Question

The tempering of ferrous martensite and the precipitation hardening of Al alloys have much in common, even though in one case the objective is to soften the material and in the other case it is to harden it:

1. (a) For both the Al alloy and for ferrous martensite plot schematically the evolution of the following parameters as a function of aging/tempering time:

   1. the expected evolution of the yield stress
   2. the expected evolution of the mean precipitate size
   3. the expected evolution of the number density of precipitates 4. the expected evolution of the precipitate volume fraction

2. (b) Plot the different contributions to the yield stress of both alloys as a function of aging/tempering time. Use one plot for the Al alloy and a different plot for the martensite. Explain briefly why aging/tempering leads to hardening of one of the materials and softening of the other.

Answer 1

Markusite B Al alloy . Yield stren ageing (tempering 2 Martansit Cal alley Mean ppt size ageing' tempering time → 3 Martensite Al-alloys pal number density Hime - 1 Aballoys vol fractions Martonote e time CS Scanned with CamScanner

b yield strar i terefesing firms - Martensite Yield stren ageing time Al-alloys CS Scanned with CamScanner

In Al alloys, Al is a soft matrix and precipitates enhance the strength by resisting the dislocation motion.

Whereas strengthening mechanism is different in martensite. Due to quench hardening, the FCC ferrite converts into BCT martensite there will be lot of dislocation density and lot of carbon entrapped in the solution. Due to this it has strength due to dislocation density as well as solid solution strengthening. Now if heat treat it, the dislocation density will be lowered due to annihilation of dislocations and the carbon could also diffuse from the solution forming precipitates but the end result would be decrease in the overall strength but increase in ductility.

That is why ageing or tempering have different effects on these two alloys