Zirconium has a hexagonal close-packed (hcp) crystal structure at room temperature, and it undergoes allotropic transformation to a body-centered cubic (bcc) structure at about 870°C (1600°F). The two microstructure characteristics result in strong anisotropy of zirconium and its alloys. Generally, the low-temperature hcp crystal structure is called alpha phase while the high-temperature bcc crystal structure is called beta phase. The lattice constants for zirconium are: a = b = 323.2 pm, c = 514.7 pm; α = β = 90°, γ = 120°.

Temperature		Phase Present at Oxygen Content of
°C	°F	1640 ppm	1370 ppm	970 ppm
955	1750	β	β	β
930	1710	α+β	β	β
925	1700	α+β	β	β
920	1690	α+β	β	β
915	1680	α	α+β	β
910	1670	α	α+β	β
905	1660	α	α+β	β
895	1640	α	α+β	α+β
890	1630	α	α	α+β
885	1625	α	α	α+β
865	1590	α	α	α
855	1575	α	α	α

*The temperature at which the metal transforms from one phase to another is called allotropic transformation temperature. Small amounts of impurities, particularly oxygen, strong affect the transformation temperatures of zirconium. The table has listed variation in allotropic transformation temperature with oxygen content for unalloyed zirconium.