GATE MT 2020 Q22. The sequence of precipitation during aging of Al- 4 wt. % Cu alloy is:

Correct answer is option A.

• Precipitation Hardening :The strength and hardness of some metal alloys may be enhanced by the formation of extremely small uniformly dispersed particles of a second phase within the original phase matrix.
• This is accomplished by appropriate heat treatments. The process is called precipitation hardening because the small particles of the new phase are termed “precipitates”.
• Mechanism of Hardening: During plastic deformation, zones or precipitates act as obstacles to dislocation motion, stress must be increased to “push” the dislocation through the distribution of precipitates. Consequently the alloy becomes harder and stronger.
•  Age hardening – A special dispersion-strengthening heat treatment. By solution treatment, quenching, and aging, a coherent precipitate forms that provides a substantial strengthening effect. Also known as precipitation hardening, it is a form of dispersion strengthening.
• Coherent precipitate : A precipitate whose crystal structure and atomic arrangement has a continuous relationship with the matrix from which the precipitate is formed.

• Mechanism of Precipitation Hardening :Formation of very small particles of a second, or precipitate, phase. During precipitation hardening, lattice strains are established at the precipitate-matrix interface. There is an increased resistance to dislocation motion by these lattice strains in the vicinity of the microscopically small precipitate particles.

• Precipitation Hardening :Small inclusions of secondary phases strengthen the material. Lattice distortions around these secondary phases impede dislocation motion. The precipitates form when the solubility limit is exceeded. Precipitation hardening is also called age hardening because it involves the hardening of the material over a prolonged time.
• Microstructural Evolution in Age or Precipitation Hardening:
• Step 1: Solution Treatment
• Step 2: Ageing Guinier-Preston (GP) zones – Tiny clusters of atoms that precipitate from the matrix in the early stages of the age-hardening process.

• Steps in Precipitation Hardening : Precipitation hardening is accomplished by two solution heat treatment.During solution heat treatment all solute atoms are dissolved to form a single-phase solid solution. Quenching or rapid cooling to room temperature to form a non-equilibrium supersaturated solid solution (to prevent diffusion and the accompanying formation of any second phase).
• Ageing : The supersaturated solid solution is heated to an intermediate temperature within the two-phase region at this temperature diffusion rates become appreciable. The precipitates of the second phase form as finely dispersed particles.

1. Precipitation Hardening :Solution treatment, in which the alloy is heated to a temperature above the solvus line into the alpha phase and held for a period sufficient to dissolve the beta phase. Quenching to room temperature to create a supersaturated solid solution. Precipitation Treatment: alloy is heated to a temperature below Ts to cause precipitation of fine particles of beta phase.

• Steps in Precipitation Hardening :By quenching and then reheating an Al-Cu (4.5 wt%) alloy, a fine dispersion of precipitates form within the Grains. These precipitates are effective in hindering dislocation motion and consequently, increasing alloy hardness and strength.

• Precipitation Hardening: Aging can also occur at room temperature for some alloys (natural aging). Data represented as hardness or tensile strength vs aging time (log scale) for T-constant. Yield Strength increases as zones or precipitates form.Strength reaches a peak value and then decreases (overageing).

Overaging in Precipitation Hardening:

•With increasing time, the strength or hardness increases, reaches a maximum, and diminishes.

• This reduction in strength and hardness that occurs after long time period is known as overaging.