Austenite is an alloy of iron and carbon with the iron in:
At room temperature, pure iron has a body-centered-cubic (bcc) structure. This phase is stable up to 912 °C, where it transforms to a face-centered-cubic (fcc) structure. The solubility of carbon in bcc is very low and reaches a maximum of  0.02% at 723°C. This material is known as ferrite.  
Above 723 °C, a solid solution of carbon in an fcc iron matrix called austenite is formed. The maximal solubility of carbon in fcc matrix is 2.1%.
When plain carbon steel containing 0.8% carbon is cooled slowly in the austenitic phase to 723 °C, it undergoes a solid-state eutectoid transformation to yield a microstructural constituent called pearlite, which consists of alternating fine-scale lamellae of ferrite and iron carbide (Fe3C), referred to as cementite, or simply, carbide. 
The Fe3C phase is much harder and more rigid than austenite or ferrite.

Key Concept

It is an interstitial solid of a small of a carbon dissolved in γ iron.
The maximum solubility is 2.1%C at 1147o C.
The crystal structure of Austenite (γ) is F.C.C.
Hardness – 40 HRC and toughness is high.

Ref:Phillips Ed-12th P: 406
Alpha form
Beta form
Gamma form
None of the above