The precipitation behavior of a Zr-2.5 wt pct Nb alloy
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I.
INTRODUCTION
THEZr-25 wt pct Nb alloy finds extensive use in pressure tube applications in the nuclear power industry because of its low neutron absorption cross section, high corrosion resistance, and excellent creep properties. When the alloy is quenched from the/3 phase field and aged below the monotectoid temperature of 610 ~ precipitation hardening o c c u r s . [1-51 The as-quenched alloy, c~' martensite, contains acicular plates with {1]01}~, internal twins, tL3,rl Upon aging the martensite at 500 ~ for 240 hours, Williams and Gilbert m observed both needle-like and spherical precipitates in the matrix and at twin boundaries, the precipitates being a Nb-rich bcc phase with a lattice parameter of 0.331 nm. These observations were confirmed in a later study by S a b o l , [31 who used aging temperatures from 400 to 600 ~ to study the precipitation behavior. However, Banerjee et al. c41found that at 550 and 600 ~ only the Zr-rich/31 phase (bcc) with a = 0.352 nm formed, while at 500 ~ the Nb-rich/32 phase, a = 0.334 nm, was observed. Banerjee et al. [4] also reported that both the matrix and twin-nucleated precipitates had an identical orientation relationship--the Burgers relationship. I71 In this study the results of a detailed investigation of the morphology, orientation relationship, and nature of the precipitates formed in a quenched and aged commercial Zr2.5 wt pct Nb alloy are presented. Two types of precipitates are found, in agreement with the study of Banerjee et al. [4] The r2 precipitates are needle-shaped and an invariant line strain model proposed by Dahmen I81is used to predict both the growth direction and crystallographic orientation of the particles. The coarser/31 precipitates have a more equiaxed morphology, and have the Burgers orientation relationship.
0.5 mm thick strips with intermediate anneals at 900 ~ for 1 hour in vacuum. The strips were then sealed in evacuated quartz tubes, back-filled with argon, solution treated at 1000 ~ for 20 minutes, and water quenched, followed by aging in vacuum for up to 240 hours at 500 ~ or 60 hours at 600 ~ The aged samples were chemically thinned in a solution containing 5 parts HF, 45 parts HNO3, and 50 parts water. TEM foils were prepared from the strips using a twinjet thinning device with a solution of 6 vol pct perchloric acid, 35 vol pct butanol, and 59 vol pct methanol at - 3 0 ~ and a current density of 4.8 A/cm:. All TEM observations were carried out on a Hitachi H800 200 kV transmission electron microscope equipped with a double-tilt specimen holder.
III.
RESULTS
A. As-Quenched Microstructures
In the as-quenched condition, the alloy consisted of c~' martensite plates (HCP), many of which were internally twinned (Figure l(a)). A selected area diffraction pattern taken from the twinned area (Figure l(b)) shows the twinning plane to be {1T01}~, in agreement with previous reports, rt'3'61 The twinning angle, i.e., the_an_gle between the reciprocal lattice vectors (1101)~, and (1101)~, in the [1120]~, zone depended on the c/a ratio, bein
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