TiAl alloys are potential structural high-temperature structural materials at a service temperature of ~900 °C, while poor ductility at room temperature and high creep rate at the elevated temperature limits the applications. To improve the room-temperature and high-temperature mechanical properties of Ti–44Al–5Nb–3Cr–1.5Zr, Mo and B were introduced into this system and Ti–44Al–5Nb–3Cr–1.5Zr–xMo–yB alloys were proposed. And then, we, respectively, studied the microstructures and mechanical properties of Ti–44Al–5Nb–3Cr–1.5Zr–xMo–0B and Ti–44Al–5Nb–3Cr–1.5Zr–1Mo–yB to elucidate the role for the addition of Mo and B. It is found that Mo can increase the fraction of B2 phase in the alloys and the microstructures of the alloys are greatly refined by the addition of B. The compression test results indicate that Mo has a positive influence on the high-temperature compressive properties of TiAl-based alloys, whereas B addition can improve their room-temperature compressive properties of Ti–44Al–5Nb–3Cr–1.5Zr–1Mo–yB alloys; the morphology of borides in each sample should be the structural origin for these phenomena.

Laves phase plays a positive role in improving the strength of high-entropy alloys (HEAs); Nb and Ti elements have potential to promote Laves phase formation in some HEAs. For improving the strength of the face-centered cubic (FCC) CoCrFeMnNi HEA, a series of (CoCrFeMnNi)100−xNbx (atomic ratio: x = 0, 4, 8, 12, 16) and (CoCrFeMnNi)100−xTix (atomic ratio: x = 0, 2, 4, 6, 8, 12) HEAs were prepared by melting. The effects of Nb and Ti on the microstructure evolution and compressive properties of the CoCrFeMnNi HEAs were investigated. For (CoCrFeMnNi)100−xNbx HEAs, the second-phase (Laves and σ phase) volume fraction increased from 0 to 42%. The yield strength also increased gradually from 202 to 1010 MPa. However, the fracture strain decreased from 60% (no fracture) to 12% with increasing Nb content. For (CoCrFeMnNi)100−xTix HEAs, the yield strength increased from 202 to 1322 MPa. The Laves phase volume fraction also increased from 0 to 27%. However, the fracture strain decreased from 60% (no fracture) to 7.5% with increasing Ti content. Addition of Nb and Ti has a good effect on improving the strength of FCC CoCrFeMnNi HEA.

The hydrogenation behavior of Ti–44Al–6Nb (at.%) alloy was studied at temperature range of 1373–1693 K, and the effect of hydrogen on hot deformability was tested on Gleeble-1500D thermo-simulation machine. It is found that the lnCH increases linearly with 1/T, and hydrogen content increases with increasing of hydrogen time and flow rate logarithmically. The positive heat of solution of hydrogen denotes that hydrogen absorption in TiAl alloys is an endothermic reaction. The results also show that hydrogen promotes the lamellar colony size and lamellar spacing because that hydrogen can promote the diffusion of elements. There is more residual B2 phase in the hydrogenated alloy revealing that hydrogen stabilizes the B2 phase during hydrogenation. The nanohardness and elastic modulus of the alloy are decreased from 4.4 and 213.5 GPa to 4.2 and 199.8 GPa after hydrogenation with 0.033 wt% H. Thermal simulation results show that the peak stress is decreased by 30% after hydrogenation with 0.033 wt% H which corresponds to decreasing the deformation temperature by about 50 K. This is attributed to hydrogen-promoted dynamic recrystallization and dislocation movement.

Ti–47Al–1.0W–0.5Si (at.%) alloy was directionally solidified in the range of growth rate (V) (V = 3–100 μm/s) at a constant temperature gradient (G = 18 K/mm). It was found that α phase was the primary phase of the alloy. Both primary dendritic arm spacing (λ) and interlamellar spacing (λs) decreased with increase of the growth rate (V) according to the relationship of λ ∝ V −0.356 and λs ∝ V −0.49, respectively. The Solidification segregation occurred since the enrichment of the solute element W in primary α phase during solidification. The degree of the segregation increased with the increase of the growth rate (V). The results also revealed that the lamellar orientation was not always perpendicular to the growth direction (GD) because the GD of primary α dendritic deviated from the preferred $\left\langle {0001} \right\rangle$ direction. The microhardness increased with increasing growth rate (V) according to H V ∝ 289.5V 0.12 because of the microstructure refinement.

Two kinds of heavy-alloying β-type TiAl-based alloys Ti44Al6Nb1.0Cr2.0V (A1) and Ti44Al6Nb1.0Cr2.0V0.15Y0.1B (A2) are newly designed. They are prepared by vacuum consumable melting (VCM) and cold crucible directional solidification (CCDS). Via the theoretical analysis and tentative experiment, five alternative heat treatment (HT) schedules are proposed and studied that the corresponding microstructure and room temperature (RT) tensile property are investigated, and finally the optimized HT schedules are acquired. After HT5 (heat preservation in β phase region and at 1290 °C, and then ladder cooling), A2 alloy cast by VCM exhibits a better tensile property with average elongation of 1.20%. For the two CCDS ingots, after HT3 (mainly annealing at 1280 °C), B2 phase and (B2 + γ) blocky morphology are reduced, the columnar grains and small angle lamellas are reserved, and the tensile property also has a moderate improvement.

Three-dimensional (3D) morphological evolution and growth mechanisms of primary I-phase particles have been investigated in directionally solidified Al–6Mn–2.5Be (wt%) alloy at a wide range of growth rates (100–1500 μm/s). At relatively low growth rates (100–600 μm/s), the I-phase particles exhibit faceted growth with strong anisotropy, forming a hierarchical flower-like aggregate with icosahedral morphological symmetry composed of several attached irregular polyhedrons or pentagonal dodecahedrons. At higher growth rates (e.g., 1000 μm/s), the interface of the I-phases becomes unstable along the edges and corners of the pentagonal dodecahedron, thereby arousing growth perturbations. Correspondingly, a transition from faceted to nonfaceted growth occurs with increasing growth rate. Further increase of the growth rate leads to the formation of I-phase columnar dendrites' preferential growth along the 3-fold axis. The configurations of the flower-like aggregates can be adequately illustrated by a geometrical model in terms of the perfect and elongated pentagonal dodecahedrons. A growth mechanism for the flower-like aggregates has been proposed based on the clear understanding of the 3D morphological evolution of the I-phase particles.

A comprehensive understanding of the growth pattern of intermetallic compounds (IMCs) during solidification is critical to both the crystal-growth theory and its property optimization. In this article, growth pattern and three-dimensional (3D) morphology of primary Al6Mn IMC were investigated in directionally solidified Al–3 at.% Mn alloy at a wide range of growth rates. A transition from faceted (<60 μm/s) to nonfaceted growth (>100 μm/s) was observed with increasing growth rates. Correspondingly, 3D morphologies of primary Al6Mn change from a solid polyhedron to a hollow structure, and then to a dendrite. This kind of change is associated with the competitive growths of different crystal planes determined by the crystallographic anisotropy and growth kinetics of Al6Mn. A growth model based on atomic cluster attachment is proposed to reveal the growth transition, and a growth-rate ratio between different crystal planes is used to appropriately reveal the formation mechanism of different morphologies at low rates.

During peritectic solidification, besides the longitudinal remelting of the primary phase at the temperature of peritectic reaction $\left( {T_{\rm{p}}^K} \right)$, a lateral remelting phenomenon of the primary phase below $T_{\rm{p}}^K$ is observed under high velocity in directionally solidified Cu–Ge alloys. The lateral remelting occurs continuously along a liquid channel as temperature decreases, and the lateral remelting velocity is larger than that of peritectic transformation. The lateral remelting leads to the morphological change of the primary dendrites, even the fragmentation of dendrite arms. The phenomenon also means that the classical theory calculating the volume fraction of the primary phase during peritectic transformation can need to be modified under some conditions. However, under low velocity, the phenomenon is not so significant. The phenomenon is explained by means of solidification and remelting theory.

During directional solidification of Cu–Ge peritectic alloys, a two-phase separated structure has been observed. With proper growth conditions, the peritectic ζ-Cu5Ge and primary α-Cu phases completely separate and form cylindrical layered structures. It is found that the formation of the separated structure is closely related to double diffusive convection and growth conditions. In the two-phase separated structure, a large trijunction region of peritectic reaction forms around the cylindrical α-Cu phase. During peritectic reaction, the morphological instabilities of ζ-Cu5Ge occur under high pulling velocities and are explained by the constitutional undercooling criterion. A new coupling growth between the ζ-Cu5Ge-phase and the groove of α-Cu phase near the trijunction is observed. Different from peritectic coupling growth, the diffusion coupling is established below the peritectic temperature. This two-phase separated growth process creates new opportunities for the fabrication of functionally layered materials.

Sn–40 at.% Mn peritectic alloys were directionally solidified at different growth rates (1–100 μm/s) under a steep temperature gradient (40 K/mm). The migration of secondary dendrite arm was observed in this peritectic alloy in which both the primary phase and the peritectic phase are intermetallic compounds with nil solubility. This migration is caused by coupling remelting/solidification at the hot/cold sides of the liquid pool between two adjacent secondary dendrite arms by temperature gradient zone melting. Its novel feature is that the remelting temperature of primary phase is a little higher than the solidification temperature of peritectic phase. Analytical solutions based on the assumption that the solubility of both primary and peritectic phases are nil have been proposed to describe this migration. It has also been found that the migration of secondary dendrite arm is most obvious at intermediate growth rates under steep temperature gradient in the directionally solidified Sn–40 at.% Mn peritectic alloy.

The primary dendrite arm spacing and its distribution at the solid–liquid interface has been examined in directionally solidified Sn–36 at.% Ni peritectic alloys under constant temperature gradient in a range of growth rates (2–200 μm/s). Statistical analysis of the primary dendrite arm spacing on transverse sections has been carried out using the minimum spanning tree and Voronoi polygon. The frequency distribution of the number of nearest neighbors determined by the Voronoi polygon suggested that the arrangement of dendrites at the solid–liquid interface could be visualized as hexagonal tessellation. The primary dendrite arm spacing determined by the conventional area counting method and minimum spanning tree all decreased with increasing growth rate, and a range of primary dendrite spacing was present during solidification. The range first increased with increasing growth rate, but when the growth rate exceeded 20 μm/s, it turned to decrease, which can be attributed to disorder induced by growth rate and interdendritic convection.

The effects of hydrogen on the structure of Zr-based bulk metallic glasses were investigated by positron annihilation lifetime spectroscopy. Three lifetime components are identified, indicating the presence of three distinct size ranges for open volume defects in the glass. The concentration of the smallest sites identified as tetrahedral interstitial holes in the densely packed and the intermediate sites identified as flow defects, changes with hydrogen addition. The concentration of tetrahedral interstitial holes in Zr55Cu30Ni5Al10 alloys initially increases with the increase of hydrogen content. When Zr55Cu30Ni5Al10 alloys were prepared in Ar + 10%H2 atmospheres, the concentration of tetrahedral interstitial holes reaches a maximum, which may provide a more dense random-packed structure. For Zr57Al10Cu15.4Ni12.6Nb5alloys, the increase of hydrogen content causes a decrease in the concentration of tetrahedral interstitial holes and an increase in the concentration of flow defects.