Technical Assessment: Thermodynamic Closure and Mechanism-Based Design in High-Strength Steels

1. The Thermodynamic Imperative

The fundamental limit for a composition-invariant transformation is the T₀ condition. In practice, this is refined to the T₀' limit to account for the stored energy of the transformation product (approx. 400 J mol⁻¹). The maximum volume fraction of bainite (V_b) is governed by the carbon enrichment of the residual austenite:

Analysis confirms that growth ceases at the T₀ limit rather than at paraequilibrium (Ae₃⁰). Crucially, any observed scatter in carbon concentration arises from the heterogeneous distribution in austenite as sheaves isolate regions during growth.

2. Strain Energy and Interface Mechanics

Bainite’s plate morphology is determined by an invariant plane strain (IPS) shape deformation, consisting of shear strain (s ≈ 0.22–0.26) and uniaxial expansion (ζ ≈ 0.03). The strain energy per unit volume (G_e/V) is derived as:

The shear component is the dominant energy penalty, requiring a glissile interface. Modern "Topological Models" describe this as a macroscopically planar irrational interface consisting of atomic steps and disconnections that must move conservatively.

3. Atomic Mobility and "Configurationally Frozen" States

At temperatures between 125°C and 478°C, substitutional atoms are in a "configurationally frozen" state. Atom probe profiles confirm the X/Fe ratio remains identical across the α_b/γ interface. In the "100-year experiment," it is noted that a single atomic jump of nickel at room temperature would take 10⁴ years. In contrast, bainite forms in seconds, proving the transformation must be diffusionless.

4. Strategic Synthesis Table