English
Menu

Strengthening mechanisms and dislocations

Which statement about the characteristics of dislocations shearing through precipitates with an ordered structure, such as the γ' phase (an L1₂ ordered phase) in Ni-based superalloys, is correct?

  1. AIn an ordered phase the period of the atomic arrangement is half that of the matrix, so a matrix dislocation splits into two partial dislocations inside the precipitate with a twin between them
  2. BOrdered precipitates are incoherent with the matrix and are never sheared by dislocations, so dislocations bow out and pass between particles of every size
  3. CA single dislocation that cuts the precipitate leaves an antiphase boundary with the atomic arrangement out of step, so a second dislocation follows to remove it and dislocations move in pairs
Show answer and explanation

Answer: C. A single dislocation that cuts the precipitate leaves an antiphase boundary with the atomic arrangement out of step, so a second dislocation follows to remove it and dislocations move in pairs

In an ordered phase, atoms of different kinds sit on specific sites. When a perfect dislocation of the FCC matrix, (a/2)<110>, cuts a precipitate, the pairing of atoms above and below the slip plane is shifted, creating an antiphase boundary (APB), and its energy is the resistance to shearing. In the ordered phase the original arrangement is restored only after a shift of a<110>, so a second dislocation with the same Burgers vector follows to remove the APB, and the dislocations move as a coupled pair, a superdislocation. The period of the ordered arrangement is longer than that of the matrix, not half of it. γ' is coherent with the matrix, so while small it is normally sheared, and the option claiming it is incoherent and never sheared is also wrong. Precipitating a large amount of γ' with a high APB energy is the key to the high-temperature strength of superalloys.

Report an error

Practice this topic Back to topic

More Strengthening mechanisms and dislocations questions

  1. Which statement about dispersion strengthening with oxide particles, as in copper containing fine Al₂O₃ particles or ODS steels with Y₂O₃ particles dispersed in iron, is correct?
  2. The yield stress of an alloy is estimated from contributions of 50 MPa friction stress, 60 MPa solid-solution strengthening, 200 MPa precipitation strengthening and 100 MPa dislocation strengthening. If the friction stress and solid-solution strengthening are simply added, while the contributions of precipitates and dislocations, which are obstacles of similar strength, are combined as the square root of the sum of squares (σ = σ₀ + σss + √(σp² + σd²)), which is closest to the yield stress?
  3. Which method of strengthening steel raises the yield strength while also lowering the ductile-to-brittle transition temperature, improving toughness at low temperatures?
  4. Strengthening a metal usually reduces its uniform elongation. Which statement correctly describes the idea behind TRIP and TWIP steels, which show large uniform elongation even at high strength? Assume that necking begins when dσ/dε = σ (σ and ε are true stress and true strain).
  5. Estimating the shear stress needed to displace the atomic planes above and below the slip plane all at once in a perfect crystal (the theoretical shear strength) as τ ≈ G/2π gives about 7.6 GPa for copper (shear modulus G = 48 GPa). Which is the correct reason why the shear stress at which a real copper single crystal begins to slip is orders of magnitude smaller, about 1 MPa?