Inter-martensitic transitions in Ni–Fe–Ga single crystals

The strain–temperature response of Ni–Fe–Ga single crystals underscores the role of the inter-martensitic transformation in creating intersecting heating and cooling segments; the separation of these segments occurs due to irreversibilities at high stresses and at high temperatures. An ultra-narrow tensile (1 °C) and compressive (<10 °C) thermal hysteresis are observed for the A ⇌ 10M ⇌ 14M case, accompanied by a small stress hysteresis (<30 MPa) in compressive and tensile stress–strain responses. The hysteresis levels increase and the intersecting segments disappear at high stresses and at high temperatures. This paper reports the use of a thermo-mechanical formulation to rationalize the role of inter-martensitic transformations. Plotting the transformation stress as a function of temperature indicates that inter-martensitic transformations enable a very wide pseudoelastic temperature range, as high as 425 °C. The measured Clausius–Clapeyron curve slope in compression (2.75 MPa °C⁻¹) is eight times the tensile slope (0.36 MPa °C⁻¹); the higher slope is attributed to the predominance of A ⇌ L1₀ at high temperatures.     

Size-independent stresses in Al thin films thermally strained down to −100 °C

Size and temperature dependencies of thermal strains of {1 1 1} textured Al thin films were determined by in situ X-ray diffraction (XRD) in the temperature range of −100 to 350 °C. The experiments were performed on 50–2000 nm thick Al films sputter-deposited on oxidized silicon (1 0 0) substrates. The in-plane stresses were assessed by measuring the {3 3 1} lattice plane spacing at each temperature in steps of 25 °C during thermal cycling. At high temperatures, the films could only sustain small compressive stresses. The obtained stress–temperature evolutions show the well-known increase of flow stresses with decreasing film thickness for films thicker than 400 nm. However, for thinner films, the measured stress on cooling is independent of the film thickness. This lack of size effect is caused by the flow stresses in the thinnest films exceeding the maximum stress that can be applied to these samples using thermomechanical loading down to −100 °C. Thus, the measured stresses of ∼770 MPa in the thinnest film represent a lower limit for the actual flow stresses. The observed stresses are also discussed taking microstructural information and possible constraints on dislocation processes into account.     

In situ neutron diffraction study of residual stress development in MgO/SiC ceramic nanocomposites during thermal cycling

Ceramic nanocomposites often contain large residual stresses due to differing thermal contraction between phases upon cooling from processing temperatures. Their role in affecting the mechanical properties is not fully understood, but is certainly of importance. This investigation used neutron diffraction to quantify the residual stresses in MgO/SiC nanocomposites throughout a thermal cycle to 1550 °C. The results showed that average stresses in 10 vol.% SiC samples at 100 °C approached −4 GPa in the particles and were +560 MPa in the matrix. The stresses showed good agreement with an elastic model with a stress-free temperature of 1600 °C. A small amount of inelastic relaxation (15%) was observed after cooling back to room temperature. Modelling suggested that this was due to relaxation of the stresses in grain boundary particles at a rate limited by diffusional processes in the MgO/SiC interface. The effect of particle size on stress level is discussed.     

Deformation behavior of a two-phase Mo–Si–B alloy

Mo–Si–B alloys are being considered as possible candidates for high-temperature applications beyond the capabilities of Ni-based superalloys. In this paper, the high-temperature (1000–1400 °C) compression response over a range of quasi-static strain rates, as well as the monotonic and cyclic crack growth behaviors (as a function of temperature from 20 °C to 1400 °C) of a two-phase Mo–Si–B alloy containing a Mo solid solution matrix (Mo(Si,B)) with ∼38 vol% of the T2 phase (Mo₅SiB₂) is discussed. Analysis of the compression results confirmed that deformation in the temperature–strain-rate space evaluated is matrix-dominated, yielding an activation energy of ∼415–445 kJ/mol. Fracture toughness of the Mo–Si–B alloy varies from ∼8 MPa√m at room temperature to ∼25 MPa√m at 1400 °C, the increase in toughness with temperature being steepest between 1200 °C and 1400 °C. S–N response at room temperature is shallow whereas at 1200 °C, a definitive fatigue response is observed. Fatigue crack growth studies using R = 0.1 confirm the Paris slope for the two alloys to be high at room temperature (∼20–30) but decreases with increasing temperature to ∼3 at 1400 °C. The crack growth rate (da/dN) for a fixed value of ΔK in the Paris regime in the 900–1400 °C range, increases with increasing temperature.     

Effect of Er additions on ambient and high-temperature strength of precipitation-strengthened Al–Zr–Sc–Si alloys

The effect of substituting 0.01 at.% Er for Sc in an Al–0.06Zr–0.06Sc–0.04Si (at.%) alloy subjected to a two-stage aging treatment (4 h/300 °C and 8 h/425 °C) is assessed to determine the viability of dilute Al–Si–Zr–Sc–Er alloys for creep applications. Upon aging, coherent, 2–3 nm radius, L1₂-ordered, trialuminide precipitates are created, consisting of an Er- and Sc-enriched core and a Zr-enriched shell; Si partitions to the precipitates without preference for the core or the shell. The Er substitution significantly improves the resistance of the alloy to dislocation creep at 400 °C, increasing the threshold stress from 7 to 10 MPa. Upon further aging under an applied stress for 1045 h at 400 °C, the precipitates grow modestly to a radius of 5–10 nm, and the threshold stress increases further to 14 MPa. These chemical and size effects on the threshold stress are in qualitative agreement with the predictions of a recent model, which considers the attractive interaction force between mismatching, coherent precipitates and dislocations that climb over them. Micron-size, intra- and intergranular, blocky Al₃Er precipitates are also present, indicating that the solid solubility of Er in Al is exceeded, leading to a finer-grained microstructure, which results in diffusional creep at low stresses.     

Creep- and coarsening properties of Al–0.06 at.% Sc–0.06 at.% Ti at 300–450 °C

Upon aging at 300–450 °C, nanosize, coherent Al₃(Sc_1−xTi_x) precipitates are formed in pure aluminum micro-alloyed with 0.06 at.% Sc and 0.06 at.% Ti. The outstanding coarsening resistance of these precipitates at these elevated temperatures (61–77% of the melting temperature of aluminum) is explained by the significantly smaller diffusivity of Ti in Al when compared to that of Sc in Al. Furthermore, this coarse-grained alloy exhibits good compressive creep resistance for a castable, heat-treatable aluminum alloy: the creep threshold stress varies from 17 MPa at 300 °C to 7 MPa at 425 °C, as expected if the climb bypass by dislocations of the mismatching precipitates is hindered by their elastic stress fields.     

Generalized type III internal stress from interfaces,triple junctions and other microstructural components in nanocrystalline materials

The microstructure in polycrystalline materials consists of four types of geometric objects: grain cells, grain boundaries, triple junctions and vertex points. Each of them contributes to internal stress differently. Due to experimental limitations, the internal stresses associated with the microstructural components are difficult to acquire directly, particularly for polycrystalline materials with nanometer-scale grain sizes. Using newly developed computational methods, we obtained the type III internal stress associated with each of these microstructural objects in a stress-free nanocrystalline Cu. We found significant variation of the internal stresses from grain to grain, and their magnitudes descended in the order of vertex point, triple junction, grain boundary and grain cell. We also examined the effect of grain size and temperature. The change in the internal stresses inside the grains is found to follow a scaling relation of Ad^?x, using the mean grain diameter d from our results. For pressure, we found x = 1 and the effective interface stress A ～ 1 N m^?1, and for shear stress x = 0.75 and A ～ 14.12 N m^?1. On the other hand, the directly calculated interface stress is about 0.32–0.35 GPa for hydrostatic pressure and 12.45–12.60 GPa for von Mises shear stress. We discuss issues in treating the two-dimensional interface stress and one-dimensional triple junction line tension in nanocrystalline materials, as well as the potential impact of the type III internal stress on mechanical behavior of poly- and nano-crytalline materials.     

The mechanical properties of intermetallic Ni50−xTi50Alx alloys (x = 6,7,8,9)

High-strength, heat- and oxidation-resistant low density Ti–Ni–Al intermetallic alloys have recently attracted attention competing with some conventional high temperature structural superalloy such as Ni-based superalloy. In the present study, the mechanical properties of Ti-rich Ni_50−xTi₅₀Al_x (x = 6,7,8,9) alloys were examined by compression tests at room temperature and at high temperature from 400 °C to 800 °C. X-ray diffraction, scanning electron microscopy as well as microhardness tester were utilized to characterize the microstructure as well as the structural evolution with the increasing Al additions. The systematic analyses of the mechanical behavior were made according to compression test at different temperatures. A yield stress of 1800 MPa and more than 10% of compression strain were achieved at room temperature; and a yield stress of 400 MPa at 800 °C. It is suggested that controlling the shape, the volume percent and the distribution of second phases in the matrix is most important to obtain good mechanical properties in these alloys. The strengthening mechanism of aluminum addition on the mechanical properties was discussed systemically according to the microstructure evolution and solution hardening and precipitation hardening upon Al addition.     

Shape memory behavior of high strength NiTiHfPd polycrystalline alloys

Systematic characterization of the shape memory properties of a quaternary Ni_45.3–Ti_29.7–Hf₂₀–Pd₅ (at.%) polycrystalline alloy was performed in compression after selected aging treatments. Precipitation characteristics were revealed by transmission electron microscopy. The effects of aging temperature and time on transformation temperatures, recoverable and residual strains, and temperature and stress hystereses were determined by differential scanning calorimetry, constant-load thermal cycling experiments and isothermal strain cycling (superelasticity) tests. The crystal structure and lattice parameters of the transforming phases were determined from X-ray diffraction analysis. It was revealed that precipitation hardening significantly improved the shape memory properties of the NiTiHfPd alloy. Under optimum aging conditions, shape memory strains of up to 4% under 1 GPa were possible, and superelasticity experiments resulted in full strain recovery without any plastic deformation, even at stress levels as high as 2 GPa. The NiTiHfPd polycrystalline alloy exhibited very high damping capacity/absorbed energy (30–34 J cm^?3) and work output (30–35 J cm^?3), which were attributed to the ability to operate at high stress levels without significant plastic deformation and to a high mechanical hysteresis (>900 MPa) at temperatures ranging from 20 °C to 80 °C.     

Influence of wire-EDM on high temperature sliding wear behavior of WC10Co(Cr/V) cemented carbide

This paper reports the friction and wear response of WC–10%Co(Cr/V) cemented carbide with different surface finishes, attained by grinding (G) and wire-EDM, respectively, during sliding experiments at 400 °C. For comparison, tests under the same conditions were carried out at 25 °C. The wear experiments were performed under a normal force of 14 N, which produced a Hertzian maximum pressure of 3.10 GPa, and a sliding speed of 0.3 m/s against WC–6%Co(Cr/V) balls of 6 mm diameter. At 25 °C the average values of the friction coefficients were 0.36 ± 0.04 and 0.39 ± 0.06 for the ground and wire-EDM surface finishes, respectively. The mechanical behavior of both systems at 25 °C was assessed by carrying out analytical calculations of the stress field created by a circular sliding contact under a spherical indenter, where the residual stresses were considered. The theoretical results are in agreement with the experimental data, indicating that the wire-EDM sample has a specific wear rate, which is approximately 3.1 times greater than that corresponding to the G sample at 25 °C. At 400 °C, an increase in the friction coefficients takes place up to values of 0.75 ± 0.1 and 0.71 ± 0.8, for the ground and wire-EDM surface finishes, respectively. The increase was associated to an adhesive mechanism, which is more pronounced for the G sample. However, for the wire-EDM sample this increase was more linked to a marked abrasive mechanism. The wear rates for both samples at 400 °C are similar to those obtained at 25 °C, which indicates that apparently the test temperature does not have an important effect on the wear rate. However, it is known that temperature influences considerably the residual stress nature. Therefore, these results were explained by taking into account the wear mechanisms between the tribopairs in view of the mechanical characteristics and the morphological features obtained from SEM coupled with EDS analysis.     

Effect of silicidation pretreatment with gaseous SiO on sinterability of TiC powders

Silicidation pretreatment with gaseous SiO at 1350 °C for 30 min is employed for chemically modifying commercially available TiC powder. Phase composition and microstructural features of the pretreated powder are discussed. Densification behavior of the pretreated TiC powder during hot pressing is studied in comparison with that of non-pretreated one. Significantly improved densification behavior and sinterability of TiC powder after silicidation pretreatment are explained by the effect of Ti₃SiC₂ acting as a solid lubricant. Nearly fully dense TiC-based ceramics having flexural strength of 370 MPa, fracture toughness of 5.6 MPa m^½, and microhardness of 24 GPa is obtained by hot pressing under conditions as mild as 1600 °C and 20 MPa.     

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.     

Sintering behavior of W–30Cu composite powder prepared by electroless plating

Powder metallurgy technique was employed to prepare W–30 wt.% Cu composite through a chemical procedure. This includes powder pre-treatment followed by deposition of electroless Cu plating on the surface of the pre-treated W powder. The composite powder and W–30Cu composite were characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). Cold compaction was carried out under pressures ranging from 200 MPa to 600 MPa while sintering at 850 °C, 1000 °C and 1200 °C. The relative density, hardness, compressive strength, and electrical conductivity of the sintered samples were investigated. The results show that the relative sintered density of the titled composites increased with the sintering temperature. However, in solid sintering, the relative density increased with pressure. At 1200 °C and 400 MPa, the liquid-sintered specimen exhibited optimum performance, with the relative density reaching as high as 95.04% and superior electrical conductivity of IACS 53.24%, which doubles the national average of 26.77%. The FE-SEM microstructure evaluation of the sintered compacts showed homogenous dispersion of Cu and W and a Cu network all over the structure.     

Sinterability studies of monolithic chromium diboride (CrB2) by spark plasma sintering

Spark plasma sintering (SPS) experiments were conducted to investigate the effect of the processing parameters such as temperature, mechanical pressure and dwell time on densification behavior of monolithic chromium diboride. The sintering experiments were performed at different temperatures ranging from 1100 °C to 1900 °C under the mechanical pressure of 30 MPa–70 MPa for 1 min–15 min duration. The onset temperature for the densification of CrB₂ is observed to be 1300 °C at 50 MPa. High dense chromium diboride (98.4%ρ_th) compact was obtained when processed at 1900 °C under a mechanical pressure of 70 MPa for 15 min duration. Hardness and fracture toughness of high density monolithic CrB₂ (98.4%ρ_th) sample were measured to be 15.89 ± 1.3 GPa and 1.8 ± 0.14 MPa·m^1/2 respectively.     

Effect of specimen thickness on the creep response of a Ni-based single-crystal superalloy

Creep tests on Ni-based single-crystal superalloy sheet specimens typically show greater creep strain rates and/or reduced strain or time to creep rupture for thinner specimens than predicted by current theories, which predict a size-independent creep strain rate and creep rupture strain. This size-dependent creep response is termed the thickness debit effect. To investigate the mechanism of the thickness debit effect, isothermal, constant nominal stress creep tests were performed on uncoated PWA1484 Ni-based single-crystal superalloy sheet specimens of thicknesses 3.18 and 0.51 mm under two test conditions: 760 °C/758 MPa and 982 °C/248 MPa. The specimens contained initial microvoids formed during the solidification and homogenization processes. The dependence of the creep response on specimen thickness differed under the two test conditions: at 760 °C/758 MPa there was a reduction in the creep strain and the time to rupture with decreasing section thickness, whereas at 982 °C/248 MPa a decreased thickness resulted in an increased creep rate even at low strain levels and a decreased time to rupture but with no systematic dependence of the creep strain to rupture on specimen thickness. For the specimens tested at 760 °C/758 MPa microscopic analyses revealed that the thick specimens exhibited a mixed failure mode of void growth and cleavage-like fracture while the predominant failure mode for the thin specimens was cleavage-like fracture. The creep specimens tested at 982 °C/248 MPa in air showed the development of surface oxides and a near-surface precipitate-free zone. Finite-element analysis revealed that the presence of the alumina layer at the free surface imposes a constraint that locally increases the stress triaxiality and changes the value of the Lode parameter (a measure of the third stress invariant). The surface cracks formed in the oxide scale were arrested by further oxidation; for a thickness of 3.18 mm the failure mode was void nucleation, growth and coalescence, whereas for a thickness of 0.51 mm there was a mixed mode of ductile and cleavage-like fracture.     

Study of LiMgVO4 electrical conductivity mechanism

This paper deals with impedance spectroscopy on single-phase polycrystalline LiMgVO₄ in the temperature range of 25–500 °C. Thermogravimetric measurements show a weight loss of 2.7% in the temperature range between 25 °C and 175 °C due to humidity desorption. A conductivity mechanism along the grain boundaries (agb) is identified in the specific temperature range and is attributed to a reversible humidity absorption–desorption mechanism. Equivalent circuits are drawn using the results of the impedance measurements at each temperature. A unique conduction process within the material is assigned to each element of the equivalent circuit and Arrhenius plots are plotted. The calculation of activation energy of each conduction mechanism is based on the Arrhenius plots. The activation energy E_b of the bulk conductivity mechanism was found to be 0.62 eV. The activation energy E_gb of the grain boundaries conductivity mechanism was found to be 1.03 eV up to 275 °C and 0.50 eV in the temperature range of 300–500 °C. The absence of the conductivity mechanism along the grain boundaries above 175 °C can only be due to the complete removal of water from the material's grains.     

Recovery of low-temperature flow stress in zone-refined aluminum single crystals

The recovery of flow stress in zone-refined aluminum single crystals, deformed at ?196 °C in uniaxial tension along the 〈1 1 1〉 direction, has been measured at temperatures between 0 and 200 °C after straining to flow stresses ranging from 7 to 30 ksi (48–207 MPa). The corresponding strains ranged from 1.2 × 10^?2 to 13.1 × 10^?2. The effects of pre-stress and recovery temperature on structure and flow stress after recovery were systematically evaluated. The measured activation energy for recovery was found to increase linearly with the extent of recovery. The flow stress after recovery is found to be proportional to the reciprocal of the square root of the cell diameter. These results are used to provide, in a self-consistent manner, a new model for the recovery process.     

Spark plasma sintering of pure TiCN: Densification mechanism,grain growth and mechanical properties

This study aims to disclose the densification mechanism and grain growth behaviors during the spark plasma sintering (SPS) of undoped TiCN powder. The SPS experiments were performed under temperatures ranging from 1600 °C to 2200 °C and a fixed pressure of 50 MPa. The sintering mechanisms were described in different models according to two grain growth behaviors: densification without grain growth at low temperatures (1600–1700 °C) and grain growth without apparent densification at higher temperatures (1800–2200 °C). At the constant grain stage, a creep model is applied to describe the densification process. In addition, the effective stress exponents, n, are calculated, indicating that the densification can be attributed to both grain boundary sliding (n = 1.5) and dislocation climbing (n = 3.13 or n = 4.29). During the second stage of sintering, the grain growth model reveals that the grain-growth is controlled by grain boundary diffusion. In addition, the Vickers hardness varies from 4326 Hv to 6762 Hv when the density ranges from 90% to 96.3%.     

Repair of NiCrAlYSi overlay coating on Ni3Al base alloy IC6

The effect of repair of NiCrAlYSi coating after long time use on the coherence of coating to substrate and mechanical properties of Ni₃Al base alloy IC6 has been studied. The alloy with NiCrAlYSi coating was stressed under the condition of 900 °C/100 MPa for 200 h to simulate the service condition of IC6 turbine vanes. The results showed that the coherence of base alloy/original coating, original coating/first repair coating, first repair coating/second repair coating or base alloy/second repair coating was firm. The interfaces between them had no cracks and porosity, for either partly or entirely repaired coatings. However, there were more Mo rich particles in the affected zone compared with original coating. Compared with IC6 alloy aged at 900 °C for 200 h, the yield strength at ambient temperature of IC6 alloy with first repair coating was almost equivalent, while the elongation decreased slightly; its stress rupture life under 1100 °C/90 MPa condition was about 100 h. For secondary coating repair, the room temperature tensile properties of base alloy had no obvious change, while stress rupture lives decreased, but still were rather long, compared to the alloy with first repair coating. Therefore, NiCrAlYSi coating repair is feasible to prolong the service lives of IC6 turbine vanes.     

Microstructure of the cBN/WC6Co composite produced by the pulse plasma sintering (PPS) method

The cBN/WC6Co composite with the relative density of 99.8% and hardness of 2130 HV5 was produced by sintering at a temperature of 1150 °C under a pressure of 100 MPa for 5 min. The composite was sintered using electric pulses generated periodically by discharging a capacitor battery. The constituent phases of the composite, as identified by the NBED method, were the cBN, WC, and Co phases. The HR STEM observations have shown that the interfaces between the individual phases are continuous and no pores or precipitates of other phases can be seen there. Thanks to the specific heating realized by electric pulses, the composite is heated during each current pulse to a temperature of 1950 °C at a rate of 10⁵ °C/s. As a result of these quick changes of the temperature, transient thermal compressive stresses of about 3 GPa are induced in the composite, which results in the grains of the WC composite matrix being refined and defected.