HOT DEFORMATION BEHAVIOR OF Zr-Nb ALLOYS

Abstract

Zr-Nb alloys are used to fabricate structural components in nuclear power reactors. Zr- 1Nb alloy is used to produce calandria tube with 60 mm diameter and 1 to 2 mm wall thickness for pressurized heavy water reactor (PHWR). Zr-2.5Nb alloy is used to fabricate pressure tube for CANDU water reactor having diameter less than 30 mm whereas, Zr-2.5Nb-0.5Cu is used to make garter spring. These components are exposed to corrosive and neutron radiation environment with surrounding temperature of 280 ‒ 400oC. Therefore, it is essential to optimize mechanical properties in order to improve service life. Thermo-mechanical processing is one of the best route to enhance mechanical properties without altering chemical composition. The processing conditions such as strain, strain rate and temperature are optimized to get desired microstructure without introducing any instability. Fine equiaxed grains, obtained through process of dynamic recrystallization (DRX), is the desirable microstructure. The Zr-Nb alloys exhibit α‒phase (hcp) at lower temperatures, β‒phase (bcc) at elevated temperatures and two phase region (i.e. α+β phase) in the intermediate temperature range. The flow stress curves exhibited phase dependent flow behavior. Most of the flow curves in single α‒phase showed strain hardening behavior whereas, single β‒phase showed steady-state behavior. On the other hand, in two phase region flow softening was observed. The degree of slow softening was quantified and plotted as a function of deformation temperature and strain rate. Most of the flow softening occurred in two phase region at lower strain rate (i.e. 10-2 s-1) for different Zr-Nb alloys. The softening in two phase region could be due to DRX or microstructural instabilities, either in the α‒phase or β‒phase or both phases. To identify and understand safe hot processing conditions and deformation mechanism, different processing map approaches were applied in the present work for different Zr-Nb alloys. The processing maps were developed using dynamic materials model (DMM), modified dynamic materials model (MDMM) and α-parameter. Among all the processing map approaches, DMM was found to be the most appropriate approach to delineate safe and unsafe processing conditions for Zr-Nb alloys. The predictions of the DMM were confirmed with microstructural analysis. The processing map developed using DMM approach exhibited high power dissipation efficiency in two phase region for Zr-1Nb alloy. High power dissipation efficiency suggests that the hot deformation is accompanied by dynamic recrystallization. In the two phase region, flow curves also showed softening. Therefore, to clarify the cause of high power dissipation efficiency and flow softening in the two phase region, a novel approach was proposed wherein, calculated ii flow stress was used to develop the processing map. Calculated flow stresses for the two phase region were obtained by incorporating phase information in the form of phase fractions of α and β‒phases and, corresponding extrapolated flow stress values. The processing map developed using calculated flow stress data was able to reveal high dissipation efficiency domains whereas, processing map developed using experimental flow stress data could not. The superimposed dissipation efficiency maps of the individual phases in the two phase region revealed that the single α‒phase has high power dissipation efficiency than β‒phase. Hence, the dominant phase or the cause of flow softening in the two phase region was found to be the α‒phase. Microstructural analysis of the samples deformed in the two phase region brought out that the α‒ phase has undergone DRX whereas, β‒phase exhibited coarse elongated grains. The constitutive equations were also developed for Zr-Nb alloys by relating flow stress with Zener‒Hollomon parameter ( ), the terms have their usual meaning. Activation energy, Q and stress exponent, n are calculated during the development of a constitutive equation. Values of Q and n help to predict deformation mechanism operating during hot deformation. In the present work, activation energy of the two phase region was found to be close to that of individual α‒phase. It brought out that the dominant phase in the two phase region of Zr-Nb alloy was the α‒phase. The constitutive equation developed using calculated flow stress values for α ( ) c   and β‒phases ( ) c   in two phase region (α+β) phase, clarified that the variation in the proportion of the two phases does not affect the predictability of the equation. Constitutive equations developed for the two phase (α+β) region and, for single phases α and β showed good agreement with the experimental values. However, the constitutive equation developed for the entire range of the deformation temperature (all the phases and phase mixture) predicted flow stress values with significant variation from experimental one. In this work, it was established that individual constitutive equation for each phase or phase mixture is expected to predict accurate results. The developed constitutive equations were further validated by FEM simulation of hot extrusion process using the parameters obtained from the constitutive equations. The prediction of ram force by FEM for industrial hot extrusion process were compared with the actual shop floor data. In FEM simulation, input data were either in the form of flow stress data (look-up table, LUT) obtained from hot compression tests or the parameters of a constitutive equation. The FEM simulation showed that the phase-wise constitutive equation provide same ram force as provided by the look-up table and matched quite well with the shop floor data. However, single constitutive equation for all phases could not do so. Z= ε exp (Q RT) iii Zr-Nb alloys are usually processed in the two phase region to get the desired microstructure and mechanical properties for structural applications in nuclear industries. The small punch test (SPT) was performed to evaluate mechanical properties of the samples after hot compression in the two phase region at different temperatures and strain rates. The microstructure after hot deformation was characterized by the size of primary α, secondary α lath size, prior β grain size and texture developed. In two phase region, samples deformed at low strain rate (0.01 s-1) have undergone significant DRX and the fraction of DRX increases with temperature, while for samples deformed at 1 s-1, prominent contribution from DRX was observed only from 850oC. The deformed samples were characterized with the formation of 1120  fiber, which was found to strengthen with increase in the deformation temperature. The strengthening of fiber is attributed to the transformation aspects and deformation in the β‒phase region. Recrystallized grains invariably found to have a weaker texture compared to the deformed grains. The yield load and maximum load was found maximum for sample deformed at 815oC/ 1 s-1 due to combined effect of higher dislocation density and finer α lath size.