Atomistic studies of dislocation-interface interactions in lamellar Ti-Al

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The metallurgy of high temperature alloys presents challenges both at a technical and at a fundamental level. Alloys that are used in turbines at high temperature have to resist high stresses over prolonged timescales and high flow rates of noxious and corrosive gases. Moreover they are often found in critical applications where failure may lead to human distasters. This is because as well as finding applications in electricity generation, high temperature alloys are now found in the high temperature zones of jet engines, both civil and military.The superalloy nickel aluminide was invented and first produced by the Mond Nickel Company in 1941. It is still the workhorse of the industry despite extensive searches for lighter alloys having the same mechanical and chemical properties. The difficulty clearly resides in the need to meet the most stringent criteria simultaneously: high temperature oxidation and sulphidation resistance; high temperature strength and creep resistance; and finally low temperature ductility. The latter requirement is essentially a manufacturing, not a service condition. Unless a metal has at least a few percent room temperature ductility it's just too difficult to work with in manufacture and installation. A component may not even survive being dropped on a concrete floor!Titanium aluminides have recently emerged as promising new candidates to replace the excellent, but weighty gamma / gamma-prime nickel based superalloys. Our proposed work is concerned with their plasticity and ductility at room temperature. Although monolithic TiAl intermetallic is very brittle, a remarkable alloy produced by certain heat treatments shows ductility at least at the few percent level. This alloy, called (polysynthetically twinned) PST-TiAl contains grains (crystals) with a microstructure of layers ( lamellae ) of two phases, one of these phases (gamma-TiAl) being itself separated into layers having different crystal orientations. This sandwich structure is thermodynamically stable and is now the object of intense study in academia and industry worldwide.Our remit is atomistic simulation. We take large collections of atoms in a computer, bonded together by some model of interatomic forces (this may be fully quantum mechanical, or a classical ball and spring, or anything in between). Then we observe their behaviour as they relax under the influence of interatomic forces (molecular statics) or move at some temperature following Newton's equation of motion (molecular dynamics). Here we do particularly difficult simulations, namely of crystal dislocations. These are two dimensional crystal defects, the carriers of slip (plastic deformation) in metals. Such simulations are demanding because of the complexity of these defects, greatly amplified in alloys compared to pure metals; and because of the nature of the associated long ranged stresses. The rewards of a successful simulation are very great. We may view directly in the computer the motions of dislocations. We may follow their progress through perfect crystal and as they interact and possibly penetrate the lamellar boundaries. We will learn how slip is transmitted within and between lamellae and uncover the secret of why the PST titanium aluminide is ductile. We propose to use results of atomistic simulations to construct a multiple pile up model to predict behaviour at a length scale on the order of the size of a grain. This is called bridging the length scale gap. Armed with this model one should then be able to improve ductility and we shall be able to make such suggestions to the practical alloy designers.Further reading: Alloys by Design, Tony Paxton, Physics World, vol 5, No 11, p 35(Nov 1992) Electron Theory in Alloy Design, Edited by D G Pettifor andA H Cottrell, (Institute of Metals, 1992) Interatomic Forces in Condensed Matter, Mike Finnis, (OUP 2003)