During the past decade considerable research efforts have been directed towards achieving balanced engineering properties of two-phase γ-titanium aluminide alloys for future applications as structural materials. For optimization of mechanical properties such as yield and creep strengths, tensile ductility and fracture resistance, a basic understanding of the temperature dependent micromechanisms of plasticity and fracture, and their interplay with various microstructural constituents is required. In this review article, the current knowledge on dislocation types and slip systems, the development of deformation substructures, factors controlling the mobility and multiplication of dislocations, interface related plasticity, solid solution and precipitate strengthening mechanisms as well as microscopic aspects of creep and fracture will be addressed. These topics will be related to specific microstructures and associated engineering properties.

Microstructure and deformation of two-phase γ-titanium aluminides

Extensive progress and improvements have been made in the science and technology of gamma titanium aluminide alloys within the last decade. In particular, our understanding of their microstructural characteristics and property/microstructurc relationships has been substantially deepened. Based on these achievements, various engineering two-phase gamma alloys have been developed and their mechanical and chemical properties have been assessed. Aircraft and automotive industries arc pursuing their introduction for various structural components. At the same time, recent basic studies on the mechanical properties of two-phase gamma alloys, in particular with a controlled lamellar structure have provided a considerable amount of fundamental information on the deformation and fracture mechanisms of the two-phase gamma alloys. The results of such basic studies are incorporated in the recent alloy and microstructure design of two-phase gamma alloys. In this paper, such recent advances in the research and development of the two-phase gamma alloys and industrial involvement are summarized.

Gamma Titanium Aluminide Alloys

Mechanical tests on a polycrystalline γ Ti47A151,Mn2 alloy between ‐ 150 and 1000°C and subsequent microstructure analysis clearly distinguish between three temperature domains which correspond to different deformation mechanisms. At low temperatures, the motion of superdislocations dragging faulted dipoles is rate controlling. At intermediate temperatures where a stress anomaly is observed, screw simple dislocations are observed with cusps, the density of which increases with increasing temperature. A description of the simple dislocation motion based on these observations is developed. The glide of a screw simple dislocation in two planes through a Peierls mechanism is believed to be the intrinsic source for the formation of pinning points. It is proposed that these pinning points can be erased by lateral motion of superkinks: an unzipping process. A model for this pinning-unzipping mechanism will be fully developed in part 11. At high temperatures, the climb of simple dislocations appears to c...

Modelling the flow stress anomaly in γ-TiAl I. Experimental observations of dislocation mechanisms

Effect of carbon addition on the microstructures and mechanical properties of γ-TiAl alloys

The plastic deformation behaviour of polysynthetically twinned (PST) Ti-Al with three different orientations has been studied in tension and compression as a function of temperature in a range from −196 to 1100°C. With increase in temperature, the yield stress decreases rather rapidly at low temperatures and then decreases gradually at intermediate temperatures for all orientations studied although the temperature dependence at low temperatures is less significant for an orientation where shear deformation occurs parallel to the lamellar boundaries. When the loading axis is perpendicular to the lamellar boundaries, the yield stress again rapidly decreases with increasing temperature at high temperatures. This is also the case for PST Ti-Al whose lamellar boundaries are parallel to the loading axis, although a small and broad anomalous yield stress peak is observed at 800°C for this orientation. When the lamellar boundaries are inclined at an intermediate angle to the loading axis, the yield stres...

Temperature dependence of yield stress, tensile elongation and deformation structures in polysynthetically twinned crystals of Ti-Al

Comparisons of the deformation mechanisms responsible for the flow stress dependence on temperature have been made for two alloys with atomic compositions Ti[sbnd]48Al and Ti[sbnd]47·5Al[sbnd]2·5Cr. Two different high values of flow stress have been observed, namely at −196 and 420°C. The high flow stress values measured at low temperatures have been attributed to a spontaneous blocking mechanism of the superdislocations present. However, the latter disappear at high temperatures and the anomalous strengthening effect that occurs between room temperature and 420°C has been described by the twinning activity and emission of single ½ 〈110] dislocations. An increase in twin density with increasing temperature produces increased stress concentrations at twin intersections from which a higher density of single dislocation can be emitted. An increase in the forest density produces multiplication of cutting events, leading to an increase in jog density along the dislocation line. The latter appears to b...

M. A. Morris

Papers

Dislocation configurations in two-phase Ti-Al alloys.III Mechanisms producing anomalous flow stress dependence on temperature