Masuo Hagiwara

Metastable order-disorder transition and sigma phase formation in Fe-V binary alloys

The influence of binary and ternary alloying compositions and heating rate on theσ formation reaction are studied. Several ways are developed to supress the stableα phase formation and to understand the metastable CsCl-type order-disorder transition in Fe-V alloys. The Fe-V ordered phase has a metastable critical transition temperature of between 850 and 880° C at equiatomic composition, and the so called “650° C anomaly” corresponds to the 550° C anomaly in Fe-Co alloys. Theσ formation reaction does not commence most easily at equiatomic composition but at the iron-rich side of the 40 at. % V alloy. The Influence of ternary alloying additions on theσ formation in Fe-V alloys depends on the relative stabilities of the binaryσ phase with iron or vanadium.

The CsCl-type Order-Disorder Transition Temperature in VMn Alloys

of the g rea t e r ra te of heat ex t rac t ion f rom the bulk l iquid (posit ion E 2 o n Fig. 2). This in t u rn will s t i l l fu r the r dec rea se the growth ra te of the co lumnar in t e r face and i n c r e a s e the t e m p e r a t u r e grad ien t and hence dec rea se the t e m p e r a t u r e of the co lumnar f ront (posit ion C4, Fig. 2). The r a t e s of growth and t e m p e r a t u re s of the co lumnar and equiaxed c ry s t a l s will t h e r e fore follow the a r rows shown on Fig. 2. The growth ra te of the equiaxed c r y s t a l s rap id ly i n c r e a s e s and the growth ra te of the co lumnar in te r face d e c r e a s e s . Eventua l ly a condit ion is r eached where the co lumnar f ront has a lmos t stopped but where the equiaxed c ry s t a l s a re growing rapidly . This phenomena leads to the v e r y abrupt change in s t r u c t u r e observed in many cas t ings and ingots. It is much more difficult to envisage such an abrupt change if the effect of g rad ien t ahead of the co lumnar f ront is neglected.