4. (a) Medium carbon content steels (ie, 0.4 wt%C) and 2000 series Aluminium All

Question

4. (a) Medium carbon content steels (ie, 0.4 wt%C) and 2000 series Aluminium Alloys (ie, Al-4 wt.% Cu) are solution treated, quenched and aged to produce desired combinations of mechanical properties for structural applications i. At what temperatures are the two alloys solution treated? ii, what crystal structure(s) do the medium carbon content steel and Al-4 wt% Cu [2 marks] [2 marks] ii. Both alloys are rapidly quenched from the solution treatment temperature to RT [2 marks] [2 marks] [2 marks] have at the solution treatment temperature? What phase changes (including crystal structures), if any, take place in both iv. Briefly describe the microstructural changes (including crystal structures) during V. Briefly describe the microstructural changes during the ageing of Al-4 wt-% Cu. SECTION B, Question 4, continued over page alloys? tempering of the medium carbon content steel TURN OVER

Explanation / Answer

Ans 4 (a) i. The two solutions of alloy should be treated above 1000OC and between 500 and 580 OC

ii. Both have the FCC crystal structure.

iii. After rapid quenching the a coarse CuAl2 phase precipitates from the solid solution. The resulting two-phase structure would be moderately harder ans stronger than pure Al.Quenching to a low temperature prevents the acheivemnet of equilibrium condition and the solid solution of Cu is frozen in.

iv. The low temperature ensures a very high nucleation rate and precipitates a very large number of small particles known as mocrostructures and the thus various microstructural changes are observed.The strength and ductility changes. Initially the hardness of the material increases because of the formation of very fine particles.Optimum strengthning is acheived after 1 day,at 190 degree celcius and 60 days at 130 degree celcius. The strenght of the material decreased by over ageing i.e by heating for longer times. Diffusion of the Cu atoms leads to particle coarsing, the smaller precipitates particle disappear, and the structure contains fewer, larger particles that are less effective in stopping the dislocations.

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