PSI - Issue 14

Priti Kotak Shah et al. / Procedia Structural Integrity 14 (2019) 242–250 Priti et. al. / Structural Integrity Procedia 00 (2018) 000–000

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(a) (b) Fig. 7. J-tested, heat tinted and fractured (a) axial notched and (b) transverse notched specimens.

900

Trans RT

800

Axial RT

700

600

Axial 300 0 C

500

400

Trans 300 0 C

300 Stress (MPa)

200

100

0

0

0.05

0.1

0.15

0.2

0.25

Strain (mm/mm)

Fig. 8. Stress strain plot for axial and transverse tensile specimens at room temperature and 300 0 C .

2. Results and Discussion

Zr-2.5Nb contains 2.4–2.8 wt% of niobium, and other major alloying elements in form of 650-1000 ppm iron and 900–1300 pm oxygen. Zr-2.5 wt% Nb pressure tube alloy has two phase ( α + β ) microstructure. Oxygen stabilizes the HCP α -phase, whereas niobium stabilizes the body-centred cubic (BCC) β -phase (Cheadle et al., 1975). It is predominantly (more than 90% volume fraction) elongated α - Zr phase (HCP) which is separated by thin filament of β phase (BCC). The alpha Zr phase has predominantly tangential basal pole texture which gets formed during hot extrusion process and undergoes little change in subsequent tube forming. Pressure tubes have dominant transverse basal pole texture, with fraction of basal pole of HCP α -Zr ( f t ) along this direction being around 0.58-0.6. The fraction of radial and axial basal poles being around 0.35 and 0.05 respectively. A typical stress-strain plot obtained in room temperature and 300 0 C testing of axial and transverse specimen for the Zr-2.5Nb alloy pressure tube material is shown in Figure 8 The transverse specimens show higher yield strength (YS) and ultimate tensile strength (UTS) as compared to the longitudinal specimens . The tensile strength of the transverse specimens is relatively closer to the yield strength as compared to that in the axial specimens. That means transverse specimens show less strain hardening as compared to the longitudinal specimens. The uniform and total elongation also depends on specimen orientation. The axial specimens showed higher uniform and total elongation compared to the transverse specimens. Plastic deformation occurs in Zr-2.5 Nb occurs by dislocation glide upon either prismatic or pyramidal slip system (Bose and Klassen, 2011). The primary slip system in alpha-Zr is on the prism planes 101¯ 0 and in 112¯ 0 directions, commonly referred as prismatic slip. Slip has also been observed along the basal { 0001 } planes in the 12¯ 10 direction, and is known as basal a slip.