INFLUENCE OF MICROSTRUCTURE ON TENSILE PROPERTIES OF THE TITANIUM ALLOY IMI 834 FROM RT TO 650 DEG C.

摘要

The microstructures of the alloy 834 in the four heat-treated conditions, are clearly shown by the optical micrographs in Fig. 1. The beta structure has been shown by TEM~9 to retain beta between the alpha platelets in both the conditions, solution treated in the alpha + beta phase field as well as in the beta phases field. It has recently been established by Nidhi~6 that there is precipitation of the ordered Ti_3Al phase, both in the alpha + beta as well as beta solution treated samples cooled at slow rate like that in air and furnace. However, the Ti_3Al phase does not precipitate if the specimens are subjected to effective quenching in water, following solution treatment. It is known that this alloy 834 is subjected to the so called stabilization treatment, by aging at 700 deg C for 2 hours to relieve the stresses developed due to solution treatment and stabilize the microstructure. It has been shown~6 that the stabilization treatment leads to precipitation of Ti_3Al phase even in the water quenched specimen, in which initially it was absent. Ramachandra et al~9 have studied in detail precipitation of silicides and also of the Ti_3Al phase in this alloy and established the range of temperature for the precipitation of silicides in the alpha + beta treated and beta treated material. Since the material in the present investigation, following four different solution treatments, was subjected to stabilization treatment, i.e. aging at 700 deg C for 2 hours, it invariably contained Ti_3Al precipitates and silicides. However, characterization of these phases has not been carried out in the present investigation. The highest yield strength in the alpha + beta ST-OQ-A condition may be attributed to fine size of transformed alpha platelets. It is relevant to mention here that volume fraction of the primary phase is almost comparable in both alpha + beta ST-OQ-A and alpha + beta ST-AC-A condition. However, the size of the transformed alpha platelets is finer in the more rapidly cooled condition, resulting from quenching in oil. Further, in view of the difference in the solution treatment of 20 deg C for the alpha + beta ST-AC-A (1010 deg C) and alpha + beta ST-OQ-A (990 deg C), the concentration of the alpha and beta stabilizing elements is to be different in the two conditions because of partitioning of the alpha, beat stabilizing elements.