Hydrogen-enhanced dislocation emission, motion and nucleation of hydrogen-induced cracking for steel

The change in dislocation configuration ahead of a loaded crack tip before and after charging with hydrogen was in situ investigated in TEM using a special constant deflection loading device The results showed that hydrogen could facilitate dislocation emission, multiplication and motion The change in displacement field ahead of a loaded notch tip for a bulk specimen before and after charging with hydrogen was in situ measured by the laser moire interferometer technique. The results showed that hydrogen could enlarge the plastic zone and increase the plastic strain The in situ observation in TEM showed that when hydrogen-enhanced dislocation emission and motion reached a critical condition, a nanocrack of hydrogen-induced cracking ( HIC) would nucleate in the dislocation-free zone (DFZ) or at the main crack tip. The reasons for hydrogen-enhanced dislocation emission, multiplication and motion, and the mechanisms of nucleation of HIC have been discussed     

An investigation of corrosion-induced stress during SCC

TEM (Transmission Electron Microscope) observations show that corrosion process during stress corrosion cracking (SCC) enhances dislocation emission and motion; and microcrack of SCC initiates when the corrosion-enhanced dislocation emission and motion reaches a certain condition. The passive film or dealloyed layer formed during corrosion or SCC can induce a large tensile stress, which can assist the applied stress to enhance dislocation emission and motion, and then SCC occurs. Experiments show that the variation of SCC susceptibility of brass,α-Ti and stainless steel with the applied potential and pH value of the solution is consistent with that of the corrosion-induced additive stress. Molecular dynamics simulations show that a dealloyed layer can generate a tensile stress; and the corrosion (dealloyed layer)-induced tensile stress can assist the applied stress to enhance dislocation emission and crack propagation.     

Film-induced stress enhancing stress corrosion cracking of austenitic stainless steel

A constant deflection device designed for use within a transmission electron microscopy (TEM) was used to investigate the change in dislocation configuration ahead of a crack tip during stress corrosion cracking (SCO of type 310 austenitic stainless steel in a boiling MgCI2 solution, and the initiation process of stress corrosion microcrack. Results showed that corrosion process during SCC enhanced dislocation emission, multiplication and motion. Microcracks of SCC were initiated when the corrosion-enhanced dislocation emission and motion reached critical state.A passive film formed during corrosion of austenitic stainless steel in the boiling MgCI2 solution generated a tensile stress. During SCC, the additive tensile stress generated at the metal/passive film interface helps enhance dislocation emission and motion.     

Chemisorption-facilitated dislocation emission and motion, and induced nucleation of brittle nanocrack

Using a special TEM constant deflection device, the change in dislocation configuration ahead of a loaded crack tip before and after adsorption of Hg atoms and the initiation of liquid metal-induced nanocracks (LMIC) have been observed. The results show that chemisorption of Hg atoms can facilitate dislocation emission, multiplication and motion. Nanocracks will be initiated in the dislocation-free zone (DFZ) or at the crack tip when chemisorption-facilitated dislocation emission, multiplication and motion reach a critical condition. On the basis of the available experimental evidence concerning liquid metal embnttlement (LME), a new mechanism for this phenomenon is considered. This involves the fact that the decrease in surface energy induced by chemisorption of Hg atoms results in a reduction in the critical stress intensity factors for dislocation emission and the resistance for dislocation motion. On the other hand, the plastic work and KIC will decrease with the decrease in the surface energy.     

TEM Study on Hydrogen-Induced Cracking of Fe3Al Alloy

Hydrogen-induced cracking (HIC) of Fe3AI alloy was studied by in silo transmission electron microscope (TEM). Electron transparent specimens were moaned onto a constant displacement device. Stress was applied to the specimen by using a bolt through the device. The results showed that hydrogen enhanced the dislocation emission and motion in Fe3Al alloy. A dislocation free zone (DFZ) was formed following the dislocation emission. Microcrack initiated in the DFZ or at the main crack tip when the emission reached a critical extension. Hydrogen played an important role in the process of brittle fracture of Fe,AI alloy.     

Stress distribution and effective stress intensity factor of a blunt crack after dislocation emission

The stress fields induced by a dislocation and its image dislocations around a narrow elliptic void are formulated. Based on the solution, the stress distribution and effective stress intensity factor of a blunt (elliptic) crack were calculated under mode I constant loading. The results show that a dislocation-free zone (DFZ) is formed after dislocation emission. There exists a second stress peak in the DFZ except a stress peak at the blunt crack tip. With an increase in the applied stress intensity factor Kla or the friction stress T, of the material, the DFZ size and the peak stress at the crack tip decrease, but the peak stress in the DFZ and the effective stress intensity factor Klf presiding at the crack tip increase. Because of dislocation shielding effects, shielding ratio Kla/Klf increases with increasing Kla, but it decreases with increasing Tf.     

Effect of fine precipitates on initiation and propagation of micro-cracks in low alloy Cr-Mo-V steel

0　ＩＮＴＲＯＤＵＣＴＩＯＮＴｈｅｃｏａｒｓｅｐｒｅｃｉｐｉｔａｔｅｓ,ｐａｒｔｉｃｕｌａｒｌｙｔｈｅｇｒａｉｎｂｏｕｎｄａｒｙｃａｒｂｉｄｅｓ,ｃａｎｃｒａｃｋｄｕｒｉｎｇｄｅｆｏｒｍａｔｉｏｎａｎｄｒｅｓｕｌｔｉｎｃｌｅａｖａｇｅｆａｉｌｕｒｅ,ｂｒｉｎｇｄｏｗｎｔｈｅｔｏｕｇｈｎｅｓｓｏｆｓｔｅｅｌｓ[1].Ｈｏｗｅｖｅｒ,ｔｈｅｉｎｆｌｕｅｎｃｅｏｆｔｈｅｆｉｎｅｓｃａｌｅｐｒｅｃｉｐｉｔａｔｅｓ(ｌｅｓｓｔｈａｎ100ｎｍｉｎｓｉｚｅ)ｏｎｄｅｆｏｒｍａｔｉｏｎａｎｄｆｒａｃｔｕｒｅｂｅｈａｖｉｏｒ…     

Molecular Dynamics Simulation and Experimental Proof of Hydrogen-enhanced Dislocation Emission in Nickel

A quasi three dimensions molecular dynamic method was used to simulate the effect of hydrogen on dislocation emission and crack propagation in nickel. In situ observation in a transmission electron microscope (TEM) was used to confirm the simulation results. The simulation result indicated that hydrogen solubilized in nickel decreased the critical stress intensity for the dislocation emission, i.e., hydrogen enhanced dislocation emission. In situ observation in TEM showed that hydrogen enhanced dislocation emission and motion before the initiation of hydrogen-induced crack.     

Liquid metal embrittlement mechanism

Liquid metal embrittlement was studied in the following two aspects. First the first principle and ChenNanxian three-dimensional lattice reverse method were employed to obtain the effective potentials for Al-Ga and GaGa. Then with the molecular dynamics simulation, the influence of liquid metal adsorption on dislocation emission was studied. The simulated result shows that after Ga atoms are adsorbed on the crack plane in Al crystal, the critical stress intensity factor decreases, which changes from 0.5 MPam~(1/2) (without adsorption) to 0.4 MPam~(1/2) (with adsorption). The reason for the reduction in the critical intensity stress factor is that Ga adsorption reduces the surface energy of the crack plane. Moreover, 7075 Al alloy adsorbing liquid metal (Hg 3atm％Ga) was in-situ studied in TEM by using a special constant deflection device. The experimental result showed that liquid metal adsorption could facilitate emission, multiplication and motion of dislocations. When this process reached a critical     

Microscopic study of mode I crack tip deformation

By simulating edge dislocation emissions from a mode I crack tip along multiple inclined slip planes, the plastic zone and dislocation-free zone around the crack tip are obtained. It is found that the shape of the mode I plastic zone consists of two leaning forward loops which is better agreement with experimental observations. Except at the crack tip there are also stress peaks in front of the crack tip. A formula of the maximum peak stress as a function of the applied stress intensity factor and the friction stress has been regressed.     

Localized Corrosion and Stress Corrosion Cracking Behavior of AA7003 in a 3.5 wt% NaCl Aqueous Solution

The microstructure,localized corrosion (LC) and stress corrosion cracking (SCC) behavior of 7003 aluminum alloy (AA7003) under various aging treatments (peak aging (PA),double peak aging (DPA),regression and re-aging (RRA)) were investigated by means of transmission electron microscope (TEM),scanning electron microscopy (SEM),electrochemical impendence spectroscopy (EIS) and slow strain rate tensile test.The results of TEM showed a discontinuous distribution of grain boundary precipitates of AA7003 under DPA and RRA treatments,which is beneficial for increasing the resistance of LC and SCC.Meanwhile,LC was found initiating firstly on intermetallics which caused the dissolution of surrounding matrix,then pitting holes were formed and developed into matrix.In addition,the SCC process of AA7003 could be divided into two stages,i e,initial pre-cracking and breeding cracking.The EIS analysis,cross-section morphologies and fracture surfaces of specimens indicated that DPA and RRA treatments significantly decreased the crack growth rate during breeding cracking stage,especially for RRA treatment.     

Molecular dynamics simulation of hydrogen enhancing dislocation emission

Ｍａｎｙｅｘｐｅｒｉｍｅｎｔｓｓｈｏｗｔｈａｔｈｙｄｒｏｇｅｎｃａｎｅｎｈａｎｃｅｄｉｓｌｏｃａｔｉｏｎｅｍｉｓｓｉｏｎ,ｍｕｌｔｉｐｌｉｃａｔｉｏｎａｎｄｍｏｔｉｏｎ[1,2].Ｉｎｏｒｄｅｒｔｏｐｒｏｃｅｓｓｔｈｅｍｏｌｅｃｕｌａｒｄｙｎａｍｉｃｓｓｉｍｕｌａｔｉｏｎｏｆｈｙｄｒｏｇｅｎｅｎｈａｎｃｉｎｇｄｉｓｌｏｃａｔｉｏｎｅｍｉｓｓｉｏｎ,ｗｅｎｅｅｄｎｏｔｏｎｌｙｔｈｅｐｏｔｅｎｔｉａｌｓｏｆｐｕｒｅｍｅｔａｌａｎｄｈｙｄｒｏｇｅｎ,ｂｕｔａｌｓｏｔｈｅｉｎｔｅｒａｔｏｍｉｃｐｏｔｅｎｔｉａｌ…     

基体界面屈服强度和残余应力失配对界面裂纹扩展的影响

对基体界面屈服强度和残余应力失配情况下的界面裂纹扩展机制进行了力学理论分析,并通过位错与裂纹的关系理论计算了界面裂纹尖端塑性区尺寸。结果表明:当界面裂纹由低强度区向高强度区扩展时,高强度区的残余压应力和屈服强度减少了裂纹尖端的塑性区长度和裂纹尖端张开位移,从而起到了抑制界面裂纹扩展的作用。     

Influence of material＇s cyclic deformation behaviour on fatigue crack growth threshold

To investigate the relation between material＇s cychc plastic behaviour and fatigue crack growth, a new model is proposed. The model incorporated the two intrinsic properties of material＇ s cyclic plastic and crack tip＇ s deformation dislocation to interpret fatigue crack threshold. The relation between material＇ s cyclic hardening parameters （cyclic hardening amplitude and cyclic hardening rate） and fatigue threshold is studied. Fatigue threshold is determined based on the dislocation-free zone （DFZ） model, the theory of cohesive zone and the cyclic deformation behaviour. The results show that fatigue threshold increases with the decrease of the amplitude of cyclic hardening and is independent of cyclic hardening rate, but fatigue crack growth rate increases with the increase of cyclic hardening rate.     

Crack initiation ahead of piled—up of dislocations emitted from a mode Ⅱ blunt crack

In situ tensile tests in a transmission electron microscope (TEM) show that dislocations emitted from a mode II crack tip will form a inverse piled-up group after equilibrium or a double piled-up group when they meet a obstruction, e.g., grain boundary or second phase. A microcrack can initiates in front of the piled-up group of dislocations. Micromechanics analysis shows that dislocations emitted from a mode II blunt crack tip can form a inverse piled-up or double piled-up group, depending upon the applied stress intensity factor K_(lla), lattice friction stress τ_ f and the distance of the obstruction from the crack tip L. The maximum normal stress in front of the double piled-up group which is located at the direction of α = -64° increases with the increase in the stress intensity K_(lla) and the obstruction site L, and the decrease in the friction stress τ_f When it increases to equate the cohesive strength, a microcrack will initiate in front of the piled-up group.     

Relativity between corrosion—induced stress and stress corrosion cracking of brass in an ammonia solution

The susceptibility to stress corrosion cracking (SCC) of brass in an ammonia solution with various pH values or under various applied potentials was measured at slow strain rate tests. The additive stress in the same solution was measured using two methods. The results indicate that the variation of the susceptibility to SCC with pH value or with potential is in an excellent agreement with the corrosion (passive film or dezincification layer)-induced stress. When pH≥7, the corrosion-induced tensile stress andthe susceptibility to SCC have maximum values and hardly change with increasing the pH value. However, when pH < 7, both the corrosion-induced tensile stress and the susceptibility to SCC reduce rapidly with decreasing the pH value. Both the corrosion-induced tensile stress and the susceptibility to SCC have maximum values at the open-circuit potential, decrease slightly under the anodic polarization, and reduce gradually to zero under the cathodic polarization.     

裂纹尖端的位错密度及其应用

本文为弹性断裂力学分析提供了一个新的变量：位错密度．并论证了这一变量在裂纹尖端具有1√r所阶的奇异性．分别给出了三类裂纹问题裂尖位错密度与应力强度因子的关系，对这一变量在数值计算中的应用进行了探讨．     

应力在16MnR钢—饱和硫化氢溶液应力腐蚀体系中的作用

通过分析16MnR钢在饱和硫化氢溶液中的应力腐蚀现象,发现了应力与材料自腐蚀电位的关系,分析了应力在不同应力腐蚀类型的裂纹孕育、扩展过程的作用。     

In situ observation of transmission and reflection of dislocations at twin boundary in CoCrNi alloys

The deformation mechanism of CoCrNi alloy with high density of annealing twins was studied by in situ transmission electron microscopy. Dislocation transmission and reflection at the twin boundary were observed during in situ loading. We characterized these reaction processes by combining TEM, dislocation theory and crystallography of twin. Twin boundary not only strengthens the material by impeding the motion of dislocation, but also acts as dislocation source to produce large of slip bands. These processes generate large of slip bands to accommodate the plastic deformation or strengthening material.     

SCC Susceptibility of Steel 16Mn in Nitrate Solution and Its Mechanism

The SCC(stress corrosion cracking )susceptibility of steel 16Mn in nitrate solution was studied.The results showed that applied potential polarization would accelerate (anodic polarization)or retard(cathodic polarization)the SCC process.The study on phase electrochemistry revealed that there was significant difference in electrochemical performacnce between ferrite and pearlite of steel 16Mn.Pearlite preferentially corroded under the action of galvanic cell.The observation on time and in situ showed that corrosion started first at the phase boundary between ferrite and pearlite,and the pearklite gradually corroded until disappared.and then corrosion corssed the phase boundary extending into the ferrite phase.According to this,an anodic dissolution mechanism of SCC was proposed ,on which pre-existing active path and phase electrochemistry(PEAP-PEC) jointly came into action (SCC mechanism of PEAP-PEC).