Saxs and tem study of the kinetics of phase separation in Al-Zn

Kinetics of phase separation and coarsening in Al-22 at.% Zn-0.1 at. % Mg, from very early to very late annealing times, has been studied by correlating the time evolution of the small-angle X-ray scattering spectrum and the microstructure, as revealed by transmission electron microscopy. Three kinetic regimes were recognized. The dominant coarsening mechanisms were interpreted as arising from cluster coagulation at early times and single atom diffusion at late times. The intermediate times probably combined both. The microstructure consisted of interconnected spherical clusters at early times and regularly spaced platelets at late times; with a mixture of both during the transition period. At later annealing times, the intensity spectra exhibited a “shoulder” whose appearance was associated with the change of morphology. The intensity spectra from different times could all be made to coincide on one curve through appropriate scaling, provided the microstructure was not changing during that time period. Simple analytical forms described the shape of the scaled intensity curves.     

Creep and rupture properties of a squeeze-cast Mg-Al-Ca alloy

A squeeze-cast Mg-Al-Ca alloy (MRI153) was creep tested at 150, 175, and 200 °C under applied stresses in the range of 30 to 120 MPa. The creep curves were characterized by an extended tertiary stage in which the creep rate increases progressively with the creep strain. Microstructural examinations revealed the precipitation and coarsening of new particles during creep. The stress dependence of the minimum creep rate suggests a transition from power-law creep at low stresses to power-law breakdown at high stresses. Creep rupture of this alloy occurred as a result of cavitation damage at dendritic grain boundaries, with the creep rupture time and the minimum creep rate following the empirical Monkman-Grant equation. A comparison is made between the creep and rupture properties of MRI153 and those of a squeeze-cast Mg-Al alloy (AZ91).     

Study of creep crack growth in 2618 and 8009 aluminum alloys

Creep crack growth (CCG) has been investigated in an 8009 (Al-Fe-V-S) P/M alloy at 175 °, 250 °, and 316 ° and in a 2618 ingot alloy at 150 °, 175 °, and 200 °. Under sustained load, subcritical crack growth is observed at stress intensity levels lower thanK _ic ; for 2618, at 200 °, crack growth is observed at stress intensities more than 40 pct lower thanK _ic . Alloys 8009 and 2618 exhibit creep brittle behavior,i.e., very limited creep deformation, during CCG. The CCG rates do not correlate with CCG parameters C* and C but correlate with the stress intensity factor,K, and theJ integral. Generally, crack growth rates increase with increasing temperature. Micromechanisms of CCG have been studied with regard to microstructural deg-radation, environmental attack, and creep damage. Although theoretical estimation indicates that CCG resistance decreases with second-phase coarsening, such coarsening has not been observed at the crack tip. Also, no evidence is found for hydrogen- or oxygen-induced crack growth in comparing test results in moist air and in vacuum. Creep deformation and cavitation ahead of crack tip are responsible for observed time-dependent crack growth. Based on the cavitation damage in the elastic field, a micromechanical model is proposed which semiquantitatively explains the correlations between the creep crack growth rate and stress intensity factor,K.     

The influence of microstructure and environment on the crack growth behavior of powder metallurgy nickel superalloy RR1000

A study of crack growth in vacuum and air at 725 °C (T/T _m=0.6) highlights the relative importance of creep and environmental crack-tip damage mechanisms in Powder Metallurgy (P/M) disc alloy RR1000. Both of these mechanisms are associated with a transition to intergranular fracture during fatigue crack growth at 0.25 Hz. Crack growth under sustained loads reveals the precise nature of these mechanisms in RR1000. The severity of creep and environmental mechanisms is controlled by the grain-boundary microstructure and the crack-tip stress. Near-tip cavitation leads to fracture in vacuum. Sigma-phase precipitation causes an increase in crack growth rate through increased crack-tip cavity nucleation. Rapid near-tip stress relaxation induced by γ′ coarsening has a beneficial effect on the severity of this type of damage. In air, increases in crack growth rates are associated with near-tip intergranular oxidation. It is proposed that the extent of this damage and subsequent growth rates are increased by sigma-phase precipitation through enhanced oxidation due to chromium depletion and subsequent decreased passivation. Again, a beneficial effect of rapid near-tip stress relaxation due to selective γ′ coarsening is apparent and environmental damage is reduced under these conditions.     

Remaining life assessment issues in high Cr martensitic steels and development of new innovative tools for damage monitoring and integrity assessment

The relatively newer high Cr martensitic steels such as P91 have now been in use in power industry for over twenty years. Over this time, there have been a number of incidents of cracking and failure in components made from P91 steel, both in thick and thin section equipments mainly due to creep damage. The thick section components have been usually failing due to Type IV cracking associated with the weldments while thin section components have been failing due to higher than expected levels of steam oxidation resulting in enhanced metal loss, increase in metal temperature above design, creep cavitation and cracking. However, it has not been possible to detect early stage creep damage/cavitation in high Cr martensitic steels using conventional replication type methods. This is because unlike the low alloy CrMoV steels, spherodisation of microstructure does not occur in high Cr martensitic steels and cavitation clearly visible by traditional methods only appears later in life when the material is about to fail. Thus there has been a need to develop new tools and more sensitive methods for integrity and damage assessment in these steels. European Technology Development (ETD) together with its industrial collaborators from Europe, Japan and North America have been looking at the development of tools and methodologies for early stage damage detection and life prediction as a part of its international multi-client project ‘P91 Integrity’. The tools which have shown successful results are portable Scanning Force Microscopy, laser guided hardness tester and more innovative use of ultrasonic probes for detecting early stage creep damage. This paper discusses the issues involved and describes some of the developments in this project.     

Kinetics of Initial Coarsening During Sintering of Nanosized Powders

Grain growth during sintering is a critical issue for the manufacture of nanocrystalline bulk materials from nanosized powders. The grain growth process during sintering can be viewed as consisting of two parts: initial coarsening during early and intermediate stages of sintering and latter stage grain growth during the final stage of sintering. The latter stage grain growth is the normal grain growth that has been well studied and reported in the literature. The initial coarsening, which often inevitably causes a material to lose nanoscaled grain size characteristics, however, is not well studied at all. In this investigation, the initial coarsening during sintering of nanosized powders was studied by both nonisothermal and isothermal experimental techniques using tungsten as an example material. The results show that the initial coarsening during the heat-up process of a sintering cycle is sufficient to increase the grain size beyond the nanoscale. The kinetics of initial coarsening is found to be linear rather than polynomial, as predicted by the conventional power law of grain growth. The analysis of activation energies showed that surface diffusion is the primary mechanism for interparticle mass transport during the initial coarsening. The linear kinetic behavior could be attributed to the pinning of grain boundaries by surface grooves and high concentration of defects as the result of the synthesis of nanosized powders.     

Evolution of Precipitate Microstructure During Creep of an AA7449 T7651 Aluminum Alloy

Creep forming is a process where plastic deformation is applied at the material’s aging temperature. It enables to obtain parts of complex shape with reduced internal stresses and finds applications, for instance, in the aerospace industry. In this article, we report in-situ small-angle X-ray scattering measurements during creep experiments carried out on an AA7449 Al-Zn-Mg-Cu alloy in the T7651 temper. In the range of temperatures of 413 K to 453 K (140 °C to 180 °C), we show that the initial microstructure is not stable with respect to the applied stress/strain. Accelerated precipitation coarsening is shown to occur, clearly related to the plastic deformation. This strain-induced microstructure evolution is shown to happen even at temperatures well below the aging temperature that has led to the initial temper.     

Microstructural Evolution and Change in Hardness of S30432 Heat‐resistant Steel during Creep at 650 °C

The microstructural evolution of S30432 heat‐resistant steel during creep at 650 °C and its effect on the change in hardness was investigated. The change of hardness during creep of S30432 at 650 °C can be divided into three stages. These are related to the precipitation and coarsening of ε‐Cu and M₂₃C₆ carbides, decrease in the number of twins and increase in grain size. The precipitation of ε‐Cu dominantly contributes to the significant hardening at stage I, and the coarsening of ε‐Cu is the key factor to decrease the hardness at stage II. At stage III, the hardness hardly changes since the microstructure of S30432 tends to be stable in the long‐term creep range.     

The influence of prior ageing on creep damage development in two variants of Alloy 800

The influence of high temperature thermal ageing treatments on the development of intercrystalline creep damage in two variants of Alloy 800 has been investigated. Ageing up to 3000 h and creep testing were carried out at 800 and 900°C. The high temperature behaviour of the 800HT variant is discussed with reference to the effect of heat treatments on the microstructure. The metallographic methods by which the creep damage was quantitatively determined are described. The growth rate of intercrystalline microcracks was described using a statistical model and the dependence of crack growth rate on the thermal history for both 800HT and 800H was determined. The carbide precipitation and growth processes were determined as functions of the exposure temperature and duration. The results showed the three characteristic stages, precipitation, growth and coarsening (Ostwald ripening). The largest increase in the intergranular creep damage was found in Alloy 800HT within the first 1000 h, in which the precipitation of chromium carbides on the grain boundaries is substantially completed. The comparison between the two variants showed significant differences in the damage development. 800HT exhibited at 800 and 900°C higher creep crack growth rates with increasing exposure duration, but in 800H this effect was not clearly identified.     

Continuum damage mechanics of constrained intergranular cavitation

A three-dimensional micromechanical model for polycrystalline materials undergoing constrained intergranular cavitation is proposed. The model combines advantages of phenomenological and micromechanical approaches, and can be calibrated from standard uniaxial creep data. When applied to creep analysis of two ferritic steels, the model accurately predicts the creep life under multiaxial stresses as well as the amount of creep damage.     

Mechanical alloying of IN-738

Mechanical alloying has been applied to produce a dispersion-strengthened superalloy IN-738 containing 1.5 wt pct Y₂O₃. Annealing of extrusion bars above the recrystallization temperature of 1160°C can be described by three stages of recrystallization:finegrain; isotropic coarse-grain; and fibrous coarse grain growth. A maximum grain length of 550 μm and a maximum grain aspect ratio of 4.8 have been obtained for an alloy, which had been extruded at 1100°C and annealed at 1280°C and 1270°C for 3 h, respectively. The three stages of grain growth are explained in terms of recovery, differences in nucleation rate and dispersoid concentration in the two normal directions and release in stored cold work. Secondary recrystallization can be excluded as a mechanism for fibrous grain coarsening. Dispersion-strengthened IN-738, heat treated to a coarse elongated grain structure, has both high intermediate temperature strength and high elevated temperature strength. The creep strength at 1000°C exceeds that of cast or directionally solidified IN-738 after 300 h service life. The failure mechanism at elevated temperature is intergranular fracture along transverse grain boundaries, nucleated by cavities that form during grain boundary sliding. Nucleation of voids is retarded in the creep specimens due to diffusional accommodation of grain boundary sliding. A depletion of surface zones of chromium, aluminum and titanium contributes to initiation of creep failure at 1000°C.     

Mechanical alloying of IN-738

Mechanical alloying has been applied to produce a dispersion-strengthened superalloy IN-738 containing 1.5 wt pct Y₂O₃. Annealing of extrusion bars above the recrystallization temperature of 1160°C can be described by three stages of recrystallization:finegrain; isotropic coarse-grain; and fibrous coarse grain growth. A maximum grain length of 550 μm and a maximum grain aspect ratio of 4.8 have been obtained for an alloy, which had been extruded at 1100°C and annealed at 1280°C and 1270°C for 3 h, respectively. The three stages of grain growth are explained in terms of recovery, differences in nucleation rate and dispersoid concentration in the two normal directions and release in stored cold work. Secondary recrystallization can be excluded as a mechanism for fibrous grain coarsening. Dispersion-strengthened IN-738, heat treated to a coarse elongated grain structure, has both high intermediate temperature strength and high elevated temperature strength. The creep strength at 1000°C exceeds that of cast or directionally solidified IN-738 after 300 h service life. The failure mechanism at elevated temperature is intergranular fracture along transverse grain boundaries, nucleated by cavities that form during grain boundary sliding. Nucleation of voids is retarded in the creep specimens due to diffusional accommodation of grain boundary sliding. A depletion of surface zones of chromium, aluminum and titanium contributes to initiation of creep failure at 1000°C.     

Decomposition in Al-Zn alloys: Part I. Isothermal decomposition in an Al-22 at. pct Zn-0.1 at. pct Mg alloy

A small-angle X-ray scattering (SAS) study has been made on solution treated and isothermally annealed specimens of an Al-22 at. pct Zn-0.1 at. pct Mg alloy. The changes in peak position and integrated area of the SAS spectra with time and temperature indicate that de composition is nearly complete immediately after quenching, in agreement with the earlier interpretation that Gerold and Merz placed on the results of Rundman and Hilliard in the binary Al-22 at. pct Zn alloy. Furthermore, structural changes occurring during annealing are consistent with a coarsening or maturation of the fluctuations in the solution. The domi nant wavelength varies ast ^1/3 over a large time span and the temperature dependence of the coarsening process yields an activation energy of 94.2 kJ/mole. The effect of Mg is to re tard the formation of the equilibrium phases while having a small effect on the growth of the composition fluctuations during the coarsening process.     

Failure mechanisms in superplastic AA5083 materials

The mechanisms of tensile failure in four 5083 aluminum sheet materials are evaluated under conditions of interest for superplastic and quick-plastic forming. Two mechanisms are shown to control failure of the AA5083 materials under uniaxial tension at elevated temperatures: cavitation and flow localization (i.e., necking). Conditions for which failure is controlled by cavitation correspond to those under which deformation is primarily by grain-boundary-sliding creep. Conditions for which failure is controlled by flow localization correspond to those under which deformation is primarily by solutedrag creep. A geometric parameter, Q, is used to determine whether final failure is controlled by cavitation or by flow localization. Differences in elongations to failure between the different AA5083 materials at high temperatures and slow strain rates are the result of differences in cavitation behaviors. The rate of cavitation growth with strain is nearly constant between the AA5083 materials for identical testing conditions, but materials with less tensile ductility evidence initial cavitation development at lower strain levels. The rate of cavitation growth with strain is shown to depend on the governing deformation mechanism; grain-boundary-sliding creep produces a faster cavitation growth rate than does solute-drag creep. A correlation is found between the early development of cavitation and the intermetallic particle-size population densities of the AA5083 materials. Fine filaments, oriented along the tensile axis, are observed on fracture surfaces and within surface cavities of specimens deformed primarily under grain-boundary-sliding creep. As deformation transitions to control by solute-drag creep, the density of these filaments dramatically decreases.     

The effect of thermal exposure on microstructural stability and creep resistance of a two-Phase TiAl/Ti3Al lamellar alloy

During annealing of a two-phase TiAl/Ti₃Al lamellar alloy at 1273 and 1323 K, the lamellar microstructure evolves into a coarse, globular microstructure. For short annealing times (less than about 1000 hours), microstructural evolution occurs predominantly by intrapacket termination migration coarsening. For longer annealing times, cylinderization and conventional Ostwald ripening coarsening mechanisms are observed. The activation energy for the rate-controlling diffusion process governing intrapacket termination migration coarsening of the lamellar microstructure was determined to be 215 kJ/mol. Compression creep tests reveal that the minimum creep rate and primary creep strain of the lamellar alloy increase with increasing prior annealing time. Furthermore, in contrast to the lamellar microstructure, the globular microstructure is not susceptible to deformation-induced spheroidization during compression creep testing. Modeling demonstrates that the increase of the minimum creep rate and primary creep strain as a consequence of annealing of the lamellar alloy can be accounted for by consideration of two factors: the decrease in the work-hardening rate of the lamellar alloy in response to the overall decrease in interphase interfacial area and the decreased mechanical strengthening effect associated with transformation from a lamellar to a globular microstructure. Formerly Graduate Student, Department of Materials Science and Engineering, University of Virginia     

On Characterizing a Composite Microstructure in 316LN Stainless Steel Weld Metal and a New Damage Micromechanism During Creep

The austenitic stainless steel weld metal fabricated by multipass welding exhibits a composite microstructure. Microstructural characterization of the weld metal revealed that there are two distinct regions on either side of the weld-pass interface. The variations in dislocation substructure and delta ferrite morphology are the two microstructural attributes which delineate these regions. The generation of subsequent thermal cycles during the fabrication of multipass weld joint is the paramount factor influencing the formation of the composite microstructure. During creep exposure, the extent of creep cavitation and propagation varies substantially in these two regions due to differences in their microstructures. This results in preferential damage during creep exposure of austenitic stainless steel weld metal.     

Decomposition in Al-Zn alloys: Part I. Isothermal decomposition in an Al-22 at. pct Zn-0.1 at. pct Mg alloy

A small-angle X-ray scattering (SAS) study has been made on solution treated and isothermally annealed specimens of an Al-22 at. pct Zn-0.1 at. pct Mg alloy. The changes in peak position and integrated area of the SAS spectra with time and temperature indicate that de composition is nearly complete immediately after quenching, in agreement with the earlier interpretation that Gerold and Merz placed on the results of Rundman and Hilliard in the binary Al-22 at. pct Zn alloy. Furthermore, structural changes occurring during annealing are consistent with a coarsening or maturation of the fluctuations in the solution. The domi nant wavelength varies ast ^1/3 over a large time span and the temperature dependence of the coarsening process yields an activation energy of 94.2 kJ/mole. The effect of Mg is to re tard the formation of the equilibrium phases while having a small effect on the growth of the composition fluctuations during the coarsening process. Now on sabbatical leave to Caterpillar Tractor Co., Mapleton Plant, Mapleton, Ill. 61554.     

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. formerly at Oak Ridge National Laboratory, Oak Ridge, TN     

Evolution of fiber fragmentation in a short-fiber-reinforced metal-matrix model composite during tensile creep deformation—An acoustic emission study

The objective of this work is to obtain deeper insight into the damage evolution occurring during creep in short-fiber-reinforced metal-matrix composites. Uniaxial tensile creep experiments were performed on a model composite with a lead (Pb) matrix. This system was chosen because it allowed the performance of all creep tests at room temperature, thus facilitating the detection of fiber fragmentation by acoustic emission measurements. By this experimental approach, for the first time, quantitative information about the spatial and temporal evolution of microfractures in creeping metal-matrix composite of this kind was obtained. The acoustic emission results show that fiber fragmentation sets in early in the creep life and continues to operate up to macroscopic failure, thus affecting the creep behavior in all stages including the steady-state regime. During the whole creep process, the fracture sites are homogeneously distributed in the specimen volume. These findings largely support the micromechanical damage model proposed by Dlouhy and co-workers, in which the creep process in short-fiber-reinforced metal-matrix composites is described as an interplay of work hardening and recovery in the matrix as well as fragmentation of the fibers.     

Engineering approach to modelling the multiaxial creep and damage behaviour of compact tension geometry specimens of a 12 Cr steel at 550° C

Theoretical creep equations including tertiary creep were determined by fitting uniaxial constant load creep test results. Additionally, a new strain/time hardening rule was established. This theoretical approach was implemented in a finite‐element routine, and the load line displacement of a Cs20‐specimen of a 12 Cr steel under constant load was calculated as a function of time. The results were compared to experimental findings and correlated with creep damage by cavitation.