Question

Problem 4 (20 pts): An alloy was subjected to age hardening test at different precipitation heat treatment temperatures. Consider the graph shown below, relating the variation of yield strength of the tested alloy with the precipitation heat treatment durations. 1 min 1 h 1 day 1 month 1 year 70 121°C (250°F) 400 60 50 300 40 149°C (300°F) Yield strength (MPa) Yield strength (ksi) 200 30 204°C (400°F) 20 260°C (500°F) 10 100 0 SS 0 10-2 1 103 104 10-1 10 102 Duration of precipitation heat treatment (h) a) (4) Provide TWO other names for the Age Hardening heat treatment. b) (6) Graphically summarize the steps of the heat treatment applied in the age hardening test. c) (6) According to the given graph (above), what are the most efficient (shortest duration) heat treatment temperatures you would choose in order to reach a yield strength of: 1) 370 MPa 2) 60 ksi 3) 45 ksi d) (4) Explain why all the curves show an increase followed by a decrease in yield strength over time.

Answer 1

a) Two other names: Precipitation hardening, Particle hardening

b)

600 500 Quench Solution heat treatment Temperature (°C) T1 400 Precipitation heat treatment - T2 - 300 200 100 Time

The process contains three major steps:

1. Heat the alloy to a temperature T1 such that it is in single phase solid solution, for long enough so that the complete alloy is in one phase. This is called solution heat treatment.

2. Rapidly quench from this state.

3. Reheat the alloy to another temperature T2 (<T1) and hold depending upon the strength and ductility requirement.

c)

1 min 1h 1 day 1 month 1 year 70 121°C (250°F) 750 2 400 1 50 3 300 40 149°C (300°F) Yield strength (MPa) Yield strength (ksi) 200 30 204°C *(400°F) 20 260°C (500°F) 100 10 0 10-2 1 103 104 10-1 101 102 Duration of precipitation heat treatment (h)

1) 370 MPa ==> Point-1: about 0.2 hours

2) 60ksi --> Point-2: about 12 hours

3) 45ksi ==> Point-3: about 1.8 seconds (0.5E-3 hours)

d) The strength of the alloy during age treatment is related to the size of intermetallic particles formed during precipitation heat treatment step. With time, the size of particles increases. The strength of the alloy increases until a critical value of particle size (peak strength value) is reached and beyond that, due to large particle size, the strength starts decreasing.