Microstructure of Ti-Al-Si alloys prepared by vacuum induction melting and hot isostatic pressing

Session

PSA.2 - Metals & Alloys

Authors

Ing. Anna Knaislová (1), Bc. Jiří Linhart (1), doc. Pavel Novák (1), doc. Ivo Szurman (2), Ing., Ph.D. Tomáš Čegan (2), Ing. Jan Juřica (2), prof. Dalibor Vojtěch (1)

Affiliations

1. University of Chemistry and Technology, Prague
2. VSB - Technical University of Ostrava

Keywords

hot isostatic pressing, intermetallics, microstructure, vacuum induction melting

Abstract text

Intermetallics based on Ti-Al system are a relatively new class of high-temperature construction materials that combine unique physical and mechanical properties for use in aircraft engines, industrial gas turbines, or automotive industry [1, 2]. They are characterized by low density, good mechanical properties at high temperatures and good oxidation resistance [3, 4]. Addition of silicon into the Ti-Al alloys improves oxidation resistance and ultimate tensile strength in compression [5, 6]. 

The complicated production limits wider use of these Ti-Al-Si intermetallic alloys. Previous research showed that production of these alloys by powder metallurgy is not suitable. The Ti-Al-Si alloys prepared by reactive sintering and Spark Plasma Sintering have heterogeneous and porous microstructure with coarse grains [7]. The mechanical alloying (MA) followed by Spark Plasma Sintering (SPS) was tested for improving the microstructure [8]. The microstructure of Ti-Al-Si alloys after MA+SPS is very fine-grained and non-porous with better mechanical properties, such as ultimate tensile strength in compression and hardness. However, the biggest problem of these alloys, their fracture toughness, was not solved. The alloys are very brittle at room temperature [9]. For this reason, the preparation of these alloys by melting metallurgy was tested [10]. Nowadays, melting metallurgy is the most commonly used for the preparation of metals. The melting metallurgy of intermetallic alloys, such as Ti-Al-Si alloys, is very complicated due to the high melting point of intermediary phases, the reaction of the melt with the melting crucibles and the melting atmosphere or the occurrence of casting defects, which must be treated by subsequent hot isostatic pressing (HIP) [11, 12]. 

This work deals with intermetallic alloys based on Ti-Al-Si with various content of aluminium and silicon which are produced by centrifugal casting in the induction vacuum furnace Linn Supercast – Titan followed by hot isostatic pressing and these results will be compared with the same alloys prepared by powder metallurgy (reactive sintering, mechanical alloying, Spark Plasma Sintering).

The microstructure and phase composition of TiAl15Si15 and TiAl35Si5 (wt. %) prepared by melting metallurgy are described. Due to the high melt overheating of the TiAl15Si15 alloy, in order to achieve the desired fluidity for subsequent casting, the alloy was contaminated with carbon, which was found in the microstructure as graphite. The prepared cast samples exhibited a homogeneous microstructure consisting of a mixture of the hard phase titanium silicide Ti₅Si₃, which was preferentially formed due to the high affinity of silicon and titanium, and titanium aluminide TiAl, which was formed by reacting the residual titanium with aluminium. For this reason, the TiAl15Si15 alloy showed a high proportion of titanium silicide, which formed large sharp-edged particles in the structure, which significantly contributed to the mechanical properties of the alloy. This production method could contribute to the commercial use of these intermetallic alloys in the future.

References

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