喷射沉积Al-Zn-Mg-Cu合金双级时效态试样裂纹扩展的原位电镜观察

In Situ Electron Microscopic Observation of Crack Propagation in Two-Stage Aging Sample of Spray-Deposited Al-Zn-Mg-Cu Alloy

通常在进行材料断裂机制研究时,是在非应力状态下对试样进行观察。因此,只能获得材料变形过程中的部分塑性形变信息,而不能获得弹性变形信息。缺少了材料在裂纹扩展过程中晶界、未溶解第二相和沉淀相与裂纹尖端作用方式等重要信息。扫描电镜(SEM)和透射电镜(TEM)原位拉伸试验能够分别在微米和纳米尺度对合金裂纹的萌生及扩展过程进行多尺度观察,解决了常规材料断裂机理研究中遇到的裂纹扩展过程难以捕捉的问题。本实验利用SEM,TEM以及配套的原位拉伸设备,对双级时效态(120℃/14 h+160℃/16 h)Al-Zn-Mg-Cu合金进行了裂纹扩展的原位电镜研究。研究结果表明:在SEM原位拉伸过程中,当相邻晶粒的取向差较小时,裂纹易穿晶扩展,当相邻晶粒间的取向差较大时,裂纹易沿晶扩展。在TEM原位拉伸过程中,裂纹尖端附近会产生无位错区(DFZ),且裂纹尖端处的晶格发生了倾转和畸变。双级时效态实验合金的抗拉强度为672.4 MPa,屈服强度为635.7 MPa,断裂方式为混合(穿晶+沿晶)断裂。

Al-Zn-Mg-Cu alloy was widely used in the aerospace industry because of its low density,high strength,and high toughness.Affected by factors such as casting,processing technology,heat treatment technology,there were still a certain number of microscale second phases and nano-scale precipitates in the final state of the alloy.The micro-scale second phases and nano-scale precipitates had important impacts on the mechanical properties and fracture properties of the material.The micron-scale second phases had higher hardness and poor plasticity than matrix.When the matrix experienced plastic deformation,the crack source was likely to appear at the interface between the second phase and the matrix,which reduced the fracture toughness of the alloy.The nano-scale precipitates in the grains would hinder the dislocation slip at the crack tip,thereby greatly increasing the strength of the alloy,and would also affect the fracture behavior.Therefore,it was of great scientific significance to study the influence of the second phase and precipitates relative to crack propagation in Al-Zn-Mg-Cu alloys and to explain the failure mechanism of the alloys.Generally,when studying the fracture mechanism of materials,the specimens were observed in a non-stressed state.Therefore,only the plastic deformation information during the material deformation process could be obtained,but the elastic deformation information could not be obtained.It lacked important information about the interaction between the grain boundary,the second phase and the precipitates and the crack tip during the crack propagation process of the material.Scanning electron microscopy(SEM)and transmission electron microscopy(TEM)in-situ tensile tests could observe the initiation and propagation process of cracks at micro-scale and nano-scale,respectively,and solve the crack propagation process encountered in the research of conventional material fracture mechanism difficult to capture.The effect of misorientation of grains on crack propagation in spray deposited Al-Zn-Mg-Cu alloy was studied by means of SEM,TEM and in-situ tensile equipment.The effects of second phase and precipitates on crack propagation were discussed.It was of great significance to improve the mechanical properties of spray deposited Al Zn mg Cu alloy.In the SEM in-situ stretching process,when the misorientation between two adjacent grains was larger,the resistance of the crack to propagate through the grain boundary was greater,and the crack was prone to propagate.Because the greater the misorientation of adjacent grains,the more disorderly the atoms arranged on the grain boundaries were than those in the grains,and the worse the coordination of the grain boundaries.Therefore,when dislocations encountered large-angle grain boundaries,they were easily hindered in grain boundaries.A large number of dislocations near the boundary caused stress concentration.When the stress reached a certain level,the crack would extend along the grain boundary;when the misorientation between adjacent grains was small,most of the atoms on the grain boundary matched the lattice on both sides.It was not easy to produce dislocation plugging and stress concentration near the grain boundary,and the crack was easy to spread through the grains.In the process of SEM in-situ tension,when the orientation difference between two adjacent grains was small,the resistance of crack propagation through grain boundary was smaller,and the crack was prone to transgranular propagation.During in-situ TEM tensile test,the thickness of the matrix on both sides of the crack in the dislocation free zone(DFZ)near the crack tip was thinner,and the number of precipitates in the matrix near the crack tip was fewer,which indicated that the deformation of the matrix on both sides of the crack was larger during the crack propagation.By analyzing the selected area electron diffraction(SAED)pattern,it could be seen that the diffraction spots of the cracks and DFZ were diffused ellipse and the spots emitted and splitted.This phenomenon was caused by the symmetrical tilt and distortion of the lattice.When the stress in DFZ reached a certain degree,the atomic bond in the local area was broken and the crack source was formed.After two-stage aging treatment,the precipitates at the matrix and grain boundary were non-coherent with the matrix.The hardness of theηphase was higher than that of the matrix,and the unit cell constant of theηphase was also different from that of the matrix.Under the action of external load,theηphase will prevent the plastic deformation of the matrix,and it was easy to cause stress concentration at the interface of theηphase and the matrix.The crack propagated from the matrix to the grain boundary,and the large discontinuous precipitates at the grain boundary provided a channel for the crack propagation.Therefore,it could be inferred that the crack propagation mode of the alloy treated by two-stage aging treatment was transgranular propagation and intergranular propagation.The tensile strength(σb),yield strength(σs)and elongation(δ)were 672.4 MPa,635.7 MPa and 9.2%,respectively.The fracture surface of the alloy after two-stage aging treatment was analyzed.It was found that there was still a part of insoluble second phase in the alloy after solution treatment.During the subsequent aging process,the morphology and distribution of the second phase at the grain boundary did not change.When the alloy was plastic deformed by external force,the crack source was easy to appear at the interface of the second phase and the matrix.With the increase of matrix deformation,these micro cracks continued to expand.When the stress in the local area near the crack tip exceeded the fracture strength of the material,the matrix would fracture locally and break off from the surrounding of the second phase,forming a large pit like dimple.There was a large second phase at the bottom of the dimple.