Phase composition and in-vitro bioactivity of plasma sprayed calcia stabilized zirconia coatings

Zirconia coatings stabilized with different calcia content (12.8 mol%, 16 mol% and 30 mol%) were fabricated on titanium alloy substrates using atmospheric plasma spraying technology. The in-vitro bioactivity of coatings was evaluated by simulated body fluid (SBF) soaking test. The morphology and phase composition of the coatings were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), micro Raman spectroscopy, energy dispersive spectrometry (EDS) and infrared spectroscopy (IR). The results showed that the apatite was formed on the surface of the calcia stabilized zirconia coatings soaked in SBF for 28 days and the formation ability of apatite decreased with the increase in calcia content of the coating. The bioactivity of zirconia coatings was thought to be related to the Zr-OH formed on their surfaces during the phase transformation in the presence of water. Osteoblast-like MG63 cells were cultured on the surfaces of the coatings to evaluate their cytocompatibility. Results showed that MG63 cell grew and proliferated well on all coating surfaces, indicating that plasma sprayed calcia stabilized ZrO₂ coatings were cytocompatible.     

Microstructure and fatigue properties of plasma transferred arc alloying TiC-W-Cr on gray cast iron

In this paper, TiC-W-Cr powders were alloyed on grey cast iron by plasma transferred arc (PTA). The alloying samples were characterized the microstructure, microhardness, fatigue life and fatigue crack growth. From the results, it is indicated that two distinguishing region: alloying zone, heat affected zone are formed on the surface after PTA alloying. The alloying zone mainly consists of primary austenite, martensite, a eutectic of (Fe,Cr)₇C₃ carbide and austenite as well as the uniformly distributed un-melted TiC particles. PTA alloying TiC-W-Cr eliminates the stress concentration at the edge of graphite and produced hard carbide, resulting in frequent crack deflection. As a result, the Weibull distribution of fatigue life demonstrates that PTA alloying TiC-W-Cr exhibits longer lives compared to matrix and PTA hardening without reinforcement, but more scattered. In addition, on the basis of the careful observation of fatigue crack growth, it is shown that the fatigue crack growth rate could be retarded by PTA alloying TiC-W-Cr at low stress intensity.     

纳米ZrO2-8％Y2O3粉末的相转变及晶粒生长动力学

采用共沉淀法制备纳米ZrO2-8％Y2O3(质量分数)粉末,然后将其在大气中于1 100～1 300℃范围内高温煅烧处理2～32 h.利用XRD、SEM、TEM等方法研究纳米ZrO2-8％Y2O3粉末高温煅烧前后的相成分、形貌和晶粒粒径变化,并分析纳米ZrO2-8％Y2O3粉末的晶粒生长动力学及生长机制.结果表明:纳米ZrO2-8％Y2O3经高温煅烧后,单斜相和四方相含量随温度的升高和时间的延长而减少,立方相含量随温度的升高和时间的延长而增加;随温度的升高和时间的延长晶粒粒径逐渐增大;在1 250℃等温煅烧时,其晶粒生长指数为6,晶粒生长速率常数为7.626×1011 nm3/min:等温锻烧温度低于1 200℃时,晶粒生长活化能为64.35 kJ/mol,晶粒生长表现为以表面扩散为主的聚合生长;等温锻烧温度高于1 200℃时,晶粒生长活化能为1 16.40 kJ/mol,晶粒生长表现为以晶格扩散为主的聚合生长;另外,还可见晶粒旋转驱动的聚合生长机制;低的晶粒生长激活能归因于大量氧空位的引入和晶粒旋转驱动的聚合生长机制.     

Residual stress relaxation in typical weld joints and its effect on fatigue and crack growth

Many factors influence the fatigue and crack growth behavior of welded joints. Some structures often undergo fairly large static loading before they enter service or variable amplitude cyclic loading when they are in service. The combined effect of both applied stress and high initial residual stress is expected to cause the residual stresses relaxation. Only a few papers seem to deal with appropriate procedures for fatigue analysis and crack growth by considering the combined effect of variable amplitude cyclic loading with residual stresses relaxation. In this article, some typical welded connections in ship-shaped structures are investigated with 3-D elastic-plastic finite element analysis. The effect of residual stress relaxation, initial residual stress, and the applied load after variable amplitude cyclic loading is revealed, and a formula for predicting the residual stress at hot spot quantitatively is proposed. Based on the formula, an improved fatigue procedure is introduced. Moreover, crack growth of typical weld joints considering residual stresses relaxation is studied.     

Effect of electron beam surface hardening on fatigue crack growth rate in AISI 4340 steel

This paper studies the effect of electron beam (EB) surface hardening on the fatigue crack growth rate in AISI 4340 steel. The heat treatment conditions were varied to consider the influence of microstructure and residual stress. The results show that increasing the EB heat input increases the compressive residual stress in the hardened layer. Thus EB surface-hardening treatment improves the fatigue crack growth resistance. This effect increases with increasing EB heat input but disappears as the ΔK value increases. The fracture mechanism of the hardened layer is intergranular fracture, while that of the base material is transgranular quasi-cleavage.     

The turning point in Paris region of fatigue crack growth in titanium alloy

通过对比钛合金不同显微组织下疲劳裂纹扩展速率Paris区转折点的位 置, 发现原始β晶粒尺寸是影响Paris区转折点位置的主要因素, 晶 团尺寸以及片层厚度对转折点位置没有影响. 并且通过分析转折点前、后 疲劳裂纹扩展微观阶段的转变及断裂方式变化, 得出转折点的出现是 疲劳裂纹尖端塑性区尺寸超过晶粒尺寸所致. 具有β晶粒的马氏体组 织疲劳裂纹扩展速率Paris区并不存在转折点, 说明转折点的出现不仅仅是 由于原始β晶粒的存在, 还和显微组织类型有关. 通过分析还发现, 钛合金片层组织中裂纹尖端塑性区实际尺寸大于计算得到的单向塑性 区尺寸以及循环塑性区尺寸.     

Influence of joining on the fatigue and fracture behavior of high density polyethylene pipe

Polyethylene pipes have been used in a spectrum of corrosion-critical applications, including distribution systems for oil, gas, water, and chemicals. In this paper, the influence of joining on bending fatigue and fracture behavior of high density polyethylene pipe is presented and discussed, and performance is compared with a plain unwelded counterpart. High density polyethylene pipes were joined using electrofusion and butt-fusion techniques. Stiffness and strength of the electro-fusion joined pipe was far inferior to the butt-fusion and the plain unwelded pipe. Tensile failure of the butt-fusion joined specimen occurred at the fusion zone, while tensile failure of the electro-fusion joined pipe specimen occurred at the fusion joint. Bending fatigue resistance, quantified in terms of life to failure, of the pressurized pipe was superior to that of the unpressurized pipe. The fatigue resistance of the butt-fusion joined specimen was superior to that of the electro-fusion joined pipe specimen. The unwelded plain polyethylene pipe had bending fatigue resistance superior to that of the butt-fusion joined counterpart. Rationale for the differences observed in bending fatigue life is presented, and intrinsic differences in failure characteristics are highlighted.     

The effects of gas nitriding on fatigue behavior in titanium and titanium alloys

Fatigue behavior has been studied on gas-nitrided smooth specimens of commercial pure titanium, an alpha/beta Ti-6Al-4V alloy, and a beta Ti-15Mo-5Zr-3Al alloy under rotating bending, and the obtained results were compared with the fatigue behavior of annealed or untreated specimens. It was found that the role of the nitrided layer on fatigue behavior depended on the strength of the materials. Fatigue strength was increased by nitriding in pure titanium, while it was decreased in the Ti-6Al-4V and Ti-15Mo-5Zr-3Al alloys. Based on detailed observations of fatigue crack initiation, growth, and fracture surfaces, the improvement and the reduction in fatigue strength by nitriding in pure titanium and both alloys were primarily attributed to enhanced crack initiation resistance and to premature crack initiation of the nitrided layer, respectively.     

Phase field modeling of the tetragonal-to-monoclinic phase transformation in zirconia

The allotropic phase transformation in zirconia from the tetragonal to monoclinic double lattices is known to occur by a martensitic twinning mechanism which shows a complex dependence on temperature, stress and environment. This paper is concerned with the development of a phase field model which accounts for the main metallurgical mechanisms governing this martensitic transition. The symmetry reduction and orientation relationship between the parent and product phases were simulated using several non-conserved order parameters representing different transformation paths. Inhomogeneous and anisotropic elastic properties were considered to determine the resultant elastic stresses. Governing equations of the tetragonal-to-monoclinic transformation were solved in a finite element framework under a variety of initial and boundary conditions. It was shown that applying different initial conditions, such as seed embryo or random, did not change the twinning patterns or the final volume fractions of the parent and product phases after the relaxation period. On the other hand, enforcing different boundary conditions resulted in completely different twinning patterns and phase volume fractions. The model was able to predict both the “V” shape morphology of twinning and the surface stress relief with “gable roof” patterns, which were observed by transmission electron microscopy and atomic force microscopy to be characteristic of the tetragonal-to-monoclinic transition.     

Cold hole expansion effect on the fatigue crack growth in welds of a 6061-T6 aluminum alloy

Compact test specimens were extracted from a 6061-T6 aluminum alloy welded plate with a thickness of 9 mm to analyze the cold hole expansion effect on fatigue crack growth tests conducted in mode I cyclic loading. At R = 0.1, a sharp crack in base metal, weld metal and heat affected zone was propagated from 17 to 24 mm. The fatigue crack growth at 24 mm (α = a/W = 0.3) was delayed by drilling a hole at the crack tip and applying a cold hole expansion of 4.1%. The residual stress fields due to cold hole expansion were determined with the finite element method. The fatigue crack growth testing was continued up to a crack length of 35 mm (α ∼ 0.43) at the same R, and crack opening displacements of the post-expansion crack were also determined with the finite element method. The results were expressed in terms of crack length versus number of cycles, as well as, fatigue crack growth rate as a function of applied and effective stress intensity factor range. The cold hole expansion contributed to delay the fatigue crack growth in base metal, and to a lesser extent in the weld metal and heat affected zone. A crack closure effect was determined by means of load versus crack opening displacement curves of the post-expansion crack, which was, completely or partially closed, in welded zones with compressive residual stress fields. The fracture surfaces of each welded zone were analyzed to elucidate the crack nucleation zone and its relation with the residual stress field. In all cases the crack was initiated at the surface of the specimen where the residual stresses were positive.     

On-line monitoring of fatigue cracks using ultrasonic surface waves

During the last decade many techniques were developed to detect fatigue cracks, and estimate their location and size. Unfortunately, most of the currently available nondestructive testing methods are off-line: the operational (or fatigue) loading and the inspection are considered as two distinct stages. Mostly, the loading should be released before inspection can take place, and sometimes the device under test even has to be disassembled. In this article, an experimental methodology based on ultrasonic surface waves will be developed to continuously inspect a structure during its operation. The proposed method uses spectral information of transmitted surface waves at several working points of the operational load. Simple statistical indicators of the transmitted wave energy during loading are introduced in order to be able to monitor the structural health on-line. As a validation experiment, a propagating fatigue crack in a sinusoidally loaded beam will be considered. In addition, a comparison with an off-line method is made, showing that the on-line method is also much more sensitive.     

Wear characteristics of plasma-sprayed nanostructured yttria partially stabilized zirconia coatings

Reconstituted nanostructured and conventional yttria partially stabilized zirconia coatings were deposited by atmospheric plasma spray. The tribologic properties of the coatings against 100C6 steel were evaluated with a ball-on-disc configuration under dry friction conditions at room temperature. Microstructure and the phase composition of the powders and the coatings were examined using a scanning electron microscope, optical microscope, and x-ray diffraction. Microhardness and the Young’s modulus of coatings were measured by indentation testing. Results showed that the wear resistance of the coatings produced using the nanostructured powder is improved compared with the coating produced using the conventional powder. The wear rates of nanostructured zirconia coatings are about four-fifths of those of conventional counterparts under a load of 5 N. The wear mechanism is also discussed. The original version of this paper was published as part of the DVS Proceedings: “Thermal Spray Solutions: Advances in Technology and Application,” International Thermal Spray Conference, Osaka, Japan, 10–12 May 2004, CD-Rom, DVS-Verlag GmbH, Düsseldorf, Germany.     

Tensile overload-induced plastic deformation and fatigue behavior in weld-repaired high-strength low-alloy steel

The effects of tensile over-load (OL) on fatigue crack growth behavior of a weld-repaired high-strength low-alloy (HSLA) steel were studied by measuring both the fatigue crack growth rate and sample-thickness variation along the fatigue crack growth path. The thickness variation, indicating the degree of plastic deformation (PD), provided an indirect measurement of associated residual compressive stresses at the crack-tip. The applied tensile OL with one-hour holding period in each test generated a damage zone at the crack tip. Microscopic details of the crack-tip damage zone were characterized by scanning electron microscopy. Three groups of expanded compact-tension (E-CT) samples, 10 mm in thickness, were tested: weld-repaired HSLA without soft buffer layer (BL), and weld-repaired HSLA with 4 mm or 10 mm thick BL. The experimental results showed that the OL-induced PD, closely linked to the crack-tip residual compressive stresses, reduced the subsequent fatigue crack growth rate, and that the HSLA with a 10 mm BL had the lowest growth rate, indicating a soft BL with an adequate thickness could further improve the fatigue resistance.     

Microstructural characterisation of fatigue crack growth fracture surfaces of lamellar Ti45Al2Mn2Nb1B

Investment cast Ti45Al2Mn2Nb1B with fine lamellar microstructures was subject to fatigue crack propagation testing at 650 °C and a stress ratio of R = 0.1. The fracture surfaces were examined under SEM and the observed features are correlated with both stress intensity range (ΔK) and lamellar orientation. Translamellar fracture is primary fracture mode for most of the lamellar orientations. Interlamellar fracture is influenced by a combination of the ΔK and lamellar orientation. At low ΔK only the lamellar colonies with their lamellar interfaces almost perpendicular to the stress axis fractured via interlamellar fracture mode. At high ΔK interlamellar fracture can occur in lamellar colonies with any orientations.     

7020铝合金型材的疲劳行为及微观机制

采用光学显微镜、扫描电镜、扫描透射电镜和电子背散射衍射等分析方法研究了 7020铝合金型材疲劳行为及微观机制.结果表明在应力比R为0、疲劳极限寿命取107周次时,合金疲劳强度为232.9 MPa.疲劳裂纹尖端应力强度因子△K=8 MPa·m1/2时,裂纹扩展速率约为6.44×10-5 mm/cycle.合金中尺寸在3～...     

Influence of characteristics of stabilized zirconia electrolyte on performance of cermet supported tubular SOFCs

Ni-Al2O3 cermet supported tubular SOFC was fabricated by thermal spraying. Flame-sprayed Al2O3-Ni cermet coating plays dual roles of a support tube and an anode current collector. 4.5mol.% yttria-stabilized zirconia (YSZ) and 10mol.% scandia-stabilized zirconia (ScSZ) coatings were deposited by atmospheric plasma spraying (APS) as the electrolyte in present study. The electrical conductivity of electrolyte was measured using DC method. The post treatment was employed using nitrate solution infiltration to densify APS electrolyte layer for improvement of gas permeability. The electrical conductivity of electrolyte and the performance of single cell were investigated to optimize SOFC performance. The electrical conductivity of the as-sprayed YSZ and ScSZ coating is about 0.03 and 0.07 S·cm-1 at 1000 ℃, respectively. The ohmic polarization significantly influences the performance of SOFC. The maximum output power density at 1000 ℃ increases from 0.47 to 0.76 W·cm-2 as the YSZ electrolyte thickness reduces from 100 μm to 40 μm. Using APS ScSZ coating of about 40 μm as the electrolyte, the test cell presents a maximum power output density of over 0.89 W·m-2 at 1000 ℃.     

TC4钛合金惯性摩擦焊接头组织及断裂韧性与裂纹扩展特性分析

文章采用了XRD、SEM、EBSD等显微表征技术分析了焊态及焊后热处理态下焊接接头各区域的微观组织特征,并研究了焊接接头的断裂韧性和疲劳裂纹扩展性能。结果表明,焊缝区以再结晶组织为主,热力影响区等轴状初生α_p相转变为棒状结构,热影响区组织与母材基本相同,热力影响区与热影响区的原始β晶粒内部分区域形成了取向差角度约为60°的针状马氏体α′相,热处理促进了残余亚稳态β相分解,在片状α_s相间形成了大量断续分布组织。焊缝区α晶粒内大量的平行或交叉分布的片状α相和复杂的相界面结构可有效阻碍裂纹的扩展并改变裂纹的扩展路径,提高焊接接头的断裂韧性及抗疲劳裂纹扩展能力。     

工业结构钢疲劳极限的断裂力学研究

文章提出一种预测材料疲劳极限的简便方法。该方法仅需获得一个材料参数(如材料硬度)便可确定材料的裂纹扩展抵抗阻力曲线。由于有效应力强度因子范围门槛值ΔKeffth∞是一个与裂纹长度及应力比无关的材料常数,只需将裂纹张开应力强度因子门槛值Kopth表示为裂纹长度的函数,作出材料的疲劳断裂曲线,就可以预测任意应力比下光滑试样的疲劳极限。利用这种方法作出的预测值与试验结果一致。