Texture Evolution and Ultrafine Grain Formation in Cross-Cryo-Rolled Zircaloy-2

doi:10.1007/s40195-015-0267-z

Abstract

Abstract:

The texture and mechanical properties of cross-rolled zircaloy-2 at 77 and 300 K were investigated. Cross-rolling at 77 K was performed to impart different thickness reductions of 25% and 50%, while at 300 K with 25%, 50%, 75% and 85% reductions to the sample. EBSD analysis of deformed sample showed that near-basal orientation is not deformed completely after 50% rolling reduction. The activation of prismatic silp, contraction twin and extension twin were evident from the deformed microstructure at 77 K. The propensity for activation of basal slip <a> at 77 K was also observed. The deformation of the sample at 300 K occurs by prismatic, basal <a> and pyramidal <c + a> slips, which were predicted by pole figures. After annealing, the tensile strengths (735 and 710 MPa) are almost the same for 50% cryo-cross-rolled and room-temperature cross-rolled zircaloy-2 with almost 2.7% difference in their ductility. KAM analysis of the deformed samples was made to estimate the stored strain energy and dislocation density. Annealing of deformed sample at 673 K for 30 min results in recrystallization, which leads to the formation of ultrafine grains.

Key words: Cross-rolling, EBSD, TEM, Ultrafine, grains, Texture

Sunkulp Goel, R. Jayaganthan, I. V. Singh, D. Srivastava, G. K. Dey, N. Saibaba. Texture Evolution and Ultrafine Grain Formation in Cross-Cryo-Rolled Zircaloy-2[J]. Acta Metallurgica Sinica (English Letters), 2015, 28(7): 837-846.

Figures/Tables 13

Table 1 Chemical composition of Zircaloy-2

Element	Tin (Sn)	Iron (Fe)	Chromium (Cr)	Nickel (Ni)	Nitrogen (N)
Wt	1.3-1.6	0.07-0.20	0.05-0.16	0.03-0.08	0.006

Fig. 1 Schematic diagram showing the cross-rolling method

Fig. 2 TEM images of cryo-cross-rolled (CCR) and room-temperature cross-rolled (RTCR) zircaloy-2: a 25% CCR; b 25% RTCR; c 50% CCR; d 50% RTCR

Fig. 3 TEM images of cryo-cross-rolled (CCR) and room-temperature cross-rolled (RTCR) zircaloy-2 after annealing at 400 °C for 30 min: a 25% CCR; b 25% RTCR; c 50% CCR; d 50% RTCR

Table 2 Mechanical properties of Zircaloy-2 after cross rolling

	Hardness (HV)	Tensile strength (MPa)	Yield strength (MPa)	% Elongation at break
Mercury quenched	182	499	331	25
25% RTCR	211	693	632	13.3
50% RTCR	233	727	684	10.45
25% CCR	215	734	698	10.2
50% CCR	237	786	753	8.7
25% RTCR Annld	211	628	592	16.2
50% RTCR Annld	233	710	634	12.5
25% CCR Annld	215	659	621	12.8
50% CCR Annld	237	735	703	9.8

Fig. 4 EBSD images of 25% a and 50% b CCR zircaloy-2; average misorientation c and KAM d of CCR and RTCR zircaloy-2; KAM images of 25% e, 50% f CCR zircaloy-2

Fig. 5 {0002} and pole figure images of zircaloy-2: a mercury quenched; b 25% CCR; c 50% CCR; d 25% CCR after annealing at 400 °C for 30 min; e 50% CCR after annealing at 400 °C for 30 min

Fig. 6 {0002} and pole figure images of 25% a, 50% b, 75% c, 85% d RTCR zircaloy-2

Fig. 7 {0002} and pole figure images of 25% a, 50% b, 75% c, 85% d RTCR zircaloy-2 after annealing at 400 °C for 30 min

Table 3 Critically resolved shear stress (CRSS) values of slip system at room temperature and cryo-temperature

Slip	CRSS at 300 K	CRSS at 77 K
Prism <a> slip	0.1	0.220
Basal <a> slip	0.16	0.260
Pyramidal <c + a> slip	0.320

Fig. 8 Taylor factor a, Schmid factor b image of 25% CCR zircaloy-2

Fig. 9 Basal slip Taylor factor a and Schmid factor b values of CCR and RTCR zircaloy-2; basal slip Taylor factor image c, Schmid factor image d of 50% CCR zircaloy-2

Table 4 Energy and dislocation density of cross rolled samples

	Mercury quenched	25% CCR	25% RTCR	50% CCR	50% RTCR
Average KAM (radian)	0.008105	0.01951	0.017593	0.0276914	0.0233
Dislocation density ρ(m^-2)		9.944 × 10¹⁴	8.805 × 10¹⁴	1.818 × 10¹⁵	1.41 × 10¹⁵
Energy (J/mol)		29.02	25.70	53.06	41.15

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