Phase Transformation and Atypical Variants in an Extruded Two Phase Zirconium Tube
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INTRODUCTION
CRYSTALLOGRAPHIC texture plays a significant role in determining the in service performance of hexagonal close-packed (HCP) materials due to significant anisotropy of properties at the crystallographic level, such as elasticity, thermal expansion, and plasticity.[1–3] Therefore, understanding the development of texture through thermomechanical processing is of particular interest, with the goal of optimizing the texture of the final component. Zirconium has a hexagonal close-packed (HCP) a phase at room temperature and a high temperature b body-centered cubic (BCC) phase. The a ! b phase transformation occurs above 865 C in pure Zr. Alloying additions can affect the stability of the phases at a given temperature and produce a two phase a + b regime at intermediate temperatures. Alloys which are processed within the two phase regime usually involve a heat treatment step into the fully b regime to homogenize the microchemistry. As such, the texture inherited from the phase transformation is of importance for texture control. One such alloy processed in this manner is Zr-2.5wt pctNb, which is used in CANDU pressurized heavy water reactors due to its good mechanical strength, corrosion and creep resistance, and neutronic properties. Pressure tubes of Zr-2.5wt pctNb act as the D. KERR and M.R. DAYMOND are with the Department of Mechanical and Materials Engineering, Queen’s University, 60 Union Street, Kingston, ON, K7L 3N6, Canada. Contact e-mail: [email protected] C. COCHRANE is with the Department of Mechanical and Materials Engineering, Queen’s University and also with the Canadian Nuclear Safety Commission, 280 Slater Street, Ottawa, ON, K1P 5S9, Canada. Manuscript submitted August 2, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
primary containment vessel of the reactor coolant heavy water at high temperature inside the core of the reactor.[4] For an alloy of zirconium containing about 2.5wt pctNb, BCC b Zr is stable at temperatures above about 860 C, below which HCP a Zr is precipitated.[5] Upon cooling to about 600 C, eutectoid decomposition occurs transforming the remaining b Zr to a Zr and a BCC b Nb phase, the latter making up approximately 10 vol pct. This reaction is typically very slow, and so room temperature materials typically consist of a Zr and undecomposed b Zr, which retains the majority of Nb alloying addition (~ 20 wt pct).[6] Transformation from the HCP a phase to the BCC b phase in Zr and Ti obeys the well-known Burger’s orientation relationship, where f0002ga kð110Þb planes are parallel and h1120ia kh111ib directions are retained, which corresponds to retention of close-packed planes and nearest-neighbor directions.[7] This orientation relationship establishes 12 symmetrically equivalent variants for the b ! a transformation during cooling and 6 possible variants for the a ! b transformation. This effect is used in some cases in industry to randomize the texture of materials.[8] Several researchers have noted a texture memory effect, where after the b treatment, the a
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