Jining Qin

Microstructure evolution and mechanical properties of a Ti–35Nb–3Zr–2Ta biomedical alloy processed by equal channel angular pressing (ECAP)

In this paper, an equal channel angular pressing method is employed to refine grains and enhance mechanical properties of a new β Ti-35Nb-3Zr-2Ta biomedical alloy. After the 4th pass, the ultrafine equiaxed grains of approximately 300 nm and 600 nm are obtained at pressing temperatures of 500 and 600°C respectively. The SEM images of billets pressed at 500°C reveal the evolution of shear bands and finally at the 4th pass intersectant networks of shear bands, involving initial band propagation and new band broadening, are formed with the purpose of accommodating large plastic strain. Furthermore, a unique herringbone microstructure of twinned martensitic variants is observed in TEM images. The results of microhardness measurements and uniaxial tensile tests show a significant improvement in microhardness and tensile strength from 534 MPa to 765 MPa, while keeping a good level of ductility (~16%) and low elastic modulus (~59 GPa). The maximum superelastic strain of 1.4% and maximum recovered strain of 2.7% are obtained in the billets pressed at 500°C via the 4th pass, which exhibits an excellent superelastic behavior. Meanwhile, the effects of different accumulative deformations and pressing temperatures on superelasticity of the ECAP-processed alloys are investigated.

Investigation of Deformation Mechanisms in β -Type Ti-35Nb-2Ta-3Zr Alloy via FSP Leading to Surface Strengthening

Friction-stir processing (FSP) is used to prepare Ti-35Nb-2Ta-3Zr alloys via different processing routes. Dislocation walls and tangles, deformation-induced α″ martensite, and deformation-induced ω phase are observed. The dominant deformation mechanisms are altered from deformation-induced α″ martensitic transformation and dislocation walls to twinning upon increasing the FSP passes. A reverse deformation-induced ω to β transformation and de-twinning process are observed together with grain refinement to the nanoscale. Meanwhile, compressive distortions along [0001]ω direction are favorable for the transformation from ω to β.

Superelastic and shape memory properties of Ti x Nb3Zr2Ta alloys

The microstructure and phase constitutions of TixNb3Zr2Ta alloys (x=35, 31, 27, 23) (wt%) were studied. With a lower niobium content the grain size of β phase in TixNb3Zr2Ta alloys increased significantly, and the TixNb3Zr2Ta system was more likely to form α″ phase and even α phase. Tensile tests showed that UTS of TixNb3Zr2Ta alloys improved as the Nb content was decreased. Cyclic loading-unloading tensile tests were carried on TixNb3Zr2Ta alloys. Ti23Nb3Zr2Ta and Ti27Nb3Zr2Ta alloys featured the best superelasticity among the alloys studied. The pseudoelastic strain ratio of Ti35Nb3Zr2Ta alloy decreased a lot as the cycle number increased. Ti31Nb3Zr2Ta alloy showed only minimum superelasticity. This is because Ti23Nb3Zr2Ta and Ti27Nb3Zr2Ta alloys had higher yield strength than Ti31Nb3Zr2Ta did, which allowed martensite phase to be induced. On the contrary, Ti31Nb3Zr2Ta alloy exhibited better shape memory property than Ti27Nb3Zr2Ta, Ti23Nb3Zr2Ta and Ti35Nb3Zr2Ta titanium alloys. β phase, α phase and α″ phase were found in Ti23Nb3Zr2Ta alloy by TEM observation. The dislocation density of α phase was much lower than that of β phase due to their crystal structure difference. This may explained why Ti23Nb3Zr2Ta with α phase possessed higher tensile strength. The incomplete shape recovery of Ti23Nb3Zr2Ta alloy after unloading resulted from two sources. Plastic deformation occurred in β phase, α phase and even α″ phase under dislocation slip mechanism, and incomplete decomposition of α″ martensitic phase resulted in unrecovered strain as well.

Microstructure and Tensile Properties of In Situ Synthesized (TiB+TiC)/Ti-6Al-4V Composites

In this paper, Ti-6Al-4V matrix composites reinforced with 5% or 10% TiB and TiC were in situ synthesized by common casting and hot-forging technology utilizing the reaction between titanium and B4C. The phase constituents were identified by XRD while transus temperatures were determined by DSC and metallography. The evolution of microstructures was studied by optical microscopy. The effects of reinforcements on the microstructures, tensile properties and fractures at room temperature were discussed. The results show that yield strength and ultimate tensile strength increased significantly while ductility decreased with reinforcements increasing. Fracture type turned to brittle when reinforcements increased.

Evaluation of Weak Interface Effect on the Residual Stresses in Layered SiC/TiC Composites by the Finite Element Method and x-ray Diffraction

A symmetrically layered SiC/TiC ceramic with a gradual structure was designed by the finite element method (FEM). After sintering, proper thermal residual stress was introduced into the ceramic due to the coefficients of thermal expansion mismatch between the different layers. After different SiC + C interlayers were inserted into the layers to weaken the interface, the effect of the composition of the SiC + C interlayers between the layers on the residual stress was evaluated. It was found that the weak SiC + C interlayer had little relaxation effect on the residual stress distribution. These ceramics were then fabricated by aqueous tape casting, stacking, and hot-press sintering. An x-ray stress analyzer was used to test the surface stress conditions of the sintered materials. The tested surface stress of the layered SiC/TiC ceramic without interlayer was very close to the FEM calculation. However, there were differences between the tested and calculated results of the layered SiC/TiC ceramics with interlayers; the reason for this was analyzed.

Weld zone characteristic and mechanical performance of in situ titanium matrix composites using gas tungsten arc welding (GTAW)

Abstract In situ TiBw and La2O3 reinforced titanium matrix composites were successfully fusion welded by the gas tungsten arc welding (GTAW) process and the weldability and feasibility for composites were studied, and uniform and defect free welds were produced with sound welding parameters. Microstructural observations showed that the joint has a distinctly identified weld zone characteristic. In addition, the distribution and size of TiBw in the weld became much more homogeneous and smaller. Moreover, the welded joints exhibited good mechanical properties at ambient temperature and the strength equal to the base metal at elevated temperature. The fracture mechanism of TiBw in the weld also was investigated.

Microstructure and mechanical properties of GTA weldments of titanium matrix composites prepared with or without current pulsing

Abstract The effects of current pulsing on the microstructure, hardness and tensile properties at different temperatures of gas tungsten arc (GTA) weldments of titanium matrix composites were studied. Full-penetration butt joints were made with or without current pulsing. Optical microscopy, hardness test and scanning electron microscopy were employed to evaluate the metallurgical characteristics of welded joints. Tensile properties of weldments at different temperatures were studied and correlated with the microstructure. The results exhibit that current pulsing leads to the refinement of the weld microstructure and TiB whisker and the redistribution of reinforcements resulting in higher hardness, tensile strength and ductility of weldments in the as-welded condition.

Microstructure and mechanical properties of (TiB+La2O3)/Ti composites heat treated at different temperatures

Abstract Near-α titanium matrix composites reinforced with TiB and La 2 O 3 were synthesized by common casting and hot-working technology. The effects of β heat treatment temperature on the microstructure and the tensile properties of the in situ synthesized (TiB+La 2 O 3 )/Ti were studied. Microstructure was studied by OM and TEM, and tensile tests were carried out at room temperature and 923 K, respectively. Results show that with the increase of β heat treatment temperature, prior β phase grain size increases and α colony size decreases. Room temperature tensile strength increases with the increase of β heat treatment temperature, which can be attributed to the decrease of α colony size with the increase of β heat treatment temperature. However, high-temperature tensile strength decreases with the increase of β heat treatment temperature and the decrease of the high-temperature tensile strength is due to the increase of the prior β phase grain size.

Microstructures and mechanical properties in laser beam welds of titanium matrix composites

Abstract Laser beam welding (LBW) was used to 2 mm thick titanium matrix composites (TMCs) sheets and microstructure, hardness and tensile properties of butt joints were studied. Welded joints without defects were obtained indicating that LBW is a suitable processing method for TMCs. The results reveal that the fusion zone completely consists of α′ martensite causing an increase of more 27% in hardness compared with that of base metal. The heat affected zone consists of a mixture of α′ martensite and primary α phases. Large gradients of microstructures and hardness are found over the narrow heat affected zone dependent on the β-transus temperature during weld cooling. TiBw with smaller sizes redistribute at grain boundaries in the weld. The joints show excellent strength and they can reach the full strength compared with the base metal with sound welding parameters, which is ascribed to the presence of α′ martensite and refinement of TiBw in the weld.

Mechanical Behavior of a Hybrid Reinforced Magnesium Composite Fabricated by Pressure Infiltration Method

A hybrid reinforced composite based on magnesium alloy AZ60A reinforced with B4C particulates and SiC wisker, with volume percentage of 12% of each reinforcement respectively, was fabricated using pressured infiltration processing technique followed by hot extrusion. The microstructure of the composite and the fracture surface of tensile sample were investigated by optical microscopy and scanning electron microscopy (SEM). The results show a nearly uniform distribution of the reinforcements throughout the as-extruded magnesium matrix, and the SiC wiskers were observed with its longitudinal distributing along with the extrusion direction. Tensile experiment after extrusion revealed that the hybrid reinforcements significantly increase the elastic modulus and ultimate tensile strength of the composite material. The elastic modulus and the ultimate tensile were reached to 80.2GPa and 420 MPa, which were 100% and 34% higher than that of the matrix material. Moreover, numerical models (FEM) by an axisymmetric unit cell method were used to investigate the evolution of voids volume fraction in metal matrix during tensile process. The results show that the cracking is prone to form and grow near wisker than particulates.