Metals and alloys
Which statement about the crystal structure of pure titanium is correct?
- AIt is the HCP α phase at room temperature and changes to the BCC β phase above about 882 °C
- BIt is BCC at room temperature and changes to the FCC γ phase above about 912 °C
- CIt remains a single FCC phase at all temperatures, with no allotropic transformation
Show answer and explanation
Answer: A. It is the HCP α phase at room temperature and changes to the BCC β phase above about 882 °C
Titanium takes the HCP α phase at room temperature and undergoes an allotropic transformation to the BCC β phase above about 882 °C. Al and O stabilize the α phase, while V and Mo stabilize the β phase, so depending on the alloying elements titanium alloys are classified as α, α+β or β alloys. The well-known Ti-6Al-4V is an α+β alloy whose strength can be adjusted by heat treatment, and it is used for aircraft parts. The choice of BCC at room temperature changing to FCC above about 912 °C confuses titanium with iron, which behaves that way. Being FCC at all temperatures without transforming describes aluminum and copper. Titanium has a density of about 4.5 g/cm³, about 60% that of steel, giving it a high specific strength, and its dense surface oxide film gives it excellent corrosion resistance. The HCP α phase has few slip systems, so cold working at room temperature is not as easy as in FCC metals.