Laiqi Zhang

Microstructural stability of lamellar TiAl-based alloys at high temperature

The microstructural stability of lamellar TiAl-base alloys at high temperature was studied by conventional and high-resolution transmission electron microscopy. This paper emphasized the influence of substructures on the thermal stability of lamellar structure. These substructures produced by thermal mechanical treatments include lattice dislocations and boundary dislocations, the subgrain boundaries, the impinged T (Q) twins and misorientated multi-ledges lamellar interface boundaries. Three dual-phase TiAl based alloys containing different type and density of substructures were produced by different thermomechanical treatments. The microstructural changes in these dual phase TiAl-base alloys were compared after exposure at 800-1000 degrees C. It was found that the existence of such substructures could accelerate the degeneration of lamellar structure, leading to the rapid necking and break-up of alpha(2) plates, the coarsening of gamma plates, and the formation of new gamma grains. A detailed description of various degeneration processes was given in the paper. As a result, the lamellar structure with heavy substructures started to degenerate after thermal exposure at 800 degrees C for 4.5 h. While only slight coarsening was observed at the colony boundaries in the lamellar structure without substructures even after exposure at 900 degrees C for 7 days

Formation of stress-induced 9R structure in a hot-deformed Ti-45Al-10Nb alloy

Two-phase {gamma}-TiAl alloys composed of {gamma}-TiAl and {alpha}{sub 2}-Ti{sub 3}Al are currently under extensive investigation because their mechanical properties are superior to both single-phase {alpha}{sub 2} and {gamma} alloys. The deformation induced microstructures of two-phase {gamma}-TiAl was investigated by many researchers using scanning and transmission electron microscopy (TEM). Various deformation modes were found to contribute to plastic deformation of two-phase alloys at different temperatures. More recently, some researchers also reported stress-induced phase transformations to occur during deformation of two-phase {gamma}-TiAl alloys. However, the mechanisms of these stress-induced phase transformations in two-phase {gamma}-TiAl alloys have not been studied in detail. In this paper, the formation of 9R structure in a hot-worked Ti-45Al-10Nb alloy with nearly fully-lamellar microstructure is studied using high-resolution transmission electron microscopy. The mechanism of the stress induced {gamma} {yields} 9R phase transformation will be discussed.

Microstructure control and mechanical properties of directionally solidified TiAl–Nb alloys

Abstract The double directional solidification (DS) technique was developed to control the lamellar microstructures in primary β TiAl-Nb alloys. Polysynthetically twinned (PST) crystals with lamellar boundaries parallel to or inclined 45° to the growth direction were achieved due to the complete peritectic transformation during directional solidification of the alloys with the dendritic solid/liquid interface. The PST crystals with aligned lamellar boundaries only parallel to the growth direction were produced when lamellar grains with lamellar boundaries in the same orientation were seeded by themselves under appropriate growth conditions. Low boron addition is harmful to align the lamellar orientation because of the growth of non-peritectic α phase. Due to the larger yttria particles and boride ribbons in the directionally solidified TiAl-Nb alloys, the tensile plastic elongations of the alloys are only close to 2%.

Effects of Heat Treatment on Microstructure of Directionally Solidified Ti-45Al-8Nb-(W, B, Y) Alloy

The effects of heat treatments on typical microstructures of directionally solidified (DS) Ti-45Al-8Nb-(W, B, Y) (molar fraction, %) alloys prepared by the Bridgeman method were studied. Two typical DS microstructures including full lamellae with cellular growth morphology and massive structure with dendritic growth morphology were examined. The results show that the heat treatment of 1250℃ for 24 h+900℃ for 30 min+air cooling can efficiently eliminate the B2 phase in the DS alloys and change the massive structure of the rapid DS alloy into lamellar microstructure. Columnar lamellar colonies with widths of 150-200 μm and 50-100 μm respectively were observed in intercellular and dendritic arm regions. The heat treatment of 1400℃ for 12 h+900℃ for 30 min+air cooling could simultaneously remove the B2 phase, massive structure and solidification segregations from the DS alloys, however, it caused severe growth of grains.

Microstructure instability of fully lamellar TiAl alloy containing high content of Nb after long-term thermal cycling

Abstract Microstructure instabilities of the fully lamellar Ti–45Al–8.5Nb–(W,B,Y) alloy were investigated by SEM and TEM after long-term thermal cycling (500 and 1000 thermal cycles) at 900 °C. Two major categories of microstructure instability were produced in the alloy after the thermal cycling: 1) The discontinuous coarsening implies that grain boundary migrations are inclined to occur in the Al-segregation region after the long-term thermal cycling, especially after 1000 thermal cycles. Al-segregation can be reduced during the process of long-term thermal cycling as a result of element diffusion; 2) The α 2 lamellae become thinner and are broken after 1000 thermal cycles caused by the dissolution of a 2 lamellae through phase transformation of α 2 →γ. The γ grains nucleate within the α 2 lamellae or (α 2 +γ) lamellae in a random direction.

Effect of spark plasma sintering temperature on microstructure and mechanical properties of melt-spun TiAl alloys

Abstract A TiAl alloy from pulverized rapidly solidified ribbons with the composition of Ti-46Al-2Cr-4Nb-0.3Y (mole fraction, %) was processed by spark plasma sintering (SPS). The effects of sintering temperature on the microstructure and mechanical properties were studied. The results show that the microstructure and phase constitution vary with sintering temperature. Sintering the milled powders at 1200 °C produces fully dense compact. Higher sintering temperature does not improve the densification evidently. The dominant phases are γ and α 2 in the bulk alloys sintered at 1200 °C. With higher sintering temperature, the fraction of α 2 phase decreases and the microstructure changes from equiaxed near γ grain to near lamellar structure, together with a slight coarsening. The bulk alloy sintered at 1260 °C with refined and homogeneous near lamellar structure reveals the best overall mechanical properties. The compressional fracture stress and compression ratio are 2984 MPa and 41.5%, respectively, at room temperature. The tensile fracture stress and ductility are 527.5 MPa and 5.9%, respectively, at 800 °C.

Reaction behaviors occurring in Ti/Al foil metallurgy

Reaction behaviors occurring in Ti/Al foil metallurgy were systematically investigated. Particular emphasis was focused on the reaction between solid Al and Ti as well as subsequent reaction between TiAl3 and Ti layer. In the solid reaction between Al and Ti, the presence of residual Al is mainly caused by inhomogeneous growth of TiAl3 layer and micro-voids existing at the interface. However, through reaction between molten Al and Ti, TiAl3/Ti multilayer can be achieved with complete consumption of Al. During subsequent high-temperature heat treatment, TiAl3/Ti multilayer will eventually turn into Ti3Al/TiAl multilayer accompanying with simultaneous formation and successive disappearance of intermediate phases, such as TiAl2 and Ti2Al5. Moreover, it is found that the growth direction of TiAl layer changes as a function of annealing time between different couples in multi-intermetallics system.

Influence of Alloying Elements on Deformability of High Nb Containing Tial Alloys

The as-cast microstructures, hot deformation behaviors and as-deformed microstructures of β-γ TiAl alloys containing high niobium and various β stabilizing elements Mn, Cr, Mo and V were investigated. It is found that the increasing addition of Cr and Mo elements contributes to the continuous decrease of flow stress. On the contrary, with increasing the amount of V content, the flow stress enhances. In the Mn content addition range 0.5~2.0at%, the alloy with 1.5%Mn possesses the minimum value of flow stress. The optimum composition for hot working is Ti-44Al-8Nb-1.5Mn-0.2B-0.2Y. The morphology of β phase as well as the amount of β phase plays an important role in controlling the formability of β-;γ TiAl alloys containing high niobium.