Question

b) An aluminum alloy is left at a furnace for the aging treatment by an operator. By referring to Figure 5(a), i) Compare the peak yield strength and its effect of the aging time for the aging temperatures of 121°C and 260°C. ii) Briefly explain the mechanism of precipitation hardening. 1 min 55 ח 1 1 day 1 month 1 year 70 121°C 1250°F) 400 50 50 300 - 40 1490 (300°F) Yield strength (MPa) Yield strength (i) 200 30 100 204C 1400F) 20 250°C (500°F) - 10 o 0 10-2 10-1 102 202 104 1 10 Aging time (h)

Answer 1

Answer(i) - Peak yield strength at 121^°C is greater than yield strength at 260^°C . Intially yield strength at 121^°C is lesser than yield strength at 260^°​​​​​​​C but as the aging time increases yield strength at both temperature increase till peak value and after that at 121^°C decreases continuously  while yield strength at 260^°​​​​​​​C decreases slightly after peak value .

Answer(ii) - Precipitation hardening, or age hardening, widely used mechanisms for the strengthening of metal alloys.The fundamental understanding and basis for this technique was established in early work at the U. S. Bureau of Standards on Duralumin.The strength and hardness of some metal alloys may be enhanced by the formation of extremely small uniformly dispersed second-phase particles within the original phase matrix in a process known as precipitation or age hardening. The precipitate particles act as obstacles to dislocation movement and thereby strengthen the heat-treated alloys.

The precipitation-hardening process involves three basic steps:

1) Solution Treatment, or Solutionizing, is the first step in the precipitation-hardening process where the alloy is heated above the solvus temperature and soaked there until a homogeneous solid solution (α) is produced. The θ precipitates are dissolved in this step and any segregation present in the original alloy is reduced.

2) Quenching is the second step where the solid α is rapidly cooled forming a supersaturated solid solution of α_SS which contains excess copper and is not an equilibrium structure. The atoms do not have time to diffuse to potential nucleation sites and thus θ precipitates do not form.

3) Aging is the third step where the supersaturated α, α_SS, is heated below the solvus temperature to produce a finely dispersed precipitate. Atoms diffuse only short distances at this aging temperature. Because the supersaturated α is not stable, the extra copper atoms diffuse to numerous nucleation sites and precipitates grow. The formation of a finely dispersed precipitate in the alloy is the objective of the precipitation-hardening process. The fine precipitates in the alloy impede dislocation movement by forcing the dislocations to either cut through the precipitated particles or go around them. By restricting dislocation movement during deformation, the alloy is strengthened.

The 3 main mechanisms are:

1.Coherency strain hardening: Coherency strain hardening results from the interaction between dislocations and the strain fields surrounding GP zones and/or coherent precipitates

2. Chemical hardening: Chemical hardening results from the increase in applied stress required for a dislocation to cut through a coherent (or semi-coherent) precipitate.

3. Dispersion hardening: Dispersion hardening occurs in alloys containing incoherent precipitates or particles - i.e. typically those that have been overaged. This hardening results from the increased shear stress required for dislocations to by-pass these obstacles.