Aspects of copper precipitation and irradiation hardening in Fe-Cu alloys

This thesis deals with several aspects of copper precipitation and irradiation hardening in Fe-Cu alloys. i) The stability of copper precipitates in a ferritic matrix has been investigated using HREM to determine the size at which the b.c.c.#->#9R transformation occurs in thermally aged Fe1.3wt%Cu alloys. This size has been shown experimentally to be #approx#12nm during ageing at 550 deg. C. Precipitates of size 4-12nm transform during cooling to room temperature or below. ii) Immediately after the b.c.c.#->#9R transformation the angle, #alpha#, between fringes in adjacent twin bands is approximately 129 deg. Plane rotations and untwinning can be induced in thermally-aged specimens by in-situ electron irradiation with 400keV electrons. After some time under the beam the planes appear to have rotated, reducing a to about 122 deg. Under electron irradiation two mechanisms were observed: i) rotation of the (009)_9_R planes without the precipitate first untwinning, ii) untwinning where one 9R twin variant grows at the expense of the other, confirming directly the untwinning mechanism proposed previously. Plane rotation and detwinning occurred simultaneously. iii) Ion irradiation of an Fe-1.3wt%Cu alloy was carried out at the Hahn-Meitner Institute in Berlin, using 300keV Fe"+ ions at temperatures between 20 deg. C - 550 deg. C at displacement rates of 10"-"3-10"-"2 dpa(s)"-"1 for doses within the range 1-30dpa. Copper precipitates were found to keep their shape but decrease in size under all irradiation conditions. The mechanism of dissolution is described as a balance between the dissolution of precipitate by cascades and the reprecipitation of the copper due to the movement of copper at the irradiation temperature. iv) High-resolution microscopy of an Fe-1.3m%Cu alloy neutron irradiated to doses of 0.008dpa and 0.06dpa at #approx#280 deg. C revealed small copper precipitates having #alpha# #approx# 122 deg. - 126 deg. It was deduced that many of these precipitates had transformed from b.c.c.#->#9R in the reactor at #approx#280 deg. C. Precipitate sizes did not depend significantly on dose, with a mean diameter of #approx#3.4#+-#0.7nm and #approx#3.0#+-#0.7nm for the 0.008dpa and 0.06dpa material respectively. The number densities of transformed precipitates were #approx#1.8x10"2"1m"-"3 and #approx#8.5x10"2"m"-"3 respectively. GIF-EELS chemical mapping analysis gave total number densities of 5.6x10"2"2m"-"3 and 6.0x10"2"2m"-"3 respectively. v) Finally, weak-beam electron microscopy was used to investigate neutron irradiated 'pure' iron 'Hawthorne' 1A and 2A alloys irradiated to a dose of 0.06dpa at #approx#280 deg. C. The dislocation loops in the 1A alloy had a mean size of #approx#2.8#+-#0.1nm, while those in the 2A alloy had a size of #approx#3.4#+-#0.1nm. About 80% had Burgers vector b=a<100> and the remainder were of type b=a/2<111>. The precipitate number densities for the 1A and 2A alloy were #approx#8.5#+-#0.9x10"2"1m"-"3 and #approx#6.6#+-#0.7x10"2"1m"-"3 respectively. The visible defect populations account for a maximum of 59% and 75% of the measured hardening values for the 1A and 2A alloys respectively. The loops are stable under thermal annealing to about 430 deg. C, which suggests they have an interstitial nature. No voids were observed. (author)SIGLEAvailable from British Library Document Supply Centre-DSC:D210120 / BLDSC - British Library Document Supply CentreGBUnited Kingdo