Microstructural characterisation of Ti/Al laminate

The literature on the processing, microstructure and properties of multilayered materials is briefly discussed. The microstructures of Ti/Al laminate materials with nanometre or micron scale layers of Ti and Al, which were prepared by physical vapour deposition using thermal evaporation and electron beam evaporation, were studied. The former laminates (designated RC) had nanometre thick layers of Ti in an Al matrix and were deposited some time ago. The latter materials (designated EBRC) were developed during the research described in this thesis using an apparatus which was designed and purpose built for the processing of laminate alloys and composites. With the exception of one EBRC laminate where yttria layers were deposited between the Ti and Al layers, all other EBRC laminates were alloys based on Ti/Al layers. The thermal stability of the microstructures was evaluated on two EBRC laminates, which were chosen to represent micron and nanometre layering. All the EBRC laminates were evaluated for their mechanical properties. The microstructure of all the EBRC laminates exhibited the typical defects found in physical vapour deposited materials such as spits, substrate initiated flaws and compositional banding. In all the RC laminates and the EBRC laminates with nanometre thick Ti and Al layers the microstructure depended on T/T.;in accordance with the model of Movchanand Demchishin, where TM represents the melting point of Al and Ti respectively for each group of laminates. For the EBRC laminates with micron thick Ti and Al layers the microstructure did not foUow the expected dependence on T/T.;suggesting that the thickness of the second layer is important in the microstructural evolution. All RC and EBRC laminates showed evidence of intermetallic formation in the as deposited microstructure, in particular TiAl3, Ti3Al and TiAl. F.c.c. Ti was also identified in RC and EBRC laminates with nanometre thick Ti and Al layers. The lattice parameter of f.c.c. Ti was 0.436nm, in agreement with earlier reports. EBRC laminates with micron thick layers of Ti and Al retained the layered structure after 2 hours at 823K while the layered structure in EBRC laminate with nanometre thick layers of Ti and Al was destroyed after Imin at 573K. The latter was accompanied by the formation of TiAls with a heat of formation substantially lower than the heat of formation of TiAl3 in bulk alloys. Stresses in EBRC laminates with nanometre thick layers of Ti and Al varied from 550MPa to l.BGPa. It is suggested that these stresses play an important role in interdiffusion between the layers and that they cannot be ignored when considering the development of the microstructures of the laminates. Earlier reports that there is an enhancement of modulus and hardness in laminates with thin layers have not been confirmed in this thesis.