U. Prakash

DO22 to L12 transition in intermetallic systems

Ternary additions of metals such as chromium, manganese, iron, cobalt, nickel, copper and zinc to tetragonal (DO22) Al3Ti are known to lead to stabilization of the cubic (L12) structure. This DO22 to L12 transition has been studied in the Al-Ti-Ni system using X-ray diffraction and scanning electron microscopy. The results show that nickel substitution has no significant effect on the lattice parameter (and therefore on the tetragonality) of the DO22 phase and that the solid solubility of nickel in the DO22 phase is very limited. The L12 phase precipitates out on addition of nickel to Al3Ti, its amount increasing with increasing nickel content of the alloy. The compositions of the DO22 and L12 phases do not change significantly with the alloy composition. These results are discussed in terms of theories of structural transitions in ordered alloys. Similar transitions have been reported in transition metal-based systems. An analysis of the transition in intermetallic systems is presented.

Formation of B2 antiphase domains in rapidly solidified Fe-Al alloys

Abstract Fe-Al-X (X = Cr, Mo) alloys in the composition range 50 to 80 at.% Fe, 0 to 20 at.% X were rapidly solidified by chill-block melt-spinning. Thin foil transmission electron microscopy of the rapidly solidified ribbons revealed that antiphase domains occur in the B2 structure in the limited composition range 22 to 37 at.% Al. The B2 antiphase domain size was found to increase with the increasing Al-content of the ribbon. The criterion for antiphase domain (APD) formation is investigated in terms of mechanisms for formation of the B2 structure.

Novel faulted structures in rapidly solidified Fe-37 at.% Al-15 at.% Mo alloy

Abstract Fe-37 at.% Al-15 at.% Mo has an equilibrium two-phase structure consisting of an ordered B2 (FeAl-based) matrix with Mo 3 Al as the second phase. Rapid solidification by chill-block melt-spinning suppresses the formation of the Mo 3 Al phase, giving instead single phase B2-FeAl supersaturated with Mo and containing a network of faults revealed by TEM. Imaging with B2 superlattice reflections revealed an anisotropic antiphase domain contrast while imaging with fundamental (b.c.c.) reflections showed typical stacking fault or thin precipitate contrast for the same faults. The faults were identified as a /2〈111〉{100} type B2 antiphase boundaries. It is proposed that the formation of such anisotropic and non-conservative antiphase boundaries is related to the retention of excess Mo within the B2 matrix by rapid solidification.

Effect of composition and heat treatment on the fracture of melt-spun Fe-Al-Cr intermetallic alloys

Abstract Fe-Al-Cr alloys in the composition range 50–78 at% Fe, 0–20 at.% Cr and 15–50 at.% Al were rapidly solidified using chill-block melt spinning. Observations of tensile fracture surfaces of as-spun and subsequently heat-treated ribbons revealed transitions from ductile to cleavage to intergranular failure with increasing Al-content. These transitions are related to hardness and to dislocation-antiphase-boundary interactions. Rapid solidification improved ductility only at compositions where there is a significant suppression of ordering and/or formation of thermal antiphase domains. Implications for ductility improvement are discussed in the light of these results.

Effect of molybdenum substitution on B2 antiphase domain formation in rapidly solidified FeAlMo alloys

Abstract Studies at Sheffield have shown that the increase in B2 ordering temperature obtained with increase in aluminium content in rapidly solidified FeAlX (X = Cr, Mo) alloys results in an increase in B2 antiphase domain (APD) size. The present contribution concerns the effect of increasing molybdenum substitution for iron in such systems, which is shown to increase the ordering temperature but decrease the B2 APD size. The results are discussed in terms of the extension of solid solubility of molybdenum by rapid solidification, molybdenum segregation to the antiphase boundaries, and molybdenum diffusion in these alloys.

The role of antiphase boundary energy in influencing intergranular failure in iron aluminides

Abstract Antiphase boundaries (APBs) may lead to stress homogenization through dislocation/APB interactions and reduce the tendency for intergranular failure in ordered intermetallics. It is shown for ordered iron aluminides that these interactions are influenced by the APB energy. The interactions are maximized at intermediate (≃32 at.%) Al-contents. A high APB energy inhibits the formation of mechanical as well as thermal APBs and contributes to intergranular failure observed at high (40 at.% or more) Al-contents.

Mechanical Properties of Ordered Fe-Al-X Alloys

Fe-Al-X alloys (X = Cr, Mo) in the composition range 50 to 80 at. % Fe, 0 to 20 at. % X were rapidly solidified by chill-block melt-spinning. As-spun and heat treated ribbons were characterised by optical and electron microscopy and X-ray diffraction. Strengthening mechanisms were identified using microhardness measurements. Tensile fracture surfaces of ribbons reveal ductile to cleavage to intergranular fracture transitions with increasing Al-content. An increasing preponderance of cleavage fracture with increasing ternary substitution for Fe was observed.

The role of elasticity in determining antiphase boundary anisotropy in ordered intermetallics

The sources of elastic stresses at an antiphase boundary (APB) are discussed as well as the contribution of these elastic effects to APB energy. Attention is focused on TEM contrast recorded from anisotropic B2 APBs in rapidly solidified Fe-Al-Mo alloys, suggesting that the elastic stresses are minimized when APBs are oriented parallel to (100) planes and that the APB anisotropy increases with increase in elastic stress at APBs. It is pointed out, however, that only in the presence of extra factors such as the formation of APBs in an Fe-Al-Mo matrix supersaturated with Mo the elastic effects become important. 10 refs.

A comparison between containerless melting/twin-piston splat quenching and chill-block melt spinning as techniques for rapid solidification of late transition metal aluminides

Abstract Some results of comparative studies of the conditions for producing rapidly solidified samples of late transition metal aluminides by splat quenching and melt spinning are reported. These suggest that, while wettability of the substrate by the melt is a necessary condition for success in chill-block melt spinning, the reverse can be the case for splat quenching. The minimum mass for sample levitation for melting and twin-piston splatting was found to be strongly dependent on alloy composition in this group of aluminides.