parameters (A, α and n1). The activation energies for DRX in α region and DRV in β region are obtained as 384 and 251 kJ/mol. Experimental peak stress values are compared with predicted peak stress values (R2 = 96.2%) and Zener-Hollomon parameter (R2 = 94.3%). The flow stress behavior up to the peak stress is verified with Cingara equation. Finally, calculated prediction results of DRX volume fraction obtained from Avrami equation is compared with experimental observed microstructure.

Determination of Instability in Zr-2.5Nb-0.5Cu Using Lyapunov Function

Hot workability of Zr-2.5Nb-0.5Cu alloy has been investigated by means of hot compression test using Gleeble-3800®, in the temperature and strain rate range of 700 to 925°C and 0.01-10s-1, respectively. Deformation behavior was characterized in terms of flow instability using peak stress with the help of Lyapunov Function. The true stress-strain curves shows that softening occurs at all lower temperature and for entire strain rates of deformation. The instable flow was suggested by negative m value at deformation condition of 700°C (5 and 10 s-1), while s value at 925°C (10 s-1). The combined result of rate of change of m and s with respect to log strain rate suggest that the deformation condition ranges from 725-780°C (10-2- 10-1 s-1) and 700°C (1-10 s-1) representing safe domain for stable flow.

Texture studies of hot compressed near alpha titanium alloy (IMI 834) at 1000°C with different strain rates

IMI 834 Titanium alloy is a near alpha (hcp) titanium alloy used for high temperature applications with the service temperature up to 600°C. Generally, this alloy is widely used in gas turbine engine applications such as low pressure compressor discs. For these applications, good fatigue and creep properties are required, which have been noticed better in a bimodal microstructure, containing 15-20% volume fraction of primary alpha grains (αp) and remaining bcc beta (β) grains transformed secondary alpha laths (αs). The bimodal microstructure is achieved during processing of IMI 834 in the high temperature α+β region. The major issue of bimodal IMI 834 during utilization is its poor dwell fatigue life time caused by textured macrozones. Textured macrozone is the spatial accumulation of similar oriented grains in the microstructure generated during hot processing in the high temperature α+β region. Textured macrozone can be mitigated by controlling the hot deformation with certain strain rate under stable plastic conditions having β grains undergoing dynamic recrystallization. Hence, a comprehensive study is required to understand the deformation behavior of α and β grains at different strain rates in that region. Hot compression tests up to 5°% strain of the samples are performed with five different strain rates i.e. 10-3 s-1, 10-2 s-1, 10-1 s-1, 1 s-1 and 10 s-1 at 1000°C using Gleeble 3800. The resultant bimodal microstructure and the texture studies of primary alpha grains (αp) and secondary alpha laths (αs) are carried out using scanning electron microscopy (SEM)-electron back scattered diffraction (EBSD) method.

Molecular dynamics evaluation of mechanical properties of carbon nanotubes with number of Stone-Wales defects

Molecular dynamics simulation has been carried out to study the mechanical properties of a 42.59 Å long armchair (6, 6), (8, 8), (10, 10), and (12, 12) single-walled carbon nanotubes (SWCNTs) with an increase number of Stone-Wales (SW) defects, by varying their relative position and orientation. Brenner bond order potential has been employed for energy minimization. In the present work, calculations of fundamental mechanical properties of SWCNTs were performed using molecular dynamics (MD) simulations via material studio by Accelrys Inc. Maximum percentage reduction of 7.5 % in Youngs modulus and increase in potential energy which is also accountable to stabilize carbon nanotubes (CNT), observed as 26.8%. Strain amplitude 0 .003 is employed and no of steps for each strain is 4. During the simulation 0.001 kcal/mol energy, 0.5 kcal/mol/Å force, maximum number of iteration 500 with Steepest Descent Algorithm has been used. The fluctuation of the total energy and temperature during the MD were also calculated. This simulation carries an optimized structure before calculating the Youngs modulus.

Calculation of Fundamental Mechanical Properties of Single Walled Carbon Nanotube Using Non-Local Elasticity

The non-local formulation of elasticity has been introduced as a correction to the local theory in order to account for long ranged inter-atomic forces. This elasticity theory is useful to estimate the in-plane stiffness of Single Wall Carbon Nanotube. The non- local effect is present in Single Wall Carbon Nanotube through the introduction of a non-local nanoscale which depends of the material and a molecular internal characteristics length. This non-local nanoscale goes to zero at macro scale and hence the non-local effect vanishes with reference to classical mechanics. The resulting from the non-local elasticity theory is verified through molecular simulations results. For this study of non-local theory in analysis of Carbon Nanotube, the value of scale coefficient is about 0.7nm.