Deformation behaviour in advanced heat resistant materials during slow strain rate testing at elevated temperature

In this study, slow strain rate tensile testing at elevated temperature is used to evaluate the influence of temperature and strain rate on deformation behaviour in two different austenitic alloys. One austenitic stainless steel （AISI 316L） and one nickel-base alloy （Alloy 617） have been investigated. Scanning electron microscopy related techniques as electron channelling contrast imaging and electron backscattering diffraction have been used to study the damage and fracture micromechanisms. For both alloys the dominante damage micromech- anisms are slip bands and planar slip interacting with grain bounderies or precipitates causing strain concentrations. The dominante fracture micromechanism when using a slow strain rate at elevated temperature, is microcracks at grain bounderies due to grain boundery embrittlement caused by precipitates. The decrease in strain rate seems to have a small influence on dynamic strain ageing at 650℃.

Effects of Strain Rate on Stress Corrosion of S355 Steel in 3.5% NaCl Solutions

The stress corrosion of S355 steel in 3.5% NaCl solution under the different strain rates was analyzed with the slow strain rate test（SSRT）, the stress corrosion cracking（SCC） behaviors of S355 steel under the different strain rates in the solution were investigated, and the fracture morphologies and compositions of corrosion products under the different strain rates were analyzed with scanning electron microscopy（SEM） and energy dispersive spectrometerry（EDS）, respectively. The experimental results show that the SCC sensitivity index is the highest when the strain rate is 2×10-6, and the medium corrosion is the main reason resulting in the highest SCC sensitivity index. The SCC sensitivity index is the least when the strain rate is 5×10-6, and the stress is the main reason resulting in the stress corrosion. The SCC sensitivity index is the middle when the strain rate is 9×10-6, the interaction of stress and medium is the stress corrosion fracture mechanism.

SCC evaluation of a 2297 Al-Li alloy rolled plate using the slow-strain rate technique

The stress corrosion cracking(SCC) susceptibility of 2297 Al-Li alloy in 1 M Na Cl +0.01 M H2O2 solution(CP solution) and 1 M NaCl + 0.01 M H2O2+ 0.6 M Na2SO4 solution(CPS solution) was investigated by slow-strain rate tests at various strain rates ranging from 10-5s(-1) to 10-7s-1. The roles of H2O2 and SO42-in the corrosion process were estimated by potentiodynamic polarization and electrochemical impedance spectroscopy. 2297 Al-Li alloy does not fracture ascribed to SCC in CP solution, while it undergoes SCC in CPS solution. In CPS solution,with a decreasing strain rate from 10-5s(-1) to 10-7s-1, the SCC susceptibility firstly rises and then declines exhibiting a peak value at a strain rate of 10-6s-1. H2O2 promotes the active dissolution while SO42- lowers the corrosion rate. The SCC fracture is associated with a decline in the dissolution rate of the crack tip by SO42-, which leads to stress concentration. In CPS solution, a reduction in the local dissolution rate of the crack tip leads to stress concentration, resulting in SCC fracture.As the preferred initiation site for a crack, pits also show a noteworthy effect on SCC of 2297 Al-Li alloy.

Improvement of hydrogen embrittlement and stress corrosion cracking by annealing for Al-4.4Mg-0.6Mn alloy

With no annealing treatment, cathodic polarization trends in 5083F A1 alloy revealed concentration polarization and activation polarization. However, the annealed specimens have lower current densities at corrosion protection potential compared to the non-annealed specimen. The results of SSRTs conducted in seawater at the applied potential range of-l.8 V to -0.5 V indicated that the maximum tensile strength, elongation, and time-to-fracture had high values at applied potentials of -0.7 to -1.4 V. The maximum tensile strength, elongation, and time-to-f？acture decreased when the potential values were beyond this range in either anodic or cathodic direction. In general, the increased shear lip caused by annealing treatment indicates elongation. Time-to-fracture would likely increase with elongation. Potentials between -0.5 V to -0.6 V were found to be in the region of stress corrosion cracking. The corrosion protection zone was determined to be -0.7 V to -J,4 V because these potential ranges produced good mechanical properties. Potential less than -1.4 V produced a fractured surface with a mixture of dimples （ductile fractures） and a quasi-cleavage pattern resulting from the effects of hydrogen gas.

Stress Corrosion Cracking of Nitrogen-containing Stainless Steel 316LN in High Temperature Water Environments

Stress corrosion cracking （SCC） of stainless steels and Ni-based alloys in high temperature water coolant is one of the key problems affecting the safe operation of nuclear power plants （NPPs）. The nitrogen-added stainless steel is a kind of possible candidate materials for mitigating SCC since reducing the carbon content and adding nitrogen to offset the loss in strength caused by the decrease in carbon content can mitigate the problem of sensitization. However, the reports of SCC of nitrogen-added stainless steels in high temperature water are few available. The effects of applied potential and sensitization treatment on the SCC of a newly developed nitrogen-containing stainless steel （SS） 316LN in high temperature water doped with chloride at 250 ℃ were studied by using slow strain rate tests （SSRTs）. The SSRT results are compared with our data previously published for 316 SS without nitrogen and 304NG SS with nitrogen, and the possible mechanism affecting the SCC behaviors of the studied steels is also discussed based on SSRT and microstucture analysis results. The susceptibility to cracking of 316LN SS normally increases with increasing potential. The susceptibility to SCC of 316LN SS was less than that of 316 SS and 304NG SS. Sensitization treatment at 700℃ for 30 h showed little effect on the S CC of 316LN S S and significant effect on the S CC of 316 S S. The predominant cracking mode for the 316LN S S in both annealed state and the state after the sensitization treatment was transgranular. The presented conditions of mitigating stress corrosion cracking are some useful information for the safe use of 316LN SS in NPPs.

Mechanical properties and stress corrosion cracking behaviour of AZ31 magnesium alloy laser weldments

An AZ31 HP magnesium alloy was laser beam welded in autogenous mode with AZ61 filler using Nd-YAG laser system.Microstructural examination revealed that the laser beam weld metals obtained with or without filler material had an average grain size of about 12 μm.The microhardness and the tensile strength of the weldments were similar to those of the parent alloy.However,the stress corrosion cracking （SCC） behaviour of both the weldments assessed by slow strain rate tensile （SSRT） tests in ASTM D1384 solution was found to be slightly inferior to that of the parent alloy.It was observed that the stress corrosion cracks originated in the weld metal and propagated through the weld metal-HAZ regions in the autogenous weldment.On the other hand,in the weldment obtained with AZ61 filler material,the crack initiation and propagation was in the HAZ region.The localized damage of the magnesium hydroxide/oxide film formed on the surface of the specimens due to the exposure to the corrosive environment during the SSRT tests was found to be responsible for the SCC.

Behavior of Stress Corrosion Cracking in a Magnesium Alloy

Slow strain rate testing （SSRT） was employed to study the stress corrosion cracking （SCC） behavior of ZE41 magnesium alloy in 0.01 M NaCl solution. Smooth tensile specimens with different thicknesses were strained dynamically in both longitudinal and transverse direction under permanent immersions at a strain rate of 10-6 s-1. It is found that ZE41 magnesium alloy is susceptible to SCC in 0.01 M NaCl solution. The SCC susceptibility of the thinner specimen is lower than that of the thicker specimen. Also, the longitudinal specimens are slightly more susceptible to SCC than the transverse specimens. The SCC mechanism of magnesium alloy is attributed to the combination of anodic dissolution with hydrogen embrittlement.

Hydrogen-induced cracking behaviors of Incoloy alloy 825

The effect of hydrogen on the fractttre behaviors of Incoloy alloy 825 was investigated by means of slow strain rate testing （SSRT） Hydrogen was introduced into the sample by electrochemical charging. The results show that surface microcracks form gradually during ag- ing at room temperature when desorption of hydrogen takes place after hydrogen charging at a current density of 5 mA/cm^2 for 24 h. SSRT shows that the increase of ductility loss is significantly obvious as the hydrogen charging current density increases. Scanning electron microscopy （SEM） images reveal ductile fracture in the pre-charged sample with low current densities, while the fracture includes small quasi-cleavage regions and tends to be brittle fracture as the hydrogen charging current density increases to 5 mA/cm^2.

Effects of precipitation strengthening heat treatment for Al-Mg alloy

The corrosions resulting from defects in painting layers frequently occur in Al alloys, so the application of corrosion preventing systems is also very important. Optimum conditions in terms of electrochemistry in relation to solution treatment, quenching and artificial aging treatment were established in order to optimize precipitation strengthening conditions intended to enhance the strength of Al alloys. Slow strain rate tests （SSRT） at various applied potentials were conducted in potential range from -1.8 to 0.5 V. The results show that the maximum tensile strengths, elongations and time-to-fracture are shown to be high values. After precipitation strengthening heat treatment, a tendency appear that time-to-fracture increases as elongation increases. In the potential range from -1.3 V to -0.7 V, the specimens show excellent mechanical properties, and thus this range is considered to be a corrosion prevention range.

Stress-corrosion behavior and characteristics of the friction stir welding of an AA2198-T34 alloy

To better understand the stress-corrosion behavior of friction stir welding(FSW),the effects of the microstructure on the stress-corrosion behavior of the FSW in a 2198-T34 aluminum alloy were investigated.The experimental results show that the low-angle grain boundary(LABs)of the stir zone(SZ)of FSW is significantly less than that of heated affected zone(HAZ),thermo-mechanically affected zone(TMAZ),and parent materials(PM),but the grain boundary precipitates(GBPs)T1(Al2CuLi)were less,which has a slight effect on the stress corrosion.The dislocation density in SZ was greater than that in other regions.The residual stress in SZ was+67 MPa,which is greater than that in the TMAZ.The residual stress in the HAZ and PM is-8 MPa and-32 MPa,respectively,and both compressive stresses.The corrosion potential in SZ is obviously less than that in other regions.However,micro-cracks were formed in the SZ at low strain rate,which indicates that the grain boundary characters and GBPs have no significant effect on the crack initiation in the stress-corrosion process of the AA2198-T34.Nevertheless,the residual tensile stress has significant effect on the crack initiation during the stress-corrosion process.

Relationship between hydrogen concentration and mechanical properties of 5Ni-16Cr-Mo steel

The qualitative relationship between hydrogen concentration and notch tensile strength has been investigated for 5Ni-16Cr-Mo steel with different strength.The notch tensile strength was determined by means of slow strain rate test(SSRT)on circumferentially notched round bar specimens with the notch root radius of 0.15 mm after hydrogen charging.Meanwhile,the hydrogen diffusion behaviors of various strength steel were studied by thermal desorption spectroscopy(TDS)analysis.The SSRT results show that the T460 steel has higher susceptibility of hydrogen embrittlement in contrast with T520 steel.The activation energies and microstructure indicate that the dislocations and interfaces of martensitic laths are hydrogen traps in 5Ni-16Cr-Mo steel.By SSRT,the elastic limit of charged specimen loaded in air is higher than the flow stress without hydrogen charging before unloading,while the difference is defined as hydrogen-induced stress.The value of hydrogen-induced stress σ*increases linearly with hydrogen concentration:σ*=−0.622+2.015C0.The finite element analysis results of stress distributions near the notch tip have shown that the maximum principal stress increases with the notch root radius decreasing.

Microstructure,mechanical properties and stress corrosion behavior of friction stir welded joint of Al-Mg-Si alloy extrusion

Microstructure,mechanical properties and stress corrosion behavior of friction stir welded(FSWed) AlMg-Si alloy were investigated.The average grain sizes of shoulder-affected zone(SAZ),nugget zone(NZ),heat-affected zone(HAZ) and base material(BM) are 6.03,4.80,168.30 and 127.24 μm,respectively.The thermo-mechanically affected zone(TMAZ),which is generated on the edge position between HAZ and weld nugget zone,has a narrow width of 400 μm.The ultimate tensile strength(UTS) of FSWed joint is 232.20 MPa,about 91.04% with respect to that of base material of 255.06 MPa,and the joint fracture occurs at HAZ on advancing side(AS).The FSWed joint is more susceptive to stress corrosion cracking(SCC) than base material,and the SCC susceptibility increases with the rise in temperature.The residual UTS of FSWed joints in constant loaded tests at the load levels of90,105 and 120 MPa is 89.97%,67.50% and 54.75% of the UST of FSWed joint in air,respectively.The increase of the load in constant loaded tests and four-point beambent tests accelerates the SCC of FSWed joints.

Comparison of the Stress Corrosion Cracking Behaviour of AISI 304 Pipes Welded by TIG and LBW

The stress corrosion cracking(SCC)behaviour of AISI 304 pipe girth welds which were welded by a single-pass laser beam welding(LBW)and a multi-pass tungsten inert gas welding(TIG),respectively,was studied by the slow strain rate tests combined with the electrochemical corrosion tests.The results show that fracture of both the TIG joint and LBW joint occurs in the heat-affected zone(HAZ).According to the electron-backscattered diffraction observation of the micro structures,comparison of potentiodynamic polarization curves and X-ray photoelectron spectroscopy analysis of corrosion products on HAZs of the two joints after the electrochemical tests,the LBW joint exhibits better SCC resistance than the TIG joint in corrosion environments,due to the synthetic effect of more Cr_(2)O_(3) in corrosion products,finer grains,lower residual strain and higher δ-ferrite content in its HAZ.Although the TIG joint has better mechanical property,considering lower SCC susceptibility and higher production efficiency of the LBW joint,the LBW promisingly replaces the TIG for welding of AISI304 pipes in the nuclear power industry.

Investigating the stress corrosion cracking of a biodegradable Zn-0.8 wt%Li alloy in simulated body fluid

Stress corrosion cracking(SCC)may lead to brittle,unexpected failure of medical devices.However,available researches are limited to Mg-based biodegradable metals(BM)and pure Zn.The stress corrosion behaviors of newly-developed Zn alloys remain unclear.In the present work,we conducted slow strain rate testing(SSRT)and constant-load immersion test on a promising Zn-0.8 wt%Li alloy in order to investigate its SCC susceptibility and examine its feasibility as BM with pure Zn as control group.We observed that Zn-0.8 wt%Li alloy exhibited low SCC susceptibility.This was attributed to variations in microstructure and deformation mechanism after alloying with Li.In addition,both pure Zn and Zn-0.8 wt%Li alloy did not fracture over a period of 28 days during constant-load immersion test.The magnitude of applied stress was close to physiological condition and thus,we proved the feasibility of both materials as BM.

Influence of Zn Content on Microstructures,Mechanical Properties and Stress Corrosion Behavior of AA5083 Aluminum Alloy

To enhance the stress corrosion cracking(SCC)resistance,Zn was utilized as an alloy element to add in the AA5083 aluminum alloys.The effects of Zn content on the microstructures,mechanical properties and SCC resistance were systematically evaluated.The results demonstrate that in the studied range adding Zn can significantly improve the SCC resistance of the AA5083 alloys.This is related to the relatively low amount of continuous β(Al3Mg2)phase along grain boundary and the formation of Zn-containing phase such as Al5Mg11Zn4 phase.Based on the results,the optimal Zn content with respect to SCC resistance is approximately 0.50 wt.%.Further increasing Zn content results in coarse precipitates discontinuously distributed along grain boundaries.

Multi-scale study of ductility-dip cracking in nickel-based alloy dissimilar metal weld

A ductility-dip-cracking(DDC)-concentrated zone(DCZ) in a width of about 3 mm was observed adjacent to the AISI 316 L/52 Mw fusion boundary(FB) in 52 Mw. The morphology, microstructure, mechanical and thermal properties and corrosion behavior in simulated primary water of DDC/DCZ were investigated by scanning electron microscopy(SEM), transmission electron microscopy(TEM), 3 D X-ray tomography(XRT), 3 D atom probe(3 DAP), slow strain rate tensile(SSRT) testing and thermal dilatometry. The results indicate that DDCs are random-shaped and disc-like cavities with corrugated structure of inner surface and are parallel in groups along straight high-angle boundaries of columnar grains, ranging from micrometers to millimeters in size. Large-size M_(23)C_6 carbides dominate on the grain boundaries rather than MC(M=Nb, Ti), and thus the bonding effect of carbides is absent for the straight grain boundaries.The impurity segregation of O is confirmed for the inner surfaces of DDC. The oxide film formed on the inner surface of DDC(about 50 nm) is approximately twice as thick as that on the matrix(about 25 nm)in simulated primary water. The yield strength, tensile strength and elongation to fracture of 52 MwDCZ(400 MPa, 450 MPa and 20 %, respectively) are lower than those of 52 Mw-MZ(460 MPa, 550 MPa and 28 %, respectively). The intrinsic high-restraint weld structure, the additional stress/strain caused by the thermal expansion difference between AISI 316 L and 52 Mw as well as the detrimental carbide precipitation and the resulting grain boundary structure all add up to cause the occurrence of DCZ in the dissimilar metal weld.

Effect of vanadium on hydrogen embrittlement susceptibility of high-strength hot-stamped steel

Based on the chemical composition of traditional hot-stamped steel(e.g.,22MnB5 and 30MnB5),Nb and V microalloying elements are added into 30MnB5 steel to meet the requirements of ultra-high strength,excellent ductility and potent resistance to hydrogen embrittlement(HE)at the same time.The influence of hot-stamped steel on HE was studied by conducting a hydrogen permeation method and pre-charged hydrogen slow strain rate test.Meanwhile,the experimental steel microstructures and corresponding fracture surfaces are observed and analyzed to characterize HE behavior.The results show that a finer microstructure,a lower apparent diffusion coefficient of hydrogen and a smaller percentage of strength and plasticity reduction are obtained due to the addition of the vanadium element into hot-stamped steel.Compared to the V free experimental steel,the steel with 0.14 wt.% V has a large number of dispersive precipitates and more grain boundary areas,which makes hydrogen atoms dispersedly distribute.

Investigation into Hydrogen Diffusion and Susceptibility of Hydrogen Embrittlement of High Strength 0Cr16Ni5Mo Steel

High strength bolt steel 0Crl6Ni5Mo was charged with hydrogen by means of electrochemical technique to evaluate the hydrogen diffusion behavior. The bolt steels were investigated by a combination of electrochemical hydrogen permeation, thermal desorption spectroscopy （TDS）, slow strain rate test （SSRT） and microstructure observation. The hydrogen concentration of both 10.9 grade （Rm=950-1 150 MPa） and 12.9 grade （Rm=1 150-1 250 MPa） bolt steels increases with increasing the hydrogen charging current densities and charging time. The 12.9 grade bolt steel has higher apparent diffusion coefficient than 10.9 grade steel, corresponding to the value of 4.7×10 7 mm^2/s. By means of TDS tests, the activation energies of the two experimental steels are 17.74 kJ/mol and 18.92 kJ/mol, respectively. The hydrogen traps of both grade bolt steels are dislocations and crystal lattice. The notch tensile strength of the steels is reduced with the hydrogen concentration carried out by SSRT. The fracture morphologies of the steels after hydrogen charging present ductile dimple and quasi-cleavage characteristic.

Influence of inclusions on hydrogen-induced delayed cracking in hot stamping steels

The hydrogen-induced delayed cracking(HIDC)behaviors of two types of 1500 MPa grade hot stamping steels(HSSs)have been investigated by the method of slow strain rate tensile test and hydrogen permeation,where one is manufactured by compact strip production(CSP)process which is a revolution to the traditional HSS and the other by the traditional cold rolling process.The results show that the performance of HSS produced by CSP is superior to that of the traditional HSS,due to lower hydrogen embrittlement index,lower hydrogen diffusion coefficient and lower hydrogen content.It has been found that HIDC behavior is closely associated with inclusions.The inclusions of HSS produced by CSP are mainly spherical Al-Ca-O and CaS,while the inclusions in the traditional HSS are TiN+AI2O3+MnS with sharp edges and corners.Based on these results,the influence of composition,shape and distribution of inclusions in HSS on HIDC and the mechanism of HIDC from the perspective of inclusions were analyzed and discussed.