Microstructure Evolution Of Alloy Research Articles

Surface Modification of Oilfield Alloys by Ultrasonic Impact Peening: UNS N07718, N07716, G41400, and S17400

Ultrasonic impact peening (UIP) is a severe plastic deformation process to induce localized surface hardening combined with compressive residual stresses which therefore extends the useful life of mechanical parts. In this investigation, UIP has been applied to four widespread alloys in use in the oilfields. These include two premium NiCrMo alloys, UNS N07718 (718) and UNS N07716 (625 Plus®), both characterized by satisfactory oilfield performance but lacking hardness and abrasive wear resistance, and two relatively low-cost alloys, UNS G41400 (4140) and UNS S17400 (17-4PH), both limited by their corrosion fatigue. To promote comparisons and determine important alloy parameters for successful UIP, all four alloys were carefully selected so that their respective yield strengths were within relative proximity (~780 to ~910 MPa), and then ultrasonically impact peened under identical conditions. Among major findings from microstructural examinations, micro-hardness indentations, and residual stress measurements, surface topological changes (roughness), alloy microstructural evolution (depth and extent of strain hardening, including mechanical twinning in the NiCrMo alloys), and compressive residual stresses were found to be well correlated. Among all four alloys, the NiCrMo alloys, in particular UNS N07716 was found to be best suited for UIP. This is explained by its FCC austenitic microstructure, relatively low stacking-fault energy (prone to mechanical twinning), and in practical terms high yield strength and high tensile-to-yield strength ratio, both related to its excellent plastic flow behavior under ultrasonic rates of plastic deformation.

Phase Field Simulations of Dendritic Crystal Growth with Focus on the Computational Efficiency

In this study, we present a numerical technique for the improvement of computational efficiency for computation of microstructural evolution in alloy during solidification process. The goal of this technique is for the computational domain to grow around the microstructure and fixed the grid spacing, while solidification advances into the liquid region. The growth around the microstructure is controlled according with the solute diffusivity for binary alloy in the liquid region. The computation showed that the microstructure with well-developed secondary arms can be obtained with low computation time and moderate memory demand.

Microstructural Evolution in Aluminum Alloy 7050 during Cooling from Homogenization

Microstructure of post-homogenization is significant to prevent particle stimulated nucleation of recrystallization. This paper investigated microstructure evolution in alloy 7050 during cooling from homogenization. Two intermetallic phases, Al7Cu2Fe and Mg2Si were insoluble during homogenization. Cooling to 460 °C and 400 °C follow by quenching, precipitates nucleated discontinuously on the grain boundaries, no precipitates was found within the grains. The precipitates were found near the grain boundaries when quenching from 350 °C, and precipitated uniformly within the center of the grains when quenching from 250 °C. The sizes of the precipitates increased with the decreased of the quenching temperature. Controlling of the post-homogenization microstructure is beneficial to prevent particle stimulated nucleation of recrystallization.

Progress on High Magnetic Field-Controlled Transport Phenomena and Their Effects on Solidification Microstructure

The recent contributions on changes induced by high magnetic fields in transport phenomena, such as convection, solute diffusion, solute or phase migration in the liquid, are reviewed and analyzed. Selectivity provided by the Lorentz, thermoelectromagnetic (a special kind of Lorentz force), and magnetic forces or combinations enables the control of transport phenomena in liquids. This possibility, together with the capability of the transport phenomena to affect solidification in alloys, allows a level of control over solidification microstructures. As a result, recent relevant work can be found dealing with the effects of high magnetic fields on transport phenomena and the corresponding solidification microstructure evolution of alloys, based on the Lorentz, thermoelectromagnetic, and magnetic forces or combinations thereof.

Nanocrystalline Microstructure in AZ91 Magnesium Alloy Prepared with Cryogenic Treatment

In order to make clear about the effect of cryogenic temperature on the microstructure evolution of alloy, the as-cast AZ91 magnesium alloy was subjected to the cryogenic treatment after solution treatment (415°C,5h) and aging treatment (190°C, 16h) with the protection of graphite power. Transmission Electron Microscope was utilized to characterize the microstructure of as-treated sample. The results show that a high volume fraction of nanocrystalline can be found in the treated alloys with the average sizes are about 55nm. The phenomenon highlights the importance of cryogenic temperature in forming nanostructures. During the continuous contraction of alloy the micro dynamic plastic deformation induces the generation, movement and accumulation of high-density dislocations that give rise to the formation of nanocrystalline during recovery and recrystallization.

Microstructural Evolution of AZ91 Magnesium Alloy during Deformation and Heat Treatment

Microstructural evolutions of AZ91 magnesium alloy, which applied to homogenizing annealing treatment, hot extrusion and ageing treatment respectively, are investigated in this paper. Results exhibit that the massive β-Mg17Al12 phase appearing in the initial structure dissolves to eliminate in α-Mg matrix mostly due to the homogenizing annealing treatment; dynamic recrystallization and consequent grain refinement occur during extrusion; banded structure, in which the β-Mg17Al12 phase precipitates parallel to the extrusion direction in α-Mg matrix, are observed in the ageing treated specimen. Furthermore, the effects of temperature, keeping time of homogenizing annealing treatment and ageing treatment, and the extrusion processing parameter on the microstructural evolution of this alloy are also discussed according to the experimental results in details. In addition, the various mechanisms of the morphology and quantity of α-Mg and β-Mg17Al12 phase are analyzed corresponded to the various states. As for the banded structure appearing in the ageing treated specimen, it can be attributed to the banded segregation which remained in the extruded one and resulted from the casting dendrite and subsequent extrusion.

MODELING AND SIMULATION OF DYNAMIC RECRYSTALLIZATION AND GRAIN GROWTH DURING HOT WORKING OF INCONEL 783 SUPERALLOY

In this study, the modeling of recrystallization and grain growth was investigated to predict the microstructure evolution during hot working of INCONEL 783 superalloy (Alloy 783). The recrystallization model was constructed on the basis of the Avrami formation. A least-square optimization technique was used to determine several important parameters within the model from isothermal heat treatment and hot compression test results. High temperature compression tests were carried out under different temperatures, strain rates and strain conditions. The model for describing the recrystallization and grain growth behaviors of Alloy 783 was implemented onto the user-subroutine of a commercial FE code. In order to demonstrate the reliability of the model, the microstructure evolution of Alloy 783 alloy during the hot compression test was simulated and was accorded well with the experimental results.

Solidified Structure Control of Metallic Materials by Static High Magnetic Fields

Recently, the studies on the effects of high magnetic fields on solidification processes have been paid much attention both from the fundamental and applied points of view. With the aid of the enhanced Lorentz force and magnetization effect caused by the remarkably increased magnetic field intensity, several interesting phenomena, such as the control of fluid flow and particle migration in a melt, crystal orientation, and phase alignment, have been obtained. Moreover, the magnetic force induced by the interaction of magnetization and high magnetic field gradient has been evidenced to show significant effects on the microstructure evolution of alloys. In this paper, the recent development of the control of the solidification process by high magnetic fields is reviewed from the view point of uniform magnetic fields and magnetic field gradients.

Thermoelectric power applied to metallurgy: principle and recent applications

Abstract Thermoelectric power is the magnitude of an induced thermoelectric voltage in response to a temperature difference across a material. The thermoelectric power of metal alloys is strongly influenced by the presence of solute atoms. Measuring thermoelectric power is therefore an accurate technique for characterizing the microstructure evolution of alloys taking place during various phase transformations. The potentialities of this technique are illustrated through four examples: (i) the measurement of copper solubility limit in iron; (ii) the characterization of the strain ageing kinetics of extra-mild steels; (iii) the determination of precipitation kinetics of MoC in low alloyed steels; and (iv) the monitoring of ageing of neutron irradiated power plant steels.

Metallurgical Effects of Solution Heat Treatment Temperatures of Alloy Haynes™ HR-120™ for Land-Based Turbine Application

ABSTRACTHaynes™ HR-120™ alloy is a solid-solution-strengthened heat resistant alloy. The main characteristics of this alloy are strength at elevated temperature combined with resistance to carburizing and sulfidizing environments. Typical solution heat treatment for this alloy is usually performed above 1100°C. Solution heat treatment promotes non-desired precipitates to dissolve and, if deformation parameters are adequate, re-crystallization after forging procedures. It is reported that the solution temperature can also promote non-controlled grain coarsening. This investigation summarizes results on the effect of solution heat treatment on the microstructure of forgings when it is performed at 1000°C, 1050°C and above 1100°C. The experimental conditions resemble industrial environments. The obtained results include the alloy microstructural evolution by optical microscopy and Scanning Electron Microscopy (SEM) and the effect of these heat treatments on mechanical properties such as tensile, hardness and stress-rupture properties.

Microstructure evolution of alloy 625 foil and sheet during creep at 750 °C

Foil and sheet forms of the nickel-based superalloy 625 have been examined in an ‘as-processed’ condition and following creep-rupture testing in air at 750 °C and 100 MPa. Both scanning and transmission electron microscopies were employed to correlate microstructures with creep behavior, and indicate the additional processing required to achieve foil form reduces creep life compared to thicker-section wrought product forms. Prior to creep testing, the microstructure consists of γ phase with M 6C precipitates. This microstructure changes during creep into one consisting of orthorhombic δ phase extending across the γ grains, and grain boundaries dominated by the presence of rhombohedral μ phase, δ phase, and a Si-rich variant of diamond-cubic M 6C (η phase). Thermodynamic modeling was also used to calculate the stable temperature ranges and compositions of equilibrium phases. The phases predicted by modeling and their compositions generally agree with those observed within alloy 625 after creep testing.

Impact Response and Microstructural Evolution of Biomedical Titanium Alloy under Various Temperatures

A compressive split-Hopkinson pressure bar and transmission electron microscope (TEM) are used to investigate the mechanical behavior and microstructural evolution of biomedical Ti alloy deformed at strain rates ranging from 8 × 102 s−1 to 8 × 103 s−1 at temperatures between 25 °C and 900 °C. In general, the results indicate that the mechanical behavior and microstructural evolution of the alloy are highly sensitive to both the strain rate and the temperature conditions. The flow-stress curves are found to include both a work-hardening region and a work-softening region. The strain-rate-sensitivity parameter, β, increases with increasing strain and strain rate but decreases with increasing temperature. The activation energy varies inversely with the flow stress and has a low value at high deformation strain rates and low temperatures. Microstructural observations reveal that the strengthening effect evident in the deformed alloy is a result primarily of dislocations and the formation of α phase. The dislocation density increases with increasing strain rate but decreases with increasing temperature. Additionally, the square root of the dislocation density varies linearly with the flow stress. Correlating the mechanical properties of the biomedical Ti alloy with the TEM observations, it is inferred that the precipitation of α phase dominates the fracture strain. Transmission electron microscope observations reveal that the amount of α phase increases with increasing temperature below the β-transus temperature. The maximum amount of α phase is formed at a temperature of 700 °C and results in the minimum fracture strain observed under the current loading conditions.

Solidification Microstructure Evolution Modeling in Nb-Si Based Intermetallics-Strengthened-Metal-Matrix Composites

AbstractFor higher fuel efficiency and greater thrust to weight ratios, there is a continuous drive for higher performance turbine engine components. Nb-silicide intermetallics, owing to their high melting point and high-temperature strength, are potential candidates for high temperature applications. These intermetallics when precipitated in the metal matrix of a (Nb) solid solution, result in intermetallic-strengthened metal matrix composites that have a good combination of room temperature toughness and high temperature strength. The microstructures of these in-situ composites can be complex and vary significantly with the addition of elements such as Ti and Hf. Hence an improved understanding of phase stability and the microstructural evolution of these alloys is essential for alloy optimization. In the present paper we describe binary alloy microstructural evolution modeling of dendritic and eutectic solidification obtained using phasefield simulations. The effect of parameters such as heat extraction rate, the ratio of the diffusivity of the solute in liquid to solid, and the liquid-solid interfacial energy, on microstructural evolution during solidification is discussed in detail.

Quantitative Phase Field Modeling of Precipitation Processes

Phase Field modelling of microstructural evolution in alloys has already a long and successful history. One of the basics of the theory is the introduction of continuous fields (concentration, long‐range order parameters) that describe the local state of the alloy. These fields have a meaning only at a mesoscopic scale. One consequence is that we can treat much larger systems than with microscopic methods such as Monte Carlo or molecular dynamics simulations. The aim of this work is to precisely analyse the status of the mesoscopic free energy densities that are used in Phase Field theories and, simultaneously, to clarify the form that the Phase Field equations should adopt.

Microstructural evolution in Zr3Al-based alloys during various long-time annealing treatments

The Zr3Al-based intermetallics have been considered as candidate materials for structural components in pressurized heavy water nuclear reactors due to their attractive properties. However, these alloys could not be used for structural application in the reactors because of their poor room-temperature ductility and irradiation-induced amorphization. Our recent studies of ternary addition of niobium in Zr3Al have shown some promising results. The present article reports the microstructural evolution in these alloys upon long-time annealing treatments. The formation of various phases, temperature regime of their stability, chemical composition, and volume fraction of these phases during prolonged annealing have also been studied. A pseudobinary phase diagram with varying niobium concentration has also been developed. The morphology and distribution of the Zr3Al phase have been explained on the basis long-range diffusion as the rate-controlling step.

Characterization of Ti-35Nb-7Zr-5Ta Alloy Produced by Powder Metallurgy

Abstract: Titanium and titanium alloys present the highest biocompatibility among metallic biomaterials. The ideal titanium alloy for orthopedic applications should have low modulus of elasticity (near the bone), excellent mechanical strength, high corrosion resistance, formability and no potential toxic elements. Among titanium alloys, the Ti-35Nb-7Zr-5Ta alloy, due its high biocompatibility and lower Young’s modulus is a promising candidate for implants material. The titanium alloys production by powder metallurgy, starting from the elementary powders, is a viable route due at the smaller costs and larger operational facilities. The Ti-35Nb-7Zr-5Ta samples were manufactured by blended elemental method from a sequence of uniaxial and cold isostatic pressing with subsequent densification by sintering between 900 at 1700 °C, in vacuum, under a heating rate of 20 °C×min-1 for 1h. The objective of this work is the analysis of alloy microstructural evolution from the powders dissolution under the increase of the sintering temperature. For the alloy microstructural characterization, scanning electron microscopy and Vickers microhardness measurements, were used. Density was measured by Archimedes method. The samples presented high densification, an homogeneous microstructural development, with complete dissolution of alloying elements in the titanium matrix with the temperature increase.

Production of new titanium alloy for orthopedic implants

The beta titanium alloys is one of the most promising groups of the titanium alloys. This fact is due to the good formability, mechanical properties and potential applications; moreover, these alloys present the highest level of mechanical, fatigue and corrosion resistance. The beta titanium alloys present the lowest elastic modulus, an interesting property for orthopedic implants. A β alloy recently developed for this application is Ti–35Nb–7Zr–5Ta. In this work, the alloy was produced by powder metallurgy, unique available alternative for obtaining parts with porous structure (until 50% of porosity), that is one important characteristic for the osteointegration. The Ti–35Nb–7Zr–5Ta samples were manufactured by blended elemental method from a sequence of uniaxial and cold isostatic pressing with subsequent densification by sintering among 900 at 1700 °C, in vacuum. The objective of this work is the analysis of alloy microstructural evolution from the elemental powders dissolution under the increase of the sintering temperature. The alloy was characterized by scanning electron microscopy, X-ray diffraction and Vickers microhardness measurements. Density was measured by Archimedes method. The results show that a β-homogeneous microstructure is obtained in the whole sample with the increase of sintering temperature. With the beginning of the β-stabilizers (Nb and Ta) dissolution, at low sintering temperatures, there is the formation of an intermediary Widmanstätten (α+β) phase.

Modeling Precipitate Microstructure Evolution in Alloys with First-Principles Energetic Information

This short article reports our recent effort to integrate the mesoscale phase-field method with first-principles total energy calculations, linear response theory, as well as mixedspace cluster expansion. A particular example of applying such a multiscale approach to the case of precipitation of semicoherent θ' particles in an Al-matrix is presented.

Quantitative interface models for simulating microstructure evolution

To quantitatively simulate microstructural evolution in real systems, we investigated three different interface models: a sharp-interface model implemented by the software DICTRA and two diffuse-interface models which use either physical order parameters or artificial order parameters. A particular example is considered, the diffusion-controlled growth of a γ ′ precipitate in a supersaturated γ matrix in Ni–Al binary alloys. All three models use the thermodynamic and kinetic parameters from the same databases. The temporal evolution profiles of composition from different models are shown to agree with each other. The focus is on examining the advantages and disadvantages of each model as applied to microstructure evolution in alloys.

High-temperature deformation behavior of a gamma TiAl alloy—Microstructural evolution and mechanisms

The present investigation was carried out in the context of the internal-variable theory of inelastic deformation and the dynamic-materials model (DMM), to shed light on the high-temperature deformation mechanisms in TiAl. A series of load-relaxation tests and tensile tests were conducted on a fine-grained duplex gamma TiAl alloy at temperatures ranging from 800 °C to 1050 °C. Results of the load-relaxation tests, in which the deformation took place at an infinitesimal level (e ≅ 0.05), showed that the deformation behavior of the alloy was well described by the sum of dislocation-glide and dislocation-climb processes. To investigate the deformation behavior of the fine-grained duplex gamma TiAl alloy at a finite strain level, processing maps were constructed on the basis of a DMM. For this purpose, compression tests were carried out at temperatures ranging from 800 °C to 1250 °C using strain rates ranging from 10 to 10−4/s. Two domains were identified and characterized in the processing maps obtained at finite strain levels (0.2 and 0.6). One domain was found in the region of 980 °C and 10−3/s with a peak efficiency (maximum efficiency of power dissipation) of 48 pct and was identified as a domain of dynamic recrystallization (DRx) from microstructural observations. Another domain with a peak efficiency of 64 pct was located in the region of 1250 °C and 10−4/s and was considered to be a domain of superplasticity.