Characterization and optimizatin of mechanical properties of massively transformed Ti-46Al-9Nb sheet material
Zusammenfassung der Projektergebnisse
2.1. general results and the scientific achievement to application The a —»^ ym massive transition is one ofthe major phase-transformation modes in TiAl alloys with Al-content of around 47 at.% [1]. This kind of reaction was observed when the alloy was rapidly cooled down from the high temperature a-phase field and transformed into a single Ym-phase with supersaturated Ti-solution, other than into the equilibrium a2 + Y two-phase mixture. Massive transformation occurs when upon undercooling a driving force for a polymorphous transformation into the Y-phase exists, and the aaivation energy of the nucleation and growth can be overcome by thermal fluctuations. When the necessary undercooling is attained, nucleation of Ym occurs as a heterogeneous process, which preferentially takes place at the grain boundaries and triple points, as has been confirmed by numerous microscopy observations [2-4]. In this project, the massive transformation ofthe so called 3"* generation TNB alloys, with the composition of Ti-46Al-9Nb (all concentrations given in at.%), was investigated on an experimental base. In this work, for instance, completely massively transformed ym-phase was achieved by air cooling of the sheet samples, which can be easily accessed technological applications. Furthermore, massive transformation experiments were also carried out under a variation of heating temperatures and cooling rates to achieve microstructures with different Ym-content and grain sizes, in order to optimise the mechanical properties by means of controlling of microstructures. Additional efforts, also oriented towards optimisation of the mechanical properties by controlling of precipitation behaviour, were realized by suitable subsequent heat treatment procedures after massive transformation. In this work, as a result of the subsequent annealing of the massively transformed samples, novel types of microstructures, namely thin crossed-lamellar and thick cross-lamellar microstructures, were obtained, which are promising for commercial utilisations. Together with the controlling of microstructures through massive transformation, the study of mechanical properties of the Ti-46Al-9Nb alloy was another focus of the project. Since this type of alloys was designed for the components that served in an elevate temperature environment, their high temperature properties, e.g. high temperature strength, ductility and creep resistance properties, would be important for application. Therefore, high temperature tensile tests and creep tests of the sheet samples with different massive transformation and subsequent annealing processes have been carried out. The aims ofthe tests were to determine fundamental properties of the massively transformed material with regard to its possible application. High temperature tensile tests (at 800°C, 900°C and 1000°C) were conduced on the sheet materials with different microstructures. Results showed that at a moderate strain rate (about 10' l/s) and a relative high temperature (900''C and above), the samples exhibited a good combination of strength and plastic deformation, which ensures the possibility in their commercial applications. The influence of microstructures on the creep behaviour of the Ti- 46Al-9Nb was studied under the stress of 225 MPa and at the temperature of 700°C. After long term creep tests, the samples with initially fiilly Ym-microstruaure showed favourable creep resistant properties, which are even better than the best values of the TiAl alloys published up to now gained by coarse-grain and fine-space fully lamellar microstructure [5]. Moreover, experimental results revealed that the creep strength can be fiirther enhanced by additional stabilising annealing. The investigation on phase transformation and mechanical experiments were supported by metallographic investigations and X-ray diffraction aiming to comprehensive characterisation of the deformation and fracture mechanisms, in particular with respect to their morphology, the grain size, the volume fraction of phases and their distribution. In addition, surface analyses were performed to determine mechanism of fi^acture. The gained information would be not only oflarge scientific importance because of their contribution to the current status of knowledge about massive transformation in TNB materials, but also of a great practical meaning for possible future high-temperature applications of TiAl-based materials. 2.2. Outlook of future work and description of possible applications The goal of this project is to investigate the potential of the massively transformed Ti-46A1- 9Nb sheet material for high temperature engineering applications. From the investigations performed so far, it has become clear that the massive transformation in TNB alloys occurs in a very large range of heat-treatment parameters and, thus, it might be of a great technical importance for the course of fabrication and deformation processes as well as for controlling ofthe microstructure-related properties. Therefore, possible applications to take advantage of the massive transformation should be further investigated. At the moment, a promising application of massive transformation seems to be the grain refinement [3,6,7]. TiAl is mainly fabricated as cast ingots due to a great potential in reducing overall costs. This method always entails problems such as large size of colonies and heterogeneity in the properties. Therefore, simple and efficient method of microstructure refinement is needed for TiAl castings. It has been shown that annealiiig of as-cast material above a-transus temperature followed by accelerated cooling leads at first to dissolution of as-cast microstructure and formation of pure a-grains, and subsequently to the massive transformation into the Ym-microstructure with much finer morphology. Moreover, since during massive transformation the orientation relationship of Ym-phase to the parent a-phase is minimised by formation of twins [8,9], it can be expected that Ym-phase does not form any privileged orientation (texture) as well. Consequently, the anisotropy of mechanical properties, which is a typical feature for the lamellar microstructure, should be significantly reduced or even completely removed in case ofthe massively transformed one. These results demonstrate that a massively transformed microstructure can be also used as a pre-cursor to obtain novel microstructures and potenfially to achieve advanced materials properties. Another method, to utilise the extraordinary susceptibility of the TNB alloys to massive transformation, can be to use the massively transformed microstructure or the subsequently treated thin crossed-lamellar or thick cross-lamellar microstructure in the finished products. We have shown in this work that these kinds of microstructures exhibit good high temperature strength, adequate ductility and very promising creep strength when compared to other finegrained microstructures. Moreover, by means of stabilisation annealing, these properties can be further increased, which is ofa great meaning for future application.
