Study on Omega Embrittlement of Welded Titanium Alloys and Omega Phase

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The as-welded materials of titanium alloys containing a large amount of β-stabilizer such as Ti-8 Mn and Ti-16 V-1 Mo alloys show considerable brittleness in the tensile and impact tests in temprature range of 400-500℃. These phenomena are due to the precipitation of omega (ω) phase in the matrix in β→α transformation process of titanium alloy and are called" ornega embrittlement of titaniurn alloy". In this work the authors investigated the crystal structure and morphology of ω phase and the effects of some alloying elements on ω embritlement of titanium alloys. The results obtained are as follows. 1. The crystal structure of ω phase is a peculiar hexagonal system, a_w=4.6 A, c_w=2.82 A, c_w/a_w=0.62 and it has atomic positions at 1/2c_w. 2. The morphology of ω phase can be classified into cubic and ellipsoidal types. The ellipsoidal ω is observed in the titanium alloys with alloying elements of atomic sizes larger than that of V, while the cubic ω is observed in the titanium alloys with alloying elements of atomic sizes smaller than tat of V. 3. In Ti-V and Ti-V-Cr system, these alloys are more strengthened with increasing V or Cr contents, but the alloys have a tendency to be come brittle. An addition of α-stabilizer such as Al or C, etc. to the alloys decreases such a tendency.
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