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.     

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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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.     

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.     

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.     

A micromechanical analysis of the yielding behavior of individual widmanstätten colonies of an <Emphasis Type="Italic">α</Emphasis> + <Emphasis Type="Italic">β</Emphasis> titanium alloy

The yielding behavior of individual Widmanstätten α (hcp) + β (bcc) colonies of Ti-8Al-1Mo-1V has been analyzed theoretically by considering the shearing of β platelets by slip in the α phase. Slip is assumed to initiate in the softer α phase and impinges on the harder β platelets. Under the combined actions of the external stress and impinging slip, yielding of the β platelets occurs when the von Mises effective stress in the β phase exceeds a critical value. The theoretical analysis indicates that the macroscopic slip planes in individual Widmanstätten colonies correspond to those that induce the highest stresses in the β platelets. In addition, the yield stress and the strain-hardening rate both increase with decreasing values of the angle, β _b , between the slip direction and the normal to the β platelets, resulting in soft and hard slip orientations. Soft orientation of the lamellar colony is associated with α slip parallel to the macroscopic, serrated α/β interface, which includes crystallographic interface and ledges, while the hard orientation is associated with slip normal to the thin platelets. This slip anisotropy appears to arise from increasing difficulty of slip transmission across the macroscopic α/β interface when the operative slip vector impinges perpendicular to the β platelets.     

Electrical properties of the LaLi<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>Co<Subscript>1 – <Emphasis Type="Italic">y</Emphasis></Subscript>O<Subscript>3 – δ</Subscript> (0 ≤ <Emphasis Type="Italic">y</Emphasis> ≤ 0.10) oxides

The effect of the Li ion concentration on the phase composition, the electrical conductivity, and the thermoelectric power of the LaLi _y Co_1–yO_3–δ (0 ≤ y ≤ 0.1) oxides synthesized by cocrystallization has been studied. It is found that the region of the perovskite-like solid solution LaLi _y Co_1–yO_3–δ is no higher than y = 0.037. In the temperature range 300–1020 K, lithium alloying leads to an increase in the electrical conductivity and a decrease in the positive thermoelectric power of the single-phase samples compared to LaCoO_3–δ. The results are discussed using the density of states model proposed by Senarus Rodriguez and Goodenough for LaCoO_3–δ and La_1–xSr _x CoO_3–δ and using the Mott theory of noncrystalline substances.     

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.     

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.     

Crystallographic Texture and Volume Fraction of <Emphasis Type="Italic">α</Emphasis> and <Emphasis Type="Italic">β</Emphasis> Phases in Zr-2.5Nb Pressure Tube Material During Heating and Cooling

The phase transformations in an as-received Zr-2.5Nb pressure tube material were characterized in detail by neutron diffraction. The texture and volume fraction of α and β phases were measured on heating at eight different temperatures 373 K to 1323 K (100 °C to 1050 °C) traversing across the α/(α + β) and (α + β)/β solvus lines, and also upon cooling at 1173 K and 823 K (900 °C and 550 °C). The results indicate that the α-phase texture is quite stable, with little change in the {0002} and { 11[`2]0 } \left\{ {11\bar{2}0} \right\} pole figures during heating to 1123 K (850 °C). The β-phase volume fraction increased while a slight change in texture was observed until heating reached 973 K (700 °C). On further heating to 1173 K (900 °C), there appears a previously unobserved α-phase texture component due to coarsening of the prior primary α grains; meanwhile the transformed β-phase texture evolved markedly. At 1323 K (1050 °C), the α phase disappeared with only 100 pct β phase remaining but with a different texture than that observed at lower temperatures. On cooling from the full β-phase regime, a different cooldown transformed α-phase texture was observed, with no resemblance of the original texture observed at 373 K (100 °C). The transformed α-phase texture shows that the {0002} plane normals are within the radial-longitudinal plane of the pressure tube following the Burgers orientation relationship of (110)<sub>bcc</sub>//(0002)<sub>hcp</sub> and [[`1]11]_\textbcc //[11[`2]0]_\texthcp [\bar{1}11]_{\text{bcc}} //[11\bar{2}0]_{\text{hcp}} with a memory of the precursor texture of the primary α grains observed on heating at 1173 K (900 °C).     

Formation of <Emphasis Type="Italic">β</Emphasis>-NiAl Phase During Casting of a Ni-Based Superalloy

A high-refractory Ni-based superalloy prototype was melted on a research scale while simulating industry practices. Ingots were vacuum induction melted and subjected to a computationally optimized homogenization heat treatment prior to fabrication which consisted of forging and hot rolling. Failure of one of the ingots at the early stage of the forging process was attributed to the precipitation of the β-NiAl phase during melting which stabilized the eutectic constituent.     

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.     

A Thermodynamic Model to Estimate the Formation of Complex Nitrides of Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Mg<Subscript>(1–<Emphasis Type="Italic">x</Emphasis>)</Subscript>N in Silicon Steel

A complex nitride of Al _x Mg_(1?x)N was observed in silicon steels. A thermodynamic model was developed to predict the ferrite/nitride equilibrium in the Fe-Al-Mg-N alloy system, using published binary solubility products for stoichiometric phases. The model was used to estimate the solubility product of nitride compound, equilibrium ferrite, and nitride compositions, and the amounts of each phase, as a function of steel composition and temperature. In the current model, the molar ratio Al/(Al?+?Mg) in the complex nitride was great due to the low dissolved magnesium in steel. For a steel containing 0.52 wt pct Al_s, 10 ppm T.Mg., and 20 ppm T.N. at 1100 K (827 °C), the complex nitride was expressed by Al_0.99496Mg_0.00504N and the solubility product of this complex nitride was 2.95?×?10^?7. In addition, the solution temperature of the complex nitride increased with increasing the nitrogen and aluminum in steel. The good agreement between the prediction and the detected precipitate compositions validated the current model.