Y.C Liu

Structural evolution of undercooled Ni-Cu alloys

The structural evolution of the whole Ni–Cu system with undercooling was systematically investigated. For pure Ni and Cu only one grain refinement was found when they were undercooled higher than the critical value ΔT*. For the alloys, however, another grain refinement could occur in a certain range of the lower undercooling. Based on current dendrite growth theory, a thermodynamic concept, dimensionless superheating, was developed to evaluate the tendency of the dendrite remelting. With undercooling increasing, the dimensionless superheating of the alloy increased first and then decreased, which suggested that the dendrite remelting should only be responsible for the grain refinement at low undercoolings although the effect of the remelting during the recalescence on the eventual structural morphology deserved special attention. The decrease of the grain size above ΔT* was attributed to the stress that originated from the impediment of the static liquid to the solidification contraction or expansion of the rapidly growing dendrite skeleton, and led to the distortion and break-up of the dendrites. The recrystallization happening in cooling could make the grain size decrease further. The measurement of the refined grain size indicated that it was a strong function of the composition.

Microstructure and mechanical properties of the hypercooled Ti50Al50 alloy

Abstract Hypercooling region of the gamma titanium aluminide alloy Ti 50 Al 50 was achieved by cyclically superheating the alloy melt in a containerless electromagnetic levitation apparatus. The maximum undercooling of the alloy melt amounted to 345 K. XRD, TEM, SEM and optical microscopy techniques were adopted to investigate the microstructure and identify the phase composition. The microstructure of the hypercooled alloy was composed of equaxied gamma phase grains with a mean diameter of 17 nm. The fine microstructure of the gamma phases produced a large strengthening effect, raising the hardness to 545 on the Vickers scale. The aluminum composition distribution in the hypercooled sample was homogenous except for a small difference between first solidified regions and boundaries of cracks. X-ray diffraction results showed that all peaks of the gamma phase shifted slightly in the direction of small angles. This can be explained by the disordering growth pattern caused by the rapid solidification process in the hypercooled melt.

Growth mode of decagonal quasicrystal phase in laser resolidified Al72Ni12Co16 alloy

Abstract The ultra-high-temperature gradient directional solidification process produced by laser beam was adopted to achieve the directionally solidified microstructure and analyze the growth pattern of the stable decagonal quasicrystal phase in Al 72 Ni 12 Co 16 alloy. The X-ray, SEM, TEM and optical microscopy techniques were used to investigate the microstructures and identify the phase. The directionally solidified decagonal quasicrystal showed the facet morphology and grew in lateral growth mode. The unusual periodic and quasiperiodic atomic structures of the decagonal quasicrystal and the epitaxial growth in the melt pool were considered to be the key factors influencing the growth morphology. The experimental results were well explained by the Toners theoretical atomistic growth model.

Icosahedral phase growth in chill cast Ti68Fe28Si4 alloy

Phase selection during growth involving the icosahedral quasicrystal and metastable crystalline β-Ti phases was investigated by means of chill cast. The metastable β-Ti phase was found near the chilling plate, while the icosahedral quasicrystal phase in the upper part. The X-ray, TEM, optical microscopy techniques were used to investigate the microstructure and identify the types of phases. The icosahedral quasicrystal phase grew in rounded dendritic pattern continuously. Aligned dendritic growth facilitated the massive transformation from metastable β-Ti phase to icosahedral quasicrystal phase.

A comparative study on the quality of protein crystals obtained using the cross-diffusion microbatch and sitting-drop vapor diffusion methods

We presented a systematic quality comparison of protein crystals grown using the cross-diffusion microbatch (CDM) and standard sitting-drop vapor diffusion methods. Eleven proteins were screened and it was found that crystals grown using CDM exhibited a better morphology. X-ray diffraction showed that the CDM method is practical and useful for obtaining high-quality protein crystals.

Recrystallization: a method to improve the quality of protein crystals

The quality of protein crystals is an important parameter for structural determination with X-ray crystallography. Indeed, a prerequisite for obtaining high-resolution diffraction data is that the crystals be of sufficient quality. However, obtaining high-quality protein crystals is a well known bottleneck to protein structural determination that remains a difficult task. In this paper, it is demonstrated that recrystallization can be an effective method of improving the quality of protein crystals. Five proteins, lysozyme, proteinase K, concanavalin A, thaumatin and catalase, were used for this investigation, and the crystal quality of these proteins was examined using X-ray diffraction before and after recrystallization. Comparisons of the crystals before and after recrystallization verified that recrystallization not only enhanced the morphology of the crystals but also improved crystal quality. Therefore, it is proposed that recrystallization might be a useful alternative method for obtaining protein crystals with enhanced diffraction.

Nucleation-controlled bulky metastable γ phase Ti–45Al–2Cr–2Nb alloy

Abstract Cyclically superheating technology was adopted effectively to undercool the Ti–45Al–2Cr–2Nb alloy melt in a containerless electromagnetic levitation apparatus. The maximum achieved undercooling of the melt was up to 252 K and phase α was detected as the primary phase of all the obtained undercooling range. TEM, SEM and optical microscopy techniques were used to investigate the microstructure and identify the phase composition. The arc-melted microstructure of it was composed of the lamellae (α 2 +γ) and retained high temperature phase α, which transformed into fully lamellar structure (α 2 +γ) after homogenization at 1273 K for 80 h. The controlling of phase selection process was attempted in the undercooled melt using an external nucleation needle with a single-phase γ structure of Ti 44 Al 56 alloy. When the undercooling of the melt was larger than a certain value, 54 K, the competitive nucleation of stable and metastable phases would be destroyed by the triggering needle with metastable γ phase crystalline structure, and the bulky metastable γ phase materials was successfully prepared in the undercooled full lamellar Ti–45Al–2Cr–2Nb alloy. The concept of the shortest incubation time of the formed phase according to the transient nucleation theory was adopted to explain the nucleation competition and metastable phase formation.

Decagonal quasicrystal growth in chill cast Al72Ni12Co16 alloy

Abstract The single-phase decagonal quasicrystal alloy Al72Ni12Co16 was melted by containerless processing and chilled on a copper substrate in an electromagnetic facility. X-ray, TEM, and SEM techniques were used to investigate the microstructure and identify the phase composition. The approximately directionally solidified decagonal quasicrystal in the chill cast sample showed facet morphology and grew in a lateral growth mode along the twofold axis normal to the plane (00002). The experimental results fit well with the prediction of Toner’s theoretical atomistic growth model.

Roughening transition of decagonal quasi-crystal in undercooled Al72Ni12Co16 alloy

Abstract High undercooling of the single-phase decagonal quasi-crystal alloy Al72Ni12Co16 was achieved by cyclically superheating the melt in a containerless electromagnetic levitation apparatus. The maximum undercooling of the said material was up to 400 K. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), and optical microscopy techniques were used to investigate the microstructure and identify the phase composition. The decagonal quasi-crystal phase in the samples with small undercoolings showed a facet morphology and grew in a lateral growth mode below the roughening transition undercooling point, ΔT=64 K. With increasing undercooling, the decagonal phase assumed an equiaxed structure and indicated a continuous growth pattern. Toners surface dynamics model of growing three-dimensional quasi-crystal was adopted to explain the roughening phenomena in the undercooled melt.