Measurement of the residual stress in a Pt–aluminide bond coat

The residual stress induced in a Pt–aluminide bond coat formed on a single-crystal superalloy due to cooling from the manufacturing temperature, has been measured with the “wafer” curvature method. This approach revealed that the bond coat is in residual tension (about 140 MPa), consistent with the thermal expansion misfit.     

Microstructure and mechanical properties of ZrB2–SiC ultrahigh temperature ceramic composite joint using TiZrNiCu filler metal

Abstract

ZrB₂–SiC ceramic composite was brazed by using TiZrNiCu active filler metal. The microstructure and interfacial phenomena of the joints were analysed by means of SEM, energy dispersive X-ray spectroscopy and X-ray diffraction. The joining effect was evaluated by shear strength. The results showed that the reaction products of the ZrB₂–SiC ceramic composite joint were TiC, ZrC, Ti₅Si₃, Zr₂Si, Zr(s,s) and (Ti, Zr)₂ (Ni, Cu), and the microstructure was separately ZrB₂–SiC/Zr(s,s)/Ti₅Si₃+Zr₂Si+TiC+ZrC+(Ti,Zr)₂(Ni,Cu)/Zr(s,s)/ZrB₂–SiC. A conceptual interface evolution model was established to explain the interface evolution mechanism. The maximum shear strength of the brazed joints was 143·5 MPa at the brazing temperature T of 920°C and the holding time t of 10 min.     

Effect of fiber distribution on residual thermal stress in titanium matrix composite

Residual thermal stresses (RTS) of SCS-6 SiC/Ti-24Al-11Nb composite were analyzed by using finite element method (FEM). Three models of fiber array in the composite and the effect of fiber distance on the RTS were discussed. In all the three models compressive stress was found in the radial direction and tensile stress in the tangential direction. It is pointed out that, in real composite system, hexagonal fiber geometry is superior because the distribution and the magnitude of the residual stress are similar to those in single fiber model. In square fiber geometry, it is easier to make the matrix crack due to the larger residual tangential stress. RTS becomes very large and changes violently when the fiber distance is less than 15μm or so, therefore too high fiber volume is apt to result in matrix crack.     

High temperature oxidation of LaB6–ZrB2 eutectic in situ composite

LaB₆–ZrB₂ eutectic in situ composites are interesting candidates for manufacture of turbine blades because of their excellent mechanical properties. The oxidation behavior of LaB₆–ZrB₂ eutectic in situ composites prepared by electric arc melting was studied in a temperature range of 916–1223°C and in a pressure range of 1.5×10⁴–7.2×10⁴ Pa. The specific weight change of the LaB₆–ZrB₂ eutectic obeys the Arrhenius equation below the oxidation temperature of 1094°C and appears to obeya paralinear equation above this temperature. The rate of weight change is described by k_p=9.71×10⁻³ exp(−31 000/RT) mg²/cm⁴ min in the range 916–1094°C. Above 1094°C, rapid oxidation kinetics were observed, La₂O₃·B₂O₃ and La₂O₃·3B₂O₃ being the primary scale constituent with ZrO₂ deficiency in the outer scale. The oxidation increases with P_O2 up to 3.9×10⁴ Pa, above which the trend is reversed due to the active oxidation of ZrB₂. The oxidation appears to be controlled by oxygen diffusion through the scale and the oxidized layers consist mainly of La₂O₃·B₂O₃, La₂O₃·3B₂O₃ and trace of ZrO₂.     

Ablation in different heat fluxes of C/C composites modified by ZrB2–ZrC and ZrB2–ZrC–SiC particles

Carbon/carbon composites modified by ZrB₂–ZrC–SiC particles (C/C–Z–SiC), C/C–Z and C/C were ablated by oxyacetylene torch using two different heat fluxes to investigate the effect of doped ceramic particles. Results indicated that C/C–Z–SiC had the best ablation property in heat flux of 2.38 MW/m² whereas their ablation rates increased fastest when heat flux rising from 2.38 to 4.18 MW/m². C/C composites had the poorest ablation property in the lower heat flux and their ablation rates increased slowest. Thermal mismatch of Z, SiC and C and evaporation of SiO₂ induced the various ablation behavior.     

Effect of heat flux on ablation behaviour and mechanism of C/C–ZrB2–SiC composite under oxyacetylene torch flame

The ablation property of C/C–ZrB₂–SiC composite under oxyacetylene torch flame with different heat flux was investigated. The ablation performance decreased with the increase of heat flux from 2400 to 4200 kW/m² and the ablation mechanism changed from chemical erosion to mechanical denudation. The good ablation resistance under 2400 kW/m² is attributed to the formation of SiO₂–ZrO₂ mixture. When heat flux is 3200 kW/m², the formation of ZrO₂ can partly prevent the diffusion of oxygen and provide ablation protection. But with heat flux increasing, the increase of mechanical denudation results in the reduction of ablation performance.     

Ablation and mechanical behavior of a sandwich-structured composite with an inner layer of Cf/SiC between two outer layers of Cf/SiC–ZrB2–ZrC

A sandwich-structured composite, which contains an inner layer of C_f/SiC between two outer layers of C_f/SiC–ZrB₂–ZrC, was fabricated by a technique that involved rapid chemical vapor infiltration and precursor infiltration and pyrolysis. In both cyclic and single ablation conditions for one thousand seconds by an oxyacetylene flame, the composites exhibited good ablation-resistant properties and low strength reductions after ablation because of the introduction of a great amount of UHTCs into the outer layers and the small thickness of the ablation-affected area, respectively. The ablation rate and strength reduction in cyclic ablation were lower than those in single ablation.     

Reactive spark plasma sintering of TiB2–SiC–TiN novel composite

The effects of adding SiC as a reinforcement and TiN as an additive on TiB₂-based composites fabricated by the spark plasma sintering (SPS) technique were investigated. SPS was implemented at the sintering conditions of 1900 °C temperature, 7 min holding time and 40 MPa pressure. Adding these two secondary phases had noticeable effects on the microstructure of TiB₂-based composites. A relative densities of 99.9% was obtained for TiB₂–SiC–TiN composite. Detection of in-situ formed phases and investigation on them were done using SEM, XRD, EDS and thermodynamic assessment. These evaluations proved the formation of in-situ phases of TiC, BN nano-platelets, TiSi and B₄C in the TiB₂-based composite codoped with SiC and TiN. Formation of these in-situ phases had fascinating effects on the sinterability and ultimate microstructure of titanium diboride.     

Improved mechanical properties of ZrB2-30 vol% SiC using zirconium carbide additive

Influence the different amount of ZrC as well as fabrication parameters on the mechanical properties such as flexural strength and densification behavior of spark plasma sintered ZrB₂-30 vol% SiC composites were investigated. The composites contained 4, 8 and 12 vol% ZrC were consolidated at 1650, 1725 and 1800 °C for 4, 9 and 14 min under 20, 30 and 40 MPa pressures. Relative density and the ratio of open porosities were measured and used to appraise the densification behavior. Three-point bending instrument applied for flexural strength measurement. Microstructural investigations were carried out using scanning electron microscopy. The results showed that the ZrC till to 4 vol%, have positive effect on grain growth and acts as grain growth inhibitor while more addition caused to grain growth happening. Also, ZrC ascent from 4 up to 12 vol%, accompanied by shrinkage temperature rising (from 1263 °C to 1389 °C and 1392 °C). Relative density reduction occurred from 94.4% to 92.2% by increasing ZrC amount from 4 up to 12 vol%. Flexural strength reached to its maximum value, 460 MPa, in the presence of 8 vol% ZrC for composite which was consolidated at temperature of 1800 °C, time of 9 min under 30 MPa pressure.     

High-precision determination of residual stress of polycrystalline coatings using optimised XRD-sinψ technique

The aim of the research is to optimise the XRD-sin²ψ technique in order to perform high precision measurement of surface residual stress. Residual stresses existing in most hard coatings have significant influence on the adhesion, mechanical properties and tribological performance. In the XRD-sin²ψ stress measurement, the residual stress value is determined through a linear regression between two parameters derived from experimentally measured diffraction angle (2θ). Thus, the precision coefficient (R²) of the linear regression reflects the accuracy of the stress measurement, which depends strongly on how precise the 2θ values are measured out of a group of very broad diffraction peaks. In this research, XRD experiments were conducted on a number of samples, including an electron beam evaporated ZrO₂ based thermal barrier coating, several magnetron sputtered transition metal nitride coatings, and shot-peened superalloy components. In each case, the diffraction peak position was determined using different methods, namely, the maximum intensity (I_max) method, the middle point of half maximum (MPHM) intensity method, the gravity centre method, and the parabolic approaching method. The results reveal that the R² values varied between 0.25 and 0.99, depending on both the tested materials and the method of the 2θ value determination. The parabolic approaching method showed the best linear regression with R² = 0.93 ± 0.07, leading to high precision of the determined residual stress value in all cases; both the MPHM (R² = 0.86 ± 0.16) and gravity centre (R² = 0.91 ± 0.11) methods also gave good results in most cases; and the I_max method (R² = 0.71 ± 0.27) exhibited substantial uncertainty depending on the nature of individual XRD scans.     

Relaxation of residual stresses in SiC wafers by annealing

Residual stresses in SiC wafers, which were introduced during production processes including sawing, lapping, mechanical polishing （MP）, and chemical-mechanical polishing （CMP）, were evaluated in terms of changes in radius of curvature and high-resolution X-ray diffractometer （HRXRD） measurements. It was found that annealing was an effective method to reduce stress fields and to improve the wafer flatness. Lapping process generated more residual stresses than other machining processes, and these stresses could be relaxed by thermal treatment. The results showed that annealing was an essential procedure following lapping in the whole production process. The molten KOH etching results accounted for the correlation between the relaxation of stresses and the creation of basal screw dislocations.     

Oxidation and ablation resistance of ZrB2–SiC–Si/B-modified SiC coating for carbon/carbon composites

ZrB₂–SiC–Si/B-modified SiC coating was prepared on the surface of carbon/carbon (C/C) composites by two-step pack cementation. The coating could efficiently provide protection for C/C composites from oxidation and ablation. The improvement of oxidation resistance was attributed to the self-sealing property of the multilayer coating. A dense glassy oxide layer could afford the high temperature up to 2573 K and efficiently protect C/C composites from ablation.     

Evaluation of residual stress relief of aluminum alloy 7050 by using crack compliance method

1　INTRODUCTION70 5 0alloyisoneofhighstrengthaluminumal loysusedmainlyas platesandforgingsintheaerospaceindustry .Itshighstrengthisachievedthrough quenchingandageing .Duringtherapidquenchingoperation ,highresidualstressesdevelopduetoseverethermalgradient .Theseinternalstress escanleadtodistortionduringsubsequentmachiningoperations ,andincreasethepossibilityofstresscor rosioncracks(SCC) .Residualstressmaybereducedwiththedecreaseofthecoolingratebyapplyingboil ingwaterororganicquenchants .H…     

Role of hot-pressing temperature on densification and microstructure of ZrB2–SiC ultrahigh temperature ceramics

This examination is aimed to study the various densification mechanisms in the ZrB₂-SiC system at different sintering temperatures. For such an objective, hot-pressing process was implemented to fabricate three ceramics at sintering temperatures of 1650, 1850, and 2050 °C under 10 MPa for 1 h. According to the results, particle rearrangement and fragmentation were the predominant densification mechanisms at the lowest sintering temperature. Additionally, it was found that the activation of the liquid phase sintering mechanism was advantageous in the particle rearrangement at low temperatures. However, rising the sintering temperature to 1850 °C changed the dominant mechanism to the plastic deformation. Such a phenomenon was accompanied by the creation of many dislocations in both ZrB₂ and SiC grains. Implementing a higher sintering temperature (2050 °C) activated another consolidation mechanism called diffusion. This occurrence, together with the evaporation of the majority of the liquid phase at elevated temperatures, resulted in emerging transgranular fracture mode in the sample.     

Fabrication and behaviour of Al–Si/SiC composite in cavitation conditions

Aluminium matrix composite (AMC) specimens were prepared using the compocasting technique. The reinforcements used were silicon carbide (SiC) particles with an average size of 30 μm. The influence of reinforced ratio of 10 wt-%SiC on cavitation behaviour was examined. The cavitation resistance of an AMC with SiC (AlSi/SiC) was evaluated using an ultrasonically induced cavitation test method. The mass loss of specimens was measured by an analytical method. The morphology of the damaged surface of tested composite was examined using scanning electron microscopy (SEM). It is shown that the cavitation rate of an AMC with SiC is almost the same as the CA6NM stainless steel, which is largely used in the production of hydraulic machinery components. Because the results show that the composites exhibited very good resistance to the cavitation erosion, this material can be successfully used under conditions where the cavitation resistance is needed.     

Densification and flexural strength of ZrB2–30 vol% SiC with different amount of HfB2

Microstructure, flexural strength and densification behavior of spark plasma sintered ZrB2–30 vol% SiC composites doped with HfB₂ were investigated. The composites contained 4, 8 and 12 vol% HfB₂ were spark plasma sintered in 1650, 1725 and 1800 °C for 4, 9 and 14 min. Relative density and the ratio of open porosities were measured and used to investigate the densification behavior. Microstructural investigations as well as the flexural strength measurements were carried out using scanning electron microscopy and three-point bending instrument, respectively.Results indicated that a part of HfB₂ may be solved in the matrix and form the solid solution of (Zr, Hf)B₂, based on the sintering temperature, time and the applied pressure. Additionally, no meaningful correlation between the amount of HfB₂ and the final grain size of the composites was found. The best outcome of the flexural strength was obtained as 470 MPa in sample containing 8 vol% HfB₂, accompanied with increasing the shrinkage start temperature from 1328 to 1255 °C.     

Thermal relaxation of residual stress in laser shock peened Ti–6Al–4V alloy

Laser shock peening (LSP) induced residual stresses in Ti–6Al–4V, and their thermal relaxation due to short-term exposure at elevated temperatures are investigated by an integrated modeling/simulation and experimental approach. A rate and temperature-dependent plasticity model in the form of Johnson–Cook (JC) has been employed to represent the nonlinear constitutive behavior under both LSP and thermal loads. By comparing the simulation results with experimental data, model parameters for Ti–6Al–4V are first calibrated and subsequently applied in analyzing the thermal stability of the residual stress in LSP-treated Ti–6Al–4V. The analysis shows that the magnitude of stress relaxation increases with the increase of applied temperature due to material softening. Most of stress relaxation occurs before 10 min to 20 min exposure in this study, and stress distribution becomes more uniform after thermal exposure. An analytical model based on the Zener–Wert–Avrami formula is then developed based on the simulation results. The activation enthalpy of the relaxation process for laser shock peened Ti–6Al–4V is determined to be in the range of 0.71 eV to 1.37 eV.