Ti-5Al-2Zr-2Sn-4Cr-4Mo合金时效强化特性

研究了Ｔｉ１７合金在相变点以下，经不同固溶温度处理、不同温度时效后亚稳β相的分解特性和硬化效应。结果表明，该合金在α＋β相区固溶处理，６３０℃附近温度时效，亚稳β相直接析出α相，具有较高的时效硬化速率。在时效初期，α相的含量随时间的增加而增加，而α相的形态经较长时间的时效方发生变化。最后，对α相的析出机制作了简要讨论。     

Frequent Occurrence of Discontinuous Dynamic Recrystallization in Ti-6Al-4V Alloy with α′ Martensite Starting Microstructure

The microstructural conversion mechanism in an α′ martensite starting microstructure during hot deformation (at 973 K (700 °C)-10 s^?1) of the Ti-6Al-4V alloy is studied through detailed microstructural observations, kinetic analysis of deformation in the microstructure, and various theoretical models. After compressing the α′ starting microstructure at 973 K (700 °C)-10 s^?1 and at a height strain of 0.8, it is observed that the α′ starting microstructure with acicular morphology evolved into an ultrafine-grained microstructure with an average grain size of 0.2 μm and a high fraction of high-angle grain boundaries. At the initial stage of deformation, subgrain formation in martensite variants and the formation of new grains with high-angle boundaries at interfaces of martensite variants, and $ \{ 10\bar{1}1\} $ twins are dominant. On increasing the height strain to 0.8, discontinuous dynamic recrystallization (DDRX) along with heterogeneous nucleation and fragmentation of grains with high-angle boundaries becomes dominant. In contrast, in the case of an (α + β) starting microstructure, continuous dynamic recrystallization (CDRX) is dominant throughout the deformation process. Thus, we found that DDRX becomes dominant by changing the starting microstructure from the conventional (α + β) to the acicular α′ martensite one. This behavior of the α′ martensite microstructure is attributed to the considerable number of nucleation sites such as dislocations, interfaces of martensite variants and $ \{ 10\bar{1}1\} $ twins, and the high-speed grain fragmentation along with subgrain formation in the α′ starting microstructure during the initial stage of deformation.     

Deformation Behavior of Ti-5.6Al-4.8Sn-2.0Zr-1.0Mo-0.35Si-0.85Nd Alloy in β/Quasi-β Forging Process

The hot deformation behavior of Ti-5.6Al-4.8Sn-2.0Zr-1.0Mo-0.35Si-0.85 Nd alloy in β/quasi-β forging process was studied using isothermal compression tests over temperature range from 1040 ℃ to 1100 ℃ and strain rates form 0.001 s~(-1)to 70 s~(-1)The results show that the flow stress and microstructure are sensitive to thermomechanical parameters.The processing maps based on the dynamic materials model at strain of 0.3 and 0.7 were established.The optimum deformation thermomechanical parameters at a strain of 0.7 have two regions that exhibit the peak of power dissipation efficiency.One is the region of 1062-1100 ℃ and 10~(-3)-10~(-1.5)s~(-1); and another which represents dynamic recrystallization is 1040-1045 ℃ and 10~(-1.8)-10~(-0.9)s~(-1)The instable region is located where the strain rate is larger than 1 s~(-1)which corresponds to the mechanical instability.     

Investigation on the Microstructure and Mechanical Properties of Ti-1.0Fe Alloy with Equiaxed α + β Microstructures

Metallurgical and Materials Transactions A - In this study, the microstructural characteristics and mechanical properties of Ti-1.0Fe alloy with equiaxed α + β microstructures...     

Microstructural Modeling of the α + β Phase in Ti-6Al-4V: A Diffusion-Based Approach

Metallurgical and Materials Transactions B - Complex heat treatment operations and advanced manufacturing processes such as laser or electron-beam welding will see the metallic workpiece experience...     

The Effect of β Stabilizers on the Structure and Energy of α/β Interfaces in Titanium Alloys

The structure and energy associated with interfaces between the BCC and HCP lattices (β and α phase, respectively) in titanium alloys with commonly used β stabilizers were analyzed. For this purpose, the crystallographic structure of the matching facets of broad, side and end faces was described using misfit dislocations and structural ledges which compensate the mismatch in atomic spacing of the α and β phases. The effect of the β/α transformation temperature due to various concentration of β stabilizers on periodicity of misfit dislocations and structural ledges was estimated. The van der Merwe approach was used to calculate energy of different matching facets. An increase in the percentage of β-stabilizing elements was found to result in a decrease in the lattice-parameter ratio (a_β/a_α) and an increase in the energy of all faces. The dependence of the interface energy on the a_β/a_α ratio was for the first time quantified, and insight into the preferred shape of α-phase precipitates was obtained.

     

Retardation Effect of Rare Earth-Rich Phase to Grain Growth of Ti-5.6Al-4.8Sn-2Zr-1Mo-0.32Si-1Nd High Temperature Alloy

Retardation Effect of Rare Earth-Rich Phase to Grain Growth of Ti-5.6Al-4.8Sn-2Zr-1Mo-0.32Si-1Nd High Temperature Alloy     

Effect of Low-Friction Composite Polymer Coatings Fabricated by Electrophoretic Deposition and Heat Treatment on the Ti-6Al-4V Titanium Alloy’s Tribological Properties

Metallurgical and Materials Transactions A - In this work, polytetrafluoroethylene/polyetheretherketone (PTFE/PEEK 708) coatings were fabricated by electrophoretic deposition (EPD) and heat...     

Understanding the Interdependencies Between Composition,Microstructure, and Continuum Variables and Their Influence on the Fracture Toughness of α/β-Processed Ti-6Al-4V

The fracture toughness of a material depends upon the material’s composition and microstructure, as well as other material properties operating at the continuum level. The interrelationships between these variables are complex, and thus difficult to interpret, especially in multi-component, multi-phase ductile engineering alloys such as α/β-processed Ti-6Al-4V (nominal composition, wt pct). Neural networks have been used to elucidate how variables such as composition and microstructure influence the fracture toughness directly (i.e., via a crack initiation or propagation mechanism)—and independent of the influence of the same variables influence on the yield strength and plasticity of the material. The variables included in the models and analysis include (i) alloy composition, specifically, Al, V, O, and Fe; (ii) materials microstructure, including phase fractions and average sizes of key microstructural features; (iii) the yield strength and reduction in area obtained from uniaxial tensile tests; and (iv) an assessment of the degree to which plane strain conditions were satisfied by including a factor related to the plane strain thickness. Once trained, virtual experiments have been conducted which permit the determination of each variable’s functional dependency on the resulting fracture toughness. Given that the database includes both K _{1 C} and K _Q values, as well as the in-plane component of the stress state of the crack tip, it is possible to quantitatively assess the effect of sample thickness on K _Q and the degree to which the K _Q and K _{1 C} values may vary. These interpretations drawn by comparing multiple neural networks have a significant impact on the general understanding of how the microstructure influences the fracture toughness in ductile materials, as well as an ability to predict the fracture toughness of α/β-processed Ti-6Al-4V.     

Microstructure and Texture Evolution During Sub-Transus Thermo-Mechanical Processing of Ti-6Al-4V-0.1B Alloy: Part II. Static Annealing in (α + β) Regime

The first part of this study describes the evolution of microstructure and texture in Ti-6Al-4V-0.1B alloy during sub-transus rolling vis-à-vis the control alloy Ti-6Al-4V. In the second part, the static annealing response of the two alloys at self-same conditions is compared and the principal micromechanisms are analyzed. Faster globularization kinetics has been observed in the Ti-6Al-4V-0.1B alloy for equivalent annealing conditions. This is primarily attributed to the α colonies, which leads to easy boundary splitting via multiple slip activation in this alloy. The other mechanisms facilitating lamellar to equiaxed morphological transformations, e.g., termination migration and cylinderization, also start early in the boron-modified alloy due to small α colony size, small aspect ratio of the α lamellae, and the presence of TiB particles in the microstructure. Both the alloys exhibit weakening of basal fiber (ND||〈0001〉) and strengthening of prism fiber (RD||〈 $ 10\bar{1}0 $ 〉) upon annealing. A close proximity between the orientations of fully globularized primary α and secondary α phases during α → β → α transformation has accounted for such a texture modification.     

Microstructure and Texture Evolution During Sub-Transus Thermomechanical Processing of Ti-6Al-4V-0.1B Alloy: Part I. Hot Rolling in (α + β) Phase Field

In the current study, the evolution of microstructure and texture has been studied for Ti-6Al-4V-0.1B alloy during sub-transus thermomechanical processing. This part of the work deals with the deformation response of the alloy by rolling in the (α + β) phase field. The (α + β) annealing behavior of the rolled specimen is communicated in part II. Rolled microstructures of the alloys exhibit either kinked or straight α colonies depending on their orientations with respect to the principal rolling directions. The Ti-6Al-4V-0.1B alloy shows an improved rolling response compared with the alloy Ti-6Al-4V because of smaller α lamellae size, coherency of α/β interfaces, and multiple slip due to orientation factors. Accelerated dynamic globularization for this alloy is similarly caused by the intralamellar transverse boundary formation via multiple slip and strain accumulation at TiB particles. The (0002)_α pole figures of rolled Ti-6Al-4V alloy shows “TD splitting” at lower rolling temperatures because of strong initial texture. Substantial β phase mitigates the effect of starting texture at higher temperature so that “RD splitting” characterizes the basal pole figure. Weak starting texture and easy slip transfer for Ti-6Al-4V-0.1B alloy produce simultaneous TD and RD splittings in basal pole figures at all rolling temperatures.     

Effect of Duplex Aging on the Initiation and Propagation of Fatigue Cracks in the Solute-rich Metastable <Emphasis Type="Italic">β</Emphasis> Titanium Alloy Ti 38-644

Aging of highly β-stabilized titanium alloys commonly leads to the formation of precipitate-free zones being susceptible to fatigue crack initiation. Duplex aging improves the fatigue properties of metastable β titanium alloys by enhancing a homogeneous α phase formation. In this study a duplex-aging cycle was designed for Ti 38-644 (β-C). Depending on the prior processing history heat treatment parameters were adapted on the basis of microstructure studies, hardness measurements and comparative tensile tests. The fatigue limit and fatigue crack growth threshold were determined for duplex-aged β-C. The results indicate that duplex aging promotes a homogeneously precipitated α phase providing excellent values of the fatigue limit. Surface-related fatigue crack initiation was observed. Comparing the fracture surfaces of direct- and duplex-aged β-C a transition of the tensile fracture mode from intergranular to predominantly transgranular was observed accompanied by a gain in ductility at comparable yield strengths. This was assumed to be the reason for the slightly improved fatigue crack growth behavior of duplex-aged as compared to direct-aged β-C. Along the entire heat treatment cycle the microstructure response was evaluated with regard to the particular effects on the fatigue properties. The results indicate clearly that key to success is a completely recrystallized β microstructure and the reasonably controlled aging response.     

Effect of Hypoeutectic Boron Addition on the <Emphasis Type="Italic">β</Emphasis> Transus of Ti-6Al-4V Alloy

In the present study, the β transus of boron-modified Ti-6Al-4V alloy was found to be almost equivalent to that of the normal alloy, although there is a difference in interstitial element content large enough to produce significant change. Compositional analysis confirms the scavenging ability of the boride particles that are present in the microstructure toward the interstitial elements. This factor can successfully retard the α → β phase transformation locally and increase the overall β transus of boron-added material.     

The Effect of Small Additions of Fe and Heavy Deformation on the Precipitation in an Al–1.1Mg–0.5Cu–0.3Si At. Pct Alloy

Metallurgical and Materials Transactions A - The effect of 0.03 and 0.08 at. pct Fe additions on the formation of secondary phases in an Al–1.1Mg–0.5Cu–0.3Si at. pct alloy was...     

Comparison of Microstructure Refinement in Wire-Arc Additively Manufactured Ti–6Al–2Sn–4Zr–2Mo–0.1Si and Ti–6Al–4V Built With Inter-Pass Deformation

Metallurgical and Materials Transactions A - The titanium alloy Ti–6Al–2Sn–4Zr–2Mo–0.1Si (Ti6242) has been deposited for the first time by a directed energy deposition...     

Effect of <Emphasis Type="Italic">β</Emphasis> Grain Size on Stress-Induced Martensitic Transformation in <Emphasis Type="Italic">β</Emphasis> Solution-Treated 51.1Zr-40.2Ti-4.5Al-4.2V Alloy

The effect of β grain size on stress-induced martensitic transformation in β solution-treated 51.1Zr-40.2Ti-4.5Al-4.2V alloy was investigated by using XRD and TEM techniques. The results show that initial β grain size has a profound effect on the triggering stress of the stress-induced martensitic (SIM) transformation. The triggering stress increases with increasing initial β grain size. The SIM transformation significantly affects the deformation behavior of the alloy. A typical double yielding is observed in the stress-strain curves due to the occurrence of the SIM transformation. The curve of work hardening rate vs. true strain is divided into three stages for the samples with small β grain size. The work hardening rate at stage ΙΙ or ΙΙΙ decreases with increasing initial β grain size, which is attributed to the effect of the SIM transformation during a tensile test.