Electron Beam Analysis of Minor Element Site Occupancies in Ternary, Quaternaryand Higher Order Duplex (gamma + alpha2) Titanium Aluminides

摘要

Duplex gamma/alpha2 Ti-A1 alloys containing minor alloying additions of V Cr, Nb,Ta and their combinations have been examined by microanalytical electron microscopy to determine quantitatively the extent to which the additional alloying atoms partition between the gamma and alpha2 phases; affect the state of atomic order, and occupy either Ti or A1 host sublattice sites. Partitioning between the gamma and alpha2 phases was determined using conventional energy-dispersive X-ray analysis. The state of atomic order for each alloy was determined from measurements of relative positions of the quantum lines of electron intensity present in diffracted beams of the higher-order Laue zones in convergent beam electron diffraction patterns. Site occupancies of alloying elements were determined by a new fully quantitative statistical method of atom location by channelling-enhanced microanalysis. The method is computerised with automated control of the electron beam, collection and analysis of the X-ray intensity data contained in incoherent channelling patterns. To cater for analysis of multiple alloying additions within a host lattice, a full energy dispersive X-ray spectrum is recorded for each pixel in incoherent channelling patterns. This has enabled the investigation of cross-elemental effects on atomic site occupancy.