Solid-state transformation in nanocrystalline Ti induced by ball milling

In the current study, we report for the first time a new Ti rhombohedral (trigonal) structure induced by HEBM and subsequent sintering. During ball milling of Ti powder, solid-state transformation does not only depend on the grain refinement but also on the successful deformation of the nano-sized crystallites due to high energy ball impacts. Thermal stability of Ti-nanocrystalline in FCC allotrope was investigated. Upon sintering, the unstable FCC restored back to the rhombohedral phase rather than to HCP. The appearance of HCP Ti after sintering could suggest that prolonged milling leads to dispersion of hard particles (HCP) into more ductile particles belonging to allotropic phases, and hence possibility of resurfacing on sintering.     

High-temperature deformation behavior of Ti60 titanium alloy

Isothermal compressions of near-alpha Ti60 alloy were carried out on a Gleeble-3800 simulator in the temperature range of 960-1110 °C and strain rate range of 0.001-10.0 s⁻¹. The high-temperature deformation behavior was characterized based on an analysis of the stress-strain behavior, kinetics and processing map. The flow stress behavior revealed greater flow softening in the two-phase field compared with that of single-phase field. In two-phase field, flow softening was caused by break-up and globularization of lamellar α as well as deformation heating during deformation. While in the single-phase field, flow softening was caused by dynamic recovery and recrystallization. Using hyperbolic-sine relationships for the flow stress data, the apparent activation energy was determined to be 653 kJ/mol and 183 kJ/mol for two-phase field and single-phase field, respectively. The processing map exhibited two instability fields: 960-980 °C at 0.3-10 s⁻¹ and 990-1110 °C at 0.58-10 s⁻¹. These fields should be avoided due to the flow localization during the deformation of Ti60 alloy.     

High temperature deformation behavior of a near alpha Ti600 titanium alloy

The high temperature deformation behavior of a near alpha Ti600 titanium alloy was investigated with isothermal compression tests at temperatures ranging from 800 to 1000 °C and strain rates ranging from 0.001 to 10.0 s⁻¹. The apparent activation energy of deformation was calculated to be 620.0 kJ mol⁻¹, and constitutive equation that described the flow stress as a function of the strain rate and deformation temperature was proposed for high temperature deformation of Ti600 titanium alloy in the α + β phase region. The processing map was calculated to evaluate the efficiency of the forging process in the temperatures and strain rates investigated and to recognize the instability regimes. High efficiency values of power dissipation over 55% obtained under the conditions of strain rate lower than 0.01 s⁻¹ and temperature about 920 °C was identified to represent superplastic deformation in this region. Plasticity instability was expected in the regime of strain rate higher than 1 s⁻¹ and the entire temperature range investigated.     

Low Temperature Thermal Conductivity of Ti6Al4V Alloy

The CUORE detector, to be installed in 2010 at LNGS, is made of 988 TeO₂ crystals to be cooled to 10 mK. It consists of a large cryogen-free cryostat cooled by five pulse tubes and one high-power specially designed dilution refrigerator (R. Ardito et al. in , [2005]). The cryostat is ∼ 3 m high and has a diameter of ∼ 1.6 m. About 5 tons of lead shielding are to be cooled to below 1 K and a mass of 1.5 ton must be cooled to 10 mK. Some tie-rods sustain the different parts of the experiment. One end of each rod is at low temperature (10 mK for the detector frame, 50 mK for the coldest radiation shield, 700 mK for the shield linked to the still) with the other end usually at room temperature. A thermalization of the rods at the temperature of the first stage of the pulse tubes will be realized. Hence the value of the thermal conductivity of the material up to room temperature is important. At the lowest temperatures, the thermal conductivity has great influence in establishing the thermal load on the dilution refrigerator. The thermal conductivity of the structural material candidates for such tie-rods is usually known down to 4.2 K. Here we present data of thermal conductivity for the Ti6Al4V alloy below its superconductive transition temperature (4.38 K). A comparison over the full temperature range of operation is also done with other materials, such as 316 stainless steel and Torlon, candidates for the realization of the tie-rods.      

Orientation dependent behavior of tensile-creep deformation of hot rolled Ti65 titanium alloy sheet

The mill products like sheet always have one or more severe textures inevitably,and its effect on mechan-ical properties is not a negligible issue.The orientation dependent tensile-creep behavior induced by rolling texture of Ti65 titanium alloy sheet has been systematically investigated at 650℃.There are some anisotropic characteristics between TD and RD of Ti65 sheet.The UTS and TYS of TD are higher than RD at 650℃.Besides,the creep endurance time of TD(172.6-174.5 h)is about three times longer than RD(55.6-65.1 h)at 650℃and 240 MPa.Moreover,the grains are inclined to form Texture Ⅲ(1 2(1)6)[1(2)11]and(01(1)3)[1(2)11]after creep along with TD,but to form Texture I((1)2-(1)0)[10-(1)0]after creep along with RD.Finally,the crack initiation site is different during creep in TD and RD.The reason for anisotropic properties of tensile and creep has been summarized in two aspects:(ⅰ)the change of the SFs(Schmid factors)value between TD and RD;(ⅱ)the difference of creep mechanism between TD(grain boundary sliding)and RD(dislocation slip).Anisotropy of Ti65 sheet should be fully considered to increase structural efficiency in the engineering design and application.     

Mechanical characterization of Ti–5Al–2.5Sn ELI alloy at cryogenic and room temperatures

An experimental campaign consisting of tensile and fracture tests at cryogenic and room temperatures has been conducted on a Ti–5Al–2.5Sn extra-low-interstitial (ELI) alloy. It has been assessed that, at decreasing testing temperature: Young’s modulus slightly increases; yield and failure strengths increase significantly; fracture toughness decreases. Since a ductile void growth to coalescence micromechanism always governs failure in the spanned temperature interval, crack growth is simulated by allowing for material nonlinearities in the process zone, where ductile tearing takes place. Numerical results have been obtained by modeling the response of the process zone through either a cohesive model or Gurson’s constitutive law for porous-ductile media. It is shown that the latter approach can accurately describe the failure mechanism at any test temperature and for any specimen geometry, whereas the former one is not able to account for stress triaxiality at the crack tip and therefore requires a new calibration anytime the specimen geometry is varied.     

Hot pressing diffusion bonding of a titanium alloy to a stainless steel with an aluminum alloy interlayer

The probability and appropriate processing parameters of hot pressing diffusion bonding (HP–DW) of a titanium alloy (TC4) to a stainless steel (1Cr18Ni9Ti) with an aluminum alloy (LF6) interlayer have been investigated. The microstructure of the bonded joints has been observed by optical microscopy, SEM, XRD and EDX, and the main factors affecting hot pressing and diffusion bonding process were analyzed. The results showed that atom diffused well and no intermetallic compound or other brittle compounds appeared at optimum parameters. The fracture way of joints was ductile fracture. With the increment of bonding temperature, large number of intermetallic compounds such as FeAl₆, Fe₃Al, FeAl₂ which were brittle appeared along the interface between the stainless steel and the aluminum alloy interlayer, as a result, the quality of joints was decreased significantly and the fracture way of joints was brittle fracture.     

A superelastic nanocrystalline Cu-Sn alloy thin film processed by electroplating

A nanocrystalline Cu-Sn alloy film was processed by electroplating, and the indentation tests and microstructural observation were conducted on the electroplated Cu-Sn alloy film. The indentation tests at room temperature showed that a large amount of strain was recovered on unloading for the electroplated Cu-Sn alloy film, in contract, such a large reversible strain was not found in an electroplated pure Cu film. Thus, the electroplated Cu-Sn alloy film exhibited superelastic behavior. The grain size of the Cu-Sn alloy film was 99 nm. In spite of the very small grain size, the austenite start and finish temperatures of the Cu-Sn alloy film were relatively high. This is suggested to be related to the presence of the α-Cu phase.     

Lowering of elastic modulus in the near-beta Ti–13Nb–13Zr alloy through heat treatment

ABSTRACT

The bio-implant devices require adequate strength and low elastic modulus, for compatibility with human bone. In this study, the near-beta Ti–13Nb–13Zr alloy was subjected to two different solutionising temperatures and quenched at different temperatures. Microstructure modifications and its influence on microhardness, elastic modulus and tensile properties were investigated. Elastic modulus was decreased with increase in cooling rate from solutioning temperature. The samples solutionised at 900°C and quenched at sub-zero temperature contained α^″ martensite along with α′ and β phases and these lowered the elastic modulus. Among all the heat-treated samples, the one solutionised at 900°C and quenched at sub-zero exhibited lowest elastic modulus of 59?GPa and adequate tensile properties for the application as bioimplants.     

Effects of hydrogenation temperature on room-temperature compressive properties of CMHT-treated Ti6Al4V alloy

Compression tests were performed at room temperature to investigate the effects of hydrogenation temperature on compressive properties of Ti6Al4V alloy treated by continuous multistep hydrogenation treatment (CMHT).Pressure-composition isotherms and microstructures were also studied.Results showed that the equilibrium hydrogen pressure increased,and the hydrogen absorption rate decreased with the increase of hydrogenation temperature.The amounts of β phase and α'martensite increased first and then decreased when Ti6Al4V alloy was treated by four times CMHT with the increase of hydrogenation temperature.Hydrogenation temperature played a different role on the compressive properties of CMHT-treated Ti6Al4V alloy.The ultimate compression of Ti6Al4V alloy treated by 11 times CMHT at 850 ℃ increased by 83.3 ％ as compared to the as-received Ti6Al4V alloy.The compressive properties of Ti6Al4V alloy were dependent on the amounts of different phases and microstructures when Ti6Al4V alloy was treated by CMHT at different temperatures.     

Ti基大块非晶合金研究进展

钛合金由于具有高的比强度，在许多领域中得到广泛应用。Ti基大块非晶合金具有更高的比强度，是极具发展潜力的机构材料。本文综述了Ti基大块非晶合金的成分设计、玻璃形成能力、制备技术及机械性能，介绍了目前该领域的研究现状。     

丁腈橡胶与钛合金粘接工艺研究

对丁腈橡胶与钛合金粘接的方法及工艺进行了研究。主要探讨了钛合金表面处理、胶粘剂种类、硫化条件、干燥处理几方面对丁腈橡胶与钛合金粘接性能的影响,并给出了最佳粘接硫化工艺。     

Formability of Ti14 alloy during semisolid deformation

Abstract

The semisolid formability for Ti14, an α+Ti₂Cu alloy, is compared with the conventional warm formability from the point of forgeability. The forgeability is evaluated by upsetting and die forging tests. The results show that excellent upsettability with the upsetting reduction in height of 70–85% and low upsetting force could be obtained in semisolid state ranging from 1000 to 1100°C, which is better than that in conventional processing. Die forging tests also show excellent workability with a forging ratio of 75% at the temperature range of 1000–1050°C. It can be concluded that the existence of liquid may serve to relax the stress concentrations caused by solid deformation, which causes low deformation resistance and results in improvement of forgeability. Furthermore, dynamic recrystallisation occurred during thixoforging, and the grain refinement was attained, which also results in the improvement of the semisolid formability.     

Homogenization kinetics of a cast Ti48A12W0.5Si alloy

The homogenization kinetics of a cast Ti48A12W0.5Si alloy with a duplex microstructure was studied in terms of γ-phase dissolution and -grain growth. It was found that the measured volume fraction of remnant γ grains can be well simulated by a model of interface-controlled dissolution in a dislocation mechanism, instead of a diffusion-controlled one. The activation energy for the /γ interface reaction was found to be Q_int = 476 kJ mol⁻¹, which is much higher than the interdiffusion activation energy in TiAl alloy. The grain growth of phase during homogenization can be categorized into three stages. During the first stage, where the volume fraction of remnant γ grains is higher than about 10%, the growth of grains follows the parabolic law D = k₁t^0.2, and the activation energy for grain growth was calculated to be Q₁ = 442 kJ mol⁻¹, very close to the Q_int for /γ interface reaction. In the second stage, where few fine γ grains (1–10 vol.%) remained, a dramatic grain growth occurs. During the final stage, as the single phase is obtained, the coarsening of grains again satisfies the grain growth law D = k₃t^0.4, with the grain growth activation energy Q₃ of 147 kJ mol⁻¹, lower than the reported interdiffusion energy of γ phase.     

Experimental studies on the cryogenic machining of biodegradable ZK60 Mg alloy using micro-textured tools

Reducing the contact area between the cutting tool rake surface and chip promotes the machining performance of the work material and increases the tool life. Magnesium alloys are ductile-lightweight materials that form continuous chips during machining. The present investigation discusses the orthogonal turning of ZK60 magnesium alloy with linearly textured cutting inserts under both dry and liquid nitrogen (LN₂) cooling conditions. Linear grooves that are parallel and perpendicular to chip flow direction were created using Nd-YAG laser on the tungsten carbide cutting inserts. The effect of texturing combined with the application of LN₂ cooling is studied by evaluating the machining temperature and forces, microhardness, surface roughness and tool wear. Textured tools considerably minimize the liaison area of the chip with the rake plane compared to non-textured tools, which resulted in favorable effects in machinability. In case of cryogenic machining, textured tools substantially minimize the friction by the coupled effect of micro-pool lubrication and the formation of thin-film lubrication between the tool–chip/tool–work interfaces. Parallel-textured tools aided with cryogenic cooling exhibit superior performance during machining among the different types of tools employed in the present investigation.     

Structure of grains and grain boundaries in cryo-mechanically processed Ti alloy

A Ti5Ta1.8Nb alloy with the major phase as α (hcp) Ti has been subjected to severe plastic deformation by means of cryo-rolling. Significant grain refinement (from ~5 μm to ~60 nm) has been observed. The mechanism of grain refinement was studied by analysis of lattice strain variations with increase in cold work using XRD technique. Various intermediate stages, such as hardening, alignment of dislocations, cell formation and criticality before new grain formation, were identified. Formation of cells with dislocations alignment at the boundaries and then finally forming an ultra-fine grain structure was confirmed by transmission electron microscopy studies. Detailed grain boundary characterisation has been carried out using high-resolution transmission electron microscopy studies and crystallographic texture analysis. The grain-refined structure was found to possess a large fraction of high angle boundaries identified also as special boundaries by evaluating the misorientation angle/axis sets for a pair of adjacent grain boundaries.     

High-cycle fatigue behaviour and mechanism analysis of a forged TiZr-based alloy

Fatigue resistance, particularly the endurance limit, is an important design consideration in engineering applications for TiZr-based alloys. The investigated Ti–20Zr–6Al–4?V (wt-%) alloy exhibited a high fatigue endurance limit of 775?MPa. Results showed that severe local stress concentration due to extensive dislocation pile-up at α/β interfaces was responsible for the crack initiation. A transition from a tensile mode to a shear mode crack was observed during crack propagation. Many striations as well as some micro-cracks which can improve the resistance to crack propagation exist in the stable crack-propagation region. A localised deviation between the crack-growth direction was also found, and this outcome combined with micro-cracks and tear ridge may be attributed to varied crystallographic orientations between different phases.     

TiO_2真空碳热反应及可溶性TiO阳极电解过程研究

通过对TiO_2碳热还原进行热力学计算,得出真空碳热还原技术能降低反应温度。采用X射线衍射以及电阻测量装置考察了TiO_2真空碳热还原过程,并将最终还原产物制备成可溶性TiO阳极进行电解。结果表明:在还原温度为1200℃,还原时间2 h,TiO_2与碳摩尔比为1:3的条件下,可以得到电阻率较低(小于0.03Ω·m)的低价氧化钛。整个还原过程TiO_2遵循逐级还原理论,反应产物会经历TiO_2→Ti_6O_(11)→Ti_4O_7→Ti_3O_5→Ti_2O_3→TiO的还原过程。最终将还原产物TiO与C混合制备成阳极,石墨为阴极时,850℃在CaCl_2-KCl熔盐体系中电解2 h后可生成TiC;相同实验条件下,TiO与C按配比压制成阳极,铁为阴极,中间以泡沫陶瓷材料相隔,产物则为金属钛及钛铁合金。     

Deformation behavior of a Co-Cr-Fe-Ni-Mo medium-entropy alloy at extremely low temperatures

We report the mechanical and microstructural characteristics of a medium-entropy alloy, Co_17.5Cr_12.5Fe₅₅Ni₁₀Mo₅ (atomic percent, at%) at cryogenic temperatures, down to a record low temperature of 0.5 K. The alloy exhibits excellent strength and ductility combined with a high strain-hardening rate in the entire temperature range investigated. Its property profile, including the yield strength, ultimate tensile strength, strain hardening capability, and absorbed mechanical energy, is better than those of most alloys and HEAs used in cryogenics. Within the interval of extremely low temperatures considered (0.5–4.2 K), the alloy exhibits several unusual features, including anomalies of the temperature dependence of the yield strength and tensile ductility, discontinuous plastic deformation (DPF), and a change in the propensity for the deformation-induced martensitic transformation. While the occurrence of these effects in the same temperature interval may be fortuitous, we hypothesize that they are interrelated and provide a tentative explanation of the observed phenomena on this basis.     

Atomic-scale dissecting the formation mechanism of gradient nanostructured layer on Mg alloy processed by a novel high-speed machining technique

Severe plastic deformation(SPD)-induced gradient nanostructured(GNS)metallic materials exhibit supe-rior mechanical performance,especially the high strength and good ductility.In this study,a novel high-speed machining SPD technique,namely single point diamond turning(SPDT),was developed to produce effectively the GNS layer on the hexagonal close-packed(HCP)structural Mg alloy.The high-resolution transmission electron microscopy observations and atomistic molecular dynamics sim-ulations were mainly performed to atomic-scale dissect the grain refinement process and corresponding plastic deformation mechanisms of the GNS layer.It was found that the grain refinement process for the formation of the GNS Mg alloy layer consists of elongated coarse grains,lamellar fine grains with deformation-induced-tension twins and contraction twins,ultrafine grains,and nanograins with the grain size of～70 nm along the direction from the inner matrix to surface.Specifically,experiment results and atomistic simulations reveal that these deformation twins are formed by gliding twinning partial dis-locations that are dissociated from the lattice dislocations piled up at grain boundaries.The corresponding deformation mechanisms were evidenced to transit from the deformation twinning to dislocation slip when the grain size was below 2.45 μm.Moreover,the Hall-Petch relationship plot and the surface equivalent stress along the gradient direction estimated by finite element analysis for the SPDT process were incorporated to quantitatively elucidate the transition of deformation mechanisms during the grain refinement process.Our findings have implications for the development of the facile SPD technique to construct high strength-ductility heterogeneous GNS metals,especially for the HCP metals.