**5. Challenge for high temperature stability of Al-Cu alloy: precipitate coarsening**

One of the major hindrances for widespread use of Al-Cu alloys, especially in the automobile and aerospace sectors, is their poor high temperature stability associated with rapid decrease in load bearing capacity above �250°C [22]. This strength degradation is because of the rapid coarsening of θ<sup>0</sup> precipitates from thickening along the broad facets, thereby resulting in a drastic decrease in their aspect ratio. At high temperature, the rate of diffusion for solute Cu atoms increases so that they segregate at the coherent and semi-coherent interfaces of θ<sup>0</sup> precipitates [23]. The broad facet (i.e. the coherent interface) thickens by ledge formation while the semicoherent interfaces having high interfacial energy grow from accommodation of Cu atoms along {100}θ0, {010}θ0, {110}θ<sup>0</sup> interfaces [24]. The thermodynamic driving force for coarsening of metastable θ<sup>0</sup> precipitates is the reduction of interfacial energy [13]. It should be noted however that the increase in diameter of θ<sup>0</sup> precipitate by the growth of semi-coherent interface does not essentially reduce the interfacial energy; rather the thickening of the broad facet i.e. growth of the coherent interface is responsible for the decrease in interfacial energy during coarsening.

*New-Age Al-Cu-Mn-Zr (ACMZ) Alloy for High Temperature-High Strength Applications… DOI: http://dx.doi.org/10.5772/intechopen.104533*

At long thermal exposure, θ<sup>0</sup> precipitates ultimately transform to stable (equilibrium) θ precipitates, which are completely incoherent with the parent α-Al matrix. Further thermal treatment leads to the growth of larger θ precipitates in expense of the smaller ones. This corresponds to an increase in the inter-precipitate distance so that coarse θ precipitates are no longer effective in restricting the motion of matrix dislocations; hence, the strength of the alloys decreases drastically [9]. It therefore appears imperative to stabilize strengthening metastable θ<sup>0</sup> precipitates against coarsening for successful high temperature application (beyond �250°C) of Al-Cu alloys.
