Question

3) For the case of 4% Al in Cu, why does the room temperature strength not decrease, while the hot aged samples show the increase then decrease in strength with time. Be sure to measure crystal structure and phase boundaries ?

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Answer 1

Natural aging has been considered to occur in heat treatable aluminum alloys together with GP-zone formation and artificial aging in occurrence with incoherent precipitation. It is though hard to point out the structural changes in aging processes. Here, the behavior of a 4% Cu aluminum alloy is presented in detail in Fig.

There are five obvious stages of structural changes during heat treatments. The first stage is representing structure before solution heat treatment. In stage II in this case precipitated copper atoms are dissolved at 500°C and possible cold work or precipitation hardening are withdrawn.After quenching the material to room temperature, stage III represents the natural aging sequence. In this stage copper-loaded coherent precipitation GP-1 zones are formed. The GP- 1 zones, which are small, do not grow any further with subsequent artificial aging but are, instead, reversed into solution for a short while at 150-200 °C. That step, where the hardness also is momentary decreased, is presented as stage IVa. Stage IVb shows the artificial aging at 150°C which makes it possible to form GP-2 zones. Further aging at elevated temperature will result in formation of metastable phase Θ´ which existing of CuAl2 as the equilibrium phase. The curve from the graph shows that maximum hardness is achieved when GP-2 zones are formed and when mixtures of GP-2 and Θ´ occur. Overaging takes place when there are only Θ´ precipitates in the structure and hence hardness, strength and elongation decreases dramatically. Further aging at temperatures between 170-300 °C will form coarse CuAl2 precipitates that causes the aluminum alloy to be soft and therefore increases in strength.

Hence, the case of 4% Al in Cu, why the room temperature strength not decrease is explained from above information as you can see around the room temperature i.e. 25 degree celcius the graph is a constant.

Now, for hot aged samples the increase then decrease in strength with time. From the below figure given is a graph representation of Mg-Al alloy after study from crystal structure which clearly shows increase and then decrease in strength with time.