Nb/NiTi复合材料预变形过程中变形行为的原位同步辐射X射线衍射研究

通过原位同步辐射X射线衍射、透射电子显微术和拉伸测试等表征手段,系统研究Nb/NiTi复合材料在预变形过程中的变形机制。结果表明:在加载过程中,Nb/NiTi复合材料经历B19′、B2和β-Nb相的弹性伸长和轻微的塑性变形,以及从B2相到B19′相的应力诱发马氏体相变。在随后的卸载过程中,Nb/NiTi复合材料的变形机制主要包括B19′、B2和β-Nb相的弹性恢复,β-Nb相的压缩变形以及不完全B19′→B2逆马氏体相变。在拉伸加载-卸载过程中,除了固有的弹性变形和应力诱发马氏体相变以外,复合材料中B19′马氏体内部的(001)复合孪晶也有助于材料实现弹性变形。因此,由于弹性变形、部分可逆且“连续”的应力诱发马氏体相变以及(001)复合孪晶的共同作用,Nb/NiTi复合材料在卸载完成后能够呈现出大的可恢复应变量。

Molecular simulation of interfacial reaction between TiAl alloy melts and different coatings

The effect of coatings(Y_2O_3, Zr O_2 and Al_2O_3) on the interfacial reaction of Ti Al alloys was studied with molecular dynamics. The binding energy of coatings and the diffusion process of oxygen in the melt were simulated, and then the simulation results were compared with the experimental results. The simulation results indicate that for each of the three simulated coatings, inordinate interfacial reactions have occurred between the coating and the melt. The binding energy results show that Y_2O_3 has the best stability and is the most difficult to break down. Zr O_2 has the greatest decomposition energy and is the easiest to break down in the melt. Besides, the molecular dynamics indicate that the diffusion coefficient of the oxygen atom in Al_2O_3 is larger than that in the other two coatings, indicating that oxygen diffusion in Al_2O_3 is the fastest at a given temperature. The experimental results show that the oxygen concentration of the melt with Al_2O_3 coating is the highest, and the oxygen diffusion is of similar magnitude to the simulation values, from which the conclusion can be obtained that the oxygen concentration is significantly influenced by the coating materials.

Morphologies and evolution of intermetallic compounds formed between Sn_(1.0)Ag_(0.7)Cu composite solder and Cu substrate

This study investigated the morphologies of the intermetallic compounds(IMC)formed during soldering reaction between Sn_(1.0)Ag_(0.7)Cu-1.0SnO_(2) composite solder and Cu substrate at various temperatures.The prismtype Cu_(6)Sn_(5) forms when the soldering temperature is 260or 2800C,while those grains transform from prism type to scallop type at the temperatures of 300 and 320℃.It can be found that the morphologies of Cu6Sn5 grains affect adsorption of Ag_(3)Sn nanoparticles during soldering reaction.The scallop-type grains with a higher growth rate need to adsorb large amounts of Ag_(3)Sn particles.In terms of mechanical behavior,the shear strength of solder joint is improved from 40 to 46 MPa at soldering temperature of 300℃.In addition,the thickness of IMC increases with the extension of aging time.During aging,the morphology of Cu6Sn5 grains remains scallop type,but the number of Ag_(3)Sn nanoparticles is reduced largely.The scallop-type Cu^(6)Sn_(5) can increase in size and flatten in morphology with the aging time increasing.

Design and fabrication of a low modulus β-type Ti-Nb-Zr alloy by controlling martensitic transformation

In this paper, high density of dislocations, grain boundaries and nanometer-scale α precipitates were intro- duced to a metastable Ti-36Nb-5Zr alloy （wt%） through a thermo-mechanical approach including severe cold rolling and short-time annealing treatment. The martensitic trans- formation was retarded, and the β phase with low content of β stabilizers was retained at room temperature after the thermo-mechanical treatment. As a result, both low mod- ulus （57 GPa） and high strength （950 MPa） are obtained. The results indicate that it is a feasible strategy to control martensitic transformation start temperature through microstructure optimization instead of composition design, with the aim of fabricating low modulus β-type Ti alloy.

Synchrotron X-ray diffraction characterization of phase transformations during thermomechanical processing of a Ti38Nb alloy

The phase transformations during thermomechanical processing can be employed to optimize mechanical properties of β-type Ti alloys.However,such understandings are still lacking for the alloy consisting of dual β+α" phases in solution-treated and quenched state.In this paper,the phase transformations in a Ti38 Nb model alloy subjected to different thermomechanical processing were investigated by using synchrotron X-ray diffraction(SXRD) experiments,and their influence on the Young’s modulus was discussed.The results indicated that highdensity dislocations introduced by cold rolling still existed after annealing at temperatures lower than 573 K,which can decrease the martensitic transformation start temperature to below room temperature.With annealing temperatures increasing,the α"→β,β→ω_(iso),and β→α phase transformations occurred successively.At annealing temperature of 473 K,the specimen consisted of a trace of α"and ω phases as well as dominant β phase which was kept to room temperature by the high density of dislocations,rather than by the chemical stabilization.As a result,an ultralow Young’s modulus of 25.9 GPa was realized.Our investigation not only provides in-depth understandings of the phase transformations during thermomechanical processing of β-type Ti alloys,but also sheds light on designing biomedical Ti alloys with ultralow Young’s modulus.

Phase transformations and mechanical properties of a Ti36Nb5Zr alloy subjected to thermomechanical treatments

Various solid state phase transformations exist in metastable β-type Ti alloys,which can be employed to optimize the mechanical properties.In this paper,synchrotron X-ray diffraction(SXRD)experiments were carried out to study the phase transformations of a Ti36Nb5Zr alloy subjected to different thermomechanical treatments.Furthermore,the correlation between the phase constitutions and the mechanical properties was discussed.The a" texture formed,and high-density defects were introduced after cold rolling of the solution treated specimen,leading to the decrease in Young’s modulus and the increase in strength.The cold-rolled specimens were then annealed at temperatures from 423 to 773 K for 30 min.Both the Young’s modulus and strength increased with annealing temperatures increasing up to 673 K,which resulted from the precipitation of the ω and/or α phases.With further increase in annealing temperatures to 773 K,the β→α precipitation replaced the β→ω_(iso) phase transformation,and the density of defects decreased,leading to the decrease in both the Young’s modulus and strength.These results provide theoretical basis for the design biomedical Ti alloys with both low Young’s modulus and high strength.

Construction of CoNi_(2)S_(4) nanocubes interlinked by few-layer Ti_(3)C_(2)T_(x) MXene with high performance for asymmetric supercapacitors

Both MXene and zeolitic imidazolate framework(ZIF)derivatives are tend to agglomerate during the compound process,which adversely affects their electrochemical properties.To alleviate this phenomenon,fewlayer MXene was stripped by mechanical method,and electrostatic self-assembly with ZIF-67 in the presence of cationic surfactants.Furthermore,CoNi_(2)S_(4)/MXene composite was synthesized by the facile hydrothermal reaction.CoNi_(2)S_(4)well retained the cube frame structure of the ZIF-67 with the sagging outer frame and rough surface.In the composite,CoNi_(2)S_(4)nanocubes were interlinked by MXene nanosheets,which can effectively improve the structural stability and make full use of the active surface.CoNi_(2)S_(4)/MXene composite electrode exhibits an outperforming specific capacitance(751 C·g^(-1)at 1 A·g^(-1)),far higher than that of pure CoNi2S4(600 C·g^(-1)at 1 A·g^(-1)).An asymmetric supercapacitor(CoNi_(2)S_(4)/MXene//reduced graphene oxide(RGO))assembling delivers high energy density of 33.8 Wh·kg^(-1)and excellent cycling performance.This study indicates the potential of MXene/ZIF derivatives in the application of supercapacitor.

Direct evidence for competition between metastable ω and equilibrium α phases in aged β-type Ti alloys

Precipitation response of a recently developed β-type Ti-25Nb-2Mo-4Sn alloy （wt%） during aging was investigated in detail. Experimental results indicate that the metastable co or equilibrium α phase can form alternatively even under the same aging condition, depending on the condition prior to the aging, i.e., solution treatment or severe cold rolling. This provides, for the first time, the direct evidence for the competition between ω and α in aged β-type Ti alloys. This peculiar aging response is found to be closely related to high-density dislocations and grain boundaries which suppress the formation of ω but favor the precipitation of fine α phase.

Strain glass in defect-containing ferroelastic Ti_(44)Ni_(51)Nb_5 alloy

An abnormal phenomenon was investigated in Ti44Ni51Nb5 alloy which exhibits an absence of martensite transformation and a negative temperature dependence of electrical resistivity with the temperature decreasing. X-ray diffraction (XRD) analysis shows the matrix phase keeps a B2 structure during cooling without a martensite transformation, and dynamic mechanical analysis displays a frequency-dependent AC modulus/loss anomaly at T g according to Vogel-Fulcher relation. Simultaneously, transmission electronic microscope (TEM) analysis manifests the superlattice spots at near 1/3 commensurate position, implying an existence of strain nanodomains with an R-like structure. And above experiment results provide evidence for the “strain glass” transition in defect-containing ferroelastic Ti44Ni51Nb5 alloy.