Xiuli Han

Fabrication of in situ Ti2AlN/TiAl composites by reaction hot pressing and their properties

Ti2AlN/TiAl composites with different volume fractions of reinforcement were successfully fabricated by hot-pressing sintering method (reaction hot pressing) using Ti, Al and TiN powders as starting materials. The synthesis process includes four stages: first, the reactions between Al and Ti powers and between Al and TiN powders respectively occur and result in TiAl3 phase; secondly, Al powders in the sample are exhausted; the remaining Ti cores react with TiAl3 layer to form Ti-Al intermetallics; moreover, a few Ti2AlN particles precipitate from the TiAl3 phase; thirdly, Ti-Al intermetallics react with the remaining Ti cores to form Ti3Al and TiAl phases. TiAl phase and original TiN powers are in direct contact each other; finally, the residual TiN powers react with TiAl phase and result in a plenty of Ti2AlN phase. Compared with TiAl matrix, the hardness, elastic modulus and high-temperature compressive strength of Ti2AlN/TiAl composite are improved obviously and they are all enhanced with increasing the volume fraction of Ti2AlN phase.

Friction and wear properties of TiAl and Ti2AlN/TiAl composites at high temperature

The high-temperature friction and wear properties of TiAl alloys and Ti2AlN/TiAl composites (TTC) in contact with nickel-based superalloy were studied. The results showed that, at 800 and 1 000 °C, the coefficient of the friction (COF) decreased with the increase of sliding velocity and the wear loss of the TTC decreased with the increase of volume fraction of Ti2AlN. The wear mechanisms of the pairs are adhesive wear and the wear debris mainly comes from the contacting nickel-based superalloy. The intergranular fracture and the cracking of the phase boundary in the lamellar structure are the wear mode of TiAl alloy. The wear mode of TTC is phase boundary fracture and adhesive spalling. The abrasive resistance of TTC is slightly higher than that of TiAl alloy.

Influence of Hydrogen on Tensile Property of Ti-6Al-4V

The influence of hydrogen content on the microstructure and the tensile property of Ti-6Al-4V alloy was studied, and the phenomenon of minimum yield stress at certain hydrogen content was discussed. The results show that Ti-6Al-4V alloy can absorb hydrogen above 600°C and the different hydrogen contents can be achieved by changing the flow rate of hydrogen. With increase of hydrogen contents, the microstructure gradually transforms from the original near basket-type to the α clusters which consist of α plates and hydrides distributed in α plates, and then to the mixture of α, β and the large amount of hydrides. When the specimens tensioned at 600°C, their strength first decreases and then increases, but their ductility changes quite the contrary as increasing hydrogen contents. There is optimum hydrogen content at which the strength is the lowest and the plasticity is the highest for the specimens tensioned at 600°C. Ti-6Al-4V alloy may gain the higher tensile strength or better ductility at 600°C through appropriate hydrogenation treatment in comparison with samples untreated. With increase of hydrogen contents, the fracture type transforms from microvoid coalescence type to “cleavage like” type for specimens tensioned at 600°C.

Residual microstructure associated with impact crater in Ti-6Al-4V meshes reinforced 5A06Al alloy matrix composite.

In this paper, TC4(m)/5A06Al composite was hypervelocity impacted by 2024 aluminium projectile with the diameter of 2mm and with the impact velocity of 3.5 km/s. The residual microstructure was observed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HREM). The TC4-Al interface before impact was composed of TiAl(3) phase and Ti(3)Al phase. Near the pithead, separation of TC4 fibers and Al matrix occurred along the impact direction. Around the middle of the crater, TC4 fibers were sheared into several sections. Near the bottom of crater, adiabatic shear band (ASB) occurred in TC4 fiber, while the angle between shear plane and cross section was 45°. The crack propagated along TC4-Ti(3)Al interface during impact and some Ti(3)Al phase at the TC4-Al interface transformed to amorphous with few nanocrystals after hypervelocity impact.

Microstructure characterization of Al matrix composite reinforced with Ti–6Al–4V meshes after compression by scanning electron microscope and transmission electron microscope

Compressive properties of Al matrix composite reinforced with Ti-6Al-4V meshes (TC4(m)/5A06 Al composite) under the strain rates of 10(-3)S(-1) and 1S(-1) at different temperature were measured and microstructure of composites after compression was characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM). Compressive strength decreased with the test temperature increased and the strain-rate sensitivity (R) of composite increased with the increasing temperature. SEM observations showed that grains of Al matrix were elongated severely along 45° direction (angle between axis direction and fracture surface) and TC4 fibres were sheared into several parts in composite compressed under the strain rate of 10(-3)S(-1) at 25°C and 250°C. Besides, amounts of cracks were produced at the interfacial layer between TC4 fibre and Al matrix and in (Fe, Mn)Al(6) phases. With the compressive temperature increasing to 400°C, there was no damage at the interfacial layer between TC4 fibre and Al matrix and in (Fe, Mn)Al(6) phases, while equiaxed recrystal grains with sizes about 10 μm at the original grain boundaries of Al matrix were observed. However, interface separation of TC4 fibres and Al matrix occurred in composite compressed under the strain rate of 1S(-1) at 250°C and 400°C. With the compressive temperature increasing from 25°C to 100°C under the strain rate of 10(-3) S(-1), TEM microstructure in Al matrix exhibited high density dislocations and slipping bands (25°C), polygonized dislocations and dynamic recovery (100°C), equiaxed recrystals with sizes below 500 μm (250°C) and growth of equiaxed recrystals (400°C), respectively.

Crystalline characteristics of alpha precipitates in Ti-15V-3Sn-3Al-3Cr alloy.

The experiment was designed to analyse the orientation relationship between alpha precipitates and beta matrix and to determine the habit plane of alpha phase in Ti-15-3 alloy using transmission electron microscopy. The orientation relationship was turned out to be 110(alpha)||111(beta) and (001)(alpha)||{110}(beta) obtained from diffraction patterns, which corresponded to Burgers orientation relationship. Based on the patterns and the crystal structure, it was determined that there were 12 possible orientation relationships between alpha precipitates with beta matrix and alpha precipitates had 12 variants. Meanwhile, the transformation matrixes of 12 orientation relationships were established. Diffraction patterns of 001(beta), 110(beta) and 311(beta) zone axes were also calibrated by these matrixes, which verified the correctness of proposed orientation relationship. Diffraction spots of lamelliform alpha precipitates spread along the [111](beta) and [1 1 1 ](beta) directions in diffraction patterns of [1 1 0](beta) zone axis, but they do not spread in diffraction patterns of [111](beta) zone axis, which indicates that the habit plane of variant 1, 2 and 3 is (111)(beta). Moreover, the habit planes of variant 4-6, variant 7-9 and variant 10-12 are (1 1 1)(beta), (1 1 1)(beta) and (1 1 1 )(beta) respectively. To sum up, the habit plane of alpha precipitates is {111}(beta).

Reaction Synthesis Mechanism of TiAl Alloy Fabricated and by Hot Pressed Sintering with Mixed Powders of TiH2 Aluminum

Ti-47Al(at.%) alloys were fabricated through hot pressed sintering using mixed powders of TiH2 and Al and mixed powders of Ti and Al, respectively, as starting materials. The microstructure of alloys were analysed by differencial thermal analysis (DTA),scanning electron microscopy (SEM) and X-ray diffraction (XRD), and the mechanism of the sintering process were studied. Through the experiment it is found that TiH2 powders have been decomposed to Ti and H2 completely before they react with Al powders during the hot press synthesis. The reaction of TiH2 and Al powders is faster than that of Ti and Al powders during the process of hot press synthesis because Ti formed from the decomposition of TiH2 powder has higher activity.

Study on Dimensional Stability of 1Cr18Ni9Ti Stainless Steel and its Mechanism

The influence of stabilizing treatments on the dimensional stability, micro structure and stress state of 1Cr18Ni9Ti austenitic stainless steel was studied by testing the real time dimensional variation of specimens through thermal-cooling cycling on-line measuring method, and then the dimension-stabilizing mechanisms were discussed. The results reveal that the dimensional stability is improved remarkably in the specimens after annealing at 500°C and then thermal-cooling cycling from -60°C to 100°C for four times. The magnitude of accumulative residual relative length change (ARRLC) at cycling end is -0.52×10−5, and the mean residual relative length change (MRRLC) is 0.13×10−5. The improvement of dimensional stability is resulted from the steady dislocation configuration and low residual stress.

Numerical Simulation of Gradient Heat Treatment Process of Cylinder Welding Component of 30Si2MnCrMoV Steel

The finite element analysis method was applied to simulate the gradient field heat treatment process of a cylinder welding component of 30Si2MnCrMoV steel by software ANSYS. In the heat treatment process，the distribution of temperature field, the variation curve of residual stress with time，the distribution of residual stress along the axial direction of cylinder, and the axial and radial deformation of the specimen were calculated and the influences of partial heating area on the distribution of temperature and residual stress were analyzed. The calculating results show that the temperature in the weld metal zone and heat affected zone of welding specimens may exceed the phase transformation points during the gradient heat temperature. The residual stress and the radial deformation in the weld metal zone were greater than those in the matrix zone. The final deformation of specimen was along the axial direction. With the increase of partial heating area, the phase transformation area was increased and the residual stress was reduced effectively.

Effect of Heat Treatment on Microstructure and Mechanical Property of 30Si2MnCrMoV Welded Joint

The effect of different heat treatments on microstructure and mechanical property of 30Si2MnCrMoV steel weld specimen was studied in this paper．A stress relieving annealing on the specimen was carried out immediately after welding. And then two heat treatments were carried out on the specimen. Observation results showed that the original microstructure of 30Si2MnCrMoV welded joints was granular pearlite and the microstructure of weld bead was coarse martensite. Lots of tempered martensite exited in the weld specimen under quenching-tempering heat treatment, the carbide was coarse and the content of retained austenite was little. Under Quenching and Partitioning (Q&P) heat treatment, the content of retained austenite increased to 7% and a small number of coarse carbides existed. The determination of mechanical properties showed that, comparing with quenching-tempering process, the plasticity and toughness of weld specimen treated by Q&P was enhanced significantly, the percentage elongation increased 30%, the impact energy increased 25% ,the intensity reduced little and comprehensive mechanical property was excellent. The tensile fracture of 30Si2MnCrMoV steel under Q&P presented dimples. The retained austenite which exited along the lath boundaries of lath martensite was the main reason of the reduction of secondary cracks and the improvement of plasticity and toughness.