Toughening and creep in multiphase intermetallics through microstructural control

The lack of engineering ductility in intermetallics has limited their structural applications, in spite of their attractive specific properties at high temperatures. Over the last decade, research in intermetallics has been stimulated by the discovery of remarkable ductilisation mechanisms in these materials. It has however often been the case that the process of ductilisation or toughening has also led to a decrease in high temperature properties, especially creep. In this paper we describe approaches to the ductilisation of two different classes of intermetallic alloys through alloying to introduce beneficial, second phase effects. The Ti2AlNb based intermetallics in the Ti-Al-Nb system can be ductilised by stabilising thebcc phase of titanium into the structure. The principles of microstructural and compositional optimization developed to achieve adequate plasticity, while retaining creep properties of these alloys, are described. An entirely different approach has been successful in imparting plasticity to intermetallics based on Fe3Al. The addition of carbon to form the Fe3AlC0.5 phase imparts ductility, while enhancing both tensile and creep strength

Processing of iron aluminides

A processing route comprising of air induction melting (AIM) with protective cover and electroslag remelting (ESR) for production of iron aluminide (Fe3Al) alloys has been developed. The use of protective cover during AIM results in the minimization of hydrogen gas porosity and a significant reduction in the impurity levels (S, O and N). Attempts have been made to further improve the ductility and hot workability through ESR. The AIM melted with high carbon ingots exhibit excellent tensile properties compared to the low (<0.074 wt.%) carbon ingots where cracks persist even after ESR. Processing maps were developed for vacuum induction melted as-cast binary and ternary (Cr and Mn) Fe3Al alloys. Processing maps of Fe3Al and Fe3Al–Cr exhibit a dynamic recrystallisation domain at temperatures >1123 K. Dynamic recrystallisation in iron aluminides is controlled by migration of interfaces. In the case of Fe3Al–Mn, dynamic recrystallisation is suppressed due to greater resistance to the migration of interfaces. Instead, large grain superplasticity is observed

Influence of hot working on electro slag remelted low alloy steel

A low alloy steel modified with vanadium addition (AISI 4340+V) has been electro slag remelted using calcium fluoride base slag. There was significant reduction of sulphur and oxygen in the metal during ESR. The size and number of nonmetallic inclusions were reduced while the remaining fine inclusions were distributed uniformly in the refined metal. The mechanical properties of cast ESR ingot were comparable to forged electrode in the longitudinal direction and much superior in the transverse direction. The influence of hot working on solidification structure and mechanical proprieties of low alloy steel has ben investigated over a reduction ratio in the of 1:1 to 7:1. Inspite of breaking the cast dendritic structure progressively by hot-forging, there was no significant improvement in tensile properties of ESR steel. However. the impact strength had shown considerable improvement with hot forging