Weijie Lu

In situ technique for synthesizing (TiB+TiC)/Ti composites

Titanium-based metal-matrix composites (MMCs), reinforced with ceramic particles have considerable potential for improvements in properties and service temperature, while retaining isotropic behavior and ease of fabrication using conventional processing methods. Traditionally, titanium MMCs have been produced by such processing techniques as powder metallurgy and casting technologies. In recent years, novel processing techniques based on the in situ production of MMCs have emerged. In the present work, the authors highlight a novel in situ process in which traditional ingot metallurgy plus SHS techniques were used to produce (TiB + TiC)/Ti matrix composites. This article gives particular attention to the reaction principles and reinforcements morphologies.

Microstructural characterization of TiB in in situ synthesized titanium matrix composites prepared by common casting technique

Abstract TiC reinforced titanium matrix composites were produced by non-consumable arc-melting technology utilizing the self-propagation high-temperature synthesis (SHS) reaction between titanium and graphite. X-Ray diffraction (XRD) was used to identify the phases in the composites. Microstructures of the composites were observed by optical microscope (OM) and transmission electron microscope (TEM). The results show that there are two phases in the composites: TiC and titanium matrix alloy. TiC has two different shapes: dendritic shape, equiaxed or near-equiaxed shape. The in situ synthesized TiC grows by dissolution–precipitation. Analysis of the binary phase diagram determined that the solidification path undertook the following three stages: primary TiC, binary eutectic β-Ti+TiC and solid transformation. Primary TiC grows in dendritic shape due to the formation of composition undercooling. Binary eutectic TiC grows in equiaxed or near-equiaxed shape. A small quantity of TiC may form twin structure during nucleation and growth. The twin plane is the (111) plane.

In situ preparation of titanium matrix composites reinforced by TiB and Nd2O3

In the present work, a novel titanium matrix composite reinforced by TiB and Nd2O3 was prepared through the synthesis reaction from Ti, B2O3 and Nd with nonconsumable vacuum arc melting. The result shows that the reinforcements are TiB and Nd2O3. TiB grows in needle shape, while Nd2O3 grows in sphere and dendritic shapes.

Microstructure and tensile properties of in situ synthesized (TiB+Y2O3)/Ti composites at elevated temperature

A novel titanium matrix composites reinforced with TiB and rare earth oxides (Y2O3) were prepared by a non-consumable arc-melting technology. Microstructures of the composites were observed by means of optical microscope (OM) and transmission electron microscope (TEM). X-ray diffraction (XRD) was used to identify the phases in the composites. There are three phases: TiB, Y2O3 and titanium matrix alloy. TiB grows in needle shape, whereas Y2O3 grows from near-equiaxed shape to dendritic shape with increase of yttrium content in the composite. The interfaces between reinforcements and titanium matrix are very clear. There is no interfacial reaction. Tensile properties of the composites were tested at 773, 823 and 873 K. Both the fracture surfaces and longitudinal sections of the fractured tensile specimens were comprehensively examined by scanning electron microscope (SEM). The fracture mode and fracture process at different temperatures were analyzed and explained. The results show that the tensile strength of the composites has a significant improvement at elevated temperatures. The predominant fracture mode of composites is cleavaged at 773 and 823 K. Fracture occurs by ductile failure at 873 K.

Microstructure and tensile properties of in situ synthesized (TiBw + TiCp)/Ti6242 composites

In the present work, (TiBw+ TiCp)/Ti6242 composites were fabricated via common casting and hot-forging technology utilizing the SHS reaction between titanium and B4C. The XRD technique was used to identify the phases of composites. The microstructures were characterized by means of OM and TEM. Results from DSC and analysis of phase diagram determine solidification paths of in situsynthesized Ti6242 composites as following stages: β-Ti primary phase, monovariant binary eutectic β-Ti + TiB, invariant ternary eutectic β-Ti + TiB + TiC and phase transformation from β-Ti to α-Ti. In situsynthesized reinforcements are distributed uniformly in titanium matrix alloy. Reinforcement TiB grows in whisker shape whereas TiC grows in globular or near-globular shape. TiB whiskers were made to align the hot-forging direction after hot-forging. The interfaces between reinforcements and Ti matrix alloy are very clean. There is no any interfacial reaction. Moreover, the mechanical properties improved with the addition of TiB whiskers and TiC particles although some reduction in ductility was observed. Fractographic analysis indicated that the composites failed in tension due to reinforcements cracking. The improvements in the composite properties were rationalized using simple micromechanics principles. The strengthening mechanisms are attributed to the following factors: undertaking load of TiB whiskers and TiC particles, high-density dislocations and refinement of titanium matrix alloys grain size.

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.

Tensile properties of in situ synthesized titanium matrix composites reinforced by TiB and Nd2O3 at elevated temperature

Abstract Titanium matrix composites reinforced with TiB and Nd 2 O 3 were prepared by a non-consumable arc-melting technology. X-ray diffraction (XRD) was used to identify the phases in the composites. Microstructures of the composites were observed by means of optical microscope (OM). There are three phases: TiB, Nd 2 O 3 and titanium matrix. TiB grows in needle shape, whereas Nd 2 O 3 grows in lath shape. Tensile properties of the composites were tested at 773, 823 and 873 K. Both the fracture surfaces and longitudinal sections of the fractured tensile specimens were comprehensively examined by scanning electron microscope (SEM). The fracture mode and fracture process at different temperatures were analyzed and explained. It shows that the tensile strength of the composites has a significant improvement at elevated temperatures compared to titanium matrix. The ductility of the composites improves with the content of neodymium and the test temperatures. The titanium composite exhibits different fracture modes at different test temperatures.

The mechanical behavior dependence on the TiB whisker realignment during hot-working in titanium matrix composites.

Low-cost TiB whiskers reinforced titanium matrix composite (TMCs) was fabricated with enhanced mechanical performances using in situ technologies and hot working. Morphologies observation indicates that needle-like TiB whiskers with a hexagonal transverse section grow along the [010] direction due to B27 crystal structure and its growth mechanism. Mechanical properties tests show that the mechanical behavior of the TiB whiskers reinforced TMCs is dependent on the deformation amplitudes applied in hot-working. The improvement in yield strength by hot-working is attributed to the TiB whiskers realignment and the refinement of microstructure. Models are constructed to evaluate the realignment of TiB whisker during deformation and the increase in yield strength of the composite at elevated temperatures. These models clarify the alignment effect of TiB whiskers under various deformation amplitudes applied in hot-workings and reveals the yield strength dependence on TiB whiskers orientation.

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.