A mechanistic study of the alpha to gamma(m) massive transformation in gamma titanium aluminides.

摘要

In this thesis, the {dollar}alphatogammasb{lcub}rm m{rcub}{dollar} massive transformation in TiAl alloys has been studied in great detail. A Ti-46.5 at.% Al alloy was chosen for study, since this composition offered the best ability to arrest the transformation at different stages by controlling the rate of cooling from the high-temperature {dollar}alpha{dollar}-phase field. Transmission Electron Microscopy (TEM) was used to characterize the microstructural features such as defects in the massively transformed gamma {dollar}(gammasb{lcub}rm m{rcub}){dollar} phase, the {dollar}rmalphasb2/gammasb{lcub}m{rcub}{dollar} interphase boundaries (IPBs) and orientation relationships (ORs) between the parent {dollar}alphasb2{dollar} (prior {dollar}alpha){dollar} and the product {dollar}rmgammasb{lcub}m{rcub}{dollar} in the Ti-46.5 Al alloy.; The results reveal that the defect structures in {dollar}rmgammasb{lcub}m{rcub}{dollar} are composed of dislocations, stacking faults (SFs) and antiphase boundaries (APBs) intimately associated with dislocations or SFs. Contrast analysis indicates that both {dollar}{lcub}1over2{rcub}{lcub}<{rcub}110rbrack{dollar} and {dollar}{lcub}1over2{rcub}{lcub}<{rcub}101rbrack{dollar} unit dislocations were present, with the latter linked by highly curved non-conservative APBs. TEM images established that wide SFs, which are created by the dissociations of {dollar}{lcub}1over2{rcub}langle 101rangle{dollar} unit dislocations, lie on {dollar}{lcub}111{rcub}{dollar} planes and are bound by b = {dollar}{lcub}1over6{rcub}langle 121rangle{dollar} Shockley partial dislocations of all possible types. In addition, APBs are found to commence or terminate at the partial dislocations with b = {dollar}{lcub}1over6{rcub}{lcub}<{rcub}121rbrack{dollar}, but not those with b = {dollar}{lcub}1over6{rcub}{lcub}<{rcub}112rbrack.{dollar} The results also reveal that besides highly curved irregular shapes, the non-conservative APBs frequently exhibit the hair-pin shape and the loop morphologies. Furthermore, these APBs are found to be composed of either partially or fully filled layer of {dollar}90spcirc{dollar} domains (c-axis {dollar}90spcirc{dollar} rotated with respect to the matrix) sandwiched between two larger domains having the same orientation (i.e. the matrix). Based on the observations and subsequent analyses, a model for the formation of these defects is proposed. Instead of the single step {dollar}rmalphatogammasb{lcub}m{rcub}{dollar} reaction, this model postulates that the overall transformation takes place in two successive steps. In the first step, the {dollar}alpha{dollar} phase transforms massively into an intermediate disordered FCC phase and dislocations and SFs are proposed to be formed during this reaction. During the following step, the intermediate FCC phase undergoes an ordering reaction into the final {dollar}rmgammasb{lcub}m{rcub}{dollar} phase. The interactions between the fast moving ordering reaction front and dislocations and SFs, as well as the formation mechanisms for APBs and thin {dollar}90spcirc{dollar} domains, are discussed.; The results also indicate that though {dollar}rmalphasb2/gammasb{lcub}m{rcub}{dollar} IPBs exhibit both faceted and curved morphologies, there are no dislocations or ledges on IPBs, and the faceted IPBs can lie on either low-index rational planes or high-index irrational planes. These results, along with the observation that there is no low-index rational OR between the {dollar}rmgammasb{lcub}m{rcub}{dollar} and {dollar}alphasb2{dollar} grains on the opposite sides of the IPB, implies that the IPBs should be incoherent. In addition, the defects are observed to be randomly distributed in {dollar}rmgammasb{lcub}m{rcub}{dollar} grains. Based on this observation, instead of the ledge mechanism, the growth mechanism of the massive transformation in TiAl alloys is postulated to be that of individual atoms randomly