Formation of Grain Boundary <Emphasis Type="Italic">α</Emphasis> in <Emphasis Type="Italic">β</Emphasis> Ti Alloys: Its Role in Deformation and Fracture Behavior of These Alloys

Beta-Ti alloys contain sufficient concentrations of β stabilizing alloy additions to permit retention of the metastable β phase after cooling to room temperature. Decomposition of the metastable β phase results in the formation of several possible phases, at least two of which are metastable. Concurrently, equilibrium α phase often forms first by heterogeneous nucleation at the α grain boundaries with an accompanying precipitate free zone observed adjacent to the grain boundary α. The grain boundary regions are softer than the precipitation hardened matrix. As a consequence, fracture follows the prior β grain boundaries, especially in high-strength conditions. This fracture mode results in low tensile ductility and/or fracture toughness. This article will describe methods of minimizing or eliminating grain boundary α formation by using metastable transition precipitates to nucleate α more rapidly. The effects on fracture behavior also will be described.     

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.     

Crystallography of the <Emphasis Type="Italic">δ</Emphasis>→<Emphasis Type="Italic">α</Emphasis> martensitic transformation in plutonium alloys

A new stress-accommodating crystallographic mechanism of the δ→α martensitic transformation in plutonium alloys is proposed. According to this mechanism, an orientation variant of the α phase is produced by a combination of a homogeneous strain and shuffling of the alternating close-packed (111) _δ planes. It is shown that the formation of stable transformation-induced twins whose twin-plane orientations and twin-shear directions do not depend on the small variations of the crystal-lattice parameters is the preferred stress-accommodating mode. Only these stable twins have dislocation-free twin boundaries, while the twin boundaries of all others are decorated by an ultradense distribution of partial dislocations. The theory predicts a crystal-lattice rearrangement mechanism involving the (205) _α stable twins. The corresponding invariant plane-strain (IPS) solutions, with special emphasis on the two simplest shuffling modes (the single and double elementary modes), are presented and compared with the existing experimental observations. It is shown that the habit-plane orientation is highly sensitive to the input values of the crystal-lattice parameters and, especially, to the accuracy of the measured volume change in the δ→α transformation. An analysis of these effects on the habit-plane orientation and orientation relations is also presented.     

Continuous cooling <Emphasis Type="Italic">β</Emphasis>-to-<Emphasis Type="Italic">α</Emphasis> transformation behaviors of extra-pure and commercially pure Ti

The in-situ continuous cooling β-to-α transformation kinetics of extra-pure (EP) Ti and of grade-4 commercially pure (CP) Ti were investigated using a fully computer-controlled resistivity-temperature realtime measurement apparatus and transmission electron microscopy. The β-to-α′ martensitic transformation occurs under near pure shear condition, and the habit plane of lath-type martensite was determined to be parallel to , which is in good agreement with the prediction of the crystallographic theory. The M _s temperature of EP-Ti was measured as 800 °C and can be raised by up to about 40 °C due to the generation of thermal stress and local deformation during rapid cooling. The massive transformation was, for the first time, observed to occur over a wide range of cooling rates in an EP-Ti. The massive start temperature and its occurrence were, unlike the martensitic transformation, hardly affected by the generation of thermal stress and local deformation during rapid cooling. The stable regime of massive transformation in a grade-4 CP-Ti was considerably shifted toward a slower cooling rate side and was significantly contracted at the same time. This is because the presence of iron impurity not only largely suppresses the massive transformation but also significantly delays a long-range diffusional transformation.     

Elasticity of Ni<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>W<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> (<Emphasis Type="Italic">x</Emphasis> ≤ 0.1875) Alloys from First Principles Calculations

The elastic properties of Ni _x W_1?x alloys up to x = 0.1875 have been determined from first principles calculations. We have used stress–strain relationships to calculate the C _ij elastic coefficients and the Voigt–Reuss–Hill approximations to determine the bulk and shear moduli of polycrystals. The W alloying increases the compression modulus while the shear modulus remains almost constant. Furthermore, the W alloying has a minor effect on the elastic anisotropy and, therefore, on its contribution to the indentation modulus.     

Differential scanning calorimetry study and computer modeling of <Emphasis Type="Italic">β</Emphasis> ⇒ <Emphasis Type="Italic">α</Emphasis> phase transformation in a Ti-6Al-4V alloy

The relationship between heat-treatment parameters and microstructure in titanium alloys has so far been mainly studied empirically, using characterization techniques such as microscopy. Calculation and modeling of the kinetics of phase transformation have not yet been widely used for these alloys. Differential scanning calorimetry (DSC) has been widely used for the study of a variety of phase transformations. There has been much work done on the calculation and modeling of the kinetics of phase transformations for different systems based on the results from DSC study. In the present work, the kinetics of the β ⇒ α transformation in a Ti-6Al-4V titanium alloy were studied using DSC, at continuous cooling conditions with constant cooling rates of 5 °C, 10 °C, 20 °C, 30 °C, 40 °C, and 50 °C/min. The results from calorimetry were then used to trace and model the transformation kinetics in continuous cooling conditions. Based on suitably interpreted DSC results, continuous cooling-transformation (CCT) diagrams were calculated with lines of isotransformed fraction. The kinetics of transformation were modeled using the Johnson-Mehl-Avrami (JMA) theory and by applying the “concept of additivity.” The JMA kinetic parameters were derived. Good agreement between the calculated and experimental transformed fractions is demonstrated. Using the derived kinetic parameters, the β ⇒ α transformation in a Ti-6Al-4V alloy can be described for any cooling path and condition. An interpretation of the results from the point of view of activation energy for nucleation is also presented.     

Morphological Stability of <Emphasis Type="Italic">δ</Emphasis>-Ferrite/<Emphasis Type="Italic">γ</Emphasis> Interphase Boundary in Carbon Steel

The morphological changes of the δ-ferrite/γ interphase boundary have been observed in situ with a high-temperature confocal scanning laser microscope (HTCSLM) during δ/γ transformations (δ → γ and γ → δ) of Fe-0.06 wt pct C-0.6 wt pct Mn alloy, and a kinetic equation of morphological stability of δ-ferrite/γ interphase boundary has been established. Thereafter, the criterion expression for morphological stability of δ-ferrite/γ interphase boundary was established and discussed, and the critical migration speeds of δ-ferrite/γ interphase boundaries are calculated in Fe-C, Fe-Ni, and Fe-Cr alloys. The results indicate that the δ-ferrite/γ interphase boundary is very stable and nearly remains absolute planar all the time during γ → δ transformation in Fe-C alloy. The δ-ferrite/γ interphase boundary remains basically planar during δ → γ transformation when the migration speed is lower than 0.88 μm/s, and the interphase boundary will be unstable and exhibit a finger-like morphology when the migration speed is higher than 0.88 μm/s. The morphological stability of δ-ferrite/γ interphase boundary is primarily controlled by the interface energy and the solute concentration gradient at the front of the boundary. During the constant temperature phase transformation, an opposite temperature gradient on both sides of δ-ferrite/γ interphase boundary weakens the steady effect of the temperature gradient on the boundary. The theoretical analysis of the morphological stability of the δ-ferrite/γ interphase boundary is coincident with the observed experimental results utilizing the HTCSLM. There is a good agreement between the theoretical calculation of the critical moving velocities of δ-ferrite/γ interphase boundaries and the experimental results.     

Structure of Zr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Pt<Subscript>100−<Emphasis Type="Italic">x</Emphasis></Subscript> (73 ≤ <Emphasis Type="Italic">x</Emphasis> ≤ 77) Metallic Glasses

The structure of hyper-eutectic Zr _x Pt_100−x (73 ≤ x ≤ 77) metallic glasses produced by melt spinning was examined with high-energy synchrotron X-ray diffraction (HEXRD) and fluctuation electron microscopy. In addition, details of the amorphous structure were studied by combining ab initio molecular dynamics and reverse Monte Carlo simulations. Crystallization pathways in these glasses have been reported to vary dramatically with small changes in compositions; however, in the current study, the structures of the different glasses were also observed to vary with composition, particularly the prepeak in the total structure factor that occurs at a Q value of around 17 nm⁻¹. Results from simulations and fluctuation electron microscopy suggest that the medium-range order of the amorphous structure is characterized by extended groups of Pt-centered clusters that increase in frequency, structural order, or spatial organization at higher Pt contents. These clusters may be related to the Zr₅Pt₃ structure, which contains Pt-centered clusters coordinated by 9Zr and 2Pt atoms. This article is based on a presentation given in the symposium entitled “Bulk Metallic Glasses IV,” which occurred February 25–March 1, 2007 during the TMS Annual Meeting in Orlando, Florida under the auspices of the TMS/ASM Mechanical Behavior of Materials Committee.

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Refining As-cast <Emphasis Type="Italic">β</Emphasis>-Ti Grains Through ZrN Inoculation

The columnar-to-equiaxed transition and remarkable refinement of β-Ti grains occur in an as-cast Ti-13Mo alloy when a new grain refiner, ZrN, was inoculated at a nitrogen level as low as 0.4 wt pct. The grain refining effect is attributed to in situ-formed TiN particles that provide active nucleation sites and solute Zr that promotes constitutional supercooling. Reproducible orientation relationships were identified between TiN nucleants and β-Ti matrix, and well explained by the edge-to-edge matching model.     

Ti-29Nb-13Ta-4.6Zr高速压制行为及其烧结性能的研究

利用自主研发的机械蓄能式高速压机成形Ti-29Nb-13Ta-4.6Zr粉末并进行真空烧结,研究冲击能量对试样的密度及力学性能的影响。结果表明:随着冲击能量的提高,试样生坯密度提高,在冲击能量为1 805 J时,获得的最大生坯密度达到5.63 g/cm~3(相对密度94.1%);径向弹性后效随着冲击能量增加而增加;经真空1 250℃烧结后,烧结坯的密度随着冲击能量的增加而增加,但烧结坯的体积发生了膨胀,最大烧结密度为5.53 g/cm~3(相对密度为92.5%);真空烧结2.0 h后,钛合金的抗拉强度和硬度达到最大值,分别为629.8 MPa和324.5 HV。     

Determination of <Emphasis Type="Italic">γ</Emphasis>/<Emphasis Type="Italic">γ</Emphasis>′ Lattice Misfit in Ni-Based Single-Crystal Superalloys at High Temperatures by Neutron Diffraction

Constrained γ/γ′ lattice misfit as a function of temperature (room temperature, 871 °C, 982 °C, 1093 °C, and 1204 °C) is measured by neutron diffraction on the first-generation Ni-based single-crystal superalloy René N4 and second-generation superalloys René N5, CMSX4, and PWA1484. All the alloys studied show negative misfit at temperatures above 871 °C. For René N4, René N5, and PWA1484, the misfit becomes less negative at temperatures above 1093 °C, possibly due to either the chemistry effect or internal stress relaxation. The magnitude of the misfit shows a qualitative agreement with Caron’s misfit model based on Vegard’s coefficients. The Re-free alloy René N4 was found to have a larger γ lattice parameter and γ/γ′ misfit due to higher fractions of Cr, Ti, and Mo. After 100 hours of annealing at high temperatures, René N5 shows a more negative misfit than the misfit after the standard heat treatment.     

Microstructure,Texture, and Mechanical Properties of <Emphasis Type="Italic">β</Emphasis> Solution-Treated and Aged Metastable <Emphasis Type="Italic">β</Emphasis> Titanium Alloy,Ti-5Al-5Mo-5V-3Cr

The current study describes the aging characteristics and mechanical properties of a metastable β titanium alloy Ti-5Al-5Mo-5V-3Cr. The aged microstructures consist of fine α-phase precipitates (lath morphology) in equiaxed β grains. The sizes of the α-phase precipitates increase with the increasing aging temperature. The β ST WQ and 823 K (550 °C)-aged material exhibits maximum hardness due to precipitation hardening. The low- and high-temperature aging conditions result in strong c-type basal and prismatic textures in the α-phase, respectively. The β-phase of the alloy aged at low temperature reveals the presence of texture with moderate intensity. In contrast, high-temperature-aged material exhibits very strong β-phase texture. The strengths of the alloy under β ST WQ- and 923 K (650 °C)-aged conditions are the maximum and minimum along TD and RD, while the ductility values are the maximum and minimum along the RD and TD direction samples, respectively. The flow curves follow typical Holloman equation along three sample directions, and the work hardening rate curves display two distinctive regimes, namely, stage I and stage II. The yield locus plots of the β ST WQ and aged materials exhibit the presence of anisotropy.