Effect of stress peening on surface layer characteristics of (TiB + TiC)/Ti–6Al–4V composite

Residual stresses and microstructure on surface layer of (TiB + TiC)/Ti–6Al–4V are investigated after stress peening. The values of domain sizes and microstrain of surface deformation layers are calculated from the integral breadth of diffractive peaks via Voigt method. The results show that the compressive residual stresses and microhardness are improved significantly after stress peening, and the variations of residual stresses are affected by both the prestresses and the directions of measurement. Microstructure investigations reveal that, the deformation amount increase after stress peening, and smaller domain grain sizes and higher density dislocations are introduced. The changes of microstructures are mainly influenced by the values of prestresses. According to these investigations, it is can be found that the stress peening is superior to the conventional shot peening treatments and it is an effective method to improve the surface properties of (TiB + TiC)/Ti–6Al–4V composite.     

Measurement of interfacial residual stress in SiC fiber reinforced Ni-Cr-Al alloy composites by Raman spectroscopy

Raman spectroscopy was used to measure Raman spectra of the inner SiC fibers and surface C-rich layers of SiC fibers, composite precursors and SiCf/Ni-Cr-Al composites. The residual stresses of the inner SiC fibers and surface C-rich layers were calculated, and the effect of the(Al + Al_2O_3) diffusion barrier layer on the interfacial residual stress in the composites was analyzed in combination with the interface microstructure and energy disperse spectroscopy(EDS) elements lining maps. The results show that the existence of(Al + Al_2O_3) diffusion barrier improves the compatibility of the SiCf/Ni-Cr-Al interface,inhibits the adverse interfacial reaction, and relieves the residual stress inside SiC fibers and at the interface of composite material. Heat treatment can reduce the residual stress at the interface. As the heat treatment time increases, the residual stress at the interface decreases.     

等离子喷涂ZrC基涂层逐道逐层沉积残余应力模拟与实验验证

采用商用ANSYS14.5软件, 依据复合梁增层力学模型, 采用逐道逐层累积模型模拟了C/C复合材料表面等离子喷涂ZrC基涂层沉积残余应力的特征, 分析了SiC过渡层、第二相(SiC, MoSi₂)和涂层厚度对ZrC基涂层残余应力的影响, 并进行了实验验证。结果表明, SiC过渡层有效缓解了涂层与基体的热失配应力。涂层体系的应力随着涂层厚度的增加逐渐减小, 符合应力松弛和叠加规律。在涂层内部的径向应力以拉应力为主, 基体中主要为压应力, 且在界面边缘存在压应力集中的极限区域, 易使涂层产生裂纹并沿界面扩展。该模拟采用逐道逐层累积的方法更逼近实际喷涂过程, 能更准确预测涂层的残余应力。     

Enhancing the high cycle fatigue life of high strength aluminum alloys for aerospace applications

Hard anodized (HA) and micro arc oxidation (MAO) coatings of identical thickness were deposited on two different high strength aluminum (Al) alloys namely, 2024‐T3 and 7075‐T6. Further, as received Al alloys were also subjected to shot peening (SP) to induce subsurface compressive residual stresses followed by the MAO coating deposition (SP + MAO). The average velocity of particle‐in‐flight during the SP process was measured and utilized to calculate the kinetic energy of the peening particles. The bare and coated alloys were subjected to completely reversed stress (R = ?1) rotating beam high cycle fatigue tests at five different stress levels. In addition, the bare and coated alloys were also evaluated for their tensile properties, elemental composition, phase constituents, surface, and cross‐sectional morphologies including the surface roughness (Ra, Rz) and correlated the same with the corresponding fatigue behavior. Irrespective of substrate alloy composition and stress levels investigated, the duplex SP + MAO treatment resulted in significant fatigue life enhancement over and above the fatigue life of corresponding bare (not shot peened) Al alloy, while the hard anodized and plain MAO (both without prior shot peening) continue to exhibit significant fatigue debit. Driven by the compressive residual stresses present beneath the subsurface region of SP + MAO coating interface, fractured surface examination of SP + MAO coatings clearly highlights the crack‐branching associated multiple crack deflection as the predominant operative mechanism responsible for diminishing the crack growth rate and therefore enhance the fatigue life as compared with the near linear crack extension without significant deflections leading to relative premature failure of plain MAO coated alloys.     

梯度陶瓷喷嘴残余应力的有限元分析

为了提高喷嘴的抗冲蚀磨损能力,将梯度功能材料理论运用于喷嘴材料的设计中,改传统的均质喷嘴材料为非均质喷嘴材料,提出在梯度陶瓷喷嘴制备中将残余压应力引入喷嘴入口的设计目标.在组成分布指数一定的条件下,针对主要设计参数对梯度陶瓷喷嘴残余应力的影响进行有限元分析,探讨了梯度层厚度、临界梯度层材料组分差对SiC/(W,Ti)C单梯度陶瓷喷嘴残余热应力的影响规律,在组成分布指数取0.5时,优化SiC/(W,Ti)C梯度陶瓷喷嘴梯度层厚和临界梯度层材料组分差.结果表明,残余应力随梯度层厚h及临界梯度层SiC体积组分差的不同产生很大差异,合理设计梯度层厚h及临界梯度层SiC体积组分差可在喷嘴入口形成有效残余压应力,最佳梯度层厚为5mm,临界梯度层SiC组分差小于5%(体积分数).     

Verhalten laserschockverfestigter und festgewalzter Randschichten der Ti‐Legierung Ti‐6Al‐4V bei schwingender Beanspruchung unter erhöhten Temperaturen

Residual stress stability and near‐surface microstructures in high temperature fatigued mechanically surface treated Ti‐6Al‐4V It is well known that mechanical surface treatments, such as deep rolling, shot peening and laser shock peening, can significantly improve the fatigue behavior of highly‐stressed metallic components. Deep rolling is particularly attractive since it is possible to generate, near the surface, deep compressive residual stresses and work hardened layers while retaining a relatively smooth surface finish. In the present investigation, the effect of deep rolling on the low‐cycle and high‐cycle fatigue behavior of a Ti‐6Al‐4V alloy is examined, with particular emphasis on the thermal and mechanical stability of the residual stress states and the near‐surface microstructures. Preliminary results on laser shock peened Ti‐6Al‐4V are also presented for comparison. Particular emphasis is devoted to the question of whether such surface treatments are effective for improving the fatigue properties at elevated temperatures up to ～450 °C, i.e., at an homologous temperature of ～0.4 T/T_m (where T_m is the melting temperature). Based on cyclic deformation and stress/life (S/N) fatigue behavior, together with the X‐ray diffraction and in situ transmission electron microscopy observations of the microstructure, it was found that deep rolling can be quite effective in retarding the initiation and initial propagation of fatigue cracks in Ti‐6Al‐4V at such higher temperatures, despite the almost complete relaxation of the near‐surface residual stresses. In the absence of such stresses, it is shown that the near‐surface microstructures, which in Ti‐6Al‐4V consist of a layer of work hardened nanoscale grains, play a critical role in the enhancement of fatigue life by mechanical surface treatment.     

Texture and residual stress in FeMn/Ni80Fe20 multilayers

The texture and residual stress in FeMn/Ni₈₀Fe₂₀ multilayers were studied using conventional sin²ψ method. The results show that the FeMn and Ni₈₀Fe₂₀ layers are both (111) textured. The sin²ψ plots are nonlinear, which indicates a strong residual stress gradient through the depth of the sample. Fitting of the sin²ψ plots give the residual stress in the samples. The in-plane residual stress is tensile and decreases from the surface of the sample to the substrate. The out-of-plane residual stress cannot be neglected because of the strong texture. In the two samples with the same total thickness but different periods, the FeMn layers have the same texture and residuals stress; while for Ni₈₀Fe₂₀ layer, as the thickness of each period increases, the texture becomes stronger and the residual stress becomes larger. It appears that the in-plane residual stress increases as the texture becomes stronger.     

Determination of residual stresses in SiC monofilament reinforced metal-matrix composites using Raman spectroscopy

Raman spectroscopy has been used to follow the deformation of chemical vapour deposition type SCS-6 and Sigma 1140+ SiC monofilaments and to determine residual stresses in these SiC monofilaments reinforced metal-matrix composites. Raman bands at 1330 and 1600 cm⁻¹ due to carbon have been observed on the monofilament surface and it has been shown that both bands shift linearly to lower wavenumbers during tensile deformation. The residual stresses in SiC monofilament reinforced composites arising from thermal expansion mismatch have also been determined by measuring the shifts of carbon bands from the same monofilaments embedded in a Ti–6Al–4V matrix. The axial residual stresses in the carbon coating are found to be around −850 MPa for the SCS-6 composite and −540 MPa for the Sigma 1140+ composite.     

Thermal relaxation of residual stresses in shot peened surface layer of (TiB + TiC)/Ti-6Al-4V composite at elevated temperatures

As an effective and important surface treatment method, shot peening can introduce high residual compressive stress and microstructure variation at near surface deformation layers. In this work, residual stresses relaxation behaviors of the shot peened layer of (TiB + TiC)/Ti-6Al-4V composite were investigated during thermal exposure, and the microstrain was calculated according to the integral breadth after isothermal annealing. The microstrain decreased fast and reached the minimum at 500 °C, which resulted from the thermal recovery and dynamic recrystallization. At elevated temperatures, the residual compressive stresses were relaxed in the whole deformation layers, which were caused by the thermally activated gliding of dislocations. The processes of relaxation can be described using a Zener-Wert-Avrami function and the activation energy of the residual stresses relaxation was higher than that of titanium self diffusion, which was ascribed to the hindrance effects of reinforcements as sink sources of dislocations during annealing.     

A functionally gradient coating on carbon fibre for C/Al composites

A functionally gradient coating on carbon fibre for casting C/Al composites with an ultimate tensile strength up to 1250 MPa (V _f=0.35) has been produced. The coating consisted of three layers: an inner pyrocarbon layer, an outer silicon layer and an intermediate gradient layer C/SiC/Si, and their optimum thicknesses were 0.1–0.15, 0.1 and 0.2 m, respectively. This coating was fabricated by chemical vapour deposition and the C/Al composite was performed by pressure-regulated infiltration. Auger electron spectroscopy and X-ray diffraction analyses confirmed that the structure of the coating was in keeping with its design. The excellent ultimate tensile strength of the C/Al composite also proves that the functionally gradient coating has many functions, including wetting agent, diffusion and reaction barrier, releaser of residual thermal stresses, and tailor of interfacial shear strength. According to the mechanical, physical and chemical coordination between fibre and matrix, the functionally gradient coating can solve nearly all the problems of the interface during fabrication and service.     

Residual Stresses in Plasma-Sprayed Coatings Al 2 O 3

Residual stresses affect significantly the quality and performance of thermally sprayed coatings. The residual stresses in alumina coatings were investigated by means of X-ray diffraction with Cr- and Cu-radiations. The sin² method could be applied to the evaluation of the stress fields. A very small stress gradient was indicated in surface layers of alumina coatings.     

Laser shock peening on fatigue crack growth behaviour of aluminium alloy

The effect of laser shock peening (LPS) in the fatigue crack growth behaviour of a 2024‐T3 aluminium alloy with various notch geometries was investigated. LPS was performed under a ‘confined ablation mode’ using an Nd: glass laser at a laser power density of 5 GW cm^?2. A black paint coating layer and water layer was used as a sacrificial and plasma confinement layer, respectively. The shock wave propagates into the material, causing the surface layer to deform plastically, and thereby, develop a residual compressive stress at the surface. The residual compressive stress as a function of depth was measured by X‐ray diffraction technique. The fatigue crack initiation life and fatigue crack growth rates of an Al alloy with different preexisting notch configurations were characterized and compared with those of the unpeened material. The results clearly show that LSP is an effective surface treatment technique for suppressing the fatigue crack growth of Al alloys with various preexisting notch configurations.     

Intrinsic stresses and stress relaxation in TiN/Ag multilayer coatings during thermal cycling

Morphology, structure and thermal behavior of magnetron sputtered TiN/Ag multilayer thin films deposited at 150 °C with a bilayer thickness Λ in the range of 75-600 nm are characterized. The films are thermally cycled and the relationship between bilayer thickness Λ, film structure and stress development is analyzed. The results indicate that the residual stresses in the as-deposited films and the behavior during heating are determined by the morphology and the mechanical properties of the Ag interlayers. The increasing crystallite size of Ag with increasing Λ and the initial porosity in the Ag layer are the reason for significant changes in the stress-temperature behavior. While coatings with Λ = 75 nm behave like a single-phase coating up to 380 °C, coatings with higher Λ show a different behavior when exceeding the deposition temperature, which is related to the densification of the Ag layers. During cooling, all coatings exhibit linear thermo-elastic behavior, where the slope of the stress-temperature curves also depends on Λ.     

Effect of aging on the onset of cracks due to redistribution of residual stresses in functionally graded environmental barrier coatings of mullite/ZrO2

Environmental barrier coatings (EBCs) are proposed as an option to reduce the high temperature water vapour corrosion in gas turbines ceramic components made of Si₃N₄ or SiC/SiC_f, which are projected to achieve further energy efficient gas turbines. These coating are commonly designed as multilayer systems firmly attached to the ceramic substrate with the aim of retarding or avoiding its degradation after exposure to environmental conditions close to those in gas turbines. Therefore, to fulfil this function crack formation/propagation in the coatings must be controlled. In present work, three types of environmental barrier coatings fabricated by air plasma spray and containing a Si layer attached to SiC substrate plus 2 to 5 layers of different mullite/Y₂O₃ stabilized–ZrO₂ mixtures are examined. To determine the level of residual stresses in the as-sprayed coating/substrate systems a three dimensional finite element model is developed and also tested for same coatings but aged under, high temperature and rich water vapour atmosphere. The model calculates the zones of maximum tensile stresses in the coatings which agree with experimental observation identifying the type, number and location of cracks. This model could be extended to similar EBC systems, and more importantly, could be use as a powerful designing tool for these complex structures.     

Influence of ion assisted deposition on interface broadening in Fe/Al multilayers investigated by medium energy ion scattering

Trilayers of Al/Fe/Al and Al/Fe multilayers produced by magnetron sputtering both with and without ion assistance have been depth profiled using Auger electron spectroscopy and medium energy ion scattering. Important differences are observed in the layer structure, with ion assisted deposition giving the narrowest Al/Fe interfaces and so maintaining the most clearly defined layer structure. Both types of sputtering result in some oxygen contamination, particularly at the surface that modeling shows to be associated with the Al layers.     

Influence of ion assisted deposition on interface broadening in Fe/Al multilayers investigated by medium energy ion scattering

Trilayers of Al/Fe/Al and Al/Fe multilayers produced by magnetron sputtering both with and without ion assistance have been depth profiled using Auger electron spectroscopy and medium energy ion scattering. Important differences are observed in the layer structure, with ion assisted deposition giving the narrowest Al/Fe interfaces and so maintaining the most clearly defined layer structure. Both types of sputtering result in some oxygen contamination, particularly at the surface that modeling shows to be associated with the Al layers.     

Surface modifications induced by shot peening and their effect on the plane bending fatigue strength of a Cr-Mo steel produced by powder metallurgy

The effect of shot peening on the plane bending fatigue strength of a 7.1 g/cm³ sintered Cr-Mo steel was investigated. Shot peening provides surface densification, strain hardening, compressive residual stresses up to −700 MPa, without impairing the dimensional and geometrical precision of specimens. Plane bending fatigue strength increases of 30%, irrespective to the different residual stress profiles obtained by changing the shot peening parameters. The improvement is mainly due to the surface densification and strain hardening.     

Machining residual stresses

As part of an extended investigation into the effects of pre-and post-processing on shot peening residual stresses, residual stresses due to a controlled turning process were investigated in the aircraft alloy 817M40. Cylindrical specimens were produced on a centre lathe with all machine parameters fixed except for feedrate. A variation in the feedrate was used to produce different surface roughnesses varying from 1·2 to 6·6 μm R_a. The residual stress field resulting from the varying feedrate was established by the well known centre hole drilling method, albeit with a modification which assisted in determining the variation of residual stresses with depth. The results indicated a definite variation in axial residual stress distribution with surface roughness, with a rough surface showing higher tensile residual stresses. This variation was however not duplicated for hoop or circumferential stresses. Maximum tensile residual stress varied from 90 MN m^?2 in the 6·6 μm R_a surface to 50 MN m^?2 in the 1·2 μm R_a surface. Although residual stresses are generally ignored in design, either because of a lack of experimental results or for ease of application, the results indicate that some consideration must be given to machining residual stresses as these can enhance or reduce the fatigue life of engineering components. The study also indicates that turning operations associated with large plastic deformations are encountered during the machining process.

MST/3230     

Effects of shot peening on bending fatigue strength of spring steel SS 2090

Abstract

The influence of prior surface condition and of a shot peening treatment on the bending fatigue strength of a standard Si–Cr spring steel (SS 2090) has been investigated. This steel was initially hardened and tempered to a hardness of 52–54 HRC. After shot peening, compressive residual stresses had been introduced into a surface layer of depth ~0·3 mm, with the maximum value of ~1000 MN m^?2 being found close to the surface. The effect of this treatment was to increase the fatigue limit by ~40% to 890 MN m^?2. Coincident with this increase was a change in the site of fatigue initiation from a surface to a subsurface location beneath the compressive residual stress layer. The initiating inclusions, which were 20–40 μm in size, were analysed and found to be Al₂O₃. At stress amplitudes greater than the fatigue limit, initiation was invariably found to occur at the surface and was not always due to inclusions. Inclusion initiated failure has been modelled using the size and spatial distribution of inclusions in the test bars in addition to the variation of applied and residual stresses through the section. A crack propagation criterion based on linear elastic fracture mechanics is used, assuming that propagation is controlled by stress intensity threshold value. It is assumed that small cracks exist at oxide inclusions from the beginning of the fatigue life and that failure is associated with the propagation of one of these cracks.

MST/1392     

A comprehensive investigation on the effects of laser and shot peening on fatigue crack growth in friction stir welded AA 2195 joints

The effects of various surface treatment techniques on the fatigue crack growth performance of friction stir welded 2195 aluminum alloy were investigated. The objective was to reduce fatigue crack growth rates and enhance the fatigue life of welded joints. The crack growth rates were assessed and characterized for different peening conditions at a stress ratio (R) of 0.1, and 0.7. The surface and through-thickness residual stress distribution were also investigated and presented for the various regions in the weld. Tensile residual stresses introduced during the welding process were found to become significantly compressive, particularly after laser peening. The effect of the compressive stresses was deemed responsible for increasing the resistance to fatigue crack growth of the welds. The results indicate a significant reduction in fatigue crack growth rates using laser peening compared to shot peening and native welded specimens. This reduced fatigue crack growth rate was comparable to the base unwelded material.