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Variant selection in stationary shoulder friction stir welded Ti-6Al-4V alloy
Xiaoqing Jiang, Bradley P. Wynne, Jonathan Martin
J. Mater. Sci. Technol.    2018, 34 (1): 198-208.   DOI: 10.1016/j.jmst.2017.11.024
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Stationary shoulder friction stir welding of Ti-6Al-4V of 7 mm thickness was conducted with varying welding speeds and rotation speeds. Variant selection analysis was carried out based on the inherited α phase texture and the reconstructed β phase texture. The weld surfaces became significantly smoother with increasing welding speed and decreasing rotation speed. Heat input decreased greatly with increased welding speed and it decreased slightly with decreased rotation speed. The orientation relationship between the prior β grains was measured based on the reconstructed electron backscattered diffraction (EBSD) data. Weak Variant selection has occurred in all the welds because most of the prior β grains did not share {110} poles. Strong links between crystal orientation of the prior β grains and hardness have been found.

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Friction-stir welding and processing of Ti-6Al-4V titanium alloy: A review
S.Mironov, Y.S.Sato, H.Kokawa
J. Mater. Sci. Technol.    2018, 34 (1): 58-72.   DOI: 10.1016/j.jmst.2017.10.018
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In this work, the current understanding and development of friction-stir welding and processing of Ti-6Al-4V alloy are briefly reviewed. The critical issues of these processes are addressed, including welding tool materials and design, tool wear, processing temperature, material flow, processing window and residual stresses. A particular emphasis is given to microstructural aspects and microstructure-properties relationship. Potential engineering applications are highlighted.

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Effect of lattice distortion on solid solution strengthening of BCC high-entropy alloys
Zhipeng Wang, Qihong Fang, Jia Li, Bin Liu, Yong Liu
J. Mater. Sci. Technol.    2018, 34 (2): 349-354.   DOI: 10.1016/j.jmst.2017.07.013
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:An analytical model is established to study the influence of lattice distortion and fraction of Hf on the yield strength of the BCC TiNbTaZrHfx multi-component high entropy alloys (HEAs). Meanwhile, the mechanism of solid solution strengthening caused by lattice distortion is also discussed in the HEA. The distorted unit cell is introduced to indicate the lattice distortion effects induced by the differences of the atomic size and shear modulus by doping other elements in Ti-based metal. The results show that the calculated values of the alloying yield strength considering the path of least resistance are obtained with regard to various grain sizes for the equiatomic TiNbTaZrHf HEA, which is well in line with the experimental results. Furthermore, it is predicted that the alloying yield strength is the largest value in the case of the same grain size for the Hf atomic fraction of 0.122. The meaningful modeling could provide a theoretical method to investigate the yield strength and alloying design of other BCC HEAs in the future.

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Microstructure and Mechanical Properties of Electron Beam Welded Titanium Alloy Ti-6246
Guoqiang Wang, Zhiyong Chen, Jinwei Li, Jianrong Liu, Qingjiang Wang, Rui Yang
J. Mater. Sci. Technol.    2018, 34 (3): 570-576.   DOI: 10.1016/j.jmst.2016.10.007
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Electron beam welding (EBW) was applied to a 10-mm-thick plate cut from Ti-6246 compressor disk. The microstructural characteristics, microhardness and room temperature tensile properties were investigated. Microstructure observations indicated that there existed plenty of thin needle-like α platelets studding in the matrix of the columnar β grains in the as-welded fusion zone (FZ). Post-weld heat treatment (PWHT) led to the precipitation of small secondary α platelets in the β matrix in heat affected zone and FZ. The thickness and the density of α platelets increased as the temperature of PWHT increased from 545 to 645 °C. The microhardness across the Ti-6246 EBW joint exhibited a nonuniform distribution. The hardness increased with the decrease of distance to the weld center, and reached the maximum of 467 HV in FZ when PWHT was carried out at 595 °C. All the weldments tested with tension were fractured at the base material (BM) and exhibited a ductile fracture mode. The major deformation barrier in BM was the platelet α/β interfaces, however, the major deformation barrier in FZ was found to be β grain boundaries and secondary α/β interfaces. The BM with thicker platelet α phases had lower strength than the other two zones in the joint, and the BM deformed first and led to fracture in this zone.

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A state-of-the-art review on passivation and biofouling of Ti and its alloys in marine environments
Shaokun Yan, Guang-Ling Song, Zhengxian Li, Haonan Wang, Dajiang Zheng, Fuyong Cao, Miroslava Horynova, Matthew S. Dargusch, Lian Zhou
J. Mater. Sci. Technol.    2018, 34 (3): 421-435.   DOI: 10.1016/j.jmst.2017.11.021
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High strength-to-weight ratio, commendable biocompatibility and excellent corrosion resistance make Ti alloys widely applicable in aerospace, medical and marine industries. However, these alloys suffer from serious biofouling, and may become vulnerable to corrosion attack under some extreme marine conditions. The passivating and biofouling performance of Ti alloys can be attributed to their compact, stable and protective films. This paper comprehensively reviews the passivating and biofouling behavior, as well as their mechanisms, for typical Ti alloys in various marine environments. This review aims to help extend applications of Ti alloys in extremely harsh marine conditions.

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New ductile laminate structure of Ti-alloy/Ti-based metallic glass composite with high specific strength
D. Li, Z.W. Zhu, A.M. Wang, H.M. Fu, H. Li, H.W. Zhang, H.F. Zhang
J. Mater. Sci. Technol.    2018, 34 (4): 708-712.   DOI: 10.1016/j.jmst.2017.07.008
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Bulk laminate structure of Ti-alloy/Ti-based metallic glass composite (MGC) was prepared by melting a preform of alternate stack-up foils in the high vacuum atmosphere. The composite demonstrates a good combination of yield strength (~1618 MPa), plasticity (~4.3%) and specific fracture strength (384 × 103 N m kg-1) in compression. The maintained yield strength results from the unique microstructure composed of the Ti layer, the solution layer with gradient structure and the MGC layer. Such a multilayer structure effectively inhibits the propagation of shear band, leading to the enhanced plasticity. Those extraordinary properities suggest that combining ductile lamella with brittle metallic glass (MG) by such a lay-up method can be an effective way to improve mechanical properties of MG.

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Flow behavior and processing map for hot deformation of ATI425 titanium alloy
Qinggang Meng, Chunguang Bai, Dongsheng Xu
J. Mater. Sci. Technol.    2018, 34 (4): 679-688.   DOI: 10.1016/j.jmst.2017.07.015
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Flow behavior and processing map play important roles in the hot deformation process of titanium alloys. In this research, compression Gleeble tests have been carried out to investigate the stress-strain relationship at temperatures ranging from 700 to 1000 °C and strain rates ranging from 0.001 to 1 s-1 for ATI 425 titanium alloy. Arrhenius type constitutive equation was obtained to describe the compressive flow behavior with modification of additional deformation dead zone, friction model, temperature model and strain rate. The introduction of novel calculation method for α value in Arrhenius equation gives more accurate fitting than traditional one. Processing maps were drawn based on the distribution of dissipator co-content, and optimized deformation temperature and strain rate range obtained. It is proven to be accurate and effective through the experimental results. The microstructure analysis shows that more dynamic recrystallization can be achieved in the area with larger η value on the processing map.

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Distinct dendritic α phase emerging on the surface of primary α phase in a compressed near-α titanium alloy
Z. Liu, Z.B. Zhao, J.R. Liu, Q.J. Wang, R. Yanga
J. Mater. Sci. Technol.    2018, 34 (4): 666-669.   DOI: 10.1016/j.jmst.2017.10.011
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The characteristic of the surface morphology of primary α phase was studied in a deformed near-α titanium alloy. Dendritic α phase emerged on the surfaces of primary α phase when the alloy was air-cooled in α + β phase field after deformation. The dendritic α grain has the same orientation with its original primary α grain. The formation of the dendritic α phase could be explained by interface instability in epitaxial growth process of the primary α phase. The dislocations induced by deformation could facilitate the formation of dendritic α phase leading to the dendritic α phase and more obvious with the increase of strain. The growth of dendritic α phase was finally limited by the nucleation of second α phase with cooling.

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Microstructure, texture evolution and mechanical properties of cold rolled Ti-32.5Nb-6.8Zr-2.7Sn biomedical beta titanium alloy
Chunbo Lan, Yu Wu, Lili Guo, Huijuan Chen, Feng Chen
J. Mater. Sci. Technol.    2018, 34 (5): 788-792.   DOI: 10.1016/j.jmst.2017.04.017
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Ti-32.5Nb-6.8Zr-2.7Sn (TNZS, wt%) alloy was produced by using vacuum arc melting method, followed by solution treatment and cold rolling with the area reductions of 50% and 90%. The effects of cold rolling on the microstructure, texture evolution and mechanical properties of the experimental alloy were investigated by optical microscopy, X-ray diffraction, transmission electron microscopy and universal material testing machine. The results showed that the grains of the alloy were elongated along rolling direction and stress-induced α″ martensite was not detected in the deformed samples. The plastic deformation mechanisms of the alloy were related to {112}〈111〉 type deformation twinning and dislocation slipping. Meanwhile, the transition from γ-fiber texture to α-fiber texture took place during cold rolling and a dominant {001}〈110〉α-fiber texture was obtained after 90% cold deformation. With the increase of cold deformation degree, the strength increased owing to the increase of microstrain, dislocation density and grain refinement, and the elastic modulus decreased owing to the increase of dislocation density as well as an enhanced intensity of {001}〈110〉α-fiber texture and a weakened intensity of {111}〈112〉γ-fiber texture. The 90% cold rolled alloy exhibited a great potential to become a new candidate for biomedical applications, since it possesses low elastic modulus (47.1 GPa), moderate strength (883 MPa) and high elastic admissible strain (1.87%), which are superior than those of Ti-6Al-4V alloy.

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Influence of alloy element partitioning on strength of primary α phase in Ti-6Al-4V alloy
L.R. Zeng, H.L. Chen, X. Li, L.M. Lei, G.P. Zhang
J. Mater. Sci. Technol.    2018, 34 (5): 782-787.   DOI: 10.1016/j.jmst.2017.07.016
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The partitioning effect of Al (α-phase stabilizer) and V elements (β-phase stabilizer) on strength of the primary α phases in the α/β Ti-6Al-4V alloy with the bimodal microstructure was investigated. It was found that partitioning of Al and V elements took place in the Ti-6Al-4V alloy during the recrystallization process, leading to the variation of the content of Al and V elements in the primary α phases with changing the volume fraction of the primary α phase. Nanoindentation tests reveal a general trend that the strength of the primary α phases increases with decreasing the volume fraction of the primary α phases, and such trend is independent on the loading direction relative to the c-axis of the α phase. The enhanced strength is attributed to the increase of the content of Al element in the primary α phase, but it is not dominated evidently by the change of the V content. The solid solution strengthening contributed from both the elastic strain introduced by the solute atoms and the variation of the density of states was estimated theoretically.

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ISSN: 1005-0302
CN: 21-1315/TG
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