Stress relaxation during thermal cycling of particle reinforced aluminium matrix composites

Two SiC-particle reinforced composites were produced by powder metallurgy using a 2124 Al-alloy matrix and two powder blending techniques: ball milling and wet blending. The effect of the blending route on the stress relaxation during thermal cycling is studied by in situ neutron diffraction based on the determination of the average stresses in the matrix and the particles. The thermal stresses in both composites partially relax by creep at T ? 90 °C. The higher creep resistance of the composite produced by ball milling reduces relaxation in comparison with the wet blended composite. This results in average axial compressive thermal stresses of ～?50 MPa and ～?10 MPa after heating to 230–300 °C in the matrices of the ball milling and wet blending composites, respectively, which relax at rates ?5 × 10^?9–3 × 10^?8 s^?1.     

Creep behaviour of AZ61 magnesium alloy at low stresses and intermediate temperatures

Abstract

Tensile creep response was investigated for AZ61 alloy (Mg - 6.4Al - 0.9Zn - 0.2Mn, wt-%) of mean linear intercept grain size ~ 25 μm at stresses in the range 0.9 - 4 MPa over the temperature range 250 - 346°C. Bingham behaviour is obtained with strain rate ? under stress σ given by ?∝σ - σ_o with a threshold stress σ_o decreasing from 1.25 MPa at 210°C to ~ 0.5 MPa at 346°C, which is similar to earlier work on pure magnesium. The corresponding Arrhenius plot of log (Td?/d σ) versus T^-1 indicates an activation energy comparable with that expected for the grain boundary self-diffusion coefficient D _B, and values of D _Bδ (where δ is the effective grain boundary thickness) derived from the Coble equation are also similar to those for pure magnesium. Grain elongation in the direction of the tensile stress is also consistent with the key indicative feature of diffusional creep: deposition of material at grain boundaries nearly transverse to the axis of tensile stressing. Strain rates versus stress are shown to be continuous with published results for superplastic flow of AZ61 at comparable temperatures but higher stresses.     

Influence of Ingot and Powder Metallurgy Production Route on the Tensile Creep Behavior of Mo–9Si–8B Alloys with Additions of Al and Ge

Refractory metals and their alloys show potential for high temperature applications, due to the elevated melting points often paired with very good creep resistance. Spark plasma sintering (SPS) as well as arc‐melting is used here to prepare quaternary and quinternary Mo–9Si–8B–xAl–yGe (x is 0 or 2; y is 0 or 2, all numbers in at%) samples. All samples consist of a Mo solid solution (Mo_ss) and two intermetallic phases: Mo₃Si (A15) and Mo₅SiB₂ (T2). Aluminum and germanium reduce the melting point and slightly decrease the density of the material. The specimens are homogenized and coarsened by a subsequent heat‐treatment in vacuum at 1850 °C for 24 h. The resulting microstructure is investigated using scanning electron microscope (SEM), energy‐dispersive X‐ray spectroscopy (EDS), X‐ray diffraction (XRD), X‐ray fluorescence spectroscopy (XRF), and inductively coupled plasma optical emission spectrometry (ICP‐OES) analysis. A vacuum creep testing device for small tensile creep specimens is presented. It is heated by graphite radiation heaters usable up to 1500 °C in vacuum of 2 · 10^‐4 Pa with an oil diffusion pump. Tensile creep tests are performed at 1250 °C and stresses from 50 MPa up to 250 MPa. Specimens produced by ingot metallurgy feature superior creep properties compared to powder metallurgy samples.     

Microstructure of MgAl5Ca3Sr alloy after creep deformation and high‐temperature heat treatment

The paper presents results of microstructural investigations of MgAl5Ca3Sr magnesium alloys in the as‐cast condition, after creep tests at 180 °C, and after heat treatment at 450 °C for 4.5 hours. The microstructure of MgAl5Ca3Sr alloy is composed of α‐Mg solid solution, irregular shaped (Mg,Al)₂Ca phase with C36 crystal structure, bulky (Mg,Al)₁₇(Sr,Ca)₂ phase, fine lamellar Mg₂Ca phase with C14 structure, needle‐shaped Al₂Ca precipitates with the C15 crystal structure. The precipitation of the needle‐shaped Al₂Ca phase in the α‐Mg grains and spheroidization of the C14 phase were found after heat treatment at 450 °C in argon atmosphere. The (Mg,Al)₂Ca (C36) and (Mg,Al)₁₇(Sr,Ca)₂ phases seems to be stable at 450 °C, however, the increasing of aluminum content in C36 compound was observed suggesting the initial stage of C36 → C15 transformation. After creep deformation at 180 °C precipitates of the Al₂Ca phase were found in α‐Mg phase. The intermetallic compounds are stable at 180 °C. The MgAl5Ca3Sr alloy exhibits good creep resistance up to 75 MPa. Tensile properties are comparable to those of Mg‐RE‐Zn–Zr alloys.     

Comparison of superplastic forming abilities of as‐cast AZ91 magnesium alloy prepared by twin roll casting and WE43 magnesium alloy

This paper describes and compares the superplastic behaviour and microstructural evolution of twin roll cast AZ91 and WE43 rolled sheet alloys. Tests were carried out in uniaxial tension on both alloys across a range of temperatures (300 °C–525 °C) and strain rates (1?10^‐4 s^‐1–1?10^‐1 s^‐1). In the case of WE43 gas bulge testing was employed at 400 °C and 0.6 MPa to offer a better analogy to superplastic forming than uniaxial tensile testing. Elongations of over 400 % were observed within WE43 when tested at 450 °C and 1?10^‐3 s^‐1 strain rate, and over 200 % within AZ91 when tested at 350 °C and 1?10^‐3 s^‐1 strain rate. A peak cone height of 41 mm was achieved with WE43 at a temperature of 400 °C and pressure of 0.6 MPa. Electron back scattered detection technique was employed to analyse the microstructural evolution of the two alloys during the forming process. Both WE43 and AZ91 were observed to undergo dynamic recrystallization during elevated temperature tensile testing and failed at low strain rates mainly by means of coalescence of cavitation, in the case of AZ91 at high strain rates cracking of Al₁₂Mg₁₇ intermetallic particles was the dominating failure mechanism. Both alloys were seen to achieve good levels of superplastic ductility over 200 % elongation, which would be industrially useful in niche vehicle and aerospace manufacturing.     

Creep of silicon nitride-titanium nitride composites

The effect of particulate TiN additions (0–50 wt%) on creep behaviour of hot-pressed (5 wt%Y₂O₃ + 2 wt%Al₂O₃)-doped silicon nitride (HPSN)-based ceramics was studied. Creep was measured using a four-point bending fixture in air at 1100–1340 °C. At 1100 °C, very low creep rates of HPSN with 0–30 wt% TiN are observed at nominal stresses up to 160 MPa. At 1200 °C the creep rate is slightly higher, and at 1300 °C the creep rate is increased by three orders of magnitude compared to 1100 °C and rupture occurs after a few hours under creep conditions. It was established that the formation of a TiN skeleton could detrimentally affect the creep behaviour of HPSN. An increase in TiN content leads to higher creep rates and to shorter rupture times of the samples. Activation energies of 500–1000 kJ mol^?1 in the temperature range of 1100–1340 °C at 100 MPa, and stress exponentsn?4 in the stress range 100–160 MPa at 1130–1200 °C were calculated. Possible creep mechanisms and the effect of oxidation on creep are discussed.     

Nanostructured and Nonsymmetrical NiO–SDC/SDC Composite Anode Performance via a Microwave-Assisted Route for Intermediate-Temperature Solid Oxide Fuel Cells

This work investigates the electrical properties of NiO–SDC/SDC anode sintered at approximately 1200°C for 1 h via the microwave method. Nanopowders Sm_0.2Ce_0.8O_1.9 (SDC—samaria-doped ceria) and NiO were mixed using a high-energy ball mill and subsequently co-pressed at three different compaction pressures of 200, 300, and 400 MPa. This study determines the effect of compaction pressure on the electrochemical performance of Ni–SDC/SDC anode, with no binder used between layers. The electrical behavior of the prepared anode was studied via electrochemical impedance spectroscopy in controlled atmospheres, operating at high temperatures (600–800°C). The results indicate that decreasing the compaction pressure and increasing the operating temperature lead to a high electrochemical performance of the nonsymmetrical NiO–SDC/SDC anode. The mechanism for manufacturing NiO–SDC/SDC involves ball milling, dry pressing, and microwave furnace sintering processes.     

Synthesis of LiMn2O4 via high-temperature ball milling process

Spinel LiMn₂O₄ powder was prepared by a novel process of high-temperature ball milling. For comparison, the spinel LiMn₂O₄ powder was also synthesized by the traditional method of solid state reaction. It was found that high-temperature ball milling significantly decreased the synthesis temperature and time. LiMn₂O₄ with pure spinel phase could be successfully synthesized only by 2?h high-temperature ball milling at 500°C and 600°C. However, pure spinel LiMn₂O₄ could not be completely synthesized by 2?h solid state reaction at 800°C. The LiMn₂O₄ particles prepared by high-temperature ball milling are nano-sized (<100?nm) and much smaller than that prepared using solid state reaction. The electrochemical tests results indicated that the as-synthesized LiMn₂O₄ by 2?h high-temperature ball milling at 600°C showed a favorable initial discharge capacity of 124.2 mAh g^?1 at current rate of 0.1 C and still retained a capacity of 119.8 mAh g^?1 at 0.1 C after 80 continuous cycles from 0.1 to 2.0 C.     

Effect of transition metal on the hydrogen storage properties of Mg–Al alloy

Mg–Al alloys were prepared via sintering combined with ball milling, and the effect of a transition metal (TM = Ti, V, Ni) on the hydrogen storage properties of these alloys was investigated; the alloys were characterized via X-ray diffraction, pressure composition isotherms, and differential scanning calorimetry. The results showed that the alloys were mainly composed of Mg and the Mg₁₇Al₁₂ phase, and the cell volume of these phases decreased after the addition of TM (TM = Ti, V, Ni), which is attributed to the improved hydrogenation kinetics of Mg–Al alloy. Moreover, the hydrogenation/dehydrogenation temperature of the Mg–Al alloy decreased with the addition of TM (TM = Ti, V, Ni). Ti, Ni, and V acted as a catalyst, thereby lowering the reaction barrier for dehydrogenation and promoting the reversible hydrogenation reaction of the Mg–Al alloy. The onset temperature of dehydrogenation of the Mg–Al–V alloy was ~244 °C, which was 66 °C lower than that of the Mg–Al alloy (~310 °C). And the apparent activation energy of the Mg–Al–V alloy was 80.1 kJ mol^?1, where it was 34.6 kJ mol^?1 lower than that of Mg–Al alloy.     

Effects of CCEP and Sc on superplasticity of Al–5.6Mg–0.7Mn alloys

Trace amount (0.3?wt%) of scandium is added to Al–5.6Mg–0.7Mn alloy to form uniformly distributed Al₃Sc precipitates for producing a fine-grained and stable microstructure at high temperature through cross-channel extrusion process. Superplasticity and hot workability of the Sc-containing Al–5.6Mg–0.7Mn alloy, after extrusion, are also examined. The result indicates that Al–5.6Mg–0.7Mn alloys with and without 0.3?wt% Sc after extrusion of six passes at 300°C, fine-grained structures were observed with grain sizes of 1–2?µm and improvement of mechanical properties. Furthermore, Al₃Sc phase can effectively retard recrystallization to increase the thermal stability and remain equiaxed. The elongation of Al–5.6Mg–0.7Mn alloy with Sc addition to failure is extended to 873% maximum at high temperature of 450°C at strain rate of 1?×?10^?1?s^?1after six passes in the CCEP.     

Effects of Steam Environment on Creep Behavior of Nextel™610/Monazite/Alumina Composite at 1,100°C

The tensile creep behavior of a N610™/LaPO₄/Al₂O₃ composite was investigated at 1,100°C in laboratory air and in steam. The composite consists of a porous alumina matrix reinforced with Nextel 610 fibers woven in an eight-harness satin weave fabric and coated with monazite. The tensile stress-strain behavior was investigated and the tensile properties measured at 1,100°C. The addition of monazite coating resulted in ~33% improvement in ultimate tensile strength (UTS) at 1,100°C. Tensile creep behavior was examined for creep stresses in the 32–72 MPa range. Primary and secondary creep regimes were observed in all tests. Minimum creep rate was reached in all tests. In air, creep strains remained below 0.8% and creep strain rates approached 2 × 10⁻⁸ s⁻¹. Creep run-out defined as 100 h at creep stress was achieved in all tests conducted in air. The presence of steam accelerated creep rates and significantly reduced creep lifetimes. In steam, creep strain reached 2.25%, and creep strain rate approached 2.6 × 10⁻⁶ s⁻¹. In steam, creep run-out was not achieved. The retained strength and modulus of all specimens that achieved run-out were characterized. Comparison with results obtained for N610™/Al₂O₃ (control) specimens revealed that the use of the monazite coating resulted in considerable improvement in creep resistance at 1,100°C both in air and in steam. Composite microstructure, as well as damage and failure mechanisms were investigated.     

Hot-pressed ceramic SiC–YAG materials

Dense SiC-based ceramic materials containing yttrium aluminum garnet (YAG) as an oxide sintering aid have been prepared by hot pressing in the temperature range 1750–1850°C. As a result of melting, the oxides fill spaces between the SiC particles, contributing to the densification of the material and mass transport during the hot pressing process. The present results demonstrate that relatively small amounts of the oxides (≤5 wt %) are needed to ensure a high degree of densification of the SiC–YAG materials. The best physicomechanical properties are offered by the SiC + 3 wt % YAG material sintered at a temperature of 1850°C: ρ = 3.24 ± 0.01 g/cm³, П = 1.1 ± 0.1%, σ_b = 640 ± 10 MPa; K_Ic = 6.4 ± 0.2 MPa m^1/2, Еel = 410 ± 20 GPa, and HV = 26.0 ± 0.2 GPa. This material experiences predominantly intercrystalline fracture.     

Preparation and characterization of dense sodium superionic conducting Na5YSi4O12 (NYS)

Na₅YSi₄O₁₂ has been prepared from spray-frozen/freeze-dried precursor powders calcined at 620°C for three hours. Sintering at 1140°C for 30 minutes gave a ceramic of 97.7% theoretical density. The material was single phase NYS with a 300°C resistivity of 6.5 Ωcm, an activation energy for Na⁺ ion conduction of 20.1 kJ/mol, a flexural strength of 75 MPa, a duplex fine grain structure of (0.1–3μm) and a constant linear thermal expansion coefficient (25–470°C) of 13.1 × 10^?6/°C ±5%. The properties of the NYS ceramics are especially sensitive to small soda deficiencies.     

Creep and creep-fatigue behaviour of continuous fibre reinforced glass-ceramic matrix Composites

The flexural creep and creep strain recovery behaviour during creep-fatigue tests of a cross-ply SiC fibre reinforced Barium Magnesium Aluminosilicate glass-ceramic matrix composite was investigated at 1100°C in air. Only heat-treated samples (1 h at 1100°C) were tested. Stress levels of 90, 105 and 120 MPa were examined to produce low strains (?0.4”?). A continuously decreasing creep strain rate with values between 1.6 × 10^?6 s^?1 to 4.7 × 10^?8s^?i at 120 MPa was observed with no steady-state regime. Extensive viscous strain recovery was found upon the unloading period during the short-duration cyclic creep (creep-fatigue) experiments. The creep strain recovery was quantified using strain recovery ratios. These ratios showed a slight dependence on the stress and cyclic loading frequencies investigated. The crept composites retained their ?graceful”? fracture behaviour after testing indicating that no (or limited) damage in the matrix was induced during creep and creep-fatigue loading.     

Optimisation of high energy ball milling parameters to synthesize oxide dispersion strengthened Alloy 617 powder and its characterization

In the present work, ultra-fine powder of oxide dispersion strengthened Alloy 617 was synthesized by high energy ball milling. Milling parameters such as rpm and milling time were varied in the range of 500–2000 and 5–360 min, respectively. Energy applied to the powder in the milling process (Energy per unit mass per hit, E_c) was estimated using the collision model. Effect of milling parameters on the microstructure of powder and refinement of oxides was investigated using X-ray Diffraction (XRD), Scanning electron Microscopy (SEM), conventional Transmission Electron Microscopy (TEM) and High resolution Transmission Electron Microscopy (HRTEM). Desired convoluted lamellar structure with average particle size ∼33 μm was observed during milling at 1000 rpm (E_c ∼ 0.4 kJ/g.hit) for 6 h. TEM analysis of the powder showed the presence of fine oxide dispersoids in the size range 4–16 nm. HRTEM analysis substantiated the presence of fine dispersoids of size ∼4 nm and showed the presence of deformation twins in the matrix. The fine dispersoids in a nanocrystalline matrix is expected to provide superior creep strength to the material at high temperatures.     

Microstructure and mechanical properties of spark plasma sintered austenitic ODS steel

In this work, austenitic oxide dispersion strengthened (AODS) steel of composition Fe–16Cr–16Ni–1.5 W–0.21Ti–0.3Y₂O₃ (wt. %) was fabricated using two–stage ball milling followed by consolidation through spark plasma sintering (SPS). In the first–stage, mechanical alloying (MA) of ferritic powder and nano sized Y₂O₃ was carried out. This was followed by the addition of Ni in second–stage milling. SPS of the milled powder was carried out at 900, 950, 1000 and 1050 °C to explore the role of SPS temperature on density, microstructure as well as mechanical properties of the consolidated samples. A relative density of ～ 99% was obtained for samples sintered at 950 and 1000 °C. The as–sintered samples were subsequently solution annealed at 1075 °C for 2 h and water quenched. X–ray diffraction studies confirmed the presence of austenite in the consolidated and solution annealed samples. Electron back scatter diffraction analysis of solution annealed samples sintered at all the temperatures revealed a bimodal microstructure. The average grain size of 1.07 ± 0.72 µm was obtained for solution annealed samples sintered at 1000 °C. Yield strength and elongation of the same was measured as 851 MPa and 18%, respectively at room temperature. These values are the best combination of strength to elongation achieved on AODS alloys processed using MA and SPS, which makes this AODS steel much promising for high temperature applications.     

Creep of mild steel at low temperatures

Creep data for mild steel at low temperatures (360–400°C) have been analysed and it has been found that the creep behaviour is similar to that at higher temperatures. The variation of secondary creep rate with stress and temperature, in the temperature range (360–538°C) may be described by two equations with a transition stress. This transition stress is 410 MPa at 360°C. For low stresses the creep equation is given by $\text{ε}{}_{\mathrm{<mtext>s</mtext>}}\text{= 250 σ}{}^{3.7}\text{exp}\text{{?(2.17 × 10}{}^5\text{/}\text{RT}\text{)}}$. Material response to stress is highly dependent on the amount of cold work; secondary creep rate decreases with increasing prior deformation.     

Hydrogen sorption properties of 90 wt% MgH2–10 wt% MeSi2 (Me = Ti, Cr)

The hydriding/dehydriding characteristics of 90 wt% MgH₂–10 wt% MeSi₂ (Me = Ti, Cr) composites, synthesized by ball milling under argon, were studied by Sievert’s type apparatus and by high-pressure differential scanning calorimetry (HPDSC). The composites have demonstrated similar absorption kinetics and capacity at temperature of 300 °C and a pressure of 1 MPa. They showed fast kinetics and achieved absorption capacity higher than 6 wt%. Slightly higher values for the enthalpy of hydriding and dehydriding were obtained at scanning conditions in the HPDSC for the composite 90 wt% MgH₂–10 wt% CrSi₂ compared to those for the composite with addition of TiSi₂.     

Compression stress–strain and creep properties of the 52In–48Sn and 97In–3Ag low-temperature Pb-free solders

Lead (Pb)-free, low melting temperature solders are required for step-soldering processes used to assemble micro-electrical mechanical system (MEMS) and optoelectronic (OE) devices. Stress–strain and creep studies, which provide solder mechanical properties for unified creep-plasticity (UCP) predictive models, were performed on the Pb-free 97In–3Ag (wt.%) and 58In–42Sn solders and counterpart Pb-bearing 80In–15Pb–5Ag and 70In–15Sn–9.6Pb–5.4Cd alloys. Stress–strain tests were performed at 4.4 × 10^?5 s^?1 and 8.8 × 10^?4 s^?1. Stress–strain and creep tests were performed at ?25, 25, 75, and 100°C or 125°C. The samples were evaluated in the as-fabricated and post-annealed conditions. The In–Ag solder had yield stress values of 0.5–8.5 MPa. The values of ΔH for steady-state creep were 99 ± 14 kJ/mol and 46 ± 11 kJ/mol, indicating that bulk diffusion controlled creep in the as-fabricated samples (former) and fast-diffusion controlled creep in the annealed samples (latter). The In–Sn yield stresses were 1.0–22 MPa and were not dependent on an annealed condition. The steady-state creep ΔH values were 55 ± 11 kJ/mol and 48 ± 13 kJ/mol for the as-fabricated and annealed samples, respectively, indicating the fast-diffusion controlled creep for the two conditions. The UCP constitutive models were derived for the In–Ag solder in the as-fabricated and annealed conditions.     

Low cycle deformation behaviour of Zircaloy-4 at elevated temperatures

The LC deformation behaviour of Zry-4 at 400°C and 600°C was examined by means of tension/compression experiments conducted in load and in strain control respectively. The main results were compared to those obtained at comparable conditions on the stainless steel type AISI 304. For both the materials the influence of the stress ratio R = σ_min/σ_max (where within one test σ_max > 0 was kept constant) upon the lifetime T_f at low and high homologeous temperature T_h was examined. Whereas at the lower T_h for R < 0 the lifetime decreased with decreasing R, the opposite was true at the higher T_h. The explanation of the influence of R upon t_f suggests that at high temperatures the fatigue damage rate Å_f drops below the rate for creep damage Å_c Two cases are considered. If the above damage mechanisms are sequentially independent the resulting damage rate Å ≈? Å_c and hence Å_c is the failure (rate) determining mechanism. In the case that the mechanisms are sequentially dependent then Å ≈? Å_f. TEM investigations conducted on Zry-4 cycled at 600° C have shown that the typical dislocation pattern revealed is a band structure consisting of dense dislocation walls separating denuded zones. The habit and the crystallographic characteristics of the band structure resemble the structure associated with PSBs observed in fee metals. The comparison of the values of the saturation stress τ_s and the wall spacing d for different fee and hep metals shows that there is a proportionality between τ_s and 1/d which is independent of stress and temperature.