TY - THES

T1 - Phase transformations and grain growth in TNM based alloys

AU - Klein, Thomas

N1 - embargoed until null

PY - 2013

Y1 - 2013

N2 - Titanium aluminides represent an attractive option to replace Ni-base super¬alloys in various components of aero and automotive engines. Major reasons are a low density, good mechanical properties at elevated temperatures, good oxidation resistance and resistance against ‘titanium fire’. The alloys under investigation were the TNM alloy containing the alloying elements niobium, molybdenum and boron as well as special TNM alloys which additionally contain carbon. Aim of this study was to investigate phase transformations and grain growth in-situ in a high temperature laser scanning confocal microscope. The β to α phase transformation was investigated regarding the morphology of the precipitating phase under different cooling rates. Low cooling rates yielded equiaxed structures, whereas high cooling rates yielded lath-like Widmanstätten structures. From the captured data continuous cooling transformation diagrams for the onset of β to α phase transformation were derived. On annealing a carbon containing TNM alloy at high temperatures precipitation of second phase particles occurred. These precipitates were identified as Ti2AlC car-bides. An orientation relationship of these carbides and the γ-phase was observed. Alloying effects on the formation of the γ-phase were studied. It was shown that carbon slows down the transformation process. Additionally, the occurrence of a cellular reaction in quenched and annealed samples was studied in the TNM alloy. Grain boundary motion was investigated at a temperature of 1300°C. It could be shown that boundary migration in alloys containing a single phase field region at this temperature follows a parabolic kinetic law. Grain boundary motion in alloys containing a two phase field region is comparably slow and follows a linear law. Additionally, grain growth was studied in the β single phase field region in β-solidifying alloys. The classic TNM alloy showed a smaller coarsening rate than the carbon containing TNM alloy as the β single phase field region is shifted to higher temperatures by alloying with carbon.

AB - Titanium aluminides represent an attractive option to replace Ni-base super¬alloys in various components of aero and automotive engines. Major reasons are a low density, good mechanical properties at elevated temperatures, good oxidation resistance and resistance against ‘titanium fire’. The alloys under investigation were the TNM alloy containing the alloying elements niobium, molybdenum and boron as well as special TNM alloys which additionally contain carbon. Aim of this study was to investigate phase transformations and grain growth in-situ in a high temperature laser scanning confocal microscope. The β to α phase transformation was investigated regarding the morphology of the precipitating phase under different cooling rates. Low cooling rates yielded equiaxed structures, whereas high cooling rates yielded lath-like Widmanstätten structures. From the captured data continuous cooling transformation diagrams for the onset of β to α phase transformation were derived. On annealing a carbon containing TNM alloy at high temperatures precipitation of second phase particles occurred. These precipitates were identified as Ti2AlC car-bides. An orientation relationship of these carbides and the γ-phase was observed. Alloying effects on the formation of the γ-phase were studied. It was shown that carbon slows down the transformation process. Additionally, the occurrence of a cellular reaction in quenched and annealed samples was studied in the TNM alloy. Grain boundary motion was investigated at a temperature of 1300°C. It could be shown that boundary migration in alloys containing a single phase field region at this temperature follows a parabolic kinetic law. Grain boundary motion in alloys containing a two phase field region is comparably slow and follows a linear law. Additionally, grain growth was studied in the β single phase field region in β-solidifying alloys. The classic TNM alloy showed a smaller coarsening rate than the carbon containing TNM alloy as the β single phase field region is shifted to higher temperatures by alloying with carbon.

KW - titanium alumindes

KW - high temperature microscopy

KW - confocal microscopy

KW - phase transformations

KW - grain growth

KW - Titanaluminide

KW - Hochtemperatur Mikroskopie

KW - Phasenumwandlungen

KW - Kornwachstum

M3 - Diploma Thesis

ER -