Grain boundary engineering for enhancing intergranular damage resistance of ferritic/martensitic steel P92

Ferritic/martensitic(F/M)steel is widely used as a structural material in thermal and nuclear power plants.However,it is susceptible to intergranular damage,which is a critical issue,under service conditions.In this study,to improve the resistance to intergranular damage of F/M steel,a thermomechanical process(TMP)was employed to achieve a grain boundary engineering(GBE)microstructure in F/M steel P92.The TMP,including cold-rolling thickness reduction of 6%,9%,and 12%,followed by austenitization at 1323 K for 40 min and tempering at 1053 K for 45 min,was applied to the as-received(AR)P92 steel.The prior austenite grain(PAG)size,prior austenite grain boundary character distribution(GBCD),and connectivity of prior austenite grain boundaries(PAGBs)were investigated.Compared to the AR specimen,the PAG size did not change significantly.The fraction of coincident site lattice boundaries(CSLBs,3≤Σ≤29)and Σ3^(n) boundaries along PAGBs decreased with increasing reduction ratio because the recrystallization fraction increased with increasing reduction ratio.The PAGB connectivity of the 6%deformed specimen slightly deteriorated compared with that of the AR specimen.Moreover,potentiodynamic polarization studies revealed that the intergranular damage resistance of the studied steel could be improved by increasing the fraction of CSLBs along the PAGBs,indicating that the TMP,which involves low deformation,could enhance the intergranular damage resistance.

Microstructure and corrosion resistance of sintered NdFeB magnet modified by intergranular additions of MgO and ZnO

Microstructure and corrosion resistance of sintered Nd15Dy1.2Fe77Al0.8B6 and Nd22Fe71B7 magnets modified by intergranular addition of MgO and ZnO were investigated. Both the remanence and sintering density of the magnets increased slightly with intergranular additions of MgO and ZnO. There was a remarkable increase in coercivity of Nd22Fe71B7 after addition. Besides, the effects on magnetic properties and an improved corrosion resistance were observed. Compared with the native magnets without addition, corrosion potential of the magnets with MgO and ZnO additives was more positive and the current density in the anodic branch of the polarization curve was reduced. Corrosion resistance resulting from autoclave testing （2×10^5 Pa of steam pressure, 120 ℃） showed that the corrosion rate of NdFeB magnets reduced with the increase of additive amount. Microstructure observation revealed that MgO and ZnO additives were incorporated into the intergranular phases in the magnets. With the introduction of MgO and ZnO, more intergranular phase with high oxygen content was formed while keeping the volume fraction of all the intergranular phases almost unchanged, which may contribute to improved corrosion resistance. Furthermore, addition of MgO and ZnO refined the grain size of Nd22Fe71B7.

Intergranular corrosion behavior of high nitrogen austenitic stainless steel

The intergranular corrosion （IGC） behavior of high nitrogen austenitic stainless steel （HNSS） sensitization treated at 650-950℃ was investigated by the double loop electrochemical potentiodynamic reactivation （DL-EPR） method. The effects of the electrolytes, scan rate, sensitizing temperature on the susceptibility to IGC of HNSS were examined. The results show that the addi-tion of NaCl is an effective way to improve the formation of the cracking of a passive film in chromium-depleted zones during the reactivation scan. Decreasing the scan rate exhibits an obvious effect on the breakdown of the passive film. A solution with 2 mol/L H2SO4＋1 mol/L NaCl＋0.01 mol/L KSCN is suitable to check the susceptibility to IGC of HNSS at a sensitizing temperature of 650-950℃ at a suitable scan rate of 1.667 mV/s. Chromium depletion of HNSS is attributed to the precipitation of Cr2N which results in the susceptibility to IGC. The synergistic effect of Mo and N is suggested to play an important role in stabilizing the passive film to prevent the attack of IGC.

The effects of sulfide stress cracking on the mechanical properties and intergranular cracking of P110 casing steel in sour environments

Variation and degradation of P-110 casing steel mechanical properties, due to sulfide stress cracking （SSC） in sour environments, was investigated using tensile and impact tests. These tests were carried out on specimens, which were pretreated under the following conditions for 168 hours： temperature, 60 ℃; pressure, 10 MPa; H2S partial pressure, 1 MPa and CO2 partial pressure, 1 MPa; preload stress, 80% of the yield strength （os）; medium, simulated formation water. The reduction in tensile and impact strengths for P-110 casing specimens in corrosive environments were 28% and 54%, respectively. The surface morphology analysis indicated that surface damage and uniform plastic deformation occurred as a result of strain aging. Impact toughness of the casing decreased significantly and intergranular cracking occurred when specimens were maintained at a high stress level of 85% %.

Diversity of intergranular corrosion and stress corrosion cracking for 5083 Al alloy with different grain sizes

5083 Al alloy sheets with different grain sizes(8.7-79.2 μm) were obtained by cold rolling and annealing. Their microstructures, intergranular corrosion(IGC), stress corrosion cracking(SCC), and crack propagation behaviors were investigated. The results showed that samples with coarse grains exhibit better IGC resistance with a corrosion depth of 15 μm. The slow strain rate test results revealed that fine-grained samples exhibit better SCC resistance with a susceptibility index(ISSRT) of 11.2%. Furthermore, based on the crack propagation mechanism, grain refinement can improve the SCC resistance by increasing the number of grain boundaries to induce the corrosion crack propagation along a tortuous path. The grains with {011} orientation could hinder crack propagation by orientating it toward the low-angle grain boundary region. The crack in the fine-grained material slowly propagates due to the tortuous path, and low H;and Cl;concentrations.

Effects of Aging Treatment on Intergranular Corrosion and Stress Corrosion Cracking Behavior of AA7003

In order to study the effects of aging treatment on the intergranular corrosion（IGC） and stress corrosion cracking（SCC） of 7003 aluminum alloy（AA7003）, the intergranular corrosion test, electrochemical test and slow strain rate test（SSRT）, combined with optical microscopy（OM） and scanning electron microscopy（SEM） as well as transmission electron microscopy（TEM） observations have been carried out. The IGC and electrochemical test results showed that the IGC resistance of AA7003 for peak aged（PA） temper is the lowest, with double peak aged（DPA） the moderate, and retrogression and re-aging（RRA） the highest among three tempers, which is attributed to the continuous feature of precipitation on grain boundary of PA temper and the interrupted feature of precipitation on grain boundary of DPA and RRA tempers, as well as the wide precipitation free zones（PFZ） of RRA temper. In addition, the SSRT results indicated that all three tempers AA7003 are susceptible to SCC in IGC solution, and the change tendency of SCC susceptibility（ISCC） of AA7003 for three tempers follows the order： ISCC（RRA）

Simulation of PFZ on intergranular fracture based on XFEM and CPFEM

A unit cell including the matrix, precipitation free zone(PFZ) and grain boundary was prepared, and the crystal plasticity finite element method(CPFEM) and extended finite element method(XFEM) were used to simulate the propagation of cracks at grain boundary. Simulation results show that the crystallographic orientation of PFZ has significant influence on crack propagation, which includes the crack growth direction and crack growth velocity. The fracture strain of soft orientation is larger than that of hard orientation due to the role of reducing the stress intensity at grain boundary in intergranular brittle fracture. But in intergranular ductile fracture, the fracture strain of soft orientation may be smaller than that of hard orientation due to the roles of deformation localization.

Numerical Simulation on Intergranular Microcracks in Interconnect Lines Due to Surface Diffusion Induced by Stress-,Electro-and Thermo-migration

Based on the weak formulation for combined surface diffusion and evaporation-condensation,a governing equation of the finite element is derived for simulating the evolution of intergranular microcracks in copper interconnects induced simultaneously by stressmigration,electromigration and thermomigration.Unlike previously published works,the effect of thermomigration is considered.The results show that thermomigration can contribute to the microcrack splitting and accelerate the drifting process along the direction of the electric field.The evolution of the intergranular microcracks depends on the mechanical stress field,the temperature gradient field,the electric field,the initial aspect ratio and the linewidth.And there exists a critical electric fieldχ_c,a critical stress field■,a critical aspect ratioβ_c and a critical linewidth■.When■or■,the intergranular microcrack will split into two or three small intergranular microcracks.Otherwise,the microcrack will evolve into a stable shape as it migrates along the interconnect line.The critical stress field,the critical electric field and the critical aspect ratio decrease with a decrease in the linewidth,and the critical linewidth increases with an increase in the electric field and the aspect ratio.The increase of the stress field,the electric field or the aspect ratio and the decrease of the linewidth are not only beneficial for the intergranular microcrack to split but also accelerate the microcrack splitting process.

Transformation mechanism of secondary phase and its effect on intergranular corrosion in laser wire filling welding Ni-based alloy/304 stainless steel

To clarify the transformation mechanism of secondary phase and the mechanism of intergranular corrosion in laser welding Ni-based alloy (Hastelloy C-276)/304 stainless steel with filler wire,the secondary phase was analyzed by electron probe micro-analysis (EPMA) and transmission electron microscopy (TEM).The evaluation of intergranular corrosion resistance of the welded joints was conducted by double-loop electrochemical potentiokinetic reactivation(DL-EPR) method,and at the same time the chemical compositions of the corrosion surface were analyzed by energy-dispersive spectrometry (EDS).The results show that p phase has complete coherence relationship withμphase,and the coherent relationship is described as[001]p//■and[430]p//[0001]μ.Theμphase is rapidly transformed from p phase,which is the inhomogeneous phase transformation.The transformation of secondary phase will increase the susceptibility to intergranular corrosion.Therefore,the transformation of secondary phase should be avoided in the welding process.

GRAIN BOUNDARY SEGREGATION AND INTERGRANULAR BRITTLENESS IN HIGH STRENGTH ALUMINIUM ALLOY

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Effects of interrupted ageing and asymmetric rolling on microstructures, mechanical properties, and intergranular corrosion behavior of Al-Mg-Si-Zn alloy

Effects of interrupted ageing(T6I6) and asymmetric rolling on microstructures, mechanical properties, and intergranular corrosion(IGC) behaviors of Al-Mg-Si-Zn alloy were investigated. Results showed that the T6 alloy has the lowest strength and the worst IGC resistance, while the T6I6 alloy has higher strength and better IGC resistance.What’ s more, the alloy treated by pre-rolling deformation has higher strength and better IGC resistance;and the alloy treated by the pre-asymmetry rolling achieves the highest strength, the best IGC resistance and lower elongation. The mechanical properties depend on microstructures such as the grain size, texture, dislocation density and precipitation, the grain boundary misorientation and grain boundary microstructure are responsible for the IGC resistance.

Improvement of Intergranular Stress Corrosion Crack Susceptibility of Austenite Stainless Steel through Grain Boundary Engineering

Intergranular stress corrosion crack susceptibility of austenite stainless steel was evaluated through threepoint bending test conducted in high temperature water. The experimental results showed that the frequent and efficient introduction of low energy coincidence site lattice boundaries through grain boundary engineering resulted in an apparent improvement of the intergranular stress corrosion crack resistance of austenite stainless steel.

Effects of Spinning Process on Intergranular Corrosion Behavior of 5A06 Aluminum Alloy

Aluminum alloy tubes were prepared by tube spinning.The intergranular and electrochemical corrosion tests were used to investigate the intergranular corrosion behavior of the 5A06 aluminum alloy blank sample and the spinning sample.Results showed that the intergranular corrosion resistance of the spinning sample was higher than that of the blank sample.In addition,the electrochemical corrosion resistance of the spinning sample was higher than that of the blank sample.The EDS maps indicated a uniform element distribution pattern of aluminum and magnesium.Moreover,the phase composition and lattice constant of the samples were obtained by XRD analysis.The differences in microstructure between the aluminum alloy subjected to the spinning process and the untreated aluminum alloy were determined by EBSD.The differences were mainly attributed to the complex interactions among grain size,dislocations and grain boundaries.

PREVENTION OF THE INTERGRANULAR FRACTURE AT ELEVATED TEMPERATURES BY ADDITION OF PHOSPHORUS TO A HIGH PURITY Fe0 002% SALLOY

Theeffectof phosphorusonthehotductility andthesegregation ofsulfuratgrain boundariesin high purityironcontaining 0 002 % sulfur werequantitativelyinvestigated bytensiletestat973 K, SEMobservation of fracture surface and scanning Auger electron spectroscopy.Theexperimental resultscan besum marized as follows:(1) addition of phosphorustothe iron remarkablysuppressestheintergranular fractureinduced bythesegregation of sulfur at973 Kandincreasesthe hot ductility;(2) phosphorusstrikingly decreasesthesegregation ofsulfur atgrain boundaries, whichisresponsibleforthesuppressionoftheintergranularfrac tureat973 K;(3) theremarkably decreasedsegregation ofsulfur atgrain boundariesbytheaddition of phosphoruscan beexplained bythesitecompetitioneffectandtherepulsiveinter action at grain boundariesbetweensulfuratomsand phosphorusatoms;(4) sulfurstrikinglysegregatestothevoid surfacesformed on grain boundaries, while phosphorushaslittlesegre gationtothevoid surfaces.

Intergranular Corrosion of UNS S31803 Heat Treated at 800 ℃ Varying Range Times

The aim of this work is evaluate the intergranular corrosion on UNS S31803 steel, with heat treatments at 800 ℃, varying treatment times of 30 mins, 360 mins and 1,440 mins. The results confirm the formation of o phases and secondary austenite （γ2）. For the analysis of the influence of o and γ2 phases, metallographic analysis were conducted through optical microscopy, potentiokinetic reactivation electrochemical techniques and potentiodynamic polarization in NaCI 3.5% solution. Microstructural analysis has shown a formation of γ2 and o phase in heat treatment, due to diffusion of chromium and molybdenum from δ phase to y phase, precipitating on δ/γ and δ/δ interfaces. The DL-EPR （Double loop electrochemical potentiokinetic reactivation） results have shown an increase of the DOS （degree of sensitization） for long periods of time on heat treatment. The results of potentiodynamic polarization showed a reduction of the corrosion and pitting potentials, followed by an increase of the current density when the UNS S31830 steel is heat treated during long periods of time.

Intergranular Corrosion of AISI 304 Heat Treated at 800℃ Varying Range Times

Austenitic stainless steels, when exposed to welding conditions or aging for length of service, it＇s observed the formation of numerous deleterious phases, such as several kinds of carbides type MC, M6C, M7C3, M23C6, and intermetallic secondary phases （sigma, chi, laves）, which cause the process of intergranular corrosion. The aim of this work was verifying the formation of the types of carbides and/or intermetallic phases existing in the stainless AISI 304 at 800 ℃, varying the timing of heat treatment between 30, 360 and 1,440 min. The optical microscopy analysis revealed the predominant formation of the carbide type M23C6. The results of DL-EPR （double loop electrochemical potentiokinetic reactivation） tests showed a gradual increase in the precipitation of this carbide with the increase of treatment time. The potentiodynamic polarization showed that the precipitation of this carbide reduce the formation of the Cr2O3 passive layer, suggesting that the precipitate carbide to be predominantly of the Cr23C6 type.

Acoustic emission detection of 316L stainless steel welded joints during intergranular corrosion

This study analyzes acoustic emission （AE） signals during the intergranular corrosion （IGC） process of 316L stainless steel welded joints under different welding currents in boiling nitric acid. IGC generates several AE signals with high AE activity. The AE technique could hardly distinguish IGC in stainless steel welded joints with different welding heat inputs. However, AE signals can effectively distinguish IGC characteristics in different corrosion stages. The IGC resistance of a heat-affected zone is lower than that of a weld zone. The initiation and rapid corrosion stages can be distinguished using AE results and microstructural analysis. Moreover, energy count rate and amplitude are considered to be ideal parameters for characterizing different IGC processes. Two types of signals are detected in the rapid corrosion stage. It can be concluded that grain boundary corrosion and grain separation are the AE sources of type 1 and type 2, respectively.

Intergranular Fatigue Cracking Behaviour in Aluminium

In order to understand the basic mechanism of intergranular cracking in pure metals during fatigue, stress-controlled push-pull fatigue tests were carried out with high purity aluminium. Tests were interrupted frequently so as to study the grain boundary (GB) cracking behaviour by the surface observation. The results show that crack initiation at GB was a process controlled by multi-factors, such as boundary structure, GB-slip interaction, GB sliding and so on. If these factors are varied so that the incompati- bility at a GB increased, the possibility of cyacking at the boundary will be raised. Some inteygranular cracking phenomena are not able to be explained by the GB stepping mechanism.

On the intergranular fracture behavior of high-temperature plastic deformation of 1420 Al-Li alloy

The tensile deformation hot simulation test of as-cast 1420 Al-Li alloy was performed on Gleeble-1500 Thermal Simulator in the deformation temperature range from 350 to 450 ℃ and the strain rate range from 0.01 to l0.0s-1.The tensile fracture behavior of the 1420 Al-Li alloy at high temperature was studied experimently. The results show that the tensile fracture mode of the 1420 Al-Li alloy at high temperature is changed from typical transgranular ductile fracture to intergranular brittle fracture with the increase of the deformation temperature and the strain rate. It is made out that the precipitation of LiH is the fundamental reason for the intergranular brittle fracture of the 1420 Al-Li alloy at high temperature. The mechanism of hydrogen embrittlement of the 1420 Al-Li alloy at high temperature was discussed, and it was proposed that the hydrogen embrittlement at high temperature is an integrated function of the dynamic and the static force, which enrichs the theories of hydrogen embrittlemen t.

INTERGRANULAR AND EXFOLIATION CORROSION BEHAVIOUR OF 8090Al-Li ALLOY

The A1-2.79Li-1.3Cu-0.7Mg-0.12Zr(8090) alloy has excellent resistance to intergranular and exfoliation corrosion as naturally aged, worst as peak aged and rather better as underaged than as overaged. The corrosion behaviour of the alloy is closely dependent on its microstructure. Under the peak aging condition, the coarse T_2 phase, which is continuously distributed along grain boundaries, has so low corrosion potential in comparison with the ma- trix, this makes the corrosion resistance of the alloy to be deteriorated by anodic dissolution along grain boundaries.