چكيده به لاتين
Co-Al-W cobalt-base superalloy containting gamma-prime (γ/) intermetallic phase was recently introduced along with superior enhanced mechanical properties. Moreover, this group of alloys to be considered as a next generation superalloys and as an alternative for Ni-base one. The primary studies have shown appropriate hot working and safe thermomechanical circumistances for this alloys. However, the lack of detailed hot working dataset is seen based on literature survey done. The purpose of this research is according to following: firstly, the deformation response and potential of as-cast Co-Al-W alloy to be listed as wrought and concurrent phe-nomena during the thermomechanical process were assessed. Secondly, study on aging heat treatment of wrought-type sample and evaluating its yield strength form room throughout high temperatures were focused. For this aims, an alloy was designed and casted in vacuum induction melting furnace (VIM) with composition of Co-9.8Al-7.3W-20.4Ni-2.7Ti-0.4C-0.1B (at%). The microstrucrure of as-cast alloy consists of high volume fraction of ordered γ/ phase (~70%), coarse MC carbides (more than 2 µm) and moderate elemental segregation due to non-eqilubrium solidification. Effect of elemental segregation was investigated on the microstructure and characteristic of γ/ particles such as size and chemical compositios during homogenization treatment at 1300ºC and various times i.e. 2, 4, 8, 16 and 24 hours. In addition, the hot compression tests was performed at temperature interval of 950-1200ºC and strain rates from 0.001s-1 to 1s-1, for evalutetion of hot deformation behavior. The results illustrated that the hot deformation at low temperature regime (950-1050ºC) was led to crack formation, fracture with no evidence of dynamic recrystallization (DRX), which was affected by precipitats of the high volume fraction of fine γ/ particles (less than 200 nm). Moverover, at this regime, slip bands of γ/ pricipitates and γ-γ/ rafted struecture were occured corresponding to the interacation of hot deformation related phenomena and particles. In the high temperature regime (1100-1200ºC) that was characterized by electron back scater difraction (EBSD) technique, it is found that the new grains were formed through discontinuous dynamic recrystallization (DDRX) at three different locations in the microstructure: original grain boundaries, vicinity of MC carbides and interior of the relatively coarser grains. These may be attributed to the mechanisms of strain induced boundry migration (SIBM), particle stimulation nucleation (PSN) and sub-grain growth, respectively. Taking the advantage of derived hot deformation concepts, processing maps of the Co-based alloy were developed to determine the safe and appropriate domains were extracted for thermo-mechanical processe. Furthermore, the hot rolling process was carried out on the cast and homogenized samples leading to wrought-type one. The aging heat treatment was performed over the various temperature (720-900ºC) and time (1-48 hours) on the previously solutioned wrought sample, along with revealing and further discustion about the main phenomena accosiated with γ/ pricipitates. To conclude, the optimal condition for aging treatment was achived by combination of the micro-hardness expremental data and available models. Finally, compressive yield strength of wrought alloy under optimal aging treatment was evaluated. The results illustrated that, the alloy yield strength over the applied temperature range was better than ones for special wrought Ni-base superalloys, such as Waspalloy and Udimet 720. This study implis that the Co-Al-W superalloys can be catogorized in wrought from cast-type using conventional routes and they may introduced as a new class of wrought superalloys along with high temperature applications.
