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Iron-carbon phase diagram, showing the conditions under which austenite (γ) is stable in carbon steel. Allotropes of iron; alpha iron and gamma iron. Austenite, also known as gamma-phase iron (γ-Fe), is a metallic, non-magnetic allotrope of iron or a solid solution of iron with an alloying element. [1]
Convergent beam electron diffraction (CBED) transmission electron micrograph of a [111] zone axis of austenitic stainless steel. Austenitic stainless steel is one of the five families of stainless steel (along with ferritic, martensitic, duplex and precipitation hardened). [1] Its primary crystalline structure is austenite (face-centered cubic).
Austenite is slightly undercooled when quenched below Eutectoid temperature. When given more time, stable microconstituents can form: ferrite and cementite. Coarse pearlite is produced when atoms diffuse rapidly after phases that form pearlite nucleate. This transformation is complete at the pearlite finish time (P f).
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Iron allotropes, showing the differences in structure. The alpha iron (α-Fe) is a body-centered cubic (BCC) and the gamma iron (γ-Fe) is a face-centered cubic (FCC). At atmospheric pressure, three allotropic forms of iron exist, depending on temperature: alpha iron (α-Fe, ferrite), gamma iron (γ-Fe, austenite), and delta iron (δ-Fe).
In steel it produces a bainite microstructure whereas in cast irons it produces a structure of acicular ferrite and high carbon, stabilized austenite known as ausferrite. It is primarily used to improve mechanical properties or reduce / eliminate distortion. Austempering is defined by both the process and the resultant microstructure.
Manganese and nickel lower the eutectoid temperature and are known as austenite stabilizing elements. With enough of these elements the austenitic structure may form at room temperature. Carbide-forming elements raise the eutectoid temperature and stabilize ferrites. [1]: 395–396
The R-phase is essentially a rhombohedral distortion of the cubic austenite phase. Figure 1 shows the general structure, though there are shifts in atomic position that repeat after every three austenitic cells. Thus the actual unit cell of the actual R-phase structure is shown in Figure 2. [2]