QB_AE_STRUC_NEW_038
Section Question Bank
MSQ · 2 Marks
Aerospace Structures • Aerospace Materials - Creep Deformation Stages
Which of the following statements regarding high-temperature creep deformation of turbine blade superalloys are **CORRECT**?
(A) Primary (transient) creep is characterized by a continuously decreasing creep strain rate due to strain hardening of the crystal lattice
(B) Secondary (steady-state) creep features a constant minimum strain rate governed by a dynamic balance between strain hardening and thermal recovery
(C) Tertiary creep is characterized by accelerating strain rate leading rapidly to rupture due to necking and grain boundary micro-void coalescence
(D) Single-crystal nickel-based superalloys exhibit lower creep resistance than equiaxed polycrystalline alloys because they eliminate strengthening grain boundaries
Answer Key:
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Step-by-Step Verified Solution
### 1. Strategy & Theoretical Principles 1. **Creep Curve Stages:** - **Stage I (Primary/Transient):** Creep rate $d\epsilon/dt$ decreases over time as dislocation density increases and strain hardening impedes slip. - **Stage II (Secondary/Steady-State):** Longest regime. Creep rate is constant (steady-state creep rate $\dot{\epsilon}_{ss}$), representing an exact equilibrium between strain hardening and dislocation climb / recovery. - **Stage III (Tertiary):** Creep rate accelerates exponentially as internal voids, microcracks, and localized necking reduce effective load-bearing area until ductile rupture occurs. 2. **Superalloy Metallurgy:** - At high operating temperatures ($T > 0.5 T_m$), grain boundaries are regions of weakness subject to sliding and cavitation. - Eliminating grain boundaries by casting **single-crystal superalloys** (e.g. CMSX-4) dramatically **increases** creep rupture life, making Statement D completely false. ### 2. Statement Evaluation - **Statements A, B, C:** **CORRECT.** - **Statement D:** **INCORRECT.** Single crystals have vastly superior creep resistance. ### 3. Exam Trap & Standard Reference - **Trap Alert:** Single crystal turbine blades are used specifically because eliminating grain boundaries eliminates high-temperature grain boundary sliding and cavitation. - **Standard Reference:** Callister, W.D., *Materials Science and Engineering*, Chapter 8.