Reaction sintering of cold- extruded
- PDF / 3,883,466 Bytes
- 10 Pages / 597 x 774 pts Page_size
- 54 Downloads / 183 Views
I.
INTRODUCTION
T I T A N I U M aluminide alloys based on the intermetallic compound TiA1 have become more interesting owing to their potential applications as high-temperature materials. These alloys exhibit low densities, high melting temperatures, good mechanical properties, and environmental resistance at elevated temperatures. They are mostly produced using ingot metallurgy. However, earlier work has shown that it is possible to prepare TiA1based alloys by reactive powder metallurgy (RPM). r~,21 In the RPM processing, the cold-extrusion technique is used to consolidate the elemental Ti- and Al-powder mixtures with or without additives. The as-extruded material exhibits no intermetallics and can therefore be easily machined to complex shapes, which are then reactively sintered to get the desired intermetallics. In this way, the poor workability of titanium aluminides can be evaded. During reaction sintering, more A1 atoms move into Ti particles and lead to formation of pores if no pressure is applied. The final pore size is proportional to the size of A1 regions. Normally, hot isostatic pressing (HIP) is applied to compress pores. An HIP treatment requires encapsulation of extruded pieces due to their open porosity and is therefore relatively expensive. It is thus of practical importance to reduce the size of A1 regions and the open porosity of extruded pieces. This can be achieved by increasing the extrusion ratio ~, which is defined as the area ratio of the specimen cross sections before (A0 and after extrusion (A2). In this study, we report on the influence of the extrusion ratio q~ on pore formation in Ti-48A1 during both pressureless reaction sintering and HIP without encapsulation. Two-step sintering, i.e., combination of pressureless sintering and HIP, was also conducted.
II.
EXPERIMENTAL
Elemental titanium and aluminium powders of sizes smaller than 100/~m, with purities of 99.8 pct and 99.9 pct, respectively, were mixed in air to the desired composition Ti-48 at. pct A1. The powder mixture was uniaxially pressed at room temperature with 500 MPa to a green compact. This compact was cold extruded (at room temperature) using different extrusion ratios of ~ = 17 and 25. A considerably higher extrusion ratio was obtained by bundling pieces of a previously extruded rod (Mueller et al. t31) and by extruding the bundle again to give a total extrusion ratio of 350. Altogether, three extrusion ratios were applied in this study: q~ = 17, 25, and 350. The extruded specimens were reactively sintered under different conditions: in a vacuum ( ~ 10 -3 N / m 2) furnace for 6 hours at 600 ~ 1000 ~ and 1350 ~ and in HIP equipment at 1350 ~ h / 2 0 0 MPa. A heating rate of 20 ~ was used in both cases. A combination of both pressureless sintering and HIP, i.e., first pressureless sintering and then HIP or vice versa, was also conducted. As-extruded specimens as well as specimens reactively sintered under different conditions were examined using light microscopy (LM). A quantative determination of the porosity, as well
Data Loading...
