2025 Plasticity GPT o3

Anihilation of opposing dislocations and rearanging of dislocations to small angle grain boundaries

healing of 0-dim defects

A, p, q=material constants

d_0=initial grain size

Z=Zener-Hollomonn Parameter

Q=activation energy

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1. BCC Metals (e.g., Ferritic steels): Exhibit irregular slip due to a lack of well-defined slip planes.

2. FCC Metals (e.g., Austenitic steels, Cu, Al): Allow easy slip but experience strong strain hardening due to dislocation interactions.

3. HCP Metals (e.g., Mg, Ti): Deformation is mostly limited to basal plane slip and requires favorable crystal orientations.

• BCC Metals: Show pronounced strain-rate sensitivity and are highly temperature-dependent.

• FCC Metals with High Stacking Fault Energy (SFE) (e.g., Al, Cu): Are less sensitive to temperature but can undergo twinning at lower temperatures.

• FCC Metals with Low SFE (e.g., Austenitic stainless steels): May exhibit strain-induced phase transformations (TRIP effect), enhancing strength.

• DSA occurs in the temperature range 0.15 Tm to 0.4 Tm

• It leads to serrated (oscillatory) flow stress due to solute-dislocation interactions, resulting in:

  • Negative strain-rate sensitivity (m<0)

  • Localized plastic deformation

  • A drop in ductility known as “blue brittleness” (notably in low-carbon steels).

1. Dislocation Climb: Enables dislocations to overcome obstacles by moving out of their slip plane.

2. Dynamic Recovery (DRV): Occurs in high-SFE metals (like Al) where dislocations rearrange to reduce internal stresses, softening the material.

3. Dynamic Recrystallization (DRX): In low-SFE metals, new, strain-free grains form during deformation, reducing dislocation density.

1. Void Nucleation:

   • Initiated at inclusions or second-phase particles (via particle cracking or decohesion).

2. Void Growth:

   • Modeled by the Rice & Tracey approach.

3. Final Coalescence:

   • Voids merge, leading to the formation of a crack and subsequent fracture.

ASBs are narrow zones of intense plastic deformation formed under conditions of extreme strain localization and adiabatic heating. They can serve as precursors to ductile fracture.

1. Superplasticity is the ability of some materials to undergo extreme plastic elongation (over 400% strain).

2. Key Mechanisms:

   • Grain Boundary Sliding (GBS): Dominates at high temperatures and small grain sizes.

   • Diffusion Creep: Atomic diffusion allows material flow at low stresses.

• Al–Mg–Sc–Zr Alloys: Operate at 450–525°C with strain rates of 10^-3 - 10^-1 s^-1

• Ti-6Al-4V Alloy:Operates at 840–900°C with strain rates of 10^-4 - 10^-3 s^-1

Key factors include:

• Pre-heating temperature

• Applied strain and strain rate

• Tool velocity

• Overall process control Together, these determine the formability and the risk of ductile fracture.

1. Formability Windows: Define the safe operating conditions (temperature, strain rate, etc.) for hot working, ensuring sufficient ductility and minimal fracture risk.

2. Ductility Trough: A temperature region where the material’s ductility is reduced, potentially leading to processing difficulties.

1. Strain Aging: Occurs when solute atoms diffuse to dislocations, temporarily increasing the yield stress.

2. Discontinuous Yielding: Can result in localized deformation features like Lüders bands, where the material yields in a jerky or stepped manner.

1. TRIP (Transformation-Induced Plasticity): Strain-induced transformation of metastable austenite to martensite, which increases strength.

2. TWIP (Twinning-Induced Plasticity): Mechanical twinning in low SFE FCC metals enhances plasticity and strain hardening.

1. Red Brittleness: Occurs around 0.6 Tm​ due to grain boundary embrittlement, because of elements migrating to the grain boundaries.

2. Hot Shortness: Seen in Cu-containing steels near 0.85 Tm​ where intergranular cracking occurs due to weakened grain boundaries.