Application of the square root diffusivity to diffusion in Ni-Cr-Al-Mo alloys

The square root diffusivity matrix [r] and diffusivity matrix [D] have been measured for a Ni-4.0 at. pct Cr-6.3 at. pct Al-3.5 at. pct Mo quaternary alloy at 1100 °C. Using a constant diffusivity analysis, the diffusivities were obtained from three diffusion couples having small initial concentration differences of 5 at. pct or less. When elements of [r] were substituted into the error function solution of the diffusion equation, it was found that the predicted concentration profiles were accurate to within ±0.1 at. pct of measured values. Also, it was found that Mo tends to reduce the diffusion kinetics of Cr and Al in Ni-base alloys. These results suggest that these methods of analyzing diffusion data and predicting interdiffusion data can be applied to alloys containing more than four components. Formerly Graduate Student, University of Connecticut.     

Microstructural Evolution of the Interdiffusion Zone between U-9 Wt Pct Mo Fuel Alloy and Zr-1 Wt Pct Nb Cladding Alloy Upon Annealing

Diffusion couple formed between U-9 wt pct Mo and Zr-1 wt pct Nb alloys, proposed as fuel and clad materials, respectively, in nuclear research reactors, was annealed to investigate the microstructural evolution of the interdiffusion zone (IZ) as a function of temperature. A layered-type IZ microstructure was observed, the mechanism of development of which was elucidated. Mo₂Zr phase, present as dispersoids, in the U-rich part of the as-bonded IZ evolved into a continuous layer and into a “massive” morphology upon annealing. The discontinuous precipitation reaction in the matrix adjoining the Mo₂Zr phase, instigated by Mo depletion, generated lamellae of α-U phase within the γ-U(Mo,Zr) matrix. Zr-rich α-Zr(U) precipitates were observed in U-rich U-Mo-Zr matrix in the IZ next to the U-9Mo base material due to the clustering tendency of the matrix phase. The IZ next to Zr-1Nb base material comprised a “basket weave” microstructure of α-Zr laths with β-Zr(Nb,U) interlath boundaries, wherein an omega like transformation of the latter to δ-UZr₂ was also noticed. The growth rates of the IZ were orders of magnitude lower when compared with the ones reported between the compositionally similar U-10 wt pct Mo alloy and the presently used Al or Al-Si cladding alloys.     

Solidification Reaction Sequence of Co-Rich Nb-Al-Co Alloys

The freezing reaction sequence of Co-rich Nb-Al-Co ternary alloys with emphasis on the formation of Laves and Heusler phases has been examined. For Co-rich alloys, the solidification reaction sequence is observed as primary freezing of α-Co and CoAl phases, subsequent [Co + C36] and [CoAl + C36] eutectics, and the final ternary eutectic reaction [L → α-Co + C36 + CoAl]. The compositions of solidified α-Co and C36 phases agree with the corresponding vertices of the tie-triangle at the solidus temperatures. When the Nb concentration is over 20 at. pct in Co-rich alloys, the quasi-peritectic reaction [L + Co₂AlNb → C36 + CoAl] does not occur as equilibrium prediction. The formation of C36 and CoAl phases occurs through solid precipitation and must be distinguished from a solidification reaction.     

Co Effect on As-cast and Heat-Treated Microstructures in Ru-Containing Single-Crystal Superalloys

The effect of Co on the as-cast and heat-treated microstructures was investigated in two experimental Ni-based single-crystal superalloys containing low levels of Re and Ru. The experimental results indicated that increasing the Co content from 7.9 to 15.8 wt pct decreased the volume fraction of (γ + γ′) eutectic and the solidification segregation ratio of W. High levels of Co additions were also found to decrease the solvus temperatures of the γ′ phase and (γ + γ′) eutectic as well as the solidus temperature. During the long-term thermal exposure at 1373 K (1100 °C), no TCP phases precipitated in either alloy. However, the coarsening and coalescence of γ′ precipitates in the alloy containing 15.8 wt pct Co was slower than that in the other alloy with 7.9 wt pct Co. In the current study, high levels of Co additions decreased the equilibrium volume fraction of γ′ phase, leading to a change in the partitioning ratios of TCP-forming elements Cr, Mo, Re, and W between the γ and γ′ phases. This change resulted in a lower degree of elemental supersaturation in the γ matrix and improved the phase stability of the γ/γ′ microstructure. These experimental results were then compared with those obtained from multi-component thermodynamic calculations, and good agreement was observed.     

Atomic Species Associated with the Portevin–Le Chatelier Effect in Superalloy 718 Studied by Mechanical Spectroscopy

In many Ni-based superalloys, dynamic strain aging (DSA) generates an inhomogeneous plastic deformation resulting in jerky flow known as the Portevin–Le Chatelier (PLC) effect. This phenomenon has a deleterious effect on the mechanical properties and, at high temperature, is related to the diffusion of substitutional solute atoms toward the core of dislocations. However, the question about the nature of the atomic species responsible for the PLC effect at high temperature still remains open. The goal of the present work is to answer this important question; to this purpose, three different 718-type and a 625 superalloy were studied through a nonconventional approach by mechanical spectroscopy. The internal friction (IF) spectra of all the studied alloys show a relaxation peak P₇₁₈ (at 885 K for 0.1 Hz) in the same temperature range, 700 K to 950 K, as the observed PLC effect. The activation parameters of this relaxation peak have been measured, E_a(P₇₁₈)?=?2.68?±?0.05 eV, τ₀?=?2·10^{?15 ± 1} s as well as its broadening factor β?=?1.1. Experiments on different alloys and the dependence of the relaxation strength on the amount of Mo attribute this relaxation to the stress-induced reorientation of Mo-Mo dipoles due to the short distance diffusion of one Mo atom by exchange with a vacancy. Then, it is concluded that Mo is the atomic species responsible for the high-temperature PLC effect in 718 superalloy.     

New Equation for Prediction of Martensite Start Temperature in High Carbon Ferrous Alloys

Since previous equations fail to predict M _S temperature of high carbon ferrous alloys, we first propose an equation for prediction of M _S temperature of ferrous alloys containing > 2 wt pct C. The presence of carbides (Fe₃C and Cr-rich M ₇C₃) is thermodynamically considered to estimate the C concentration in austenite. Especially, equations individually specialized for lean and high Cr alloys very accurately reproduce experimental results. The chemical driving force for martensitic transformation is quantitatively analyzed based on the calculation of T ₀ temperature.     

Atom-Probe Tomographic Investigation of Austenite Stability and Carbide Precipitation in a TRIP-Assisted 10 Wt Pct Ni Steel and Its Weld Heat-Affected Zones

Newly developed low-carbon 10 wt pct Ni-Mo-Cr-V martensitic steels rely on the Ni-enriched, thermally stable austenite [formed via multistep intercritical Quench-Lamellarization-Tempering (QLT)-treatment] for their superior mechanical properties, specifically ballistic resistance. Critical to the thermal stability of austenite is its composition, which can be severely affected in the weld heat-affected zones (HAZs) and thus needs investigations. This article represents the first study of the nanoscale redistributions of C, Ni, and Mn in single-pass HAZ microstructures of QLT-treated 10 wt pct Ni steels. Local compositions of Ni-rich regions (representative of austenite compositions) in the HAZs are determined using site-specific 3-D atom-probe tomography (APT). Martensite-start temperatures are then calculated for these compositions, employing the Ghosh-Olson thermodynamic and kinetics approach. These calculations predict that austenite (present at high temperatures) in the HAZs is susceptible to a martensitic transformation upon cooling to room temperature, unlike the austenite in the QLT-treated base-metal. While C in the QLT-treated base-metal is consumed primarily in MC and M₂C-type carbide precipitates (M is Mo, Cr, V), its higher concentration in the Ni-rich regions in the HAZs indicates the dissolution of carbide precipitates, particularly M₂C carbide precipitates. The role of M₂C carbide precipitates and austenite stability is discussed in relation to the increase in microhardness values observed in the HAZs, relative to the QLT-treated base-metal. Insights gained from this research on austenite stability and carbide precipitation in the single-pass HAZ microstructures will assist in designing multiple weld cycles for these novel 10 wt pct Ni steels.     

Deformation Microstructure and Deformation-Induced Martensite in Austenitic Fe-Cr-Ni Alloys Depending on Stacking Fault Energy

The deformation microstructure of austenitic Fe-18Cr-(10-12)Ni (wt pct) alloys with low stacking fault energies, estimated by first-principles calculations, was investigated after cold rolling. The ?-martensite was found to play a key role in the nucleation of α′-martensite, and at low SFE, ? formation is frequent and facilitates nucleation of α′ at individual shear bands, whereas shear band intersections become the dominant nucleation sites for α′ when SFE increases and mechanical twinning becomes frequent.     

Improvement of Creep Resistance at 950 °C and 400 MPa in Ru-Containing Single-Crystal Superalloys with a High Level of Co Addition

Microstructural features, including γ′ volume fraction and size, γ-γ′ lattice misfit, γ channel width, and dislocation substructure, are known to significantly influence the creep performance in Ni-base single-crystal superalloys. In this study, the microstructural characteristics of Ru-containing single-crystal superalloys with different levels of Co, Mo, and Ru additions were quantitatively investigated after ruptured and interrupted creep tests conducted at 1223 K (950 °C) and 400 MPa. The creep lifetime was slightly increased with the high level of Co addition and significantly increased with the coadditions of Mo and Ru. A minor effect of Co content on the γ channel width and γ′ volume fraction was found in experimental alloys. The alloy with high levels of Mo and Ru additions was determined to possess a more negative γ-γ′ lattice misfit, and a high density of stacking faults (SFs) was formed in the γ channels during creep. The combined effects of the SFs in the γ matrix serving as the barriers to dislocation movement, as well as the dense interfacial dislocation networks preventing dislocation to shear the γ′ phase, were considered as the main mechanism responsible for the improvement of creep resistance. Results from this study are helpful to understand the effect of microstructural features on creep performance and contribute to the knowledge of physical metallurgy in Ru-containing single-crystal superalloys.     

Effects of Growth Rates and Compositions on Dendrite Arm Spacings in Directionally Solidified Al-Zn Alloys

Dendritic spacing can affect microsegregation profiles and also the formation of secondary phases within interdendritic regions, which influences the mechanical properties of cast structures. To understand dendritic spacings, it is important to understand the effects of growth rate and composition on primary dendrite arm spacing (λ ₁) and secondary dendrite arm spacing (λ ₂). In this study, aluminum alloys with concentrations of (1, 3, and 5 wt pct) Zn were directionally solidified upwards using a Bridgman-type directional solidification apparatus under a constant temperature gradient (10.3 K/mm), resulting in a wide range of growth rates (8.3–165.0 μm/s). Microstructural parameters, λ ₁ and λ ₂ were measured and expressed as functions of growth rate and composition using a linear regression analysis method. The values of λ ₁ and λ ₂ decreased with increasing growth rates. However, the values of λ ₁ increased with increasing concentration of Zn in the Al-Zn alloy, but the values of λ ₂ decreased systematically with an increased Zn concentration. In addition, a transition from a cellular to a dendritic structure was observed at a relatively low growth rate (16.5 μm/s) in this study of binary alloys. The experimental results were compared with predictive theoretical models as well as experimental works for dendritic spacing.     

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.     

The Friction Stress of the Hall–Petch Relationship of Pure Mg and Solid Solutions of Al,Zn, and Gd

Temperature and solute concentration effects on the friction stress, σ_o, of cast (texture-free) polycrystals of pure Mg, and of Mg-Al, -Zn and -Gd binary solid solutions are discussed using phenomenological arguments. The temperature effects on the pure metal suggest that σ_o relates to the ratio between the CRSS of prism and basal slip, against early suggestions that it should only relate to the CRSS for basal slip. Solid solution softening upon prism slip accounts for the minima in σ_o at ~ 0.5 at. pct in Mg-Zn and Mg-Gd alloys. In the concentrated alloys, solute-specific hardening effects upon slip and twinning lead to diverging behaviors: in Mg-Al and Mg-Zn, σ_o remains below that of pure Mg. Strong short-range order by Gd leads to a steep monotonic increase, and to a value larger in compression than in tension due to the activation of {10-11} twinning at high concentrations. The negative σ_o of the dilute Mg-Zn alloys is an artifact created by the tension/compression asymmetry stemming from the polar character of {10-12} twinning.     

Kinetics of Post-dynamic Coarsening and Reverse Transformation in Ti-6Al-4V

Experiments were carried out on the post-dynamic coarsening of alpha and reverse transformation of Ti-6Al-4V. The post-dynamic coarsening followed rⁿ vs time kinetics and the n = 3 best fit indicated that it was controlled by bulk diffusion, i.e., by vanadium diffusion through the beta matrix. Its rate was one order of magnitude faster than that applicable to static coarsening. The reverse transformation was characterized using a compression dilatometer and occurred in two stages; the first was transformation on dislocations; the second involved the growth of the alpha structure.     

Effect of Continuous Galvanizing Heat Treatments on the Microstructure and Mechanical Properties of High Al-Low Si Transformation Induced Plasticity Steels

Heat treatments were performed using an isothermal bainitic transformation (IBT) temperature compatible with continuous hot-dip galvanizing on two high Al–low Si transformation induced plasticity (TRIP)-assisted steels. Both steels had 0.2 wt pct C and 1.5 wt pct Mn; one had 1.5 wt pct Al and the other had 1 wt pct Al and 0.5 wt pct Si. Two different intercritical annealing (IA) temperatures were used, resulting in intercritical microstructures of 50 pct ferrite (α)-50 pct austenite (γ) and 65 pct α-35 pct γ. Using the IBT temperature of 465 °C, five IBT times were tested: 4, 30, 60, 90, and 120 seconds. Increasing the IBT time resulted in a decrease in the ultimate tensile strength (UTS) and an increase in the uniform elongation, yield strength, and yield point elongation. The uniform elongation was higher when using the 50 pct α-50 pct γ IA temperature when compared to the 65 pct α-35 pct γ IA temperature. The best combinations of strength and ductility and their corresponding heat treatments were as follows: a tensile strength of 895 MPa and uniform elongation of 0.26 for the 1.5 pct Al TRIP steel at the 50 pct γ IA temperature and 90-second IBT time; a tensile strength of 880 MPa and uniform elongation of 0.27 for the 1.5 pct Al TRIP steel at the 50 pct γ IA temperature and 120-second IBT time; and a tensile strength of 1009 MPa and uniform elongation of 0.22 for the 1 pct Al-0.5 pct Si TRIP steel at the 50 pct γ IA temperature and 120-second IBT time.     

Room-Temperature Deformation and Martensitic Transformation of Two Co-Cr-Based Alloys

Deformation of two Co-Cr alloys was studied by in situ synchrotron X-ray diffraction. Both alloys show stress-induced martensite transformation, which is affected by phase stabilities and transformation strains. Crystal structure of WC in Co-20Cr-15W-10Ni is identified. Compared with other phases present, it is elastically isotropic, exhibits high strength, and can elastically withstand strains exceeding 1 pct. Texture change during phase transformation is explained based on the crystal orientation relationship between γ- and ε-phases.     

Application of the Thermodynamic Extremal Principle to Massive Transformations in Fe-C Alloys

The thermodynamic extremal principle was applied to propose a model in which trans-interface diffusion from the product phase to the interface, from the interface to the parent phase and interface migration are integrated for diffusion-controlled phase transformations in Fe-C alloys. Compared with the classical models with either a local-equilibrium condition or a constrained carbon equilibrium condition, the current model is able to predict massive transformations in the two-phase region. Application to isothermal phase transformations showed that the phase transformation mode is independent of (dependent on) trans-interface diffusion when the initial composition is close to the T₀ line (close to the α/α?+?γ boundary). Ascribed to the large solute diffusivity of C, the thickness of the spike upon massive transformations could be much larger than the atomic spacing and the diffusion-controlled phase transformations could be faster than the interface-controlled phase transformations. Three stages, i.e., the diffusive transformation, massive transformation and the soft impingement stages, were predicted for phase transformations upon continuous cooling, according to which the critical limit between diffusive and massive transformations was determined to be within the two-phase region, being consistent with the experimental results in ultra-low-carbon Fe-C alloys. The current work could be very useful to control diffusion-controlled phase transformations and modulate the mechanical properties of steels.     

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.     

Magnetic Hysteretic and Mechanical Properties of a 31Kh20K3M Alloy with an Increased Carbon Content

An Fe–31Cr–20Co–3Mo (31Kh20K3M) alloy containing 0.09 wt % C, which is almost twice as much as its maximum content according to GOST 24897–81, has been studied to verify the influence of the carbon content on the magnetic hysteretic properties of hard magnetic high-chromium Fe–Cr–Co alloys. The optimal heat treatment, including thermomagnetic treatment, results in the average values of residual magnetic induction B_r = 0.96 T and coercive force H_cB = 63 kA/m and the maximum energy product (BH)_max = 29 kJ/m³. Some heat treatment regimes give B_r = 1.03 T, H_cB = 72 kA/m, and (BH)_max = 31 kJ/m³. In addition, for isotropic alloy samples, the following average values are obtained: B_r = 0.71 T, HcB = 56 kA/m, and (BH)_max = 15 kJ/m³. These magnetic hysteretic properties of the 31Kh20K3M alloy with an increased carbon content are similar to those of a powder 30Kh21K3M alloy with the minimum carbon content.     

Electron-microscopic study of the slip systems in an Mg-4.5% Al-1% Zn alloy

Samples of a hot-rolled Mg-4.5% Al-1% Zn alloy plate cut normal to the rolling direction are studied to determine the density of dislocations with different Burgers vectors after warm rolling at low total strains. In the experiment, 22 different grains and their orientations are studied in a JEM-1000 high-voltage transmission electron microscope at an accelerated voltage of 750 kV using the WB DF technique (g, ng, where n = 2, 3, …) for the observation of dislocations. The main method used for the analysis of the Burgers vector of the dislocations is the invisibility criterion gb = 0. In the samples, dislocations with Burgers vectors 〈a〉, [c], and 〈a + c〉 are found. The dislocation dissociation reaction of the 〈a + c〉 dislocation into the 〈a〉 and [c] dislocations has been found for the first time, and an assumption is made that all of the found [c] dislocations result from this reaction. The density of the basal and nonbasal 〈a〉 dislocations does not depend on the orientation of the grains. Rarely found [c] and 〈a + c〉 dislocations are in grains whose orientations are near the texture maximum in the {0002} pole figure.     

Regularities of the contact interaction of titanium carbide with Ni and Ni–Mo melts

Regularities of the dissolution, the phase formation, and the structure formation implemented under the contact interaction conditions of titanium carbide of various compositions with Ni and Ni–(5–25%)Mo melts are investigated. It is originally established that the dissolution of carbide TiC_x in nickel-based melts is incongruent. Preferentially, carbon transfers into the melt at x ≥ 0.9 and titanium at x ≤ 0.8. The limiting stage of the dissolution is diffusion of metal atoms in the liquid phase. The formation regularities of carbide Ti_1–nMo_nC_x (K-phase)—the main product of the contact interaction in the TiC/Ni–Mo system—are revealed. It is established that the K-phase is formed under the relative excess conditions of the Ni–Mo melt preferentially according to the dissolution–isolation mechanism. The composition of autonomous isolations of the K-phase depending on the experimental conditions (1450°C, 0–25 h) varies in limits from Ti_0.4Mo_0.6C_0.7 (a = 4.27 Å) to Ti_0.7Mo_0.3C_0.6 (a = 4.29 Å). It is determined by the molybdenum concentration in the melt at the unsteady dissolution stage and by the concentration ratio between titanium and carbon in it at the steady-state dissolution stage.