Microstructural Changes In Alloys Research Articles

In-Situ Characterization of Microstructural Changes in Alloy 718 during High-Temperature Low-Cycle Fatigue

Components made of nickel-based alloys are typically used for high-temperature applications because of their high corrosion resistance and very good creep and fatigue strength, even at temperatures around 1000 °C. Corrosive damage can significantly reduce the mechanical properties and the expected remaining service life of components. In the present study, a new method was introduced to continuously determine the change in microstructure occurring as a result of exposure to high temperature and cyclic mechanical loading. For this purpose, the conventional low-cycle fatigue test procedure was modified and a non-destructive, electromagnetic testing technique was integrated into a servohydraulic test rig to monitor the microstructural changes. The measured values correlate with the magnetic material properties of the specimen, allowing the microstructural changes in the specimen’s subsurface zone to be analyzed upon high-temperature fatigue. Specifically, it was possible to show how different loading parameters affect the maximum chromium depletion as well as the depth of chromium depletion, which influences the magnetic properties of the nickel-based material. It was also observed that specimen failure is preceded by a certain degree of microstructural change in the subsurface zone. Thus, the integration of the testing technology into a test rig opens up new possibilities for improved prediction of fatigue failure via the continuous recording of the microstructural changes.

Study on high temperature mechanical behavior and microstructure evolution of Ni3Al-based superalloy JG4246A

Ni3Al-based polycrystalline intermetallic compound is a kind of engineering superalloy, which have been used in the adjustment flaps of aeroengine tail nozzle. In order to explore its mechanical properties and microstructure evolution in service, the high temperature flow characteristics and microstructural changes of alloys were investigated by uniaxial isothermal compression tests on Gleeble-1500. The true stress-true strain curves were obtained and the microstructures were analyzed by OM, SEM, and EBSD. Based on the experimental data, three different constitutive models, belonging to phenomenological model, physical-based model, and radial basis function (RBF) neural network model, separately, were established to predict the deformation stress. The results show that the true stress is very sensitive to temperature and stain rate. Flow instability occurred during the deformation process, which caused some decline in the stress. Microstructure evolution under different deformation conditions indicated that the γ' to γn' phase transformation was also related to the degree of deformation and the four stage evolutionary process was identified. The SEM and EBSD results showed that the DRX appeared at the later stage of the deformation. The prediction capabilities of the three models were assessed by correlation coefficient (R), the absolute value of relative error (ARE), and the absolute average error (AARE). The result demonstrated that the RBF neural network had the best ability to predict the flow stress and the coefficient.

A practical method for low-temperature hydrogen addition into zirconium alloys

A practical method to add hydrogen to zirconium alloys is described that works at low temperatures and short times. Fitness-For-Service predictions to support extended service of CANDU pressure tubes requires mechanical testing of tubes removed from service with end-of-life hydrogen concentrations and representative changes in alloy microstructure caused by irradiation. With the current method, (101 ± 5) wt.ppm hydrogen has been added homogeneously to Zircaloy sheets at (201.5 ± 4.4) °C within 22.3 h. These times and temperatures are well below values that would change any effects of irradiation in the alloy. The method has also been demonstrated with a Zr-2.5Nb micro-pressure tube.

Microstructure Changes and Improvement in the Mechanical Properties of As-Cast AlSi7MgCu0.5 Alloy Induced by the Heat Treatment and ECAP Technique at Room Temperature

The changes in the microstructure and improvement in the mechanical properties of as-cast AlSi7MgCu0.5 alloy induced by the heat treatment and technique of equal channel angular pressing (ECAP) were investigated. The heat treatment of as-cast alloy performed before the ECAP technique was required to increase the plasticity of the alloy. Therefore, the samples of analysed alloys were solution annealed at optimized temperature of 823 K for 4 hours to dissolve the particles of intermetallicπ(Al8FeMg3Si6) phase and to spheroidize the Si particles. Subsequently, water quenching and artificial ageing at optimized temperature of 573 K for 5 hours was used to obtain an overaged alloy state. The microstructure of alloy was consisted ofα(Al) solid solution, eutectic Si particles, and intermetallicβ(Mg2Si), Q-Al4Mg5Si4Cu,α-Al12(Fe,Mn)3Si, and/orα-Al15(Fe,Mn)3Si2phase particles. The crystal structure of present phases was confirmed by hard X-ray diffraction at Deutsches Elektronen-Synchrotron (DESY) in Hamburg and by the selected area electron diffraction (SAED) performed inside the transmission electron microscope (TEM). The heat-treated alloy was processed by ECAP at room temperature following route A. Repetitive ECAP of alloy homogenized the heterogeneous as-cast microstructure and formed the ultrafine subgrain microstructure with elongated subgrains of 0.2 µm in width and 0.65 µm in length and the high dislocation density. Microstructural changes in alloy induced by both heat treatment and ECAP led to the high strain hardening of the alloy that appeared in an improvement in strength, ductility, and microhardness of alloy in comparison with as-cast alloy state.

Microstructural evolution and support vector regression model for an aged Ni-based superalloy during two-stage hot forming with stepped strain rates

The high-temperature flow characteristics and microstructural changes of alloys are usually investigated by isothermal compressive experiments at constant strain rates. However, during the practice industrial production of components, the strain rate is varying. In the present work, the microstructure changes and high-temperature flow characteristics of a Ni-based superalloy containing δ phases were researched by isothermal two-stage high-temperature compression experiments with stepped strain rates. It is revealed that the true stress of the later stage distinctly increases when the true strain/strain rate of the former stage or the strain rate of the later stage are raised. Meanwhile, the dynamic recrystallization (DRX) grains become finer, and the dissolution of δ phase is weakened. But, the coarsening of DRX grains and the dissolution of δ phase are enhanced as the deformation temperature is raised. The primary DRX nucleation mechanism is the discontinuous DRX. Based on the measured flow stress, an e-insensitive support vector regression (e-SVR) model is established for depicting the flow behavior of the researched alloy. The measured results are consistent with the forecasted ones, which illustrating that the developed e-SVR model is feasible to accurately describe the flow behaviors of the researched alloy during two-stage high-temperature deformation with stepped strain rates.

Quasi in situ analytical TEM to investigate electrochemically induced microstructural changes in alloys: AA2024-T3 as an example

A quasi in situ method using a specially developed holder is presented to follow electrochemically induced microstructural changes on a nanometre scale using analytical transmission electron microscopy (TEM). By following a specific specimen area after repeated exposure to oxygen bubbled through aqueous HCl of pH=3, de-alloying of a grain-boundary precipitate of ∼200nm was observed by monitoring the changes in the EELS copper L2,3-edge and EFTEM elemental maps. These results, supporting the existing models of de-alloying of copper rich precipitates in aluminium alloys, demonstrate the effectiveness of the quasi in situ specimen holder for analytical TEM studies of corrosion.

Microstructural changes in alloy 625 due to creep and characterization using NDE techniques

Alloy 625 exposed to 60,000 hours of service was re- solution annealed at 1433 K and thermally aged at various temperatures in the range of 923–1123 K to produce different microstructures. The microstructures were characterised using scanning electron microscope. Tensile properties and hardness were determined using nondestructive ball indentation technique. Precise ultrasonic velocity measurements were carried out to evaluate the microstructures. A direct correlation has been observed between ultrasonic velocity and mechanical properties. This study demonstrates the feasibility of applying ultrasonic velocity measurements on-line for nondestructive evaluation of mechanical properties of alloy 625.

Influence of T4 Heat Treatment on Tribological Behavior of Za27 Alloy Under Lubricated Sliding Condition

The effects of heat treatment on the microstructure, hardness, tensile properties, and tribological behavior of ZA27 alloy were examined. The alloys were prepared by conventional melting and casting route. The heat treatment of samples included the heating up to 370 °C for 3 or 5 h, quenching in water, and natural aging. Lubricated sliding wear test were conducted on as-cast and heat-treated ZA27 samples using block-on-disc machine. The friction and wear behavior of alloys were tested in contact with steel discs using combinations of three levels of load (10, 30, and 50 N) and three levels of linear sliding speeds (0.26, 0.50, and 1.00 m/s). To determine the wear mechanisms, the worn surfaces of the samples were examined by scanning electron microscopy (SEM). The heat treatment resulted in reduction in the hardness and tensile strength but increase in elongation. The heat-treated alloy samples attained improved tribological behavior over the as-cast ones, under all combinations of sliding speeds and contact loads. The rate of improvement increased with duration of solutionizing process before quenching in water. Obtained tribological results were related to the effects of heat treatment on microstructure changes of alloy.

Microstructural changes in alloy 625 during high temperature creep

Microstructural changes in the nickel-base alloy 625 during creep deformation have been investigated in the temperature range of 923 to 1148 K for durations from 30 to 31,800 h. Intermetallic phases such as γ″-phase and δ-phase have been found to precipitate extensively within the austenitic matrix under most of the test conditions. The γ″-phase was observed at 973 K and δ-phase was observed at all temperatures above 973 K. The morphology of the needle-shaped δ-phase was influenced by time and temperature of exposure and stress. An almost continuous decoration of M 23C 6 type carbides was observed on the grain boundaries.

‘Severe’ wear challenge to ‘as-cast’ and ‘double heat-treated’ large-diameter metal-on-metal hip bearings

The wear generation of double-heat-treated and as-cast large-diameter metal-on-metal (MOM) hip bearings was investigated using standard- and 'severe'-gait simulations. The test hypothesis was that double heat treatment would change MOM hip wear compared with the as-cast condition. Two groups of high-carbon MOM bearings of 40 mm diameter were manufactured and subjected to either hot isostatic pressing (HIP) and solution annealing (SA) or no heat treatment (as cast). The results showed no statistical difference between the two groups under both running-in and steady state conditions. Even under the most 'severe'-gait simulation published to date, the mean volumetric wear rates were 2.9 and 3.9 mm3 per 10(6) cycles for the HIP-SA and as-cast bearings respectively, showing a ten-fold increase in wear compared with walking. These differences were not statistically different; therefore our hypothesis was negated. Changes in alloy microstructure do not appear to influence the wear behaviour of high-carbon cast MOM articulations with similar chemical compositions. This is in sharp contrast with the published significance of bearing diameter and radial clearance on the wear of MOM hip bearings.

Modelling of Copper Precipitation in Fe-Cu Alloys Under Irradiation

ABSTRACTPrecipitation of copper-rich clusters is a major cause of in-service hardening of reactor pressure vessel steels and has attracted much attention. Experimental studies of microstructural changes in alloys under various conditions have revealed similarities and differences. It has been established that under ageing the precipitate ensemble experiences normal nucleation, growth and Ostwald ripening, a distinguishing feature of which is the bcc-9R-3R-fcc transformations the precipitates undergo during growth. The main effect of electron irradiation is believed to be enhancement of the diffusion of copper and hence acceleration of the kinetics. In the case of neutron irradiation, however, there are many aspects that are not clear. One is that at temperatures less than about 300°C the precipitate size is observed to be very small (∼1-3 nm), i.e. the coarsening rate is very low. In this paper we study this phenomenon by computer simulations based on the “mean-field” approach for describing microstructural evolution.

Microstructure and phase identification in type 304 stainless steel-zirconium alloys

Stainless steel-zirconium alloys have been developed at Argonne National Laboratory to contain radioactive metal isotopes isolated from spent nuclear fuel. This article discusses the various phases that are formed in as-cast alloys of type 304 stainless steel and zirconium that contain up to 92 wt pct Zr. Microstructural characterization was performed by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), and crystal structure information was obtained by X-ray diffraction. Type 304SS-Zr alloys with 5 and 10 wt pct Zr have a three-phase microstructure—austenite, ferrite, and the Laves intermetallic, Zr(Fe,Cr,Ni)2+x. whereas alloys with 15, 20, and 30 wt pct Zr contain only two phases—ferrite and Zr(Fe,Cr,Ni)2+x. Alloys with 45 to 67 wt pct Zr contain a mixture of Zr(Fe,Cr,Ni)2+x and Zr2(Ni,Fe), whereas alloys with 83 and 92 wt pct Zr contain three phases—α-Zr, Zr2(Ni,Fe), and Zr(Fe,Cr,Ni)2+x. Fe3Zr-type and Zr3Fe-type phases were not observed in the type 304SS-Zr alloys. The changes in alloy microstructure with zirconium content have been correlated to the Fe-Zr binary phase diagram.

Reply to comment on “the ductile-brittle transition of a zirconium alloy due to hydrogen”

Bai estimates that the local strain rate at the notch tip in our notch tensile tests is at least equals to 2.4x10 -3 /s. Then, he compares our results with recent results by Bai et a1.[3-5] in which the specimens were tested at 10~/s or 10-3/s. In this work it was found that for the hydrided specimens, those tested at 10-4/s exhibited an usual ductilebrittle transition behavior at room temperature, while those tested at 10-3/S were brittle with elongations less than 2 %, independent of the testing temperatures and the hydrogen contents. Therefore, Bai comments that the different hydrogen contents for ductile-brittle transition may be also attributed to the strain rate effect beside with the notch effect. We can partly agree with this point of view. It is well acknowledged that the local strain rate at a notch tip can be magnified to a large extent and the attendant increase of strain rate can induce brittle behavior of metals. However, from our results on smooth hydrided specimens tensile tested at 5x103/s, as shown in Figure 1, no ductilebrittle transition in reduction of area was found up to a hydrogen content of 340 ppm. Since the local strain rate at the notch tip, as estimated by Bai, is close to the strain rate used in tensile tests for the smooth specimens, it is clear that strain rate per se is not significant for the early occurrence of ductile-brittle transition. In fact, Gill et al.[6] performed notch tensile tests on hydrided zirconium and found an overall lowering of elongation values for notched specimens. This was attributed to the effects of triaxiality in the stress system introduced by notch. The triaxiality can reduce the magnitude of shear stress which are responsible for plastic deformation. Thus, the notched specimens exhibit brittle behavior at a lower hydrogen content. Bai's findings of brittleness of the specimens tested at 103/s were obtained from specimens containing at least 500 ppm of hydrogen. Since we did not perform any tests on specimens with hydrogen content higher than 350 ppm, no direct comparison can be made with Bai's results. The other point mentioned by Bai is the effect of hydriding temperature on the mechanical properties of hydrided specimens and the hydrides themselves. The hydriding temperature in our study was purposely chosen to avoid extraneous changes in alloy microstructure. Since the stress relief temperature of zirconium alloys is 480595 °C [7], hydriding at a temperature above 500°C may produce changes in alloy microstructure and thereby change the mechanical properties of zirconium alloys. Therefore, it is not surprising that the ductile-brittle transition on Zircaloy-4 specimens hydrided at 700°C occurs at a different hydrogen content than those hydrided at 400°C. However, for the specimens hydrided at temperatures lower than the stress relief temperature, such as 400°C or 350°C, it is not expected that any extraneous changes in alloy microstructure from the effect of temperature will occur; as a result, the mechanical properties of the zirconium matrix, regardless hydrided at 350°C or 400°C, will

Evaluation of microstructural changes in alloy 713 LC by neutron small angle scattering and analytical electron microscopy

Small angle neutron scattering (SANS) studies in combination with transmission electron microscopy (TEM) were performed on the cast nickel base superalloy Alloy 713 LC and on a Hf-modified version of the same alloy. With the aid of the TEM results the profile of the scattering curves was correlated with the M23C6 carbide and the γ′ precipitates. A coarsening of the γ′ precipitates with increasing creep deformation to a larger plate-like shape was observed. The small axis of these precipitates averaged over the grains was parallel to the stress axis. The γ′precipitates start to become anisotropic even in the primary stage of creep. In the vicinity of the fracture surface the volume fraction of the cavities and microcracks caused by creep deformation was 10−3 to 10−2 The influence of the cavities was indicated by the smaller anisotropy factor measured near the fracture surface in comparison to the rest of the cylindrical part of the specimen. The variation of the anisotropy factor as determined by the SANS-method may be used to nondestructively measure the accumulated damage in the material.