Materials Science and Engineering, A 168 (1993) 23-28 23 Structure and mechanical properties of high-temperature titanium alloys after rapid heat treatment O. M. Ivasishin Institute for Metal Physics, Kiev (Ukraine) G. Lfitjering Technical UniversityHamburg-Harburg, 2100 Hamburg 90 (Germany) Abstract In this study a new approach to optimizing the mechanical properties of high-temperature titanium alloys was developed. It is based on using rapid heating of equiaxed structures into the//-field to achieve a fine/3 grain size (less than or equal to 100/~m), transforming on subsequent cooling into a fully lamellar structure. This fine/3 grain size is an order of magnitude smaller than the grain sizes achieved by conventional furnace//-treatment. Structures and mechanical properties (tensile, fatigue and creep) of high temperature alloys after rapid and conventional furnace heat treatments were compared. The results are discussed in terms of structure-property relationships. 1. Introduction Titanium alloys in hot sections of advanced jet engines offer significant weight reductions compared with nickel-base superalloys, resulting in an increase in engine fuel efficiency and thrust-to-weight ratio. A combination of high tensile and fatigue strength values with good creep resistance is important for elevated temperature applications. Structure requirements to maximize these properties are often contradictory. Creep resistance of alloys with lamellar morphology is higher, while globular structures offer better tensile and fatigue properties. All the properties can be balanced to some extent by processing bimodal structures con- sisting of equiaxed primary a-grains in a lamellar ( a +/3)-matrix [ 1]. The aim of this study was to optimize the properties of high-temperature alloys using rapid heat treatment which included rapid heating above the fl-transus temperature and subsequent cooling. It was shown previously [2, 3] that by starting from equiaxed struc- tures it is possible to produce fully transformed lamel- lar structures with fine fl-grains, changing the morphology and the size of the lamellar structure by controlling the cooling rate. 2. Materials and experimental procedure Typical near-a type alloys developed in the USA (Ti 6242) and Russia (VT 18 Y) for high-temperature application were studied. The chemical compositions of the alloys are given in Table 1. The starting equiaxed microstructures for both alloys were produced by rolling in the (a +/3)-field and subsequent annealing for 6 h at 940 °C (Ti 6242) or 6 h at 900 °C (VT 18 Y ). Specimen blanks (7 x 7 x 60 mm 3) were taken from these materials and /3-heat treated either con- ventionally in a furnace at 1050 °C for 1 h (hereafter designated FH), or rapidly by a direct resistance method (hereafter designated RH). The parameters of rapid heating were chosen from the influence of the heating rate on the fl-transus temperature (Fig. 1) and /3-grain size. To achieve the same /3-grain sizes (approximately 100 /am) the alloys were heated at a rate of 50 °C s -1 to 1150 °C (Ti 6242) or to 1130 °C (VT 18 Y), overheating above the corresponding fl- transus temperatures by 25 °C and 70 °C respectively. After /3-treatment the blanks of Ti 6242 were either water quenched (hereafter designated WQ) or air cooled (hereafter designated AC). Water spray quench- ing was used after rapid heating while samples were immersed in water after conventional furnace heating. Only AC was done for the VT 18 Y blanks. Final heat treatments were 8 h at 590 °C (Ti 6242) or 6 h at 600 °C (VT 18 Y). After the final heat treatment specimens for mechanical testing were machined. Tensile tests were performed at room temperature and 450 °C with an initial strain rate of 8 x 10 4 s-1. Fatigue tests were done in air on electrolytically polished hourglass shaped specimens by rotating beam loading for high Elsevier Sequoia