Lattice strain evolution during creep in single-crystal superalloys

In situ neutron diffraction studies are carried out to characterize the micromechanical deformation occurring during tensile creep of a typical single-crystal nickel-based superalloy, CMSX-4. The loading responses of the matrix γ phase and the precipitate γ′ are distinct. Moreover, the behaviour in the tertiary creep regime (in which the γ′ phase remains intact) is qualitatively different from that in the primary creep regime (when γ′ is sheared). In tertiary creep, initial deformation of the matrix leads to a release of misfit between the phases in the (1 0 0), resulting in elastic compression of the γ in the loading direction. The load state then remains fairly constant during creep. During the initial stages of primary creep, elastic compression of the γ phase is observed until at around 2–4% creep strain this compression stabilizes as the (1 0 0) misfit is released. This is the point at which γ′ shearing is thought to begin. Subsequently, the load in the γ increases by around 200 MPa until a maximum is reached at around 8% creep strain. This load is then suddenly released, which may be due to the release of back-stress.     

In situ high-energy X-ray diffraction studies of deformation-induced phase transformation in Ti-based amorphous alloy composites containing ductile dendrites

The deformed-induced microstructure evolution and phase transformation behavior of Ti-based amorphous alloy composites (AACs) containing ductile dendrites in situ formed during solidification were investigated using ex situ transmission electron microscopy (TEM) and in situ high-energy X-ray diffraction (HE-XRD). In situ synchrotron-based HE-XRD experiments provide clear evidence on the deformation-induced phase transformation from β to α″ martensite initiated already in the linear elastic stage of the macroscopic stress–strain curve. Detailed analyses from the diffraction experiments show that the grains that were aligned with [0 0 1]_β along the loading direction (LD) were then easily transformed into α″ martensite, whereas the martensitic variants oriented with [1 0 0]_α″ along LD were preferentially formed under compression. The current study provides quantitative information about changes in various microstresses between the crystal phase and the amorphous matrix during deformation. Enhancement of the macroscopic plasticity of the AACs was mainly attributed to the strain relaxation in the β phase and to the formation of multiple shear bands in the amorphous matrix triggered by the deformation-induced phase transformation inside β, knowledge of which greatly deepens understanding of the complex micromechanical behaviors in advanced AACs.     

Austenite–ferrite transformation kinetics under uniaxial compressive stress in Fe–2.96 at.% Ni alloy

The effect of an applied constant uniaxial compressive stress on the kinetics of the austenite (γ) → ferrite (α) massive transformation in the substitutional Fe–2.96 at.% Ni alloy upon isochronal cooling has been studied by differential dilatometry. All imposed stress levels are below the yield stress of austenite and ferrite in the temperature range of the transformation. An increase in compressive stress results in a small but significant increase of the onset temperature of the γ → α transformation and a decrease of the overall transformation time. A phase transformation model, involving site saturation, interface-controlled growth and incorporation of an appropriate impingement correction, has been employed to extract the interface-migration velocity of the γ/α interface. The interface-migration velocity for the γ → α transformation is approximately constant at fixed uniaxial compressive stress and increases with increasing applied uniaxial compressive stress. Furthermore, the value obtained for the energy corresponding with the elastic and plastic deformation associated with the accommodation of the γ/α volume misfit depends on the transformed fraction and decreases significantly as the applied uniaxial compressive stress increases. An understanding of the observed effects is obtained, recognizing the constraints imposed on the phase transformation due to the applied stress.     

A study of the densification mechanisms during spark plasma sintering of zirconium (oxy-)carbide powders

Zirconium oxycarbide powders with controlled composition ZrC_0.94O_0.05 were synthesized using the carboreduction of zirconia. They were further subjected to spark plasma sintering (SPS) under several applied loads (25, 50, 100 MPa). The densification mechanism of zirconium oxycarbide powders during the SPS was studied. An analytical model derived from creep deformation studies of ceramics was successfully applied to determine the mechanisms involved during the final stage of densification. These mechanisms were elucidated by evaluating the stress exponent (n) and the apparent activation energy (E_a) from the densification rate law. It was concluded that at low macroscopic applied stress (25 MPa), an intergranular glide mechanism (n ? 2) governs the densification process, while a dislocation motion mechanism (n ? 3) operates at higher applied load (100 MPa). Transmission electron microscopy observations confirm theses results. The samples treated at low applied stress appear almost free of dislocations, whereas samples sintered at high applied stress present a high dislocation density, forming sub-grain boundaries. High values of apparent activation energy (e.g. 687–774 kJ mol^?1) are reached irrespective of the applied load, indicating that both mechanisms mentioned above are assisted by the zirconium lattice diffusion which thus appears to be the rate-limiting step for densification.     

Load partitioning in honeycomb-like silicon carbide aluminum alloy composites

A 50/50 vol.% Al/SiC composite was made via melt infiltration of an aluminum alloy into a porous beech wood-derived SiC preform. The honeycomb-like composite microstructure consisted of an interconnected SiC phase surrounding discrete Al “fibers” aligned in the growth direction of the beech wood. High energy synchrotron X-ray diffraction was used to measure the volume averaged lattice strains in both the SiC and Al phases during in situ compressive loading up to an applied stress of ?530 MPa. Load transfer from the Al to the SiC was observed, and the Al yielded at an applied stress of above ?213 MPa. The elastic behavior of the composite was modeled with both an isostrain rule of mixtures calculation and variational bounds for the effective elastic modulus. Furthermore, calculations of the von Mises effective stress of the SiC and Al phases showed that the wood-derived SiC was a more effective reinforcement than either SiC particle- or whisker-reinforced composites.     

Development of intergranular thermal residual stresses in beryllium during cooling from processing temperatures

The intergranular thermal residual stresses in texture-free solid polycrystalline beryllium were determined by comparison of crystallographic lattice parameters in solid and powder samples measured by neutron diffraction during cooling from 800 °C. The internal stresses are not significantly different from zero >575 °C and increase nearly linearly <525 °C. At room temperature, the c axis of an average grain is under ～200 MPa of compressive internal stress, and the a axis is under 100 MPa of tensile stress. For comparison, the stresses have also been calculated using an Eshelby-type polycrystalline model. The measurements and calculations agree very well when temperature dependence of elastic constants is accounted for, and no plastic relaxation is allowed in the model.     

Microscale-calibrated modeling of the deformation response of low-carbon martensite

Micropillar compression tests were used to determine the uniaxial compressive stress–strain response of martensite blocks extracted from a low-carbon, fully lath martensitic sheet steel, M190, with the nominal composition C = 0.18, Mn = 0.47, P = 0.007, S = 0.006, Si = 0.18, Al = 0.06, Ti = 0.045, B = 0.0014 and balance Fe (all in wt.%). Specimens with a diameter exceeding ～1 μm and consisting of a single martensite block showed elastic–nearly perfectly plastic behavior with a yield stress of the order 1200 MPa. Similar specimens which contained multiple martensite blocks showed pronounced strain hardening, arising from the geometrical constraint produced by the interface(s). No size dependence of flow stress was observed in micropillars with diameters exceeding 1.0 μm, but a significant scatter in strength and hardening rate was observed in micropillars with smaller diameters. Flow data for micropillars in the size-independent regime were used to determine parameters in a crystal-plasticity-based model of martensite. Full three-dimensional crystal plasticity simulations, with material properties determined from micropillar tests, were then used to predict the macroscopic uniaxial stress–strain behavior of a representative volume element of martensite. The predicted stress–strain behavior was in excellent agreement with experimental measurements, and demonstrates the potential for micropillar tests to determine material parameters for individual phases of a complex microstructure.     

Kinetics of austenitization under uniaxial compressive stress in Fe–2.96 at.% Ni alloy

Differential dilatometry has been employed to study the kinetics of the massive ferrite (α) → austenite (γ) transformation upon isochronal heating (i.e. austenitization) of the substitutional Fe–2.96 at.% Ni alloy subjected to a range of applied constant uniaxial compressive stresses. A phase-transformation model, involving site saturation, interface-controlled (continuous) growth and incorporating an impingement correction for an intermediate of the cases of ideally periodically and of ideally randomly dispersed growing particles, has been employed to extract the interface-migration velocity of the α/γ interface and the transformation-induced deformation energy taken up by the specimen. The value obtained for the energy corresponding with the elastic and plastic deformation associated with the accommodation of the α/γ volume misfit depends on the austenite fraction and increases distinctly with an increase in the applied uniaxial compressive stress, which is compensated by, in particular, an increase in the chemical driving force corresponding to an increase in the onset temperature. The opposite effects of an applied uniaxial compressive stress on the α → γ transformation and on the γ → α transformation can be discussed as the outcome of constrained plastic deformation due to transformation-induced strain.     

CRSS of γ/γ′ phases from in situ neutron diffraction of a directionally solidified superalloy tension tested at 900 °C

Neutron diffraction was used to monitor elastic strains during in situ tension testing of a directionally solidified (DS) superalloy at 900 °C. Changes in misfit and thermal expansion coefficients of individual phases were obtained. In the γ phase, it is demonstrated that elastic strains saturate at 350 MPa, which is well below the yield strength of the alloy. This is interpreted as the onset of dislocation glide through less stressed vertical channels. The critical resolved shear stress (CRSS) of γ is found to be 143 ± 11 MPa, in agreement with a calculated CRSS that is dominated by Orowan bowing of dislocations through nanoscale-wide γ channels. This provides confirmation of Orowan bowing in plasticity/creep of the γ phase. Implications of CRSS and misfit in a “threshold stress” for creep and rafting are discussed. The CRSS of γ′ is found to be consistent with pairwise penetration of dislocations into γ′.     

Stress–strain relationship between ferrite and martensite in a dual-phase steel studied by in situ neutron diffraction and crystal plasticity theories

The stress–strain relationship between ferrite and martensite phases in the commercial dual-phase DP980 steel was studied using in situ neutron diffraction and the crystal plasticity finite element method (CPFEM). The phase identification method based on the image quality of electron backscatter diffraction and a filtering process was used to obtain information concerning individual crystallographic orientations for ferrite and martensite phases in DP980 steel. The (2 0 0) and (2 1 1) lattice strains of ferrite and martensite phases were measured along the loading and transverse directions as a function of macroscopic stress using in situ neutron diffraction. A CPFEM based on representative volume elements (RVE) was applied to determine the microscopic hardening parameters for each phase by fitting the measured macroscopic stress and measured (2 0 0) and (2 1 1) lattice strains. The microscopic hardening parameters for each phase successfully captured the influence of the crystallographic orientation of the ferrite phase on the localization of shear strain and the behavior of ductile failure in RVE of the unit cell during uniaxial tension.     

Effects of superimposed electric field and porosity on the hydrostatic pressure-induced rhombohedral to orthorhombic martensitic phase transformation in PZT 95/5 ceramics

The hydrostatic pressure-induced martensitic transformation from the ferroelectric rhombohedral to antiferroelectric orthorhombic phase in PZT 95/5 ceramics has been studied using neutron diffraction. The transition to the orthorhombic phase initiates at a pressure of 260 MPa and is almost complete at 290 MPa. This stress range is much narrower than that observed in uniaxial loading, which starts at 200 MPa and is incomplete even at 400 MPa. The narrower stress range observed under hydrostatic loading is attributed to a lack of internal stress developed during the transformation. By contrast, the work required to start the transformation is approximately the same under both types of loading. The transformation progresses more gradually with increasing pressure when a static electric field is applied to a specimen in a pre-poled state. Tests carried out on porous specimens, having a relative density of approximately 90%, demonstrated that the transformation occurred over a narrow pressure range but with a lower transformation pressure of approximately 220 MPa.     

Effects of applied stress and long-term stability on PPy(CF3SO3) linear actuators

The actuation performance of PPy(CF₃SO₃) films, in the free-standing form, has been characterized in aqueous NaPF₆ electrolytes during potential step experiments. Actuation strains of up to 7.5% were observed due to anion insertion at more positive potentials. The actuation strain decreased as the applied stress was increased up to 12 MPa, and then remained constant at above 4% up to the maximum applied stress of 28.8 MPa. A relatively large creep was observed in the case of an applied stress of 28.8 MPa. The film could be cycled more than 1500 times with the retention of 18% of the initial strain level.     

Hydride precipitation and stresses in zircaloy-4 observed by synchrotron X-ray diffraction

The grain stresses within hydrides precipitated in rolled zircaloy-4 plates were determined by synchrotron X-ray diffraction experiments using an 80 keV photon beam and a high-speed area detector placed in transmission geometry. Results showed large compressive stresses (360 ± 20 MPa) in the hydrides along the plate rolling direction. The origin of these stresses was investigated by performing hydride dissolution/precipitation in situ for thermal cycles between room temperature and 400 °C. A large stress hysteresis was observed, with a steady decrease on heating and an abrupt change on cooling. The observed stresses are explained by the constraint imposed by grain boundaries on the growth of hydride platelets on the rolling–transverse plane of the rolled plates.     

Equilibrium shape of prismatic dislocation loops under uniform stress

The rolling, recrystallization and cooling of AgCl containing 1–5 μm glass spheres generates thermal misfit dislocations. Under stress, prismatic loops elongate in the glide cylinder defined by their line sense and Burgers vector. Using optical microscopy, the shape of dislocation loops under an applied stress of 2.3 MPa is measured. The measurements are corrected for a friction stress of 0.34 MPa and compared with a model which incorporates the orientation dependent line tension (ODLT) of a dislocation. The measured data show considerable scatter; after averaging, good agreement between theory and experiment is obtained.     

Comparison of microstructures and properties of Zr-based bulk metallic glass composites with dendritic and spherical bcc phase precipitates

In this paper, the microstructures and mechanical properties of the two BMG composites with the same composition of Zr_56.2Ti_13.8Nb_5.0Cu_6.9Ni_5.6Be_12.5, which contain in situ formed dendritic and spherical bcc β-Zr (Ti, Nb) solid solutions, are compared based on micrograph observations, XRD analysis and uniaxial compression tests at room temperature. The dendritic β phase exhibits well-developed primary dendrite axes with lengths of 20–50 μm and diameters of about 1–3 μm; the spherical β phase with the same volume fraction of about 30% as the dendritic β phase has a much larger average diameter of about 18 μm as compared to the primary dendrite axes of the β phase. Compression tests demonstrated that the yield stress, strain at the yield point, ultimate fracture strength and fracture plastic strain of the composite containing dendritic β phase are 1208 MPa, 1.78%, 1757 MPa and 8.82%, respectively. For the composites containing spherical β phase, however, a much larger fracture plastic strain of about 12% was achieved, and combined with a somewhat higher yield strength of 1350 MPa and a larger strain of 2.32% at the yield point, the ultimate fracture strength was measured to be about 1800 MPa.     

Creep behaviour of a new air-hardenable intermetallic Ti–46Al–8Ta alloy

Creep behaviour of a new cast air-hardenable intermetallic Ti–46Al–8Ta (at.%) alloy was investigated. Constant load tensile creep tests were performed at initial applied stresses ranging from 200 to 400 MPa in the temperature range from 973 to 1073 K. The minimum creep rate is found to depend strongly on the applied stress and temperature. The power law stress exponent of the minimum creep rate is n = 5.8 and the apparent activation energy for creep is calculated to be Q_a = (382.9 ± 14.5) kJ/mol. The kinetics of creep deformation of the specimens tested to a minimum creep rate (creep deformation about 2%) is suggested to be controlled by non-conservative motion of dislocations in the γ(TiAl) matrix. Besides dislocation mechanisms, deformation twinning contributes significantly to overall measured strains in the specimens tested to fracture. The initial γ(TiAl) + α₂(Ti₃Al) microstructure of the creep specimens is unstable and transforms to the γ + α₂ + τ type during creep. The particles of the τ phase are preferentially formed along the grain and lamellar colony boundaries.     

Single-crystal plasticity of the complex metallic alloy phase β-Al–Mg

We have determined the macroscopic plastic deformation behaviour of high-quality single-crystalline samples of the complex metallic alloy β-Al–Mg. Uniaxial deformation experiments at a constant strain rate of 10⁻⁴ s⁻¹ were performed at temperatures between 200 and 375 °C. The material exhibits ductile behaviour down to temperatures of 225 °C. At this temperature an upper yield stress of 780 MPa was observed, which is a very high value compared to commercial Al–Mg alloys. The upper yield point is followed by an almost constant flow stress level up to strains of about 6%. Stress-relaxation tests and temperature changes were carried out in order to determine the thermodynamic activation parameters of the deformation process.     

Tensile creep of directionally solidified NiAl–9Mo in situ composites

Well-aligned Mo fiber-reinforced NiAl in situ composites were produced by specially controlled directional solidification. The creep behavior parallel to the growth direction was studied in static tensile tests at temperatures between 900 °C and 1200 °C. A steady-state creep rate of 10^?6 s^?1 was measured at 1100 °C under an initial applied tensile stress of 150 MPa. Compared to binary NiAl and previously investigated NiAl–Mo eutectics with irregularly oriented Mo fibers, this value demonstrates a remarkably improved creep resistance in NiAl–Mo with well-aligned unidirectional Mo fibers. A high-resolution transmission electron microscope investigation of the NiAl/Mo interface revealed a clean semi-coherent boundary between NiAl and Mo, which enabled an effective load transfer from the NiAl matrix to the Mo fibers, and thus leads to the remarkably increased creep strength. The stress exponent, n, was found to be between 3.5 and 5, dependent on temperature. The activation energy for creep, Q_c, was measured to be 291 ± 19 kJ mol^–1, which is close to the value for self-diffusion in binary NiAl. Transmission electron microscopy observations substantiated that creep occurred by dislocation climb in the NiAl matrix. The Mo fiber was found to behave in a quasi-rigid manner during creep. A creep model for fiber-reinforced metal matrix composites was applied for an in-depth understanding of the mechanical behavior of the individual components and their contribution to the creep strength of the composite.     

Electro-stiffening in polypyrrole films: Dependence of Young's modulus on oxidation state,load and frequency

Electrochemically driven actuation of polypyrrole in aqueous sodium hexafluorophosphate (NaPF₆) solution has been shown to produce repeated large strains (>6%) at low voltages and with high conductivity, making it one of the most promising electroactive conducting polymers. Little is known about the voltage dependent stiffness of this version of the polymer. This information is important in determining the strain as a function of load. In this paper the complex Young's modulus (storage and loss components) of a hexafluorophosphate-doped polypyrrole film in aqueous NaPF₆ electrolyte at different oxidation states, under various loads and as a function of the frequency of the applied load, is investigated. Uniformity of doping was ensured by allowing enough time to reach steady state charge levels, and the creep during measurements was minimized by using preconditioning cycles. The results of this study show that storage modulus decreases (from 1 GPa to 0.80 GPa) as the polypyrrole oxidation potential increases (from ?0.4 V to +0.4 V versus Ag/AgCl reference electrode). The loss modulus, on the other hand, increases from 55 MPa to 80 MPa. An increasing trend in the Young's modulus is also observed with the applied load. The storage modulus increases from 0.65 GPa to 1 GPa by increasing the applied load from 0.2 MPa to 2.5 MPa. The modulus is found to increase with time through the experiment, which may be due to stretch alignment of the polymer. It is also observed that complex Young's modulus increases in proportion to the logarithm of frequency.     

Loss of pseudoelasticity in nickel–titanium sub-micron compression pillars

Nickel–titanium (NiTi) is capable of undergoing pseudoelastic deformation wherein relatively large amounts of inelastic deformation are recovered upon load removal due to a martensitic phase transformation. This study investigates pseudoelastic as well as plastic deformation in sub-micron diameter NiTi compression pillars. Pillars ranging in diameter from approximately 2 μm to 200 nm were prepared using focused ion beam micro-machining of aged [1 1 1] single crystal NiTi. Results reveal pseudoelasticity in all samples tested with diameters between 2 μm and 400 nm, although permanent strain was introduced at relatively low strains compared to bulk. Decreased sample size generally showed a smaller stress–strain hysteresis, with a full loss of recoverable pseudoelastic strain for samples with a diameter smaller than 200 nm. In addition, plastic flow stress of the martensite was shown to be independent of sample diameter for the aged NiTi material. Lastly, it is observed that crystallographic orientation has a stronger influence on martensite plastic flow strength than pillar size.