Y.Q. Yang

Transformations in sputter-deposited thin films of NiTi shape memory alloy

Abstract Amorphous NiTi films deposited by magnetron sputtering have been studied. The results show that it is easier for precipitation to occur in crystallized films and that, in the films aged at 550 °C for 1 h, the parent phase transforms to rhombohedral phase at 27 °C and then to martensitic phase at 2 °C.

A review on the research progress of push-out method in testing interfacial properties of SiC fiber-reinforced titanium matrix composites

Experimental analysis of single-fiber push-out for SiC fiber-reinforced titanium matrix composites (TMCs) is complicated by the incorporation of large thermal residual stresses, strong chemical bond of the fiber/matrix interface and matrix plastic deformation. This paper summarizes the development of push-out test and the characteristics of push-out test for TMCs such as crack initiating at the bottom face and theoretical analysis of the test. Moreover, it deeply analyzes the progresses of interfacial shear strength and fracture toughness, and work focus is pointed out in future.

The time-temperature transformation diagram concerning pure bainite formation in a Cu-Zn alloy

The particular time and temperature regime of the bainitic transformation in a Cu-Zn alloy is revealed by the time-temperature transformation (TTT) diagram determined experimentally. The areas, in which a pure or mixed phase product appears, are well exhibited in the TTT diagram. The pure bainitic transformation occurs only when the point representing temperature and time falls within a particular area marked in the TTT diagram.

Effect of quenching on the matrix microstructure of SiCf/Ti–6Al–4V composites

Heat treatment can change the matrix microstructure of metal based composites, which leads to the change of mechanical properties of the composites. In the present work, the variation of microstructure and hardness of the matrix of SiCf/Ti–6Al–4V composites prepared by foil-fiber-foil (FFF) method were investigated after quenching from 950, 980 and 990xa0°C. The results indicated that the β transus temperature of the matrix was close to 990xa0°C, which is slightly higher than that of the Ti–6Al–4V alloy, which is between 950 and 970xa0°C. This increase was attributed to the diffusion effect of carbon atoms. Some carbide particles were precipitated at the prior β grain boundaries when full martensite was obtained. Transmission electron microscopy examination observed three α′ variants that obeyed the Burgers orientation relationship with the β phase in the sample quenched from 950xa0°C. The hardness of the matrix increased with the increase in quenching temperature, and the highest hardness of 420 HV was obtained after quenching from 990xa0°C, which is much higher than that of the Ti–6Al–4V alloy. The high hardness was resulted from phase transformation strengthening by martensite, solid solution strengthening of carbon atoms and the dispersion strengthening of TiC precipitates.

Precession electron diffraction assisted orientation mapping of gradient nanostructure in a Ni-based superalloy

Surface mechanical grinding of a Ni-based superalloy can introduce a gradient microstructure in the surface layer with a grain size from nanoscale to microscale. In-depth investigation of the crystal orientation distribution of the surface nanostructured layer is more often, however, not an easy work by using the scanning electron microscope (SEM) based electron backscatter diffraction (EBSD) method due to its sensitivity to lattice distortions and spatial resolution limitation. Here we use a newly developed precession electron diffraction (PED) technique coupled with transmission electron microscopy (TEM) to investigate the microstructural and crystallographic characteristics of the surface gradient nanostructure, with particular emphasis on the topmost nanocrystalline layer. A strong shear texture and a minor Copper texture were identified according to orientation analyses of the 1.6 pm thick near-surface nanocrystalline layer. The PED technique is proved to be practical for two dimensional orientation mapping of severely deformed microstructures at the nanoscale.

The influence of interface reaction zone on interfacial fracture toughness of SiC fiber reinforced titanium matrix composites

Abstract A fiber-reaction zone-matrix three-phase model is developed to evaluate the interfacial fracture toughness of titanium alloys reinforced by SiC monofilaments. Based on fracture mechanics, theoretical equations of GIIc are presented, and the effects of several key factors such as crack length and the interface reaction zone thickness on the critical applied stress necessary for crack growth and interfacial fracture toughness are discussed. Finally, the interfacial fracture toughness of typical composites including Sigma1240/Ti-6Al-4V, SCS-6/Ti-6Al-4V, SCS-6/Timetal 834, SCS-6/Timetal 21s, SCS-6/Ti-24Al-11Nb and SCS-6/Ti-15V-3Cr are predicted by the model. The results show that the model can reliably predict the interfacial fracture toughness of the titanium matrix composites. dA is the incremental increase in crack surface area, and dUex is the work done by the loading system, and dUse is the change in strain energy of the system due to crack advance, dUfr is the work done in frictional sliding at the interface.

Fatigue behaviors of C/Mo double-coated SiC fiber-reinforced Ti6Al4V composites with varied interfacial microstructure

Fatigue crack propagation behaviors of the as-prepared and thermally exposed (700 °C/196 h, 800 °C/196 h) C/Mo double-coated SiC fiber-reinforced Ti6Al4V composites were investigated. The results show that interfacial microstructure evolution has significant effect on fatigue crack propagation behaviors. As for the as-prepared and 700 °C/196-h thermally exposed composites, fiber-bridging accompanied with interfacial debonding can decrease the crack growth rate significantly, while the latter one has lower crack growth rate at the early fatigue crack propagation stage, as the diffusion of Mo atoms makes the matrix close to the interface have more ductile β-Ti. However, fiber-bridging phenomena disappeared with the altered interface in the composite after 800 °C/196-h thermal exposure. Fatigue fracture analysis further reveals that multistep deflections of crack in interface contribute to the improvement of interface toughness. The formation of ductile β-Ti layer resulted from the diffusion of Mo atoms is beneficial to blunt the crack tip.