Y.G Zhang

Quantitative descriptions of periodic layer formation during solid state reactions

Abstract Periodic layer formation during solid state reactions is related to the stresses induced by the difference in interface growth rate of the two phases within the layer. When the elastic deformation of the slow-growing phase reaches its elastic maximum, it would be split up from the reaction front and the layer separating occurs. Theoretical model describes systematically the patterns formed in various systems and explains almost all of the experimental facts. It also predicts that the reactive diffusion system Zn/Co 2 Si should be interface controlled and system Zn/Fe 3 Si be mixed-controlled.

Aging behavior of Li containing Al–Zn–Mg–Cu alloys

Abstract The aging behaviors of three Al–Li–Zn–Mg–Cu alloys were systemically studied including one-step aging, two-step aging and pre-straining aging. It is shown that the addition of Li suppressed the homogenous precipitation of MgZn 2 phases in Al–Zn–Mg–Cu alloys, and the Zn-rich phases precipitate primarily on grain boundaries in the form of large particles. The two-step aging plays a key role in stimulating nucleation in the Li-free alloy, but has little effect on the hardening efficiency of Li containing alloys. The pre-deformation promotes the precipitation of Zn-rich phases in the matrix, and improves the yield strength greatly. These results are interpreted in terms of the effect of preferential binding of Li atoms and vacancies on the nucleation mechanism. Since Li atoms possess a high concentration and significant vacancy-binding energy, they preferentially trap the quench-in vacancies, then retard the diffusion of Zn, Mg atoms, and suppress the homogenous nucleation of Zn-rich phases in the matrix. The two-step aging shows little benefit on the precipitation of Zn-rich phases in the matrix because of the retarding effect of Li atom. While pre-deformation increases the defect concentration in the matrix, thereby causes the modification of size and distribution of Zn-rich precipitates.

In-situ TEM study of fracture mechanisms of polysynthetically twinned (PST) crystals of TiAl alloys

Abstract The fracture mechanisms of polysynthetically twinned (PST) crystals of Ti–49at.% Al in different lamellar orientations have been investigated by using in-situ transmission electron microscopy (TEM). The results indicated that the fracture mechanisms and crack propagation behavior depended strongly on the lamellar orientation of PST crystals. When the tensile axis was nearly parallel to the lamellae, the main crack propagated discontinuously by nucleation, growth and linkage of microcracks formed ahead of crack tip. Slip band decohesion, fracture along prism plane of α2 plates and twinning-induced microcracks are three modes of nucleation of microcracks. When the loading axis was perpendicular to the lamellae, the crack parallel to the lamellae propagated by tearing and shear of shear ligament. Interfacial microcrack usually occurred ahead of the main crack tip. Sometimes, the main crack propagated along (111) cleavage plane within γ lamellae.

General theory of interdiffusion growth in diffusion couples

Abstract This paper shows that it is possible to treat solid-phase growth thoroughly with methods of fluid mechanics. Based on the concept ‘flow point’ borrowed from fluid mechanics, the general equations are derived to describe atomic interdiffusion in diffusion couples. Two types of phase growth, i.e. volume and interface growths, are mathematically defined as the growing expansion of flow points within interdiffusion field and the creation of flow points at interphase interface, respectively. Mathematical discussions on the thickness–time dependence also indicate that phase growth in diffusion couples can be really interface-controlled when the interface coefficient is much smaller than the characteristic diffusion coefficient. It is also shown that with certain conditions Darkens equations can be strictly deduced from the general equations and Darkens equations should be only applicable for the binary interdiffusion system, in which system no new phase should be created. Furthermore, the Kirkendall velocity expressed by one of the Darkens equations should be defined as the velocity of flow point within that system.

Effect of internal stresses on the fracture toughness of a TiAl-based alloy with duplex microstructures

Abstract A series of fine duplex microstructures with different volume ratios of equiaxed γ and lath colonies (α2+γ) were achieved from an isothermally forged Ti–46.5Al–2Cr–2Nb alloy. The internal stress determination using X-ray diffraction techniques and plane-strain fracture toughness tests were carried out at room temperature, and the effects of the internal stresses on the fracture toughness were investigated. It was found that the internal stresses play an important role in determining the fracture toughness of the TiAl-based alloy. The compressive internal stresses in γ phase and the tensile internal stresses in α2 phase may be responsible for the delamination along the γ/α2 boundaries. For the microstructures with different volume fraction of lamellar grains, the fracture toughness increases with the internal stresses, which facilitate the formation of shear ligaments; and the effects of the α2 phase and equiaxed gamma grains on the internal stresses and the fracture toughness were discussed.

The fracture mechanism of a fully lamellar γ-TiAl alloy through in-situ SEM observation

Abstract Fracture mechanisms in fully lamellar γ-TiAl alloys were studied by investigating interactions between cracks and lamellae within grain or grain boundaries using in situ SEM technique. The results showed that the fracture mechanisms depended on not only lamellar orientation to loading axis but also the type of grain boundary encountered.

A study of mechanical properties and microscopic stress of a two-phase TiAl-based intermetallic alloy

In order to study the effects of the microstructures on the mechanical properties and internal stresses of a TiAl-based alloy, a series of fine duplex microstructures with different volume ratios of equaxied grains and +2 lath colonies of a TiAl-based alloy were produced by isothermal forging and heat treatment. Tensile tests were carried out at room temperature, and X-ray elastic constants were determined experimentally. The mechanical state of a given phase in the alloy, induced by uniaxial tensile loading, was characterized by X-ray diffraction. It was found that the macroscopic mechanical properties and microscopic stresses strongly depend on the microstructures, and there is a difference between the macroscopic properties and the microscopic properties, and the reasons are discussed.

X-ray elastic constant determination and microstresses of α2 phase of a two-phase TiAl-based intermetallic alloy

Abstract The elastic constants of α 2 phase of a two-phase TiAl-based intermetallic alloy were determined experimentally by X-ray diffraction method. From these results, the residual stresses and the mechanical state of α 2 phase in the alloy, induced by a uniaxial tensile loading, were characterized by X-ray diffraction. It was found that the X-ray elastic constants of α 2 phase differ from those of γ phase and the macroscopic ones, the mechanical state of α 2 phase shows a great difference compared with that of γ phase, and the reasons were discussed.

Serrated flow related to shearing of particles in binary Al-Li single crystals

Abstract Serrated flow was investigated in binary Al–Li single crystals treated from naturally aged to overaged conditions. Serrations were observed during the deformation of crystals in artificially aged conditions, but not in naturally aged condition. The number of serrations with small stress drops as well as short elapsed time increases in peak-aged condition as compared to underaged or overaged conditions. It is indicated that shearing of δ ′ particles plays a key role in the discontinuous deformation of Al–Li single crystals.

Thermodynamic calculated and TEM observed microstructure of Al–Li–Mg–Si alloys

Abstract The microstructures of two Al–Li–Mg–Si alloys have been studied by thermodynamic calculation and TEM observation in this paper. The results show that the addition of lithium changes the precipitation behavior of Al–Mg–Si alloys. The calculated Al–Li–Mg–Si phase diagrams are used to investigate the equilibrium phase distribution as a function of temperature for the Al–1.8Li–1.8Mg–0.8Si alloy and Al–1.8Li–2.8Mg–0.8Si alloy. According to calculation, δ′ phase (Al 3 Li) becomes the mayor precipitate in Al–1.8Li–1.8Mg–0.8Si alloy while a few amount of β′(Mg 2 Si) phase and AlLiSi phase precipitate. TEM observation show that δ′ phase and AlLiSi phase are precipitated in Al–1.8Li–1.8Mg–0.8Si and heterogeneous rod Mg 2 Si phase is also observed. Calculation on Al–1.8Li–2.8Mg–0.8Si show that both δ′ phase and Mg 2 Si phase precipitate on aging temperature. Experimental results are in agreement with calculation.