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′.     

Low strain rate deformation behavior of a Cr–Mn austenitic steel at −80 °C

The deformation behavior of a Cr–Mn austenitic steel during interrupted low strain rate uniaxial tensile testing at ?80 °C has been studied using X-ray diffraction (XRD), electron backscatter diffraction and transmission electron microscopy. Continuous γ → ε → α′ martensite transformation was observed until failure. High dislocation densities were estimated in the austenite phase (～10¹⁵ m^?2), and for the α′-martensite they were even an order of magnitude higher. Dislocation character analysis indicated that increasing deformation gradually changed the dislocation character in the austenite phase to edge type, whereas the dislocations in α′-martensite were predominantly screw type. XRD analyses also revealed significant densities of stacking faults and twins in austenite, which were also seen by transmission electron microscopy. At low strains, the deformation mode in austenite was found to be dislocation glide, with an increasing contribution from twinning, as evidenced by an increasing incidence of ∑3 boundaries at high strains. The deformation mode in α′-martensite was dominated by dislocation slip.     

Evidence of variation in slip mode in a polycrystalline nickel-base superalloy with change in temperature from neutron diffraction strain measurements

The deformation mechanisms under tensile loading in a 45 vol.% γ′ polycrystalline nickel-base superalloy have been studied using neutron diffraction at 20 °C, 400 °C, 500 °C, 650 °C and 750 °C with the results interpreted via (self-consistent) polycrystal deformation modelling. The data demonstrate that such experiments are suited to detecting changes of the γ′ slip mode from {1 1 1} to {1 0 0} with increasing temperature. Between room temperature and 500 °C there is load transfer from γ′ to γ, indicating that γ′ is the softer phase. At higher temperatures, opposite load transfer is observed indicating that the γ matrix is softer. At 400 °C and 500 °C, an instantaneous yielding increment of about 2% was observed, after an initial strain of 1.5%. This instantaneous straining coincided with zero lattice misfit between γ and γ′ in the axial direction. Predicted and experimental results of the elastic strain response of the two phases and different grain families showed good agreement at elevated temperatures, while only qualitative agreement was found at 20 °C.     

Deformation mechanisms of a 20Mn TWIP steel investigated by in situ neutron diffraction and TEM

The deformation mechanisms and associated microstructure changes during tensile loading of an annealed twinning-induced plasticity steel with chemical composition Fe–20Mn–3Si–3Al–0.045C (wt.%) were systematically investigated using in situ time-of-flight neutron diffraction in combination with post mortem transmission electron microscopy (TEM). The initial microstructure of the investigated alloy consists of equiaxed γ grains with the initial α′-phase of ～7% in volume. In addition to dislocation slip, twinning and two types of martensitic transformations from the austenite to α′- and ε-martensites were observed as the main deformation modes during the tensile deformation. In situ neutron diffraction provides a powerful tool for establishing the deformation mode map for elucidating the role of different deformation modes in different strain regions. The critical stress is 520 MPa for the martensitic transformation from austenite to α′-martensite, whereas a higher stress (>600 MPa) is required for actuating the deformation twin and/or the martensitic transformation from austenite to ε-martensite. Both ε- and α′-martensites act as hard phases, whereas mechanical twinning contributes to both the strength and the ductility of the studied steel. TEM observations confirmed that the twinning process was facilitated by the parent grains oriented with 〈1 1 1〉 or 〈1 1 0〉 parallel to the loading direction. The nucleation and growth of twins are attributed to the pole and self-generation formation mechanisms, as well as the stair-rod cross-slip mechanism.     

Evolution of orientation distributions of γ and γ′ phases during creep deformation of Ni-base single crystal superalloys

The evolution of orientation distributions of γ and γ′ phases in crept Ni-base single crystal superalloys have been investigated by theoretical calculations with elastic–plastic models and by experiments. As creep deformation proceeds, the crystallographic orientation distributions for both phases are broadened as a result of the waving of the raft structure, which occurs to reduce the total mechanical energy. The broadening of the orientation distribution occurs in such a way that the 0 0 1 pole broadens isotropically while the h k 0 poles broaden preferentially along the 〈0 0 1〉 directions. Since the extent of the broadening increases almost linearly with the number of creep deformation, the measurement of the broadening by X-ray diffraction can be utilized in non-destructive methods to predict the lifetime of Ni-base superalloys.     

Analysis of local microstructure after shear creep deformation of a fine-grained duplex γ-TiAl alloy

The present work characterizes the microstructure of a hot-extruded Ti–45Al–5Nb–0.2B–0.2C (at.%) alloy with a fine-grained duplex microstructure after shear creep deformation (temperature 1023 K; shear stress 175 MPa; shear deformation 20%). Diffraction contrast transmission electron microscopy (TEM) was performed to identify ordinary dislocations, superdislocations and twins. The microstructure observed in TEM is interpreted taking into account the contribution of the applied stress and coherency stresses to the overall local stress state. Two specific locations in the lamellar part of the microstructure were analyzed, where either twins or superdislocations provided c-component deformation in the L1₀ lattice of the γ phase. Lamellar γ grains can be in soft and hard orientations with respect to the resolved shear stress provided by the external load. The presence of twins can be rationalized by the superposition of the applied stress and local coherency stresses. The presence of superdislocations in hard γ grains represents indirect evidence for additional contributions to the local stress state associated with stress redistribution during creep.     

Observations of a〈0 1 0〉 dislocations during the high-temperature creep of Ni-based superalloy single crystals deformed along the [0 0 1] orientation

A NASAIR-100 superalloy single crystal was tested in tension creep at 1000 °C at a stress of 148 MPa, for a time period of 20 h and to a strain of 1.1%. Analysis of the resulting dislocation structures after rafting was completed reveals the frequent presence of all three types of a〈0 1 0〉 dislocations in the γ′ particles. Two of these families experience no resolved forces due to the applied stress. It is proposed that these a〈0 1 0〉 dislocations form as a result of the combination of two dissimilar a/2〈0 1 1〉 dislocations entering from γ channels. The possible driving forces for the movement of these a〈0 1 0〉 dislocations are discussed, and a novel recovery mechanism during creep of rafted microstructures is introduced on the basis of these observations.     

Microstructural analysis of thermally induced and deformation induced martensitic transformations in Fe–12.5 wt.% Mn–5.5 wt.% Si–9 wt.% Cr–3.5 wt.% Ni alloy

Martensitic transformations induced by thermally and compression deformation at room temperature in Fe–12.5 wt.% Mn–5.5 wt.% Si–9 wt.% Cr–3.5 wt.% Ni alloy were studied in detail by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). From microstructural observations, it was seen that heat treated samples exhibited regular overlapping of stacking faults and ɛ martensite plates were formed parallel to each other. Also, TEM investigations showed that the orientation relationship between γ (fcc) and ɛ (hcp) phases corresponds to Shoji–Nishiyama type. With applied low plastic deformation rate, only ɛ martensite occurred in austenite grain. As a consequence, 4 and 25% plastic deformation at room temperature caused ɛ martensite formation in austenite phase and the new ɛ (hcp) and α′ (bcc) martensite formation in martensite phases, respectively. Orientation relationship between ɛ and α′ phases was found by the electron diffraction analysis.     

The effect of annealing on the microstructure and tensile properties of a β/γ′ Ni–Al–Fe alloy

The evolution of the microstructure of a (β/γ ′) Ni–32 at.% Al–5 at.% Fe alloy during annealing has been studied by electron microscopy and X-ray diffraction. Annealing at 800°C and 1100°C causes a reverse martensitic transformation, L1₀→B2 (β), and a B2→L1₂ (γ ′) phase transformation. The lower annealing temperature leads to a higher volume fraction of the γ ′-phase but a smaller size of the γ ′-particles. The kinetic laws of the coarsening and of the increase in the volume fraction of the γ ′-phase are discussed. The orientation relationships between the β and γ ′ phases appeared to be mainly of Nishiyama–Wassermann and Bain types after 800°C annealing, while Kurdjumov–Sachs and Bain orientation relationships were predominant in the alloys annealed at 1100°C. A strong correlation between the volume fraction of the γ ′-phase and the tensile characteristics of the alloy has been established.     

In situ TEM straining of single crystal Au films on polyimide: Change of deformation mechanisms at the nanoscale

In situ transmission electron microscopy straining experiments were performed on 40, 60, 80 and 160 nm thick single crystalline Au films on polyimide substrates. A transition in deformation mechanisms was observed with decreasing film thickness: the 160 nm thick film deforms predominantly by perfect dislocations while thinner films deform mainly by partial dislocations separated by stacking faults. In contrast to the 160 nm thick film, interfacial dislocation segments are rarely laid down by threading dislocations for the thinner films. At the late stages of deformation in the thicker Au films prior to fracture, dislocations start to glide on the (0 0 1) planes (cube-glide) near the interface with the polymer substrate. The impact of size-dependent dislocation mechanisms on thin film plasticity is addressed.     

Direct observation of the NiTi martensitic phase transformation in nanoscale volumes

We report on quantitative in situ transmission electron microscopy nanocompression tests used to study the deformation behavior of NiTi pillars on the nanometer scale. By recording the diffraction patterns in real time we have obtained direct evidence that the stress-induced B2 to B19′ (austenite to martensite) transformation exists in NiTi even when the sample size is below 200 nm. Correlation of the appearance of the B19′ phase in the diffraction pattern with our quantitative data showed that the transformation starts at approximately 1 GPa. We found that the transformation occurred through a multi-step process, and that the reverse transformation did not occur due to extensive deformation of the B19′ phase. Our results have direct implications for the application of the shape memory effect in nanoscale NiTi devices.     

Microstructural evolution in copper subjected to severe plastic deformation: Experiments and analysis

The evolution of microstructure and the mechanical response of copper subjected to severe plastic deformation using equal channel angular pressing (ECAP) was investigated. Samples were subjected to ECAP under three different processing routes: B_C, A and C. The microstructural refinement was dependent on processing with route B_C being the most effective. The mechanical response is modeled by an equation containing two dislocation evolution terms: one for the cells/subgrain interiors and one for the cells/subgrain walls. The deformation structure evolves from elongated dislocation cells to subgrains to equiaxed grains with diameters of ∼200–500 nm. The misorientation between adjacent regions, measured by electron backscatter diffraction, gradually increases. The mechanical response is well represented by a Voce equation with a saturation stress of 450 MPa. Interestingly, the microstructures produced through adiabatic shear localization during high strain rate deformation and ECAP are very similar, leading to the same grain size. It is shown that both processes have very close Zener–Hollomon parameters (ln Z ∼ 25). Calculations show that grain boundaries with size of 200 nm can rotate by ∼30° during ECAP, thereby generating and retaining a steady-state equiaxed structure. This is confirmed by a grain-boundary mobility calculation which shows that their velocity is 40 nm/s for a 200 nm grain size at 350 K, which is typical of an ECAP process. This can lead to the grain-boundary movement necessary to retain an equiaxed structure.     

High-temperature strength and deformation of γ/γ′ two-phase Co–Al–W-base alloys

The high-temperature strength and deformation behavior of γ/γ′ two-phase Co–Al–W-base alloys have been studied with polycrystalline and single-crystal materials. The ternary, quaternary and higher-order alloys containing Ta, Cr and/or Re exhibit flow stress anomalies above 873 K due to slip of pairs of 1/2〈1 1 0〉 superpartial dislocations on {0 0 1} planes, in addition to {1 1 1} planes, in the γ′ precipitates. Compression tests on the single-crystal specimens reveal a true anomalous peak temperature of 1073 K for both ternary and Ta-containing quaternary alloys. Above the peak, the ternary alloy exhibits a rapid decrease in strength with temperature, as 1/2〈1 1 0〉 dislocations bypass the γ′ precipitates without significant shearing. Conversely, the Ta-containing quaternary alloy sustains strength to higher temperatures due to the activation of 1/3〈1 1 2〉 partial dislocation slip that introduces a high density of stacking faults in the γ′ precipitates.     

Strong single-crystalline Au films tested by a new synchrotron technique

Although it is well known that thin films exhibit mechanical properties very different from those of their bulk counterparts, knowledge of the underlying mechanisms is incomplete. Single-crystalline films have a favorable microstructure for investigating the scaling behavior of mechanical properties. We present a novel experimental route for preparing single-crystalline Au films on a compliant polyimide substrate. For such single-crystals, we have developed a synchrotron-based tensile testing technique to measure the isothermal stress–strain curves and average peak widths. The analysis of Laue diffraction patterns as well as a parallel transmission electron microscopy study give new insight in the initial and evolving microstructure of the films. Complex novel deformation mechanisms are found, including a transition of the dominant deformation mechanism from full to partial dislocations in films thinner than 160 nm. The scaling behavior is described in view of the coexistence of different deformation mechanisms where the nucleation stress for single dislocations very likely governs the behavior.     

The deformation of Gum Metal through in situ compression of nanopillars

The name “Gum Metal” has been given to a set of β-Ti alloys that achieve exceptional elastic elongation and, with appropriate preparation, appear to deform by a dislocation-free mechanism triggered by elastic instability at the limit of strength. We have studied the compressive deformation of these materials with in situ nanocompression in a quantitative stage in a transmission electron microscope. The samples studied are cylindrical nanopillars 80–250 nm in diameter. The deformation pattern is monitored in real time using bright-field microscopy, dark-field microscopy or electron diffraction. Interesting results include the following: (i) nanopillars approach, and in several examples appear to reach, ideal strength; (ii) in contrast to conventional crystalline materials, there is no substantial “size effect” in pillar strength; (iii) the deformation mode is fine-scale with respect to the sample dimension, even in pillars of 100 nm size; (iv) shear bands (“giant faults”) do form in some tests, but only after yield and plastic deformation; and (v) a martensitic transformation to the base-centered orthorhombic α′′ phase is sometimes observed, but is an incidental feature of the deformation rather than a significant cause of it.     

An in situ bulk Zr58Al9Ni9Cu14Nb10 quasicrystal-glass composite with superior room temperature mechanical properties

An in situ bulk Zr₅₈Al₉Ni₉Cu₁₄Nb₁₀ quasicrystal-glass composite has been fabricated by means of copper mould casting. The microstructure and constituent phases of the alloy composite have been analyzed by using X-ray diffraction, transmission electron microscopy and high-resolution transmission electron microscopy. Icosahedral quasicrystals were found to be the majority phase and the grain size is in half-μm scale. In between the I-phase grains is a glassy phase. Optical microscopy and scanning electron microscopy revealed that the as-cast alloys were pore-free. The microhardness of the composite is about 5.90 ± 0.30 GPa. The room temperature compression stress–true strain curve exhibits a 2% elastic deformation up to failure, and a maximum fracture stress of 1850 MPa at a quasi-static loading rate of 4.4 × 10⁻⁴ s⁻¹. The mechanical property is superior to the early developed quasicrystal alloys, and is comparable to Zr-based bulk metallic glasses and their nanocomposites. The quasicrystal-glass composite exhibits basically a brittle fracture mode at room temperature.     

Creep Properties and Deformation Mechanisms of a FGH95 Ni-based Superalloy

By means of full heat treatment, microstructure observation, lattice parameters determination, and the measurement of creep curves, an investigation has been conducted into the microstructure and creep mechanisms of FGH95 Ni-based superalloy. Results show that after the alloy is hot isostatically pressed, coarse γ′ phase discontinuously distributes along the previous particle boundaries. After solution treatment at high temperature and aging, the grain size has no obvious change, and the amount of coarse γ′ phase decreases, and a high volume fraction of fine γ′ phase dispersedly precipitates in the γ matrix. Moreover, the granular carbides are found to be precipitated along grain boundaries, which can hinder the grain boundaries’ sliding and enhance the creep resistance of the alloy. By x-ray diffraction analysis, it is indicated that the lattice misfit between the γ and γ′ phases decreases in the alloy after full heat treatment. In the ranges of experimental temperatures and applied stresses, the creep activation energy of the alloy is measured to be 630.4 kJ/mol. During creep, the deformation mechanisms of the alloy are that dislocations slip in the γ matrix or shear into the γ′ phase. Thereinto, the creep dislocations move over the γ′ phase by the Orowan mechanism, and the $ \left\langle { 1 10 } \right\rangle $ 〈 1 10 〉 super-dislocation shearing into the γ′ phase can be decomposed to form the configuration of (1/3) $ \left\langle { 1 12 } \right\rangle $ 〈 1 12 〉 super-Shockleys’ partials and the stacking fault.     

Microstructure changes during non-conventional heat treatment of thin Ni–Ti wires by pulsed electric current studied by transmission electron microscopy

Transmission electron microscopy, electrical resistivity measurements and mechanical testing were employed to investigate the evolution of microstructure and functional superelastic properties of 0.1 mm diameter as-drawn Ni–Ti wires subjected to a non-conventional heat treatment by controlled electric pulse currents. This method enables a better control of the recovery and recrystallization processes taking place during the heat treatment and accordingly a better control on the final microstructure. Using a stepwise approach of millisecond pulse annealing, it is shown how the microstructure evolves from a severely deformed state with no functional properties to an optimal nanograined microstructure (20–50 nm) that is partially recovered through polygonization and partially recrystallized and that has the best functional properties. Such a microstructure is highly resistant against dislocation slip upon cycling, while microstructures annealed for longer times and showing mostly recrystallized grains were prone to dislocation slip, particularly as the grain size exceeds 200 nm.     

Microstructural evolution during deformation of tin dioxide nanoparticles in a comminution process

Nanoparticles can be produced by wet grinding in stirred media mills if agglomeration is prevented by stabilization of the particles. Since the fracture mechanisms at the lower nanoscale are not yet understood, we studied the evolution of the microstructure within tin dioxide particles. Electrostatic stabilization allows the formation of tin dioxide with a mean particle size of 25 nm as measured by dynamic light scattering. High-resolution transmission electron microscopy (HRTEM) images show particles well below 10 nm and mean crystallite sizes of 9 nm were obtained from X-ray diffraction by applying the Rietveld refinement method. Additionally, TEM and HRTEM analyses were conducted to gain detailed insight into the microstructural effects governing the grinding process. Microscopy revealed surprisingly rich phenomena including the formation of shear bands, twinning and stacking faults that directly affect the grinding behavior. Interestingly the ceramic nanoparticles showed not only fracture patterns expected from brittle fracture but also many traces of plastic deformation. For comparison the uniaxial compression of particles up to 30 nm in diameter was simulated using molecular dynamics. The simulated particles shared microstructural details with the real samples, most importantly the shear bands which lead to significant plastic deformation. The internal microstructure produced during multiple particle stressing events in the mill and also observed in the simulations is directly linked to the fracture mechanism and the experimentally observed grinding limit.     

Synthesis and properties of nanostructured dense LaB6 cathodes by arc plasma and reactive spark plasma sintering

Nanostructured polycrystalline LaB₆ ceramics were prepared by the reactive spark plasma sintering method, using boron nanopowders and LaH₂ powders with a particle size of about 30 nm synthesized by hydrogen dc arc plasma. The reaction mechanism of sintering, crystal structure, microstructure, grain orientations and properties of the materials were investigated using differential scanning calorimetry, X-ray diffraction, Neutron powder diffraction, Raman spectroscopy, transmission electron microscopy and electron backscattered diffraction. It is shown that nanostructured dense LaB₆ with a fibrous texture can be fabricated by SPS at a pressure of 80 MPa and temperature of 1300 °C for 5 min. Compared with the coarse polycrystalline LaB₆ prepared by traditional methods, the nanostructured LaB₆ bulk possesses both higher mechanical and higher thermionic emission properties. The Vickers hardness was 22.3 GPa, the flexural strength was 271.2 MPa and the maximum emission current density was 56.81 A cm^?2 at a cathode temperature of 1600 °C.