Abstract
Two cast <FONT FACE="Symbol">g</font> titanium aluminides, Ti47Al and Ti50Al (at. %), were obtained by arc melting under an argon atmosphere. The as-cast microstructure modification by annealing under protective argon atmosphere and hot isostatic pressing (HIP) was analysed by optical and scanning electron microscopy with microanalysis facilities. As-cast structures presented strong microsegregations, being composed of primary <FONT FACE="Symbol">a</font> dendrites (with low Al content), which transformed into lamellar <FONT FACE="Symbol">a</font>2 + <FONT FACE="Symbol">g</font> during solid state cooling, and interdendritic <FONT FACE="Symbol">g</font> phase (with high Al content). Annealing in the <FONT FACE="Symbol">a</font> + <FONT FACE="Symbol">g</font> field was responsible for a partial reduction of microsegregation, a decreasing of the volume fraction of <FONT FACE="Symbol">a</font>2 + <FONT FACE="Symbol">g</font> lamellar dendrites, an increasing of <FONT FACE="Symbol">g</font> volume fraction and the occurrence of a small fraction of <FONT FACE="Symbol">a</font>2 particles and plates in some of the <FONT FACE="Symbol">g</font> grains. Annealing in the <FONT FACE="Symbol">a</font> field at 1400 °C for one hour was insufficient to eliminate the as-cast microstructure in the Ti50Al alloy and produced large equiaxed lamellar colonies of <FONT FACE="Symbol">a</font>2+<FONT FACE="Symbol">g</font> in the Ti47Al alloy. HIP in the <FONT FACE="Symbol">a</font> + <FONT FACE="Symbol">g</font> field (to avoid excessive grain growth) was responsible for microstructure modifications similar to the ones obtained by heat treating under similar soaking conditions.
Highlights
Titanium aluminides with low density, oxidation resistance up to 800 °C and good creep resistance are promising alloys for high temperature structural applications, the lack of room temperature ductility being the main obstacle to its utilization as engineering materials[1,2]
If the temperature is raised to 1200 °C, interlamellar spacing increases and coarse α2 particles and plates are present at non lamellar γ grains[13]
In this work two titanium aluminides were produced, Ti50Al and Ti47Al, by the arc melting technique under argon atmosphere in a water cooled copper mould
Summary
Titanium aluminides with low density, oxidation resistance up to 800 °C and good creep resistance are promising alloys for high temperature structural applications, the lack of room temperature ductility being the main obstacle to its utilization as engineering materials[1,2]. Mechanical properties of these alloys are strongly microstructure dependent. The fine lamellar structures are quite different from the coarser ones resulting from slow cooling rates. If the temperature is raised to 1200 °C, interlamellar spacing increases and coarse α2 particles and plates are present at non lamellar γ grains[13]
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