Effect of Sn content and texture on the twinning behaviour of Zr–Sn binary alloys at low temperature

Journal of Nuclear Materials 465 (2015) 71e77

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Effect of Sn content and texture on the twinning behaviour of ZreSn binary alloys at low temperature N. Keskar*, K.V. Mani Krishna, D. Srivastava, G.K. Dey Materials Science Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 November 2014 Received in revised form 28 May 2015 Accepted 30 May 2015 Available online 3 June 2015

The present study is concerned with the twinning behaviour of ZreSn alloys at low temperature as a function of Sn content and starting texture. ZreSn samples, with Sn contents in the range of 0.33e2.9 wt. % and with different initial textures were subjected to uniaxial compression tests at
80  C. Sn was found to enhance the f1012g1011 type of deformation twinning and the extent of its effect on promoting this type of twinning was observed to be texture dependent. The influence of Sn in promoting twinning was seen to be stronger when the texture of the samples was relatively less favourable for the twinning. © 2015 Elsevier B.V. All rights reserved.

Keywords: Zirconium Deformation mechanisms Twinning Texture EBSD X-ray diffraction

1. Introduction Most of the vital structural core components of the thermal nuclear reactors are essentially made of Zr based alloys. The inreactor performance of these components is largely influenced by their as-fabricated microstructure and texture [1e5]. The fabrication of these components, such as clad tubes, involves several deformation stages [6,7]. The microstructural and the textural developments during such the deformation stages are dictated by the active deformation modes and their relative contribution. Operation of multiple deformation systems is a characteristic of hcp materials [8]. For instance, in the case of deformation of a-Zr at room temperature, apart from the easy, and hence dominant, slip, other modes of deformation such as < cþa> slip and twinning were also shown to contribute to the deformation [8e15]. Deformation by slip accounts for the majority of the imposed strains and determines the formation of the microstructural features such as deformation bands and near boundary gradient zones. However, it does not result in significant orientation changes. Twinning, on the other hand, results in pronounced orientation changes and thus contributes to considerable textural evolution [12e14,16e19]. The

* Corresponding author. E-mail addresses: [email protected], [email protected] (N. Keskar). http://dx.doi.org/10.1016/j.jnucmat.2015.05.044 0022-3115/© 2015 Elsevier B.V. All rights reserved.

microstructure and texture of Zr based structural components can thus be tailored by suitable control of the relative extent of these deformation modes: slip and twinning [6,20]. Previous works have accumulated considerable knowledge on the deformation by slip and the factors influencing the same in great detail [8,9,21e25]. For instance, the roles of alloy composition and temperature on the various slip modes have been well documented [26,27]. Similarly, literature exists on the deformation twinning and factors influencing it at room temperature [1,12,17,28,29]. In general, deformation twinning is initiated when the slip gets hindered. This hindrance comes from several sources such as, unfavourable orientation of the grain with respect to imposed stress state, the presence of foreign atoms (solid solution strengthening) and low mobility of atoms on account of lower temperatures. Previous studies, in general, explored the first two factors to a large extent in understanding the deformation twinning, using different initial crystallographic texture and composition as the variables. It has been shown that the texture plays a major role in determining the extent of twinning. Presence of solid solution strengtheners, like Sn, was shown to promote twinning probability in Zr [30]. It may be emphasized that all of these studies were in a temperature domain where slip is the dominant mechanism and twinning is, at best, expected to compete with the secondary slip modes of deformation. It would thus be interesting to see, if the role of the crystallographic texture and the alloy composition get influenced, when

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Table 1 Alloy composition expressed in parts per million by weight (wppm), except for Sn and Zr, which are expressed in weight %. Sample

Cr

Fe

Hf

Mg

Mn

Mo

N

Nb

Ni

O

Pb

Si

Sn

Ta

Ti

Zr

Zr-0.33Sn Zr-1.2Sn Zr-2.9Sn

<25 <25 <25

810 560 460

<50 <50 <50

<20 <20 <20

<25 <25 <25

<20 <20 <20

40 44 45

<50 <50 <50

30 <15 15

830 930 880

<25 <25 <25

<50 60 45

0.33 1.19 2.9

<100 <100 <100

25 25 25

99.1 98.9 96.6

the ODFs. For quantitative analysis of the textural evolution, Kearns' parameter [1,31,32] was calculated for each sample. 3. Results and discussion

Fig. 1. Schematic showing the extraction of the samples in the two orientations from a cold-rolled and heat treated alloy (not to scale). RD and TD imply rolling direction and transverse direction respectively.

the temperature of deformation shifts to sub zero domain, where the twinning competes even with the primary slip modes. This is the motivation for the present study. A series of ZreSn binary alloys with varying Sn contents, were subjected to uniaxial compressive loading along different directions (to simulate different initial textures) at
80  C. The objective was to explore the influence of Sn and starting texture on the extent of twinning and microstructural and textural modifications brought by the same at this temperature, where twinning is one of the dominant modes of deformation. 2. Experimental In the present study, three binary alloys of Zr with 0.33, 1.2 and 2.9 wt. % Sn were used. The chemical compositions of the alloys are given in Table 1. Samples of these alloys were subjected to 50% reduction by cold rolling followed by annealing at 650  C for 4 h. This heat treatment resulted in a fully recrystallized microstructure with a texture characterized by a majority of basal poles {0001} aligned along the normal direction (ND). From each alloy, cylindrical samples of 5 mm diameter and 7 mm height were extracted from two orientations to simulate two different starting textures for further compression tests, as shown in Fig. 1. Each of these samples was subjected to uniaxial compression at
80  C along respective cylindrical axes. To avoid adiabatic heating, a low strain rate of 1 x 10
4s
1 was selected and load-elongation data were recorded every 0.1 s. Each deformed sample was cut along the rolling directiontransverse direction (RD-TD) plane for microstructural and textural examination. The RD-TD plane was then metallographically polished and electropolished for characterization by Electron Backscattered Diffraction (EBSD) in an FEI™ Quanta-3D Field Emission Gun Scanning Electron Microscope (FE-SEM). The bulk textures of all these samples were also recorded using a PANalytical™ PRO-MRD system. To ensure that the localized effects of friction near the sample-loading plate interface are not reflected in microstructure and texture measurements, these were recorded from the middle region of the specimen, which is a better representation of bulk material behaviour. Four incomplete pole figures ([f0111g], {[0112]}, {[0002]} and {[0113]}) were measured and used for constructing the orientation distribution functions (ODF) using Mtex software. The basal pole figures were computed from

Fig. 2aed shows that the initial microstructure and texture of all the three alloys were almost identical, owing to the similar heat treatment that these had undergone. A strong basal pole concentration along the ND, with some spread along the TD was observed in all the alloys. In addition, the alloys showed negligible basal poles in the RD. Such observations were also made by Cheadle et al. [33,34] in thermo-mechanically processed Zr-based components. Texture in Zr-alloys is quantitatively expressed by the Kearns' Fparameter, which is routinely used as a specification for the quality of Zr-alloy components [1,6,20,31,32]. In the present case, the initial F-parameters were: Fr ¼ 0.047e0.069, Ft ¼ 0.114e0.133 and Fn ¼ 0.798e0.836, where the subscripts r, t and n stand for rolling, transverse and normal direction, respectively. The initial grain size of the samples of all the three alloy compositions was in the range of 10e12 microns. The flow behaviour of the three alloys at
80  C is shown in Fig. 3 using engineering stress-strain plots. An increase in the yield strength of the alloys, with an increase in the Sn content (CSn), was observed, and was attributed to the solid solution strengthening effect of Sn. Considering the fact that the flow behavior showed a systematic trend with that of the alloy Sn content, the variation in the twinning behaviour can be attributed to the variation in the Sn content of the three alloys rather than to the impurities. It may be noted that the total variation in the impurity contents among the three alloys was mainitained in a narrow range (±400 ppm) resulting in starting microstructures which were indistinguishable with respect to second phase particles (SPP) as seen in Fig. 2bed. The resulting microstructures, due to compressions in the RD and the TD, in the three alloys, are shown in Figs. 4 and 5, respectively. The boundaries demarcated in black in these figures were identified as 85 tensile twin boundaries using TSL-OIMTM software. 85 tensile twin refers to the twinning type characterized by the orientation relationship f1012g1011 commonly known as “tensile twinning” [11]. These are the types of twins found for all compositions and levels of deformations used in the present study. However, twinning behaviour was seen to change with the CSn. As seen in Figs. 4 and 5, increase in the CSn resulted in more instances of twinning for deformation along both the RD and the TD. Additionally, an overall refinement in the twin width and grain size with CSn was observed (see Table 2 and Fig. 6). Based on these observations, therefore, one may assume a dominance of nucleation of twins over their growth. Increase in the extent of deformation (EoD) led to larger fraction of twins and a refinement in the grain size, as seen in Table 2 and Figs. 4e6. In some cases, parallel twins inside a parent grain were seen to merge together to form an apparently larger twin, but such cases retained the signatures of multiple twin nucleation. Overall, twinning was seen to be more dominant in samples compressed along the RD as compared to those compressed along the TD. For any given alloy and a particular deformation extent, the number of twins in the RD compression was about 3 times (nominal value) the number of twins in the TD compression. Fig. 6 shows the variation

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Fig. 2. (a) The initial microstructure and the texture of the three alloys in the RD-TD plane after cold rolling and heat treatment at 650  C for 4 h. Transmission Electron Microscopy (TEM) micrographs showing size and distribution of SPP in (b) Zr-0.33% Sn, (c) Zr-1.2% Sn and (d) Zr-2.9% Sn.

Fig. 3. The engineering stressestrain curves for the three alloys on compression along the (a) rolling and (b) transverse direction respectively at
80  C.

in twin width as a function of the CSn and EoD. Notwithstanding the uncertainty associated with the measurement of the widths of twins, a conspicuous refinement in these with the EoD was observed. As described in a later section, the volume fraction of the twins increased with CSn and EoD, which could be attributed to the increase in the number density of the twins, as shown in Figs. 4 and 5. Since twinning leads to sharp changes in the orientation of the lattice, observed strong change in the texture is usually observed [35]. As seen in Figs. 7 and 8, in the case of the RD compression, the basal pole intensity, which was localised near the ND initially, shifted towards the RD upon deformation. However, in the case of the TD compression, the shift in the basal pole intensity was towards the TD, and the extent of this shift varied in the two cases, i.e., RD and TD deformation. The magnitude of the shift stated above was about 45 and 90 in the cases of TD and RD compression, respectively. This is a clear indication of variation in the extent of twinning for the two sample orientations. Figs. 9 and 10 show the change in the Kearns' parameter (DF) with Sn content and extent of deformation for the two different loading directions employed. The change in the Kearns' parameter is a quantitative measure of the textural change, which, in turn,

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N. Keskar et al. / Journal of Nuclear Materials 465 (2015) 71e77

Fig. 4. The microstructure development of the three ZreSn alloys on compression to different extents. CDjjRD implies that the compression direction is parallel to the initial RD.

can be equated to the measure of the extent of twinning, in this case. The reason for this assertion is the fact that, for the compressive deformation used in the present study, the textural changes brought about by the slip are rather insignificant in comparison to the changes induced by twinning. The changes in the F-parameter along the RD and TD, denoted by DFr and DFt, respectively, were used to quantify the extent of twinning. The reason for this choice was the fact that the operation of 85 tensile twinning makes the axes of the twins to orient themselves parallel to the principal compressive stress. It is evident that DFr increased with an increase in EoD for all CSn and for the case of RD deformation, as shown in Fig. 10a. This is a signature of the operation of 85 tensile twins which makes the axis of the hcp unit cell rotate towards the compression direction, RD in this case. Similarly, activation of these 85 tensile twins in case of TD deformation should result in rotation of axes towards TD, which gets reflected in the DFt, as shown in Fig. 10b. In summary, Figs. 9 and 10 depict that increase in CSn resulted in an increase in the extent of twinning, which was reflected by the corresponding changes in the respective F-parameters. However, careful observation of the magnitude of variation of the F-parameters, as a function of CSn, reveals that, the influence of Sn is dependent on the loading direction (see Figs. 9 and 10). While deformations along RD as well as TD showed an increase in twins with increasing CSn, the effect of the variation of CSn appeared more prominent in case of deformation along TD. Contrary to this, deformation at room temperature has shown that the influence of CSn on the extent of twinning was more for RD than for TD compression, for similar initial microstructure and

texture [36]. The difference in these observations was attributed to the difference in the temperatures of deformation. It may be emphasised here, that at low temperatures of deformations, as in the present case, twinning competes with the primary slip modes. In general, the role of Sn is expected to be due to its solid solution strengthening effect on Zr [30]. Sn is known to raise the stress required for a given threshold plastic strain required for twin nucleation. This makes the driving force available for twinning higher for high CSn alloys. If this were the only factor, one would expect Sn to have similar role on extent of twinning irrespective of deformation direction. Following is the possible explanation for the observed anomaly of influence of Sn on extent of twinning in RD and TD. Two important factors that in general govern the extent of twinning are (a) crystal orientation (b) operating stress magnitude. Since the deformations being studied in this work are at
80  C, there is a significant tendency for twinning to start with as the slip is highly constrained. Further, the operation of 85 tensile twinning requires tensile stress along the axis of the parent grains, making the orientation of the grains important in determining the probability of the twinning. Based on this fact, one can easily see why the extent of twinning is more in case of the RD deformed samples. These samples have majority of their grains oriented favourably for such twinning, see Figs. 1 and 2. In such cases, twinning can be induced very easily and thus the extent of twinning need not be limited significantly by the availability of high magnitudes of stress. In other words the favourable orientation and constraints to slip (due to low temperature) are sufficient enough to induce significant twinning if the operating stress exceeds required

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Fig. 5. The microstructure development of the three ZreSn alloys on compression to different extents. CDjjTD implies that the compression direction is parallel to the initial TD.

threshold. This is the reason why additional stress brought in by higher Sn did not significantly enhance twinning (see Figs. 4, 7, 9a and 10a). Contrary to the deformation along the RD, the twinning in the case of deformation along the TD is relatively constrained by the comparatively unfavourable texture. Thus, any additional influence that can promote twinning is expected to reflect itself significantly in enhancing tendency for twinning. This is where Sn can help promote twinning by its solid solution strengthening effect. Higher CSn alloys, particularly in the absence of easy slip due to lower temperatures, can significantly increase the operating stresses, thereby promoting twinning. This can explain the observed higher effect of Sn on the extent of twinning in case of deformation along the TD (see Figs. 5, 8, 9b and 10b). While this study was carried out for ZreSn alloys under compression, a similar effect of Sn content on the twinning behaviour is expected for these alloys under tension as well. However, the influence of starting texture on the twinning behaviour may differ for alloys under tensile loading [22,23].

4. Conclusions A study was carried out to analyze the effect of CSn and texture on the twinning behaviour of ZreSn alloys with particular focus on low temperature deformation using compressive loading.

Table 2 Refining of average grain size (mm) with increasing deformation for samples compressed at
80  C parallel to the RD (CDjjRD) and to the TD (CDjjTD). Alloy

0.33 Sn 1.2 Sn 2.9 Sn

10%

20%

30%

CD jj RD

CD jj TD

CD jj RD

CD jj TD

CD jj RD

CD jj TD

9.5 ± 0.3 9.8 ± 0.3 9.3 ± 0.3

9.9 ± 0.3 9.4 ± 0.3 9.8 ± 0.3

8.0 ± 0.3 7.5 ± 0.3 6.9 ± 0.3

9.0 ± 0.3 8.9 ± 0.3 8.9 ± 0.3

7.1 ± 0.3 6.5 ± 0.3 6.4 ± 0.3

7.9 ± 0.3 7.3 ± 0.3 7.3 ± 0.3

Fig. 6. Variation of twin width with deformation for three Sn contents of 0.33, 1.2 and 2.9 wt.% for (a) compression along RD and (b) compression along TD.

Fig. 7. The texture evolution in the three ZreSn alloys on compression to different extents. CDjjRD implies that the compression direction is parallel to the initial RD.

Fig. 8. The texture evolution in the three ZreSn alloys on compression to different extents. CDjjTD implies that the compression direction is parallel to the initial RD.

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twinning was more prominent when the texture of the samples is relatively less favourable for the twinning. Acknowledgements The authors are grateful to Prof. I. Samajdar and the National Facility for Texture and Orientation Imaging Microscopy, Department of Metallurgical Engineering and Materials Science, IIT Bombay, Mumbai, India e 400076 for providing EBSD and X-ray diffraction facility. References [1] [2] [3] [4] [5] [6] [7] Fig. 9. The change in F- parameter along the ND, i.e., DFn for compression deformations of 10%, 20% and 30% for all three alloys. (a) shows the change in Fn for compression along the RD whereas (b) shows the change in Fn for compression along the TD.

[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]

Fig. 10. The change in F-parameter, i.e., DFr, along the RD and DFt along the TD for compression deformations of 10%, 20% and 30% for all three alloys. (a) shows the change in Fr for compression along the RD whereas (b) shows the change in Ft for compression along the TD.

1. The effect of Sn on the extent of twinning was seen to be deformation direction dependent, in case of ZreSn alloys. 2. In case of low temperature deformation, Sn in general promoted the twinning activity in Zr. Further, the effect of Sn in promoting

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