Alloy Design

Abstract

In the TiAl literature, alloy and suitable processing technology development has played a signifi cant role for at least the last 20 years. Advances have been made, so that the strength levels achieved in technical alloys over the last few years are around double those of 20 years ago, while maintaining the same levels of roomtemperature ductility. Improvement of other properties such as creep and oxidation resistance has been achieved through alloying and the use of halogens, although improvement of other properties, such as toughness, through composition rather than microstructure has met with less success. It now seems well acknowledged that there is no universal alloy composition that is suitable for all applications; rather alloy composition must be tailored together with processing to achieve the required properties for specifi c components.Accurate information concerning the phase relationships in both binary Ti -Al and in multicomponent systems is very important in designing alloys and suitable heat treatments. This is because for a given alloy, properties are very dependent on microstructure, and microstructure is highly sensitive to composition. The high microstructure/composition sensitivity is perhaps one of the most challenging aspects in producing components from industrially produced ingot stock; as compositional variation within ingots can lead to fi nished components with a range of microstructures and thus properties. This is obviously unacceptable, especially for safety -relevant aerospace parts. A good overview concerning the infl uence of different alloying elements was published in 1993 [1] . This chapter will briefl y outline how alloys have developed over the last 20 years and discuss the different alloydesign philosophies. Effect of Aluminum ContentIn binary alloys, the aluminum level determines the initial phase to precipitate and the subsequent phase transformations that occur on solidifi cation. According to Huang and Hall [2] who published work in 1991 on binary alloys within the range Ti -(46 -60)Al made via rapid -solidifi cation processing, ductility is highest for duplex microstructures where the heat treatment is roughly in the middle of the 13 Gamma Titanium Aluminide Alloys: Science and Technology, First Edition.