High temperature micropillar compression of Al/SiC nanolaminates

The effect of the temperature on the compressive stress–strain behavior of Al/SiC nanoscale multilayers was studied by means of micropillar compression tests at 23 °C and 100 °C. The multilayers (composed of alternating layers of 60 nm in thickness of nanocrystalline Al and amorphous SiC) showed a very large hardening rate at 23 °C, which led to a flow stress of 3.1 ± 0.2 GPa at 8% strain. However, the flow stress (and the hardening rate) was reduced by 50% at 100 °C. Plastic deformation of the Al layers was the dominant deformation mechanism at both temperatures, but the Al layers were extruded out of the micropillar at 100 °C, while Al plastic flow was constrained by the SiC elastic layers at 23 °C. Finite element simulations of the micropillar compression test indicated the role played by different factors (flow stress of Al, interface strength and friction coefficient) on the mechanical behavior and were able to rationalize the differences in the stress–strain curves between 23 °C and 100 °C.     

Reinforcement architectures and thermal fatigue in diamond particle-reinforced aluminum

Aluminum reinforced by 60 vol.% diamond particles has been investigated as a potential heat sink material for high power electronics. Diamond (CD) is used as reinforcement contributing its high thermal conductivity (TC ≈ 1000 W mK^?1) and low coefficient thermal expansion (CTE ≈ 1 ppm K^?1). An Al matrix enables shaping and joining of the composite components. Interface bonding is improved by limited carbide formation induced by heat treatment and even more by SiC coating of diamond particles. An AlSi7 matrix forms an interpenetrating composite three-dimensional (3D) network of diamond particles linked by Si bridges percolated by a ductile α-Al matrix. Internal stresses are generated during temperature changes due to the CTE mismatch of the constituents. The stress evolution was determined in situ by neutron diffraction during thermal cycling between room temperature and 350 °C (soldering temperature). Tensile stresses build up in the Al/CD composites: during cooling <100 MPa in a pure Al matrix, but around 200 MPa in the Al in an AlSi7 matrix. Compressive stresses build up in Al during heating of the composite. The stress evolution causes changes in the void volume fraction and interface debonding by visco-plastic deformation of the Al matrix. Thermal fatigue damage has been revealed by high resolution synchrotron tomography. An interconnected diamond–Si 3D network formed with an AlSi7 matrix promises higher stability with respect to cycling temperature exposure.     

Processing of ultra-high strength SiCp/Al–Zn–Mg–Cu composites

The ultra-high strength SiC_p/Al–10%Zn–3.6%Mg–1.8%Cu–0.36%Zr–0.15%Ni composite was prepared by spray co-deposition followed by extrusion process. The heat treatment processing, microstructures and mechanical properties of the as-processed composite were investigated. The well-bonded SiC/Al interfaces and fine grains of matrix alloy were obtained in the as-extruded composite. The precipitated phase MgZn₂ dissolved during solid solution treatment at 490 °C for 1 h, but the Cu-rich phase was residual in the matrix. Comparatively, the Cu-rich phase dissolved into the matrix alloy exposed at 470 °C for 1 h and then at 490 °C for 1 h. The composite heat-treated with 470 °C/1 h + 490 °C/1 h + 120 °C/28 h exhibited high modulus above 100 GPa and ultra-high strength about 785 MPa, which was 30 MPa higher than that of the same composite treated with 490 °C/1 h + 120 °C/28 h processing. The low elongation of the composite can be attributed to the breakage of SiC particulates and interfacial debonding of SiC/Al.     

Evolution of the residual stress state in a duplex stainless steel during loading

The evolution of micro- and macrostresses in a duplex stainless steel during loading has been investigated in situ by X-ray diffraction. A 1.5 mm cold-rolled sheet of alloy SAF 2304 solution treated at 1050°C was studied. Owing to differences in the coefficient of thermal expansion between the two phases, compressive residual microstresses were found in the ferritic phase and balancing tensile microstresses in the austenitic phase. The initial microstresses were almost two times higher in the transverse direction compared to the rolling direction. During loading the microstresses increase in the macroscopic elastic regime but start to decrease slightly with increasing load in the macroscopic plastic regime. For instance, the microstresses along the rolling direction in the austenite increase from 60 MPa, at zero applied load, to 110 MPa, at an applied load of 530 MPa. At the applied load of 620 MPa a decrease of the microstress to 90 MPa was observed. During unloading from the plastic regime the microstresses increase by approximately 35 MPa in the direction of applied load but remain constant in the other directions. The initial stress state influences the stress evolution and even after 2.5% plastic strain the main contribution to the microstresses originates from the initial thermal stresses. Finite element simulations show stress variations within one phase and a strong influence of both the elastic and plastic anisotropy of the individual phases on the simulated stress state.     

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.     

Transient liquid phase diffusion bonding of 6061-15 wt% SiCp in argon environment

Extruded 6061-15 wt% SiCp composite was joined by transient liquid phase diffusion (TLPD) bonding process in argon environment using 50-μm thick copper foil interlayer. The bonding was carried out at 560 °C with two different applied pressures (0.1 and 0.2 MPa) and five different holding times (20 min, 1, 2, 3 and 6 h). Kinetics of the bonding process was significantly accelerated in the presence of reinforcement (SiC). This acceleration is attributed to the increased solute diffusivity through defect-rich SiC particle/matrix interface and porosity. Adequate bond strength (90% of the original composite strength) was achieved for bonding at 0.2 MPa pressure with 6 h of holding. This is very close to the reported highest bond strength achieved (92% of the original composite strength) for joining aluminium-based metal matrix composite by TLPD process in vacuum followed by isostatic pressing. The rejection of oxide at periphery on completion of isothermal solidification, and elimination of void at bond interface through solid state diffusion at higher pressure (0.2 MPa) were the main reasons of achieving high bond strength.     

Effect of thermal cycling on the expansion behavior of Al/SiCp composite

The coefficient of thermal expansion (CTE) and accumulated plastic strain of the pure aluminum matrix composite containing 50% SiC particles (Al/SiC_p) during thermal cycling (within temperature range 298–573 K) were investigated. The composite was produced by infiltrating liquid aluminum into a preform made by SiC particles with an average diameter of 14 μm. Experiment results showed that the relationship between the CTE of Al/SiC_p and temperature is nonlinear; CTE could reach a maximum value at about 530 K. The theoretical accumulated plastic strain of Al/SiC_p composites during thermal cycling has also been calculated and compared with the experimental results.     

Effect of Al addition on SiC–B4C cermet prepared by pressureless sintering and spark plasma sintering methods

SiC–B₄C–Al cermets containing 5, 10 and 20 wt.% of Al were fabricated by high-energy planetary milling followed by conventional sintering and spark plasma sintering (SPS) techniques separately. The average particle size reduced to ~ 3 μm from an initial size of 45 μm after 10 h of milling. The as-milled powders were conventionally sintered at 1950 °C for 30 min under argon atmosphere and SPS was carried out at 1300 °C for 5 min under 50 MPa applied pressure. The formation of Al₈B₄C₇ and AlB₁₂ phases during conventional sintering and SPS were confirmed by X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses. The formation of Al₈B₄C₇ at 700 °C and AlB₁₂ at 1000 °C was well supported by XRD and differential scanning calorimetry (DSC). The maximum relative density, microhardness and indentation fracture resistance of SiC–B₄C–10Al consolidated by SPS are 97%, 23.80 GPa and 3.28 MPa·m^1/2, respectively.     

In situ synthesis,microstructure, mechanical properties and thermal shock resistance of (ZrB2 + SiC)/Zr2[Al(Si)]4C5 composites

Dense (ZrB₂ + SiC)/Zr₂[Al(Si)]₄C₅ composites with adjustable content of (ZrB₂ + SiC) reinforcements (0–30 vol.%) were prepared by in situ hot-pressing. The microstructure, room and high temperature mechanical and thermal physical properties, as well as thermal shock resistance of the composites were investigated and compared with monolithic Zr₂[Al(Si)]₄C₅ ceramic. ZrB₂ and SiC incorporated by in situ reaction significantly improve the mechanical properties of Z₂[Al(Si)]₄C₅ by the synergistic action of many mechanisms including particulate reinforcement, crack deflection, branching, bridging, “self-reinforced” microstructure and grain-refinement. With (ZrB₂ + SiC) content increasing, the flexural strength, toughness and Vickers hardness show a nearly linear increase from 353 to 621 MPa, 3.88 to 7.85 MPa·m^1/2, and 11.7 to 16.7 GPa, respectively. Especially, the 30 vol.% (ZrB₂ + SiC)/Zr₂[Al(Si)]₄C₅ composite retains a high modulus up to 1511 °C (357 GPa, 86% of that at 25 °C) and superior strength (404 MPa) at 1300 °C in air. The composite shows higher thermal conductivity (25–1200 °C) and excellent thermal shock resistance at ΔT up to 550 °C. Superior properties render the composites a promising prospect as ultra-high-temperature ceramics.     

Adhesion properties of tungsten and SiC ceramic-bonded carbon

A newly developed carbon-based composite, SiC ceramic bonded carbon (30 vol% SiC) was directly clad with tungsten (W) at 1700 °C by spark plasma sintering. There were no voids or cracks in the fabricated material. The adhesion strength between W and SiC/CBC is 90 MPa and 33 MPa by the bending and tensile test, respectively. The interface between W and SiC/CBC was analyzed and the cladding mechanism was discussed.     

Fabrication of Al6061 composite with high SiC particle loading by semi-solid powder processing

Aluminum alloys reinforced with silicon carbide (SiC) particles have been studied extensively for their favorable properties in structural and thermal applications. However, there has been only limited research into investigating the loading limit of a reinforcement phase of a metal matrix composite. In this paper, semi-solid powder processing (SPP), a fabrication method that exploits the unique behavior of a solid–liquid mixture, was used to synthesize SiC particle-reinforced Al6061. A high volume loading (>45 vol.%) of SiC in Al6061 matrix was investigated by varying the SiC loading volume fraction, forming pressure, SiC particle size and Al6061 particle size. The compaction and synthesis mechanism of the composite by SPP was discussed based on reinforcement phase compaction behavior and processing parameters. Microstructure, hardness, fracture surface and X-ray diffraction results were also analyzed. Results showed that SPP can achieve over 50 vol.% loading of SiC in Al6061 matrix with near theoretical density.     

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.     

Measurement of scratch-induced residual stress within SiC grains in ZrB2–SiC composite using micro-Raman spectroscopy

An analytical framework for determination of scratch-induced residual stress within SiC grains of ZrB₂–SiC composite is developed. Using a “secular equation” that relates strain to Raman-peak shift for zinc-blende structures and the concept of sliding blister field model for scratch-induced residual stress, explicit expressions are derived for residual stress calculation in terms of phonon deformation potentials and Raman peak shift. It is determined that, in the as-processed composite, thermal expansion coefficient mismatch between ZrB₂ and SiC induces compressive residual stress of 1.731 GPa within the SiC grains and a tensile tangential stress of 1.126 GPa at the ZrB₂–SiC interfaces. With increasing scratch loads, the residual stress within the SiC grains becomes tensile and increases in magnitude with scratch load. At a scratch load of 250 mN, the calculated residual stress in SiC was 2.6 GPa. Despite this high value, no fracture was observed in SiC grains, which has been rationalized based on fracture strength calculations from Griffith theory.     

Microstructure and mechanical properties of TiB2–SiC ceramic composites by Reactive Hot Pressing

TiB₂–SiC ceramic composites with different contents of Ni as additive were prepared by the Reactive Hot Pressing (RHP) process at 1700 °C under a pressure of 32 MPa for 30 min. For comparison, a monolithic TiB₂ ceramic and TiB₂–SiC ceramic composite were also fabricated under the identical temperature, pressure and holding time by the Hot Pressing (HP) process. The effects of the fabrication process and Ni on the microstructure and mechanical properties of the composites were investigated. About 8 vol.% of elongated TiB₂ grains with an aspect ratio of 3–6 and a diameter of 0.5–1 μm were produced in the composite prepared by the RHP process. The improvement of the fracture toughness was attributed to the toughening and strengthening effects of SiC particles and the elongated TiB₂ grains such as crack deflection. The TiB₂–SiC–5 wt.% Ni ceramic composite had the optimum mechanical properties with a flexural strength of 858 ± 87 MPa, fracture toughness of 8.6 ± 0.54 MPa·m^1/2 and hardness of 20.2 ± 0.94GPa. The good mechanical properties were ascribed to the relatively fine and homogeneous microstructure and the strengthening effect of Ni. Ni inhibited the anisotropic growth of TiB₂.     

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.     

Interfacial phenomena in thermally sprayed multiwalled carbon nanotube reinforced aluminum nanocomposite

The interfacial phenomena in thermally sprayed (plasma and high-velocity oxyfuel spraying) hypereutectic Al–Si composite with multiwalled carbon nanotube (MWCNT) reinforcement have been analyzed both theoretically and experimentally. The formation of an ultrathin β-SiC reaction layer at the interface is confirmed. Plasma sprayed composite exhibits a thicker SiC layer (∼5 nm) than the high-velocity oxyfuel sprayed composite (∼2 nm). The presence of SiC layer formation is also corroborated in a chemical vapor deposition experiment where Si was deposited on MWCNTs. The formation of β-SiC is responsible for the improved wettability of the molten Al–Si alloy matrix with MWCNT reinforcement.     

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′.     

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.     

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.     

Mechanical and tribological properties of electrically conductive SiC based cermets

Three different types of SiC based cermets with various content (30, 40, 50 wt.%) of electrically conductive TiNbC phase have been fabricated by hot-pressing without sintering additives. The effect of TiNbC content on the basic mechanical, electrical and tribological properties of SiC-TiNbC cermets was investigated. Tribological properties have been characterized by the ball-on-disc method at the ambient temperature and dry wear conditions with air humidity 35–40% at the load of 5–30 N, sliding distance of 500 m, with the static partner made from SiC. Corresponding wear rate was calculated and wear mechanisms were identified. Resulting materials were relatively hard, with increasing amount of TiNbC the hardness increased from 19.8 ± 1 GPa for 30 wt.% of TiNbC up to 25.4 ± 0.9 GPa at 50 wt.% of TiNbC. The fracture toughness values were independent on TiNbC phase and varied between 2.7 and 2.9 MPa.m^1/2. Similarly, Young's modulus increased from 354 GPa to 435 GPa. It was found that electrical conductivity of SiC cermets was rapidly improved with increased fraction of metallic phases and was three orders of magnitude higher at 30 wt.% TiNbC addition and around four order of magnitude higher at 50 wt.% of TiNbC metallic phase comparing to conventional semiconductive SiC ceramics with electrical conductivity ~ 10 Sm^− 1. Coefficient of friction (between 0.3 and 0.5) and wear resistance (10^− 6–10^− 7 mm³/Nm) were comparable with the wear resistant SiC materials.