Research On Microscopic Mechanism Of Stress Corrosion Cracking And Corrosion Fatigue Behavior Of Nickel Aluminum Bronze Alloys

Nickel aluminum bronze?NAB?alloy is broadly applied in marine components,such as large ship propeller,pressure pump body and hydraulic valve,due to its high strength,good fracture toughness and excellent corrosion resistance properties.The as-casted NAB alloys are the most commonly used states in its applications.Their internal microstructures are complex,and contain various second-phase precipitates.They also present lots of uneven coarse grains,porosity and segregation and other casting defects.Meanwhile,complex marine environment and changeable stress loading profoundly challenge the service performance of NAB components with a series of problems like the stress corrosion cracking,corrosion fatigue and other failure forms.Although China began to produce nickel-aluminum bronze alloy in industry as early as the 1980s,the scientific research about NAB alloy only focuses on microstructural characterization and static corrosion performance.The mechanical properties in the corrosive environment,especially the stress corrosion cracking and corrosion fatigue behaviors,have not been systematically studied.Considering these issues,in this paper,the slow strain rate tensile test and fatigue crack growth test are conducted to explore the stress corrosion and corrosion fatigue behavior of NAB alloys with different microstructure.According to the experimental results and scientific analysis,the response relation between microstructure and service performance is established,and the microscopic mechanisms of various failure forms in corrosive environment are revealed.This contribution provides extremely important theoretical guidance for microstructure designing and service life improvement.The main experimental results and conclusions are as follow:?1?Using different heat treatments process,micro-level design and control can be realized to obtain the target NAB alloy with different microstructures,whose stress corrosion susceptibility was subsequently tested under different strain rate conditions.The results show that the NAB alloys with different microstructure present different stress corrosion cracking susceptibility,which is negatively related to the strain rate.The continuous lamellar?+?_IIIII eutectoid structure is the sensitive area of stress corrosion cracking as it is easily selectively corroded,which makes the annealed NAB alloy present the highest susceptibility.The thin and overlapped Widmanstatten?grain can hinder extension of the cracks,thus the susceptibility of stress corrosion cracking of normalized NAB presents some decline.The quenched+aged NAB alloy exhibits lowest susceptibility both at low and high strain rates because of its homogeneous microstructure without any large continuous preferential corrosion structure.?2?Stress corrosion cracking is considered as a phenomenon appears only when metals are under the combined action of mechanical stress and corrosive medium.Tensile deformation induces the inhomogneously distributed residual stress into the alloy,which increases the corrosion tendency and oxide film thichness.However,the residual stress has a negative effect on the long-term stability of the oxide film.Furthermore,the stress corrosion crack,affected by corrosive medium,will initiate at the NAB alloy surface when the applied stress reaches 450 MPa and it will propagate inward along the hard?phase boundaries in which the residual stress is higher.?3?Corrosion fatigue behavior of NAB alloys with different microstructures in air and in 3.5%NaCl solution was studied by fatigue crack growth test.It turns out that all the NAB alloys show higher crack growth rate and lower threshold under the influence of corrosive medium.The martensite?'phase becomes a high incidence area to crack propagation in 3.5%NaCl solution although it present a strong resistance of crack propagation in air.Meanwhile,the?_III phase and?+?_IIIII eutectoid structure are easily corroded to be the channels for crack propagation,and finally accelerate the fatigue crack growth rate.?4?Stress loading conditions such as stress intensity factor and loading frequency,are important factors affecting the corrosion fatigue behavior of alloy.The results show that with the decrease of stress intensity factor,the corrosion effect on crack propagation tend to enhance and make the propagation mode transform from transgranular to intergranular.The cracks mainly appeared around the?phase and extended toward the dissolved?_II phase particles or lamellar eutectoid structure.The crack growth rate increases with the decreasing frequency and there is a critical loading frequency of 0.05Hz.Only the loading frequency is lower than this critical value,the corrosion fatigue crack growth rate is correlated with loading frequency.?5?The crack tip is the direct influence zone of corrosion medium and stress alternating load on the alloy,which determines the fatigue performance of alloys to a great extent.It is found that the dissolved oxygen near the crack tip is consumed continuously to form an oxygen-poor zone as the crack propagation,which results in the hydrolysis reactions and acidification occurred in the solution environment near the crack tip.It also makes the?phases rapidly dissolved and generated continuous pores,which further induces fatigue crack propagation.On the other hand,acidification makes the hydrogen diffusion and enrichment around the crack tip,which promotes the local plastic deformation of the crack tip rapidly and leads to crack propagation by local hydrogen embrittlement effect.Beside,the dealumination and copper deposition degenerate the mechanical properties of crack tip including hardness and modulus,accelerating the fatigue crack growth for NAB alloy.