Abstract
The phase transformation types operative in the decomposition of partially stabilised and fully stabilised B Zr-Mo binary and B Zr-Mo-Al ternary alloys have been investigated with hardness, x-ray diffraction and metallographic methods. The report is presented in two volumes. In all these alloys, decomposition of the ft phase involves the metastable hardening phase w whose temperature stability is of great interest to reactor technologists. It is demonstrated that although quenched alloys containing to yield higher hardness than either a or B , ageing renders such alloys harder by 100-150 V.P.N. Furthermore, although in both binary and ternary alloys the increase in ageing hardness is inversely proportional to the temperature of ageing, the increase is markedly greater in the ternary alloys containing aluminium (c.p. 423 and 480 V.P.N. at 500 C for 7% Mo binary and 7% Mo-3% Al ternary alloy). Thus ternary alloys compounded by the addition of a stabilising substitutional solutes to ft -binary alloys enhance the temperature stability by raising the (w+B)/(Be+a) boundary and allowing larger proportions of w to coexist with B during ageing. The part played in the transformations by B platelike precipitates has been found to be unimportant in water-quenched specimens. The precipitates are however induced by hydrogen and are most probably metastable hydrides. Larger quantities of hydrogen (> 0.5 at.%) do affect the decomposition of the B phase, causing higher as-quenched hardness but lower ageing hardness than in normal water-quenched alloys. In terms of the (w + B) metastable constitution alone this behaviour is inconsistent and puzzling. An extensive literature survey has enabled the experimental contribution to be assessed in terms of the behaviour of other B -zirconium and B -titanium alloys. Critical compositions - e.g. all- w peak, complete stabilisation of B and (w + B) mixture yielding the highest ageing hardness appear to be directly related to the ft stabilising capacity of the solute and consequently reflect an electronic factor.