| The present study investigated the deformation behaviour, microstructure evolution and fracture behaviour under hot working conditions of alloys designed for elevated-temperature applications. For this purpose, iron-aluminum and titanium-aluminum alloys were selected and their compositions are: Fe-8.5wt%Al-5.5Cr-2.0Mo-0.2Zr-0.03C, Fe-16.5Al-5.5Cr-1.0Nb-0.05C and Ti-33.3Al-2.8Mn-4.8Nb. These alloys were tested in the as-cast condition and in the form of hot-rolled + annealed plate for the iron-aluminum alloys and in the HIP'ed condition for the titanium-aluminum alloy. Isothermal compression tests were carried out with a Gleeble 2000 over a range of temperatures from 800 to 1250°C and constant strain rates from 10-3 to 10 s-1.; In general, the flow curves are marked by a peak stress and softening which decline as temperature rises, and a flow stress which diminishes with rise in temperature and decrease in strain rate. The flow behaviour at peak stress (sigmap) and 0.5 true strain of these materials was described well by the Zener-Hollomon parameter Z=3&d2;exp /RTQHW , where Z=K3sinha sn . A numerical curve-fitting method was used to yield values of the following parameters: (i) stress exponent, n and (ii) activation energy, QHW . The dynamic material modeling approach was performed to extract from hot compression data: (i) the strain rate sensitivity parameter, m, (ii) the efficiency of power dissipation, eta, and (iii) the instability parameter, xi. The microstructure evolution and fracture behaviour were assessed using optical and electron microscopy.; The deformation processes occuring were determined by correlation of the sigma-epsilon curves, m and microstructural observations. The resulting deformation map indicates that at lower temperatures and higher strain rates, the dominant restoration occurs by dynamic recovery, while at lower strain rates and higher temperatures dynamic recrystallization is the operative mode. At the highest temperatures, dynamic recrystallization followed by grain coarsening occurs.; Hot workability maps were constructed based on xi, cracking observations and sigmap. These maps indicate good workability can be achieved over a wider range of temperatures and strain rates with wrought alloys compared with as-cast alloys. The beneficial effect of dynamic recrystallization on hot workability has been clearly demonstrated.; Finally, the application of hot workability maps is demonstrated and approaches to improving the hot workability of selected materials are presented. |
