Synthesis of in-situ TiAl-based composites from elemental powders
Alloys and composites based on the intermetallic compound TiAl are emerging as an important class of light-weight, high-temperature structural materials. Recently, it has been recognized that these alloys have applications in industries, such as the automotive industry, where cost is frequently a major concern in materials selection. However, for these alloys to be used in this type of application, new low cost methods for high volume component fabrication are required. One potential fabrication approach is reactive synthesis (also termed combustion synthesis). This technique involves initiating an self-propagating, high-temperature synthesis (SHS) reaction within an intimate mixture of elemental powders. This process has been used to fabricate intermetallics, ceramics and in-situ composites in the form of powders and dense monoliths. SHS reactions tend to initiate at low homologous temperatures of the forming compound (for aluminides near or at the melting point of Al, 660°C), and tend to go to completion in a short period of time (i.e., a few seconds). For some compounds, particularly aluminides, the reaction is ac companied by the formation of transient liquid phases. These factors can reduce the required processing parameters (time, temperature and pressure) needed to produce dense products by reactive synthesis techniques compared to conventional powder metallurgical approaches. This paper characterizes the reactions that occur and resultant microstructures of TiAl based composites fabricated from ternary mixtures of elemental Ti, Al and B or Si powders. Mixtures of the elemental powders were prepared corresponding to TiAl reinforced with 0, 10 25, 60 and 100 vol. pct. Ti5Si3 or TiB2. The powders were consolidated by reactive hot-pressing (at 1000°C and 20 MPa for 1 hr). It was found that the composites produced from Ti, Al and Si powders were dense, and the elemental powders transformed to the target phases of TiAl and Ti5Si3. Whereas, composites produced from the Ti, Al and B powders were porous and inhomogeneous, that is several aluminide (TiAl, Ti3Al and TiAl3) and boride phases (TiB2, AlB12, TiB) formed during hot-pressing. The different behavior observed by the two ternary systems can be attributed to both reaction sequence and phase diagram considerations. First, Differential Thermal Analysis (DTA) revealed that an endothermic reaction associated with the formation of Al-Si eutectic occurs prior to the initiation of an SHS reaction within the mixtures of Ti, Al and Si powders. No such pre-reaction melting occurred within the mixtures of Ti, Al, and B powders. Thus, the "extra" transient liquid phase that formed during the reaction between Ti, Al and Si systems enhances diffusion (hence homogenization) and densification within this system during reaction processing. Also, an examination of phase diagrams reveals that there exists no Al-Si compounds to compete with the formation of titanium-aluminide and titanium-silicides during reactions between Ti, Al and Si powders. However, there are several aluminum-boride phases that can compete with the formation of titanium-aluminide and titanium-boride during reactions between Ti, Al and B powders. The implications of this study is that TiAl-based composites can be designed for densification during reactive processing.
- Research Organization:
- Albany Research Center (ARC), Albany, OR (United States)
- Sponsoring Organization:
- USDOE - Office of Fossil Energy (FE)
- DOE Contract Number:
- None cited
- OSTI ID:
- 923263
- Report Number(s):
- DOE/ARC-1997-023; TRN: US200804%%1033
- Resource Relation:
- Conference: 1997 TMS Annual Meeting, Orlando, FL, Feb. 9-13, 1997
- Country of Publication:
- United States
- Language:
- English
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