Microstructural and thermal stability of a Ti-43AI alloy containing dispersoids of titanium di-boride
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RESULTS AND D I S C U S S I O N
A. As-Cast Condition
The microstructure of the as-cast material consists of grains of a2 + y lamellae with a dispersion of randomly oriented particles of TiB2. The microstructures of the Ti-43A1 alloys without and with the addition of TiB2 particles are shown in Figures 2(a) and (b), respectively. The microstructures differ with respect to grain size. Grains in the TiB2-containing alloy are much smaller than in the alloy without TiB2. The TiB2 particles act as an innoculant and provide extra nucleation sites during solidification and, therefore, a finer grain size. B. As-Extruded Condition
Extrusion of the alloy changes the microstructure of the alloy in various ways. The TiB2 particles have a randomly oriented rod-type morphology in the as-cast condition. After extrusion, these rods are aligned with the major axis (predominantly [0001]) along the extrusion direction (shown with an arrow in Figure 3).
Extrusion in the c~ + y temperature range allows the formation of proeutectoid 3/phase having an equiaxed grain morphology (for example, see A through C in Figure 4). Such equiaxed 3/grains were not present in the cast material, an observation consistent with those of other researchers. [5-8] The proeutectoid y is the equilibrium phase and should be expected to form under equilibrium cooling, but since the kinetics of precipitation of y in an a matrix are extremely slow, t7] no proeutectoid y was observed in the as-cast condition. Shong et al. r81 reported the absence of proeutectoid 3' when the cast material was cooled at rates as slow as 1 K/min. The formation of the proeutectoid y-phase grains observed in the as-extruded material may have been facilitated by the increased diffusivities of atoms during thermomechanical working of the composite. Using an SEM, the microstructure of the matrix was shown to consist of (a) lamellar a2 + y regions separated by chains of y-phase grains and (b) equiaxed y- and a2-phase grains. The two kinds of structures are marked as A and B, respectively, in Figure 5. Transmission electron microscope micrographs of regions similar to A and B in Figure 5 are shown in Figures 6(a) and (b). The d E and y lamellae have the conventional orientation relationship: (0001)~2 II (111}v and (1120)a 2 II (110) w The lamellar regions are separated by chains or "necklace"-type regions of 3' phase. The y grains have roughly the same orientation as the y lamellae in the a2 + y regions. The equiaxed 3/grains in the chains are separated by low-angle boundaries. The matrix/TiB2 particle interfaces are faceted and sharp with no visible evidence of the presence of any reaction zone. The TiB2 particles contain a moderate number of stacking faults. Figures 7(a) and (b) show bright- and dark-field TEM images from one of the TiB2 particles. No attempt was made to analyze the nature of these stacking faults. C. Extruded and Aged Condition 1. Microstructural changes in the matrix
Fig. 1 - - S E M micrograph of TiB2-containing alloy at an intermediate stage of dissolution of tita
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