Microstructure Evolution and Analysis of A [011] Orientation, Single-Crystal, Nickel-Based Superalloy During Tensile Creep

By means of the elastic?Cplastic finite-element method (FEM) for calculating the distribution features of the von Mises stress and strain energy density, the influences of the applied stress on the von Mises stress of the ????/?? phases and the rafting of the ???? phase for the [011] orientation, single-crystal, nickel-based superalloy are investigated. The results show that, after being fully heat treated, the microstructure of the [011] orientation, single-crystal, nickel-based superalloy consists of the cuboidal ???? phase embedded coherently in the ?? matrix, and the cuboidal ???? phase on (100) plane is regularly arranged along a 45?deg angle relative to the [011] orientation. Compared with the matrix channel of [010] orientation, the bigger von Mises stress is produced within the [001] matrix channel when the tensile stress is applied along the [011] orientation. Under the action of the larger principal stress component, the bigger expanding lattice strain occurs on the (001) plane of the cuboidal ???? phase along the [010] direction, which may trap the Al, Ti atoms with a bigger atomic radius for promoting the directional growth of the ???? phase into the stripe-like rafted structure along the [001] orientation. The changes of the interatomic potential energy, misfit stress, and interfacial energy during the tensile creep are thought to be the driving forces of promoting the elements?? diffusion and directional growth of the ???? phase.     

Refinement of Ferrite Grain Size near the Ultrafine Range by Multipass, Thermomechanical Compression

Plane-strain compression testing was carried out on a Nb-Ti-V microalloyed steel, in a GLEEBLE3500 simulator using a different amount of roughing, intermediate, and finishing deformation over the temperature range of 1373?K to 1073?K (1100?°C to 800?°C). A decrease in soaking temperature from 1473?K to 1273?K (1200?°C to 1000?°C) offered marginal refinement in the ferrite (??) grain size from 7.8 to 6.6???m. Heavy deformation using multiple passes between A _e3 and A _r3 with true strain of 0.8 to 1.2 effectively refined the ?? grain size (4.1 to 3.2???m) close to the ultrafine size by dynamic-strain-induced austenite (??) ?? ferrite (??) transformation (DSIT). The intensities of microstructural banding, pearlite fraction in the microstructure (13?pct), and fraction of the harmful ??cube?? texture component (5?pct) were reduced with the increase in finishing deformation. Simultaneously, the fractions of high-angle (>15?deg misorientation) boundaries (75 to 80?pct), beneficial gamma-fiber (ND//??111??) texture components, along with {332}??133?? and {554}??225?? components were increased. Grain refinement and the formation of small Fe₃C particles (50- to 600-nm size) increased the hardness of the deformed samples (184 to 192?HV). For the same deformation temperature [1103?K (830?°C)], the difference in ??-grain sizes obtained after single-pass (2.7???m) and multipass compression (3.2???m) can be explained in view of the static- and dynamic-strain-induced ?? ?? ?? transformation, strain partitioning between ?? and ??, dynamic recovery and dynamic recrystallization of the deformed ??, and ??-grain growth during interpass intervals.     

Analysis of Phase Distribution in Thin Surface Layers Comparable to the Penetration Depth of X-Rays

After hydrogen concentration, gradients in austenitic-type stainless steels, formed during electrochemical charging and followed by hydrogen loss during aging at room temperature, surface stresses, and martensitic phases ????-bcc and ??-hcp, developed. Phase quantitative X-ray surface analysis of distributions of martensitic phases in a thin layer, comparable to the penetration depth of X-rays, based on diffraction data taken for various diffraction reflections (2??, Bragg??s angles) and with various radiations (??-wavelengths) was applied for various degrees of the type steel in the surface layers. An examination of the relationships between ??-phase transitions in a number of stainless steels and their ?? stability revealed that the stability of the ?? phase increased (S stability factor changed from 26.5 in AISI 321 to 44 in AISI 310), the amount of ????-martensites (from 25?pct in AISI 347 to 0?pct in AISI 310) decreased, and ??-martensites (from 48?pct in AISI 310 to 77?pct in AISI 321) increased, while the depth (from 6.2???m in AISI 321 to 3???m in AISI 310) of the martensitic phases decreased. Deformation and fracture experiments were carried out at room temperature in a high-resolution transmission electron microscope with single-axis tilt tensile stage and environmental cell. The principal effect of hydrogen was to decrease the stress required for dislocation motion, for phase transformation of the austenite, and for crack propagation. Formation of ??- and ????-martensite was noted along the fracture surfaces and in front of the crack tip. The cracks propagated through the ??-martensite plates, which formed along the active slip planes, while ???? phase was always found in the high stress regions.     

Unbiased estimates of the parameter rate sensitivity of superplastic materials

The procedure for determining the rheological superplasticity parameters K and m entering the standard power relation ?? = K?? ^m by the input data set describing the dependence of the flow stress (??) on the strain rate (??) established by the results of standard uniaxial tests is suggested. The procedure is based on minimizing the deviations of the computed values of ?? from the corresponding experimental data. In contrast with the standard procedure, the suggested approach does not involve the linearization of the material model ?? = K?? ^m . The procedure is approved by the experimental data for alloys Al-12Si, Al-33Cu, Al-Cu-0.4Zr, and SURPAL known from the references. It is established that the linearization of the nonlinear superplasticity model ?? = K?? ^m can cause a shift in the estimates of parameter m by 5?C10%, which in turn leads to an overestimation of the duration of superplastic forming by 10?C30%.     

In Situ Measurement of the γ/γ′ Lattice Mismatch Evolution of a Nickel-Based Single-Crystal Superalloy During Non-isothermal Very High-Temperature Creep Experiments

The evolution of the ??/???? lattice mismatch of the AM1 single-crystal superalloy was measured during in situ non-isothermal very high-temperature creep tests under X-ray synchrotron radiation. The magnitude of the effective lattice mismatch in the 1273?K to 1323?K (1000?°C to 1050?°C) temperature range always increased after overheatings performed at temperatures lower than 1403?K (1130?°C). In contrast, a decrease of its magnitude was observed after overheatings at temperatures greater than 1453?K (1180?°C) due to massive dislocation recovery processes occurring at very high temperature.     

HT9钢的高温蠕变性能研究

??The tensile creep of HT9 steel was measured at 700 and 800?? with different stress levels. Stress exponent was fitted by power law relation. Rupture time vs. minimum creep rate of HT9 steel was fitted by M- G relationship and modified M- G relationship. The fracture morphology after creeping and the creep mechanism and damage mechanism were analyzed by scanning electron microscopy, transmission electron microscopy and X- ray diffraction. The results showed that the minimum creep rate and creep rupture time of HT9 steel obeyed a linear relationship with the stress in double logarithmic coordinates, which could be described by M- G and modified M- G relationship. The stress exponent increased with the temperature. The dislocations bypassed the second phase particles during the creep process according to the Orowan mechanism. The fracture had a distinct dimple structure, and some of the second phase particles coarsened. The oxidation of HT9 steel was obvious during the creep at 800??. The main precipitates were M23C6 during the creep, which showed different forms, with significant differences in the size of the precipitated phases. The damage mechanism of HT9 steel included external cross- sectional area loss, material microstructure degradation, environmental damage, etc. There may also be internal sectional area loss.     

Liquid-Metal-Induced Embrittlement of Zn-Coated Hot Stamping Steel

Liquid-metal-induced embrittlement (LMIE) of galvanized hot stamping steel occurs due to the simultaneous application of stress and the presence of a liquid Zn surface layer during the hot stamping process. The mechanism specific to the liquid metal induced embrittlement occurring during hot stamping was investigated in detail. It was found that when a tensile stress was applied, liquid Zn could penetrate along grain boundaries in the steel matrix at temperatures above the Liquid?+???-Fe (Zn) ?? ??₁ peritectic transformation temperature of 1055?K (782?°C). The results show that an increase of the annealing time prior to hot stamping is an effective way to prevent LMIE by the elimination of the liquid phase.     

Evaluation of Microstructural, Mechanical Properties and Corrosion Behavior of AISI Type 316LN Stainless Steel and Modified 9Cr-1Mo Steel Exposed in a Dynamic Bimetallic Sodium Loop at 798 K (525?°C) for 16,000 Hours

Changes occurring in the chemical composition, microstructure, mechanical properties, and carburization behavior of type 316LN stainless steel and modified 9Cr-1Mo steel on exposure to flowing sodium at 798?K (525?°C) for 16,000?hours in a bimetallic loop are discussed in this article. Type 316LN stainless steel revealed a degraded layer of approximately 5???m depth. No significant microstructural changes were observed in the case of modified 9Cr-1Mo steel exposed to sodium. The carburization depth in type 316LN stainless steel was approximately 100???m and the surface carbon concentration was 0.374?wt?pct. In the case of modified 9Cr-1Mo steel, the carbon concentration at the surface was approximately 3.50?wt?pct and the depth of carburization was nearly 75???m. The concentration of nickel and chromium decreased from the bulk to the surface of type 316LN stainless steel, leading to the formation of a ferrite layer. The concentration of these two elements reached the original matrix concentration at around 30???m. Sodium-exposed material indicated an increase in yield strength by 10?pct and reduction in ductility by 34?pct vis-à-vis annealed material. No such changes in strength and ductility were observed in the case of modified 9Cr-1Mo steel. A decrease in impact energy was noticed for sodium-exposed type 316LN stainless steel and modified 9Cr-1Mo steel vis-à-vis as-received material.     

High Strain-Rate Behavior and Transformation-Induced Plasticity of a High-Strength FeCrMoVWC Alloy Manufactured by Rapid Solidification Technique

By employing the rapid solidification technique and special manufacturing conditions, e.g., using only pure elements, an alloy was produced that exhibits major differences to conventionally produced high-speed steels. Within this study, the strain rate dependent material behavior of the examined alloy was characterized under compressive loading for a strain rate range from 10^?3?seconds^?1 to 10³?seconds^?1. The aim was to understand the inherent mechanisms when low and high strain rates are applied. Initially, microstructural observations of the base material, which revealed ????-martensite, retained austenite, and complex carbides, were conducted. The material exhibits extraordinarily high ultimate compression strength of up to 4800?MPa, a high work-hardening behavior, and a good deformability of 15?pct. Moderate strain rate sensitivity was detected. Furthermore, a strain-induced transformation (transformation induced plasticity [TRIP]-effect) from retained austenite to ????-martensite occurred. Interrupted compression tests at different strains and strain rates were carried out to understand the microstructural evolution. The examinations showed that adiabatic heating decreases the transformation rate of retained austenite to ????-martensite and counteracts the work-hardening behavior. For higher strain rates higher ????-martensite contents in the initial deformation region as well as a pronounced saturation behavior of ????-martensite was detected. A hypothesis is given for the strong work-hardening behavior of the alloy.     

A Novel Metastable Ti-25Nb-2Mo-4Sn Alloy with High Strength and Low Young’s Modulus

A novel metastable ?? Ti-25Nb-2Mo-4Sn (wt pct) alloy with high strength (1113?MPa) and low Young??s modulus (65?GPa) was prepared. Nanometer-scaled ?? precipitates as well as the dislocations play a key role in strengthening. The ?? phase containing low content of ??-stabilizers is stabilized with the assistance of thermomechanical treatment, resulting in a relatively low elastic modulus. Therefore, the current alloy performs high strength-to-modulus ratio and could be a potential candidate for biomedical applications.     

High Temperature Plasticity of Bimetallic Magnesium and Aluminum Friction Stir Welded Joints

The high temperature deformation of a bimetallic AZ31/AA6061 Friction Stir Welded joint was investigated in the present study by constant load creep experiments carried out at 473 K (200 °C). The microstructural analysis revealed the strongly inhomogeneous nature of the weld, which was characterized by an extremely fine grain size in the magnesium-rich zones and by the extensive presence of intermetallic phases. In the high stress regime, the creep strain was concentrated in the refined and particle-rich microstructure of the weld zone, while the AA6061 base metal remained undeformed. In the low stress regime, deformation became more homogeneously distributed between the AZ31 base metal and the weld zone. The creep behavior of the weld was found to obey the constitutive equation describing the minimum creep rate dependence on applied stress for the base AZ31, slightly modified to take into account the finer microstructure and the role of secondary phase particles, i.e., the retardation of grain growth and the obstruction of grain boundary sliding.     

Improving the Mechanical Properties of Fe-Nb-(Ni-Mn) Dendrite-Ultrafine Eutectic Composites via Controlling the Primary Phase Features

Tuning of microstructure by addition of austenite stabilizers effectively enhances the mechanical properties in Fe-Nb-(Ni-Mn) dendrite-ultrafine eutectic composites. The Fe₉₃Nb₇ alloy displays the improved plasticity up to 10?pct due to the introduction of a ductile ??-Fe dendrite into the ultrafine eutectic matrix. Meanwhile, the Fe₇₈Nb₇Ni₁₀Mn₅ alloy, which forms an in-situ martensitic ????-Fe dendritic phase reinforced ultrafine eutectic composite exhibits excellent combination of a high fracture strength of 1.6?GPa and a large plastic strain of 11?pct. The investigations reveal that the characteristics of the modulated primary dendrites in the dendrite-ultrafine eutectic composites play an important role in manipulating the generation and propagation of shear bands, thus resulting in the improved mechanical properties and plastic deformation behavior.     

Microstructure and Creep Behavior of High-Pressure Die-Cast Magnesium Alloy AE44

The microstructure and creep behavior of a high-pressure die-cast AE44 (Mg-4Al-4RE) alloy have been studied. The creep properties were evaluated at 423?K and 448?K (150?°C and 175?°C) under stresses in the range 90 to 110?MPa. The microstructures before and after creep were examined by transmission electron microscopy (TEM). After creep, AE44 exhibits anomalously high stress exponents (n ?= 67 at 423?K [150?°C] and n ?= 41 at 448?K [175?°C]) and stress-dependant activation energies ranging from 221 to 286?kJ/mol. The dislocation substructure developed during creep is characterized by extensive nonbasal slip and isolated but well-defined subgrain boundaries. It is shown that the anomalously high stress exponents cannot be rationalized by the threshold stress approach that is commonly adopted in analyzing the creep behavior of dispersion-strengthened alloys or metal matrix composites. A comparison in creep resistance is also made between AE44 and AE42 (Mg-4Al-2RE).     

Analysis of Tensile Stress-Strain and Work-Hardening Behavior in 9Cr-1Mo Ferritic Steel

Detailed analysis on tensile true stress (??)-true plastic strain (??) and work-hardening behavior of 9Cr-1Mo steel have been performed in the framework of the Voce relationship and Kocks-Mecking approach for wide range of temperatures, 300 K to 873 K (27 °C to 600 °C) and strain rates (6.33 × 10^?5 to 6.33 × 10^?3 s^?1). At all test conditions, ??-?? data were adequately described by the Voce equation. 9Cr-1Mo steel exhibited two-stage work-hardening behavior characterized by a rapid decrease in instantaneous work-hardening rate (?? = d??/d??) with stress at low stresses (transient stage) followed by a gradual decrease in ?? at high stresses (stage III). The variations of work-hardening parameters and ??-?? as a function of temperature and strain rate exhibited three distinct temperature regimes. Both work-hardening parameters and ??-?? displayed signatures of dynamic strain aging at intermediate temperatures and dominance of dynamic recovery at high temperatures. Excellent correlations have been obtained between work-hardening parameters evaluated using the Voce relationship and the respective tensile properties. A comparison of work-hardening parameters obtained using the Voce equation and Kocks-Mecking approach suggested an analogy between the two for the steel.     

The Role of Local Microstructure on Small Fatigue Crack Propagation in an α?+?β Titanium Alloy, Ti-6Al-2Sn-4Zr-6Mo

Microstructural origins of the variability in fatigue lifetime observed in the high- and very-high-cycle fatigue regimes in titanium alloys were explored by examining the role of microstructural heterogeneity (neighborhoods of grains with similar crystallographic orientations or microtexture) on the initiation and early growth of fatigue cracks in Ti-6246. Ultrasonic fatigue of focused ion beam (FIB) micronotched samples was used to investigate long lifetime (10⁷ to 10⁹) behavior for two microstructural conditions: one with microtexture and one without microtexture. For specimens containing notches of nominally 20???m in length, fatigue crack initiation in the microtextured material was most likely to occur from notches placed in neighborhoods with a microtexture favorably oriented for easy basal slip. Initiation lifetimes in the untextured material with similar sized notches were, on average, slightly greater than those for the microtextured condition. In both materials, the crack-initiation lifetime from micronotches of length 2c?>?20???m was a very small fraction (<1?pct) of the measured fatigue lifetime for unnotched specimens. Furthermore, in the microtextured condition, small fatigue crack propagation rates did not correlate with the microtextured regions and did not statistically differ from average small crack growth rates in the untextured material. As the micronotch size was reduced below 20???m, fatigue crack initiation was controlled by microstructure rather than by FIB-machined defects. Finally, predictions of the fraction of life consumed in small and long fatigue crack growth from preexisting cracks nominally equivalent in size to the micronotches was compared with the measured fatigue life of unnotched specimens. The predicted range of lifetimes when factoring in the experimentally observed variability in small fatigue crack growth, only accounted for 0.1?pct of the observed fatigue lifetime variability. These findings indicate that in the high-and very-high-cycle fatigue regimes, fatigue life is dominated by crack initiation and that the variation in the initiation lifetime is responsible for the observed variation in total fatigue life.     

High-Temperature Mechanical Behavior of Extruded Mg-Y-Zn Alloy Containing LPSO Phases

The high-temperature mechanical behavior of extruded Mg_97?3x Y_2x Zn _x (at. pct) alloys is evaluated from 473 K to 673 K (200 °C to 400 °C). The microstructure of the extruded alloys is characterized by Long Period Stacking Ordered structure (LPSO) elongated particles within the magnesium matrix. At low temperature and high strain rates, their creep behavior shows a high stress exponent (n = 11) and high activation energy. Alloys behave as a metal matrix composite where the magnesium matrix transfers part of its load to the LPSO phase. At high-temperature and/or low stresses, creep is controlled by nonbasal dislocation slip. At intermediate and high strain rates at 673 K (400 °C) and at intermediate strain rates between 623 K and 673 K (350 °C and 400 °C), the extruded alloys show superplastic deformation with elongations to failure higher than 200 pct. Cracking of coarse LPSO second-phase particles and their subsequent distribution in the magnesium matrix take place during superplastic deformation, preventing magnesium grain growth.     

Analysis for Free Dendritic Growth Model Applicable to Nondilute Alloys

A steady-state free dendritic growth model applicable to concentrated alloys was proposed as an extension of Wang et al.??s model.[14] The present model adopted a realistic thermodynamic model to replace the Baker?CCahn equation and included a generalized marginal stability criterion and a nondilute solute trapping model to completely eliminate the dilute alloy limitation. Comparative analysis shows that Wang et al.??s model is a very close approximation to the present model at low undercoolings for dilute alloys. However, the difference appears at high undercoolings even for dilute alloys. Furthermore, the difference of the model predictions for both models increases with nominal composition of alloys due to the inherent limitation of dilute alloys in Wang et al.??s model. A comparison with the experimental data for Cu₇₀Ni_30?alloy demonstrates the applicability of the present model to nondilute alloys.     

Creep of Single Crystals of Nickel-Based Superalloys at Ultra-High Homologous Temperature

The creep behavior of single crystals of the nickel-based superalloy CMSX-4 was investigated at 1288 °C, which is the temperature of the hot isostatic pressing treatment applied to this superalloy in the industry. It was found that at this super-solvus temperature, where no γ′-strengthening occurs, the superalloy is very soft and rapidly deforms under stresses between 4 and 16 MPa. The creep resistance was found to be very anisotropic, e.g., the creep rate of [001] crystals was about 11 times higher than that of a [111] crystal. The specimens of different orientations also showed a very different necking behavior. The reduction of the cross-sectional area ψ of [001] crystals reached nearly 100 pct, while for a [111] crystal ψ?=?62 pct. The EBSD analysis of deformed specimens showed that despite such a large local strain the [001] crystals did not recrystallize, while a less deformed [111] crystal totally recrystallized within the necking zone. The recrystallization degree was found to be correlated with deformation behavior as well as with dwell time at high temperature. From the analysis of the obtained results (creep anisotropy, stress dependence of the creep rate, traces of shear deformation, and TEM observations), it was concluded that the main strain contribution resulted from 〈\( 0 1\bar{1} \)〉{111} octahedral slip.     

Phase Equilibria of Sn-Co-Cu Ternary System

Sn-Co-Cu ternary alloys are promising lead-free solders, and isothermal sections of Sn-Co-Cu phase equilibria are fundamentally important for the alloys?? development and applications. Sn-Co-Cu ternary alloys were prepared and equilibrated at 523?K, 1073?K, and 1273?K (250?°C, 800?°C, and 1000?°C), and the equilibrium phases were experimentally determined. In addition to the terminal solid solutions and binary intermetallic compounds, a new ternary compound, Sn₃Co₂Cu₈, was found. The solubilities of Cu in the ??-CoSn₃ and CoSn₂ phases at 523?K (250?°C) are 4.2 and 1.6?at. pct, respectively, while the Cu solubility in the ??-Co₃Sn₂ phase is as high as 20.0?at. pct. The Cu solubility increases with temperature and is around 30.0?at. pct in the ??-Co₃Sn_2?at 1073?K (800?°C). The Co solubility in the ??-Cu₆Sn₅ phase is also significant and is 15.5?at. pct at 523?K (250?°C).     

Microstructure and deformation rate during long-term cyclic creep of the martensitic steel X22CrMoV12-1

The microstructure of three specimens of the martensitic steel X22CrMoV12-1 which had been subjected to long-term cyclic creep at 873 K with intermittent phases of unloading (stress ratio R = 0) and compression (R = ?1) was quantified by electron microscopy with regard to carbides, dislocations and pores. The laws of time dependent coarsening of carbides and strain controlled growth of subgrains found for monotonic creep hold also for cyclic creep. The longer time it takes cyclic creep to reach a given strain leads to a growth advantage of carbides compared to monotonic creep. The microstructural model of plastic deformation previously developed for monotonic creep on X20(22)CrMoV12-1 allows to calculate the cyclic creep acceleration due to this advantage in carbide growth.