Hot deformation behavior of Zr-1Nb alloy in two-phase region –microstructure and mechanical properties

Accepted Manuscript Hot deformation behavior of Zr-1Nb alloy in two-phase region –microstructure and mechanical properties K.K. Saxena, K.S. Suresh, R.V. Kulkarni, K.V. Mani Krishna, V. Pancholi, D. Srivastava PII:

S0925-8388(18)30008-2

DOI:

10.1016/j.jallcom.2018.01.008

Reference:

JALCOM 44472

To appear in:

Journal of Alloys and Compounds

Received Date: 3 November 2017 Revised Date:

27 December 2017

Accepted Date: 2 January 2018

Please cite this article as: K.K. Saxena, K.S. Suresh, R.V. Kulkarni, K.V. Mani Krishna, V. Pancholi, D. Srivastava, Hot deformation behavior of Zr-1Nb alloy in two-phase region –microstructure and mechanical properties, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.01.008. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Roorkee-247667,Uttarakhand, India

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Post Irradiation Examination Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India

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Materials Science Division, Bhabha Atomic Research Center, Mumbai-400085, India

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Department of Mechanical Engineering, Institute of Engineering and Technology, GLA University, Mathura-281406, India

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Hot Deformation Behavior of Zr-1Nb Alloy in Two-Phase Region – Microstructure and Mechanical Properties K. K. Saxenaa,d, K. S. Suresha, R.V. Kulkarnib, K.V. Mani Krishnac, V. Pancholia*, D. Srivastavac a

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Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Roorkee-247667,Uttarakhand, India b Post Irradiation Examination Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India b Materials Science Division, Bhabha Atomic Research Center, Mumbai-400085, India d Department of Mechanical Engineering, Institute of Engineering and Technology, GLA University, Mathura-281406, India * [email protected]

Abstract:

Zr-Nb alloys are usually processed in two phase region to get desired microstructure and

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mechanical properties for structural applications in nuclear industries. In the present work, 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

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microstructure after hot deformation was characterized by the size of primary α, secondary α lath size, prior β grain size and texture developed. The deformed samples were characterized with the formation of <112ത0> fiber, which was found to strengthen with increase in deformation temperature. 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

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size.

Keywords: Two phase alloy, Hot deformation, Small punch test, Mechanical properties Introduction:

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Zr-Nb alloys are extensively used for structural applications in nuclear industries due to their excellent mechanical properties, low neutron absorption cross-section, corrosion resistance

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and low hydrogen pickup rate compared to Zircoloy-2 and Zircaloy-4 [1]. In particular, Zr1Nb alloy is employed as clad material in thermal nuclear reactors (VVERs). In Zr-Nb and Ti-alloys, microstructural features like geometrical arrangement of the grains/plates of the two phases and their volume fraction and, crystallographic texture influence the mechanical properties up to a large extent [2-6]. It was reported that the features of α and a ' (βtransformed) phases affect the strength and ductility to a great extent [2, 7-12]. Similar observation was reported by Nag et al. [13] for a heat treated Ti alloy using micro hardness measurements. Xiao et al. [14] investigated high temperature mechanical behavior of zirconium over a range of strain rates (10−4 s−1–103 s−1) and temperatures (298–1073 K) using compression tests. They reported an increase in the twin fractions with strain and strain rate

ACCEPTED MANUSCRIPT whereas, a decrease with increase in temperature. Hammad et al. [15] conducted tensile tests on Zr-1Nb alloy in the temperature range 295 to 773 K, and reported that the precipitates in the α-Zr matrix are responsible for the variation in ductility. Escobedo et al. [16] performed dynamic tensile extrusion test on high purity Zr and found that increase in ductility at elevated temperature was due to enhanced dislocation activity.

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Hot compression of small cylindrical samples is extensively used to simulate and design thermo-mechanical processing route to identify deformation mechanisms like dynamic recrystallization (DRX), dynamic recovery (DRV), superplasticity and shear band formation [17-19].

Approach of processing maps in conjunction with microstructural studies has

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proved to be a potent tool to predict these deformation mechanisms. However, there are limited studies to understand the effect of thermo-mechanical deformation parameters on the mechanical properties in the hot compressed cylindrical samples of Zr-Nb system. It was

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expected that dislocation density, grain refinement, texture development and features of second phase will affect the mechanical properties to a great extent. It is felt that although the processing and microstructural aspects are captured in the hot compression tests, there is limited work on the accompanied changes in mechanical properties of these samples. Since the dimensions of the cylindrical samples which are subjected to hot compression tests are

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not suitable for preparing the standard samples for mechanical testing, miniature mechanical test methods need to be employed. Small punch test is one such useful miniature method, which allows extraction of mechanical properties form the small sized samples in a faithful manner.

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Therefore, the aim of the present study is to understand the effect of different hot deformation conditions on microstructural features such as morphology of constituent phase in two phase

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region, their texture and subsequently their effect (i.e. microstructural features) on mechanical properties using small punch test. 2. Materials and methods 2.1 Specimen preparation: The starting material was a swaged rod of Zr-1Nb alloy

with 11 mm in diameter. The rod was then annealed at a temperature of 700oC (single phase (α) region) in a controlled atmosphere of argon and then slowly cooled to room temperature. The microstructure of the annealed sample is shown in Fig.1 and is characterized by the presence of equiaxed α grains surrounded by a network of thin β regions. Cylindrical samples of 10 mm in diameter and 15 mm in height were prepared for hot compression tests from

ACCEPTED MANUSCRIPT these heat treated samples. The hot compression tests were performed on Thermo-mechanical simulator (Gleeble 3800®) at different temperatures and strain rates under argon atmosphere. The method for friction reduction, heating rate and deformation were kept similar as described in the literature [20]. The compression tested samples were cooled to room temperature at a rate of 1oC/s.

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2.2 Microstructural characterization: The compression tested samples were cut parallel to compression axis for microstructural examination. Samples were polished using different grid papers, followed by diamond paste and/ or colloidal silica. For optical microscope, polished samples were chemically etched with a solution of HNO3, HF and

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water (%vol. 45, 10 and 45 respectively). The etched and dried samples were then observed for optical microstructure, using Leica DMI 5000M optical microscope. For electron backscattered diffraction (EBSD) measurement, compressed samples were polished in similar

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method as for optical, and then polished samples were electro polished using an electrolyte of perchloric acid and methanol (20:80) at a temperature of -40oC at 20 volts for 20 seconds. The EBSD scans were performed on FEI model Quanta™ 3D FEG. A fix scan size i.e. 250 µm × 250 µm was scanned with a step size of 0.5 µm. The data obtained after EBSD scans were analysed using TSL OIM software. At least a minimum of 2000 grains were selected for

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calculating the microstructural parameters such as grain size, distribution of grain boundaries, misorientations, grain orientation spread, and texture. Grains were defined as regions that have a minimum misorientation of 5° and at least 5 pixels. Grain orientation spread is the deviation between orientations of each pixel in a grain to the average orientation of the grain.

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Inverse pole figures were plotted using Harmonic series expansion method (with L=22) and

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are presented normal to compression axis.

Figure 1: Optical micrograph showing the dark etching network of β phase in the Zr-1Nb alloy after slow cooled from 700oC. Equiaxed α grains are clearly visible surrounded by β grain boundaries.

ACCEPTED MANUSCRIPT 2.3 Mechanical properties: Mechanical properties were evaluated using small punch test (SPT). Samples for SPT were cut from the compressed samples at the same location, where microstructural characterizations were done. Samples for SPT were 3 mm in diameter and 0.26 ± 0.01 mm in thickness. These samples were prepared using conventional polishing method. The SPT test on prepared samples were performed at room temperature as per

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guidelines provided for metallic material testing in ASTM handbook [21]. SPT was performed on a Universal Testing Machine (UTM) with a load cell capacity of 500 kg and Linearly Variable Differential Transformer (LVDT) with a resolution of 0.001 mm. The disc specimen was circumferentially gripped between the upper and lower dies of a jig and a 1.0

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mm diameter hardened steel ball was positioned at the centre of the test specimen. The steel ball was forced at a constant cross head velocity of 0.1 mm/s using a plunger. As the plunger pushes the steel ball, a typical load (P) vs central deflection (δ) curve recorded during the test

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until the specimen attains the fracture. For selected hot compression conditions, at least four samples were tested and among these tests, three reproducible results were considered for evaluation of mechanical properties. A typical load (P) vs central deflection (δ) curve recorded for one of the tests is shown in Fig. 2 along with the method of identifying yield load and the maximum load from the curve. Using the value of yield load (Py) and the

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maximum load (Pm) from each test, the mechanical properties, yield strength (σy) and ultimate tensile strength (σUTS) were evaluated incorporating the procedure recommended in

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ref. [22-25].

Figure 2: Typical load vs deflection curve after small punch test (SPT). Intersection of two red lines represents the value of yield load (Py), whereas highest value of load indicated by top arrow is maximum load (Pm).

ACCEPTED MANUSCRIPT 3. Results: The swaged Zr-1Nb alloy followed by annealing was subjected to uniaxial hot compression tests at different temperatures and strain rates. Based on the processing map prediction and flow curves analysis, compression tested samples were characterized extensively for two phase region. The microstructural characterization, texture development after hot

properties are described in the subsequent sections.

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compression test, and the influence of different thermo-mechanical parameters on mechanical

3.1 Processing map: The constitutive behavior of the processed materials is usually studied using the processing map approaches [26]. Among different processing map

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approaches [4, 27-29], dynamic materials model (DMM) was shown to be most suitable approach for Zr-Nb alloys [20, 30-33]. In the DMM approach, energy dissipation during hot

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deformation occurs in two different forms: one is used for plastic deformation (G) and other one is absorbed in microstructural changes (J). The strain rate sensitivity of flow stress (m) is a main factor that partitions total power (energy) in to two complementary parameters or directions.

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& d ln σ ∆ log σ dJ ε& dσ εσ = = ≈ ≡m dG σ d ε& σε& d ln ε& ∆ log ε&

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η = 2m (m +1)

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(energy) dissipation efficiency ratio η during hot working is defined as [4]: (3)

Prasad and Seshacharyulu [4] have confirmed that the flow instability may occur during hot working, and can be identified when following relation is satisfied:

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ξ = ∂ ln(m (m +1)) ∂ lnε& + m < 0

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The negative value of ξ represents the instability during the hot working. The instability parameter varies with strain rate and temperature. The superimposition of power dissipation map and instability map on the axis of temperature and strain rate represents the processing

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Figure 3: Processing map at strain of 0.6. The contours with high efficiency of power dissipation represent safe processing conditions, whereas the shaded portion represents the unsafe processing conditions. The processing conditions under red colour area are further of experimental interest.

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The processing map of Zr-1Nb alloy at a strain of 0.6 is shown in Fig.3, and developed using experimental flow stress after adiabatic temperature rise (ATR) correction. From Fig.3, it is

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clear that high power dissipation efficiency contours are existing at strain rate of 10-2 s-1 at two different temperatures: one at 850oC belongs to two phase region and other one around 950oC belongs to single phase‒β, whereas a moderate value of power dissipation efficiency is appeared at both the strain rates of 10-1 s-1 and 1 s-1 for the temperature range of single α‒ phase and two (α+β) ‒ phase. In processing map (Fig.3), instability occurs at higher strain rate of 10 s-1 at higher temperature of 850oC in two phase and expands to the single β‒phase (i.e. up to 1025oC). The high efficiency at processing condition of 850oC and 950oC at strain of 10-2 s-1 suggests that the dynamic recrystallization and the moderate efficiency of power dissipation at lower strain rates (i.e. 10-1 s-1 and 1 s-1) could be an indication of dynamic recovery in the microstructure. The occurrence of instability in processing map (Fig.3) at

ACCEPTED MANUSCRIPT higher strain rate of 10 s-1 in two phase and single β‒phase point towards instabilities in microstructures such as flow localisation, void formation, wedge cracking and shear band formation. As observed from processing map (Fig. 3), that the high efficiency of power dissipation exhibits in two phase region and single β‒phase region. The suggested high efficiency in two

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phase region is quite relevant with industrial processing, as in the industries Zr-Nb alloy is usually processed in two phase region [34]. Therefore, based on the correlation of processing map and processing industries, a high value power dissipation efficiency range representing specific deformation conditions, is considered (a fix area, red area marked in processing map,

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Fig.3) for microstructural analysis and corresponding mechanical properties evaluation. 3.2 Flow behavior:

The Fig. 4 shows true stress ‒ true strain curves in two phase region starting from

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temperature of 815oC to 885oC and strain rates range of 10-2 to 10 s-1. All flow stress curves are corrected for adiabatic temperature rise (ATR) [35]. The shape of true stress– true strain curves stipulates some features that help to identify the hot deformation mechanisms. All flow curves in two phase region represent strain hardening and then flow softening, depending upon deformation conditions except at strain rate of 10-2 s-1. At strain

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rate of 10-2 s-1, flow curves at all three temperatures (i.e. 815, 850 and 885oC) show steady state behavior. At strain rate of 1 and 10 s-1 flow curves represent strain hardening (high flow stress) up to a true strain of 0.2 and thereafter they exhibit either softening (lower flow stress) or steady state behavior. In the present study, the starting material has an equi-axed

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microstructure (Fig. 1) with β‒phase along the grain boundaries of α and between the α plates. Therefore, the strain hardening processes at low temperature deformation (i.e. 815oC),

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could be mostly associated with dislocation interactions within the α‒phase and pile-up at the grain and/or phase boundaries [36]. Increase in the fraction of β‒phase with deformation temperature, could effectively change the strain partitions between the phases. At initial stages of deformation, the softer phase β accommodates more strain and subsequent strain hardening might increase the load bearing capacity similar to α phase. At this stage more fractions of α phase accommodates the imposed strain. Therefore, competitive contribution from deformation of α and β phases decides the strain hardening at higher temperatures (i.e. 885oC). On the other hand, mechanisms of softening are able to balance the rate of work hardening after true strain of 0.2 and suggest that mechanisms like dynamic recrystallization and/or dynamic recovery are operating beyond strain of 0.2 [37]. Similar phenomenon of

ACCEPTED MANUSCRIPT flow softening was also reported for Ti-6Al-4V alloy [38] and Zr-Nb alloys [39] in two phase region. Out of four strain rates reported here, two strain rates i.e. 10-2 and 1 s-1 were selected for further analysis of microstructure and mechanical properties based on two considerations; i) these two strain rates represent different deformation mechanisms as explained above, ii)

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both strain rates show efficiency of more than 30% in the processing map.

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Figure 4: The true stress- true strain curves of Zr-1Nb alloy after uniaxial compression at different strain rates and deformation temperatures of (a) 815oC, (b) 850oC, (c) 885oC in two phase region. All flow curves are adiabatic temperature rise (ATR) corrected. At higher strain rates of 1 s-1 and 10 s-1, dashed lines are the experimental values of flow stress, while solid lines are the adiabatic temperature rise (ATR) corrected flow stress.

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3.3. Microstructure evolution after hot deformation: 3.3.1. SEM micrographs: Fig. 5 shows scanning electron microscopy (SEM) images

using in-lens detector of the SEM. These microstructures correspond to samples deformed in two phase region. In SEM images, bright (white) portion represents the β‒phase boundaries, whereas dark (black) portion is α‒phase. The combined presence of the α‒phase grains (primary α) and α plates (transformed from β‒phase) are clearly visible in Fig. 5, whereas a small fraction of β‒phase along the grain boundaries of α and between inter-lamellar locations are also noticed.

ACCEPTED MANUSCRIPT Fig. 5a clearly reveals very fine primary α with scattered β phase at grain boundaries corresponding to material deformed at initial temperature of two phase region and at lower strain rate (815oC/10-2 s-1). At higher strain rate of same deformation temperature (i.e. 815oC/ 1s-1) in Fig. 5b, microstructure reveals coarser primary α grains with fine features of α plates. At temperature 850oC (Fig. 5c and 5d), clear appearance of transformation of β‒phase into α

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plates and primary α at both the strain rates are visible. Microstructural morphology exhibited at temperature of 885oC (very close to β transus temperature) for both the strain rates (see Fig. 5e and 5f) have similar features compared to the microstructure of sample deformed at 850oC, the only difference could be in shape and size of grains/plates. It can be noticed that

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the grain size of primary α increased and the length and thickness of α plates decreased with increase in strain rate, whereas on increasing deformation temperature length and thickness of α plates increased and primary grain size decreased, at least for samples deformed at 1 s-1.

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The fragmentation of β‒phase and fine primary α‒phase at 815oC/10-2 s-1 could have resulted from large strain partitioning to the softer β‒phase, while the harder α‒phase carries more stress. Owing to the bcc structure of β‒phase, it is softer than the hcp structured α‒phase. Due to the soft nature of β‒phase, it starts to deform more by accommodating most of the imposed strain at initial stages of deformation. However, the low volume fraction of β‒phase ensures

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that all the imposed strain cannot be accommodated by itself. With increasing plastic strain, as the β‒phase strain hardens, the flow stress may become comparable to both the phases and strain could be partitioned by both the phases. The microstructure of the sample at the same temperature (815oC), but at higher strain rate (1 s-1), is characterized by the presence of

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continuous β‒phase. During high temperature deformation, with increase in strain rate, the time available for recovery or recrystallization reduces and as a consequence, in spite of large

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strain accommodated by the phases, they do not undergo substantial restoration processes. Such differences in the extent of DRV/DRX may explain the formation of fine features of β‒ phase at lower strain rate while continuous β‒phase at higher strain rate [40, 41]. To understand the effect of deformation temperature on phase transformation, Radical® commercially available image analysis software is used for quantitative analysis to get the volume fractions of primary α‒phase and transformed β and β‒phase and shown in Table I. It is clear from Table I that the volume fraction of α‒phase decreases as temperature increases and correspondingly volume fraction of α plates increases. At higher temperatures in two phase region (i.e. 850 and 885oC), strain rates have no significant influence in phase transformation and shows more or less similar values of volume fractions.

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primary α grain

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Figure 5: Backscatter electron images depicting the variation in microstructure at different deformation conditions in two phase region: (a) 815oC/ 10-2 s-1 , (b) 815oC/ 1 s-1, (c) 850oC/ 10-2 s-1, (d) 850oC/ 1 s-1, (e) 885oC/ 10-2 s-1 and (f) 885oC/ 1 s-1. Higher magnification images are shown in insight. The compression direction is vertical to image.

The quantified results of α plate size are shown in Table II, that are calculated using line intercept method from SEM images. The fraction and thickness of transformed α plates increased with deformation temperature, whereas the thickness of the plates were not much affected by the strain rate. It is worth mentioning that a constant soaking time of 5 mins was provided to all the samples. Therefore, there is no influence of soaking time on different deformation conditions. Only the soaking temperature is a parameter that affects the

ACCEPTED MANUSCRIPT morphology of microstructure along with the rate of deformation. It is clear from Fig. 5 and Table II, that for two phase microstructure, decrease in α platelet thickness increases the peak stress (see Fig. 4). Similar observation were reported by Semiatin and Bieler [42]. At lowest strain rate (i.e. 10-2 s-1), the same trend was observed for microstructure of 850oC and 885oC and the peak stress decreases with increasing thicker α plates. More clear morphology can be

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seen in image quality map shown in Fig. 6. It is well reported that the higher soaking temperature (deformation temperature) increases the β‒phase transformation and size of

Table I: Volume fractions (%) of primary α‒phase and transformed β (α plate) and β‒phase at different deformation conditions of two phase region

α‒phase

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α‒plates [9]. Only at the deformation conditions of 815oC/10-2 s-1, microstructure seems to have negligible effects of transformation and predominantly governed by the high temperature deformation mechanisms. The volume fraction of transformed β‒phase (α plate)

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affect the volume of α/β phase boundaries.

Figure 6: Image quality map of small area of EBSD at: (a) 850oC/ 10-2 s-1, (b) 885oC/ 10-2 s-1, and (c) 885oC/ 1 s. Showing bimodal type microstructures in which primary α‒phase and transformed α plates from β‒phase.

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From Table II, it is clear that the width of transformed β phase (α plate) is affected with deformation temperatures and strain rates. To understand the quantitative relationship between strain rate and temperature with width of transformed β phase (α plate), ZenerHollomon parameter  Z = ε& exp ( Q RT )  is considered. The Zener-Hollomon parameter (Z) is a parameter which can incorporate effect of both temperature and strain rate. Plot of width of transformed β phase (α plate) as a function of Z is shown in Fig. 7. From Fig. 7, it is clear

ACCEPTED MANUSCRIPT that Z has a linear relation with width of α plate for hot deformation of Zr-1Nb alloy. Increase in value of Z i.e. decreasing the deformation temperature or increase in strain rate, led to

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Width of α plate= -0.3021(ln Z) + 17.722 R² = 0.99

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decrease in width of transformed β phase (α plate).

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Figure 7: Plot showing the quantitative relationship between Zener-Hollomon parameter (Z) and width of transformed β phase (α plate). Table II: Width of transformed β‒phase (α plate) at different deformation conditions of two phase region

Strain rate (s-1)

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Width of transformed β phase (α plate) (µm) 10-2 1 0.73 1.19 1.76

The trend of current results with respect to the volume fraction of each phase and the

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thickness of α plates obtained in this work is consistent with the reported work of Kulkarni et al. [43] for Zr-2.5Nb alloy. At 885oC, close to the β‒transus, microstructure has lower fraction of primary α‒phase, as compared to all other of 815oC and 850oC. This effect can be attributed to the fact that closer to β transus temperature, the fraction of transformed microstructure increases with temperature [44]. Seshacharyulu et al. [45] also found the refined microstructure as an increase of strain rate and coarse plate size on increasing temperature during the hot deformation of two phase Ti-6Al-4V alloy, whereas Chen and Cao [46] reported that the α grain size as well as volume fraction of α‒phase decreases with increasing temperature in two phase region of Ti-alloy.

ACCEPTED MANUSCRIPT 3.3.2. EBSD analysis: For better understanding of the occurrence of DRX, DRV and the evolution of deformation microstructure, the grain orientation spread (GOS) along with high and low angle grain boundaries at different deformation conditions in two phase region are presented in Fig. 8. It is to be mentioned that the value of GOS is an indicator for the intra-grain misorientation that could be directly correlated to the fraction of geometrically

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necessary dislocations (GND) in a deformed sample [47, 48]. A higher GOS value is representative of deformed microstructure and a very low value that of recrystallized microstructure. The higher volume fraction of GOS<1 represents the dynamic recrystallization (DRX) in the microstructure, whereas GOS>2.5 represents only deformed microstructure after hot deformation. At strain rate of 10-2 s-1, the volume fraction of GOS<1

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first decreases from 0.629 to 0.489 for increase in temperature from 815oC (see Fig. 8a) to 850oC (see Fig. 8c), and subsequently increased to 0.626 at 885oC (see Fig. 8e), whereas at

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strain rate of 1 s-1 it continuously increases from 0.146 to 0.607 for temperature of 815oC (see Fig. 8b) and 885oC (see Fig. 8f). A bar graph representing GOS<1 is shown in Fig. 9a, reveals the distribution of recrystallized regions after hot deformation. Fig. 9b shows a bar graph to represent the fractions of deformed regions (volume fraction of GOS>2.5), which is more or less complimentary to the distribution of grain with GOS<1. At deformation

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condition of 815oC/1 s-1, the volume fraction of GOS>2.5 (Fig. 9b) is maximum (i.e. 0.603) and it decreases as the deformation temperature increases at same strain rate. Whereas at lower strain rate 10-2 s-1, it is maximum with value of 0.273 at 850oC and for lower and higher temperature it decreased to 0.115 and 0.076 respectively. While the difference

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between the recrystallized fractions reduces with temperature, the maximum difference observed at 815oC, between the two strain rates warrants some explanation. The amount of

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recrystallized fraction is expected to be increased with increase in temperature and decrease in strain rate.

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Figure 8: EBSD images showing grain orientation spread (GOS) with high angle grain boundaries (HAGB) and low angle grain boundaries (LAGB) at different deformation conditions: (a) 815oC/ 10-2 s-1, (b) 815oC/ 1s-1, (c) 850oC/ 10-2 s-1, (d) 850oC/ 1 s-1, (e) 885oC/ 10-2 s-1 and (f) 885oC/ 1 s-1. Keys for GOS, HAGB and LAGB are shown at right bottom.

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Figure 9: Volume fraction of grain orientation spread (GOS) at different deformation temperatures in two phase region at strain rate of 0.01 and 1 s-1 (a) GOS<1, (b) GOS>2.5. GOS<1 is the representation of dynamic recrystallization (DRX), whereas GOS>2.5 represents neither DRX nor dynamic recovery in the microstructures.

Fig. 10a shows the fraction of high angle grain boundary (HAGB) at different deformation temperatures and for the strain rates of 10-2 s-1 and 1 s-1. The fraction of HAGB at strain rate

ACCEPTED MANUSCRIPT of 1 s-1 increases with temperature, while that of 10-2 s-1 only a small change is noticeable. However, the grain growth seems to be higher with temperature for samples deformed at 10-2 s-1, while high strain rate deformation does not exhibit any significant change. The changes in the fractions of HAGB with respect to temperature is very similar to the changes in low GOS values (<1), as depicted in Fig. 9(a). Both these changes along with the microstructures in

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Fig. 5(f) suggest that there is a sudden increase in the fraction of recrystallized grains at 885oC for high strain rate, while low strain rate deformation doesn’t show such a variation. With increase in deformation temperature, the fraction of β‒phase increased and at 885oC the microstructure will majorly comprised of β‒phase. The significant enhancement of

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recrystallized fractions only at high strain rate indicates a possibility for change in the mechanism of softening with increased fractions of β‒phase. The deformation microstructure, at lower strain rate does not imply any drastic change in the fraction of DRX grains, however,

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driven by the thermal effects the average grain size increase with temperature. Therefore, restoration mechanisms at lower strain rate are not influenced by the increase in β‒phase fraction. 0.9 0.8

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0.7 0.6

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10 8 6 4 2

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0 815

850

Temperature (oC)

885

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815

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0.2

12

Figure 10: (a) Fraction of high angle grain boundary (HAGB), (b) average grain size at different deformation conditions.

The influence of hot deformation at different processing condition on grain size is shown in Fig. 10b. It is clear from Fig. 10b that the size of grain monotonically increased with the temperature in two phase region at strain rate of 10-2 s-1, whereas at higher strain rate of 1 s-1 a marginal decrease in the grain size was observed. The influence of hot deformation on grain size at strain rate of 1 s-1 was minimal in this temperature regime. Fig. 11 shows the inverse pole figure (IPF) of sample compressed in two phase region at different temperature and strain rates, to show the fibre direction in different orientation along

ACCEPTED MANUSCRIPT compression direction. It is clear from Fig. 11 that mostly the <11‒20>||CD fiber is observed at all the deformation conditions. The tendency for the formation of <11‒20>||CD fiber increases with temperature and strain rate which is demonstrated through the strengthening as well as decrease in the spread of the orientations (Fig. 12). The strengthening of texture could arise from two factors, either through deformation and recrystallization or due to β
α

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transformation. In order to identify the major source of strengthening of texture at high deformation temperatures, the EBSD data were partitioned based on the GOS values to represent deformed (GOS>2.5) and recrystallized microstructures (GOS<1) and the texture of corresponding regions were calculated. The micro-texture of the deformed and recrystallized

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grains, irrespective of deformation temperature and strain rate, are qualitatively similar in

Te mp ./ S. rat e

815oC

GOS <1

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0.0 1/s

1/s

GOS >2.5

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0.0 1/s

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terms of

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1/s

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Figure 11: Inverse pole figures (IPF) images showing orientation of fiber in three different directions at [001] for different deformation temperature and strain rates. GOS<1 represents that the IPF image shows only the fibre having GOS <1, and GOS<2.5 represents that the IPF image shows only the fibre having GOS <2.5.

formation of texture components, however, they exhibit drastic changes close to the transformation temperature in terms of quantitative description. The deformed regions

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demonstrate a formation of stronger <11-20>||CD fiber compared to recrystallized region and strength of this fiber significantly increases after deformation at 885oC. A weak contribution

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of <2-1-14>||CD fiber is also noticed at certain deformation conditions. With increase in temperature, the fraction of recrystallized grains is expected to increase and the same is observed through the microstructural evidences. Albeit a concomitant strengthening of texture of the recrystallized regions, a sudden strengthening of texture of deformed regions at 885oC is observed which suggest that transformation aspects and/or deformation in the β‒ phase field are playing a greater role in strengthening of this texture. Strengthening of <111>

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β fiber is reported for hot deformation of two phase titanium alloys [49, 50], and which up on transformation is expected to transform to <11-20> α fiber. With increased volume fraction of transformed microstructures with temperature, fraction of grains following Burgers orientation relationship increases. Therefore, a direct correlation between textures of α and β‒

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phases are possible. While at 815oC, sufficiently below the β‒transus, only the deformation of α‒phase is predominantly contributing to the texture of α, and at higher temperature,

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deformation within β and subsequent transformation to α contributes towards texture formation. The formation of weak texture at 815oC could be attributed to multiple slip activity at high temperature. With increase in processing temperature, the difference between the critical resolved shear stresses of different slip systems are expected to reduce and as a result, different slip systems take part in deformation, which otherwise were restricted at low temperature. With the activation of multiple slip systems, texture migrates towards randomization, rather than towards a stable end orientation [51].

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0.40

0.30 0.25 0.20

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Number Fractions

0.35

0.15 0.10

0.00 815

850

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Temperature (oC)

885

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Figure 12: Volume fraction of <11‒20>||CD fiber at different temperatures and strain rates.

3.4. Mechanical Properties obtained from small punch test (SPT): The small punch test was conducted at room temperature. The hot deformed samples in two phase region at different deformation temperatures and strain rates were considered for SPT. The obtained results of SPT in terms of yield load (Py) vs displacement (δ) and

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maximum load (Pm) vs displacement (δ) are shown in Fig. 13a and 13b respectively. The yield load of undeformed (as received) sample is 38.33 N. The yield load value (Fig. 13a) of hot deformed samples had increased for all deformation conditions except 815oC/ 10-2 s-1, where this value got reduced to 34.7 N. In spite of having low yield value, it had higher

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maximum load value compared to the as received sample. An appreciable high value of yield (Fig. 12a) and maximum load (Fig. 13b) was obtained for the sample deformed at condition

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of 815oC/ 1s-1. The samples deformed at other conditions exhibited moderate increment in the value of yield and maximum load compared to as received sample. Fig. 13a and 13b reveals that as the temperature increases in two phase region, the value of yield load and maximum load decreases at strain of 1 s-1, whereas at strain rate of 10-2 s-1 it increases. The yield load and maximum load is higher at strain rate of 1 s-1 compared to strain rate of 10-2 s-1 for all temperatures, but at temperature of 885oC, lower strain rate (10-2 s-1) shows higher value in yield and maximum load. The similar value of maximum load is observed on the sample deformed at 850oC and strain rate of 10-2 s-1 and 1s-1. The strengthening of hot deformed samples compared to the as received condition might have contribution from different factors, such as strengthening from grain boundaries,

ACCEPTED MANUSCRIPT dislocation density, morphology of α‒phase, and crystallographic texture. Among the samples deformed at different conditions, a remarkable increase in yield point was observed for samples compressed at 815oC and 1 s-1 (Fig. 13a). The changes in the average grain size after hot compression was negligible and do not seems to have significant impact during SPT. The strengthening of texture fibers with respect to deformation temperature or strain rate is

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not as significant compared to the rise in the yield load of sample deformed at 815oC and 1 s-1 and due to this the texture strengthening might not be a major factor affecting the yield load during SPT. Among the different morphologies of Zr sample, fine lamellar structure provides higher strength compared to equiaxed grains. While the fraction of lamellar α that results

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from the transformed microstructure is higher after deformation at high temperature (885oC), the yield point increment was insignificant. As noticed from Fig. 9a, the fraction of grains with GOS<1, is higher for all testing temperatures at 10-2 s-1 and indicate possible occurrence

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of DRX. Because of high fraction of DRX grains, the strengthening contribution from dislocation density for these samples are only moderate and only lamellar microstructures contribute towards strengthening, which is in accordance with the observed increase in strength. The remarkable increase in yield point at 815oC and 1 s-1 could be attributed to the combined effect of high dislocation density and finer size of α lath. As noticed from Fig. 9(b), the highest fraction of grains with GOS>2.5 is observed for samples deformed at 815oC and 1

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s-1, which could be directly correlated with the presence of GNDs. Further, this sample is characterized by finer α lath size (Table II). The hardness of α with hcp structure is higher than that of β with bcc structure. The α plates tends to align parallel to the external force

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during mechanical testing such as tensile test. When the aspect ratio (the ratio of length to diameter) of the secondary α is higher than the critical aspect ratio, the load can be transferred

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to α through α/β interface according to shear lag theory. Moreover, the obstacle of α plates to dislocation slip is favorable for promoting the strength [52]. The strain partitioning between the two phases and the temperature of deformation decides the extent of DRV/DRX. At low temperature and high strain rate, the chances of restoration are always lower in both the phases. Because of high dislocation density in both the phases combined with finer size of α lath, any slip transfer, during room temperature SPT, across the lamellar microstructure will be harder. While samples deformed at higher temperatures, have both higher β‒phase fraction and sufficient thermal driving force for DRV/DRX. With sufficiently higher aspect ratio of

α‒phase and lower dislocation density, slip transfer becomes easier for other processed conditions.

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(a)

Maximum Load

Yield load 500

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70

50 40 30 20

300

200

100

10 0

0

As recieved 780

815

850

Temperature (oC)

885

As recieved 780

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Maximum load (N)

Yield load (N)

400

60

815

850

Temperature (oC)

885

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Figure 13: Small punch test (SPT) results of samples deformed at different processing conditions: (a) Yield load, (b) Maximum load.

3.5. Fractured Surface of SPT specimens: The SEM image of fractured sample

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deformed at 815oC/1 s-1 is shown in Fig. 14a, depicts the pure tensile fractured surface with high ductility, as shown in Fig. 14a and 14b. The fractures surface of sample deformed at 850oC/10-2 s-1 (Fig. 14c), shows similar features of dimple pattern as of sample deformed at 815oC/1 s-1 (Fig. 14a). The appeared dimple pattern in both the conditions represent that the samples had fractured in ductile mode. Failure normally takes place by void nucleation and

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growth in ductile materials. The fracture surfaces exhibit two kinds of dimple pattern, i.e. equiaxed and conical dimples. The failure mode under the high strain rate at room temperature is quite different as compared at low strain rate conditions. From Fig. 14, it can be seen that the dimples at high strain rates (see Fig. 14a and 14b) are more, indicating better

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plasticity at a higher strain rate, as represented in Fig. 13. The observed results are similar to

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the results of Feng et al. [53].

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Figure 14: Fractured surface after small punch test at room temperature of the samples deformed at; (a) 815oC/ 1 s-1, (b) higher magnification of figure a, and (c) 850oC/10-2 s-1.

Conclusion: Hot deformation of Zr-1Nb alloy in two phase region, microstructural analysis

of deformed samples and the influence of deformed microstructure on mechanical properties •

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lead to the following conclusions.

Samples deformed at low strain rate (0.01 s-1) have undergone significant DRX and the fraction of DRX increases with temperature in the range of 815 to 885oC, while

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for samples deformed at 1 s-1, prominent contribution from DRX is observed only

from 850oC.

•

All the deformed samples were characterized with the formation of <11-20> fiber and their volume fraction increases with temperature. The strengthening of fiber is attributed to the deformation and transformation aspects of β‒phase. Recrystallized grains invariably found to have a weaker texture compared to deformed grains.

•

A notable increase in the yield point of sample deformed at 815oC and 1 s-1 was attributed to the combined effect of higher dislocation density of the material and finer α lath size.

ACCEPTED MANUSCRIPT Acknowledgments

The authors acknowledge the financial support (Grant no. 2011/36/15) from Board of Research in Nuclear Science (BRNS). The National Facility of Texture & OIM, IIT Bombay (a DST-IRPHA facility) was utilized for this study and is duly acknowledged.

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References:

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List of Figures and Tables:

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1. Figure 1: Optical micrograph showing the dark etching network of β phase in the Zr-1Nb alloy after slow cooled from 700oC. Equiaxed α grains are clearly visible surrounded by β grain boundaries. 2. Figure 2: Typical load vs deflection curve after small punch test (SPT). Intersection of two red lines represents the value of yield load (Py), whereas highest value of load indicated by top arrow is maximum load (Pm).

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3. Figure 3: Processing map at strain of 0.6. The contours with high efficiency of power dissipation represent safe processing conditions, whereas the shaded portion represents the unsafe processing conditions. The processing conditions under red colour area are further of experimental interest.

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4. Figure 4: The true stress- true strain curves of Zr-1Nb alloy after uniaxial compression at different strain rates and deformation temperatures of (a) 815oC, (b) 850oC, (c) 885oC in two phase region. All flow curves are adiabatic temperature rise (ATR) corrected. At higher strain rates of 1 s-1 and 10 s-1, dashed lines are the experimental values of flow stress, while solid lines are the adiabatic temperature rise (ATR) corrected flow stress.

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5. Figure 5: Backscatter electron images depicting the variation in microstructure at different deformation conditions in two phase region: (a) 815oC/ 10-2 s-1, (b) 815oC/ 1 s-1, (c) 850oC/ 10-2 s-1, (d) 850oC/ 1 s-1, (e) 885oC/ 10-2 s-1 and (f) 885oC/ 1 s-1. Higher magnification images are shown in insight. The compression direction is vertical to image.

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6. Figure 6: Image quality map of small area of EBSD at: (a) 850oC/ 10-2 s-1, (b) 885oC/ 10-2 s-1, and (c) 885oC/ 1 s-1. Showing bimodal type microstructures in which primary α‒phase and transformed α plates from β‒phase.

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7. Figure 7: Plot showing the quantitative relationship between Zener-Hollomon parameter (Z) and width of transformed β phase (α plate). 8. Figure 8: EBSD images showing grain orientation spread (GOS) with high angle grain boundaries (HAGB) and low angle grain boundaries (LAGB) at different deformation conditions: (a) 815oC/ 10-2 s-1, (b) 815oC/ 1s-1, (c) 850oC/ 10-2 s-1, (d) 850oC/ 1 s-1, (e) 885oC/ 10-2 s-1 and (f) 885oC/ 1 s-1. Keys for GOS, HAGB and LAGB are shown at right bottom. 9. Figure 9: Volume fraction of grain orientation spread (GOS) at different deformation temperatures in two phase region at strain rate of 0.01 and 1 s-1 (a) GOS<1, (b) GOS>2.5. GOS<1 is the representation of dynamic recrystallization (DRX), whereas GOS>2.5 represents neither DRX nor dynamic recovery in the microstructures.

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10. Figure 10: (a) Fraction of high angle grain boundary (HAGB), (b) average grain size at different deformation conditions

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11. Figure 11: Inverse pole figures (IPF) images showing orientation of fiber in three different directions at [001] for different deformation temperature and strain rates. GOS<1 represents that the IPF image shows only the fibre having GOS <1, and GOS<2.5 represents that the IPF image shows only the fibre having GOS <2.5. 12. Figure 12: Volume fraction of <11‒20>||CD fiber at different temperatures and strain rates.

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13. Figure 13: Small punch test (SPT) results of samples deformed at different processing conditions: (a) Yield load, (b) Maximum load.

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14. Figure 14: Fractured surface after small punch test at room temperature of the samples deformed at; (a) 815oC/ 1 s-1, (b) higher magnification of figure a, and (c) 850oC/10-2 s-1. 15. Table I: Volume fractions (%) of primary α‒phase and transformed β (α plate) and β‒ phase at different deformation conditions of two phase region.

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16. Table II: Width of transformed β‒phase (α plate) at different deformation conditions of two phase region.

ACCEPTED MANUSCRIPT Highlights •

Mechanical properties of hot compresses samples were calculated using small punch test. Microstructure was characterized by the size α, prior β grain size and texture

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developed. •

Yield load and maximum load was found maximum for sample deformed at 815oC/ 1

Maximum values are due to combined effect of higher dislocation density and finer α

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lath size.

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s-1.