Mechanical Anisotropy of a Gamma Titanium Aluminide Alloy After Hot Extrusion
- PDF / 1,606,081 Bytes
- 7 Pages / 393.3 x 625.5 pts Page_size
- 101 Downloads / 223 Views
extruded at 1380 'C was investigated in the as-worked condition. The detailed processing history of the three different material conditions obtained is given in Table I. The 02 and N 2 contents amounted to 600 J-g/g and 80 gg/g, respectively, in the as-cast state, and did not change over the entire processing route. For microstructural characterisation optical microscopy using polarized light, scanning electron microscopy in the backscattered electron mode on electrolytically polished specimens and transmission electron microscopy were applied. For TEM a Philips CM30 electron microscope was used operating at 300 kV. RESULTS AND DISCUSSION Microstructureof Extruded Materials
The microstructure of the cast starting material consisted predominantely of lamellar colonies with a size of some hundred gtm. In addition, small fractions of borides with a "lacy-like" morphology and y grains were observed in this material. After extrusion at 1250 'C an equiaxed and obviously fully recrystallized microstructure was obtained which however exhibited stripes of small and coarser y grains (Fig. 1). Similar banded microstructures often were observed after hot-working of TiAl alloys and were attributed to originate from the segregation of Al on peritectic solidification [3 - 5]. By a subsequent annealing treatment at 1030 'C the microstructure did not change, except for an increase of the grain size (material 1, table I). In this state, mean linear grain sizes of 3.7 gtm and of 2.9 gtm were determined parallel and perpendicular to the extrusion direction, respectively. The material extruded at 1250 'C was also subjected to a heat treatment at 1380 'C (material 2, table I). Although this temperature is well above the a transus temperature the obtained microstructure was not fully-lamellar but contained a significant amount of y grains in particular at colony boundaries (Fig. 1). These grains often showed a typical elongated morphology. The formation of 03phase between a grains at high tem-
material I 2
Table 1: Processing history and microstructure of the materials investigated. processing microstructure extrusion at 1250 *C, banded microstructure, fine and coarser grains,
HT 2 h/1030 °C 0
+6 3
0.9 vol.% a, phase, mean lin. grain size: 3.3 gtm
HIP 4 h/I 180 C/ 140 MPa, predominantely lamellar microstructure, extrusion at 1250 'C, mean fin. colony size: 150 gm HT 30 min/1380 TC h/900 °C
extrusion at 1380 IC
predominantely lamellar ms, 5.6 vol.% ot phase, mean lin. colony size: 58 g.tm, lam. spacing: 340 nm
Figure 1: Microstructure of the investigated materials processed by hot extrusion: a) material 1, b) material 2, c) material 3. The extrusion direction is horizontal in the micrographs. KK5.13.2
peratures could be one possible reason for this microstructural feature. On cooling, the j phase might directly transform to y which would result in intercolony y grains as observed and discussed in detail by Cheng and Loretto [10] and Krishnan et al. [11]. The third material investigated here (see table I) which was extruded at
Data Loading...
