Dislocation Slip and Crack Nucleation Mechanism in Dual‑Phase Microstructure of Titanium Alloys:A Review

The study on the deformation mechanism of titanium alloys is beneficial to revealing the influence of microstructure on mechanical properties,and then providing guidance for the optimization of microstructure and properties.For most near-αandα+βtitanium alloys,slip is the dominant deformation mechanism.Therefore,investigating the slip initiation and slip transfer behavior,as well as crack nucleation mechanism,is essential to reveal the fundamental relationship between microstructure and mechanical properties.However,due to the coexistence of grain boundary and phase boundary in dualphase microstructure of titanium alloys,the phase content,grain size,grain boundary misorientation andα/βorientation relationship would affect the slip initiation and transfer behavior,resulting in a very complex plastic deformation mechanism.Based on the previous investigations of deformation mechanism of near-αandα+βtitanium alloys,this review first analyzed the sequence of slip initiation betweenαandβphases and discussed the main factors affecting the slip initiation inαphase.Secondly,the basic rule of slip transfer and the influence of different interfaces on slip transfer were reviewed.Finally,the mechanism of crack nucleation and effect of microstructure on crack nucleation were analyzed based on slip transfer behavior.

Crack nucleation and propagation simulation in brittle two-phase perforated/particulate composites by a phase field model

Fracture is a very common failure mode of the composite materials,which seriously affects the reliability and service-life of composite materials.Therefore,the study of the fracture behavior of the composite materials is of great significance and necessity,which demands an accurate and efficient numerical tool in general cases because of the complexity of the arising boundary-value or initial-boundary value problems.In this paper,a phase field model is adopted and applied for the numerical simulation of the crack nucleation and propagation in brittle linear elastic two-phase perforated/particulate composites under a quasi-static tensile loading.The phase field model can well describe the initiation,propagation and coalescence of the cracks without assuming the existence and the geometry of the initial cracks in advance.Its numerical implementation is realized within the framework of the finite element method(FEM).The accuracy and the efficiency of the present phase field model are verified by the available reference results in literature.In the numerical examples,we first study and discuss the influences of the hole/particle size on the crack propagation trajectory and the force-displacement curve.Then,the effects of the hole/particle shape on the crack initiation and propagation are investigated.Furthermore,numerical examples are presented and discussed to show the influences of the hole/particle location on the crack initiation and propagation characteristics.It will be demonstrated that the present phase field model is an efficient tool for the numerical simulation of the crack initiation and propagation problems in brittle two-phase composite materials,and the corresponding results may play an important role in predicting and preventing possible hazardous crack initiation and propagation in engineering applications.

3D analysis for pit evolution and pit-to-crack transition during corrosion fatigue

This paper presents a deterministic model to predict the pit evolving morphology and crack initiation life of corrosion fatigue.Based on the semi-ellipsoidal pit assumption,the thermodynamic potential including elastic energy,surface energy and electrochemical energy of the cyclically stressed solid with an evolving pit is established,from which specific parameters that control the pit evolution are introduced and their influence on the pit evolution are evaluated.The critical pit size for crack nucleation is obtained from stress intensity factor criterion and the crack nucleation life is evaluated by Faraday's law.Meanwhile,this paper presents a numerical example to verify the proposed model and investigate the influence of cyclic load on the corrosion fatigue crack nucleation life.The corrosion pit appears approximately as a hemisphere in its early formation,and it gradually transits from semicircle to ellipsoid.The strain energy accelerates the morphology evolution of the pit,while the surface energy decelerates it.The higher the stress amplitude is,the smaller the critical pit size is and the shorter the crack initiation life is.

Analysis of crack initiation location and its influencing factors of fretting fatigue in aluminum alloy components

To promote the development of fretting fatigue assessment and control technology for aircraft components,this paper uses the Crystal Plasticity Finite Element(CPFE)method and sub-modeling technology to study the Crack Initiation Location(CIL)of fretting fatigue in Aluminum Alloy(AA)specimens.The effects of external excitations such as normal load,tangential load,and axial stress on the CIL are investigated.It is found that the Most Likely Cracked(MLC)site revealed in a specimen and the CIL may always be consistent after a limited number of cyclic loadings,and they are both located at the hotspot on the contact surface or in the subsurface.The MLC site may also migrate from the hotspot on the contact surface to the hotspot in the subsurface with an increase of the cyclic number,and finally transform into a CIL.The relationship between the MLC site and the CIL of fretting fatigue and its influencing factors have also been described,as well as the identification method of the CIL of fretting fatigue,which provide theoretical and technical supports for anti-fretting fatigue design of AA components in service.

A pathway to improve low-cycle fatigue life of face-centered cubic metals via grain boundary engineering

To probe a pathway to improve the low-cycle fatigue life of face-centered cubic(FCC)metals via grain boundary engineering(GBE),the tension-tension fatigue tests were carried out on the non-GBE and GBE Cu-16 at.%Al alloys at relatively high stress amplitudes.The results indicate that the cyclic strain localiza-tion and cracking at grain boundaries(GBs)can be effectively suppressed,especially at increased stress amplitude,by an appropriate GBE treatment that can result in a higher resistance to GB cracking and a greater capability of compatible deformation.Therefore,the sensitivity of fatigue life to stress amplitude can be weakened by GBE,and the low-cycle fatigue life of Cu-16 at.%Al alloys is thus distinctly improved.

Cyclic oxidation behavior of NiCoCrAlY/YSZ@Ni composite coatings fabricated by laser cladding

Composite coating on GH4169 alloy is prepared by laser cladding yttria-stabilized zirconia(YSZ)@Ni core-shell powdersmixed with NiCoCrAlY alloy powders.The cyclic oxidation behavior of the coatings,especially the growth process of theoxide layer,is investigated based on experimental research and first-principle calculations.The results indicate that theoxidation resistance of coated GH4169 alloy is beer than that of uncoated GH4169 alloy.The coating has three layers:acellular dendrite outer layer,a planar YSZ interlayer,and an inner layer composed of Cr2O3 formed during laser clading.After oxidation at 1000 and 1050℃,as the oxidation time increases,the cellular dendrite outer layer becomes thicker,andthe planar yttria-stabilized zirconia interlayer becomes thinner.Between the planar interlayer and Cr2O3 inner layer,and Al2O3 layer formed.Notably,cracks formed in the intcrface of Al2O3/Cr2O3 owing to thcir weak interface strength,whichled to the failure of the composite coating.