激光冲击强化对Ti-6Al-3Nb-2Zr-1Mo钛合金组织与性能的影响

Effect of Laser Shock Peening on Microstructure and Properties of Ti-6Al-3Nb-2Zr-1Mo Titanium Alloy

激光冲击强化(Lasershockpeening,LSP)是一种新颖的非弹丸撞击式表面改性技术。利用LSP对片层组织Ti-6Al-3Nb-2Zr-1Mo(Ti80)钛合金进行表面处理,并研究了LSP对其微观组织、力学性能及耐腐蚀性能的影响。LSP处理后Ti80钛合金发生了严重的塑性变形,表层片层组织中形成了位错缠结、形变孪晶及层错等晶体亚结构缺陷,晶粒发生细化。LSP处理后表层显微硬度提高了21.7%,残余压应力达到最大值(-334 MPa),且显微硬度和残余压应力都随深度的增加呈现梯度变化特征。LSP处理后产生的加工硬化、细晶强化和残余压应力的作用改善了拉伸性能,断口形貌中的韧窝变得大而深且解理面变少。晶粒细化为钝化膜的形成提供更多的形核位置,同时杂质不容易在晶界偏析,延缓晶间腐蚀;高密度位错阻碍电子转移,降低腐蚀电流密度,使Ti80钛合金在5mol/L HCL溶液中的耐腐蚀性能得到明显提高。

Objective Ti-6Al-3Nb-2Zr-1Mo(Ti80)alloy has been widely used in pressure pipelines and structural parts subjected to relatively large loads in the marine environment such as pressure hulls of bathyscaphs.However,high pressure and salinity of deep seawater can easily cause corrosion and premature failure of the parts due to the surface damage of the materials.Hence,it is very urgent to ameliorate the mechanical and corrosion properties of such an alloy to extend its service life.Recently,the studies on surface modification technologies,such as ultrasonic shot peening(USP),sliding friction treatment(SFT),ultrasonic surface rolling(USL),and laser shock peening(LSP),to improve the properties of Ti alloys have been intensively done.In this work,the microstructural evolution and mechanical and corrosion properties of Ti80 alloys treated by LSP are investigated with the hope to broaden the surface treatment methods of Ti80 alloys and satisfy the property requirements of pressure hulls of bathyscaphs.This study aims at providing decisive insights for the surface structural enhancement of the pressure hulls of bathyscaphs and the application of the key components of Ti80 alloys treated by LSP,and bringing obvious social and economic benefits to the shipbuilding industry.Methods In this work,the Ti80 alloy is first heat-treated to obtain the lamellar microstructure consisting of lamellarαclusters with different phases parallel to each other andβphases between them.Then it is subjected to the LSP surface modification treatment,and the Ti80 alloys before and after LSP are subjected to a physical phase analysis with the aid of an X-ray diffractometer(XRD).In addition,the metallographic microstructure is observed using an optical microscope(OM),and subsequently the Ti80 alloy is mechanically ground to 50μm with different types of sandpapers,punched into discs ofφ3mm,and thinned using a Gatan 691ion thinning instrument.The microstructure is investigated using aJEM-2010transmission electron microscope(TEM).The mechanical properties(strength,elongation,compressive residual stress,and microhardness)are characterized using a tensile tester,an X-ray stress analyzer,and a microhardness tester.Fiannly,the electrochemical impedance spectroscopy and the potentiodynamic polarization curves of the Ti80 alloys before and after LSP are measured using an electrochemical workstation,the corrosion rate is measured by an immersion test,and the corrosion morphology is analyzed by scanning electron microscopy(SEM)to reveal and analyze the corrosion mechanism.Results and Discussions The lamellar microstructure of the Ti80 alloy surface layer after LSP undergoes severe plastic deformation,forming crystal substructure defects such as dislocation tangles[Fig.5(c)],stacking faults[Fig.5(f)]and deformation twins[Fig.5(e)],and grain refinement.After LSP,microhardness increases substantially by21.7%(Fig.6),and the maximum compressive residual stress is-334 MPa(Fig.7).Both microhardness and compressive residual stress present a gradient change with the increase of the depth.The tensile properties are improved by the action of work hardening,fine grain strengthening,and the compressive residual stresses after LSP(Fig.8).The dimple becomes bigger and deeper,and the cleavage plane becomes less in the fracture morphology(Fig.9).Grain refinement provides more nucleation sites for the formation of passivation films,while the impurities are not easily segregated at grain boundaries,retarding intergranular corrosion.The high-density dislocations hinder electron transfer and reduce the corrosion current density(Table 3),resulting in a significant improvement in the corrosion resistance of the Ti80 alloy in the 5mol/L HCL solution.Conclusions In this study,the LSP surface treatment of a Ti80 alloy is implemented,and the corresponding microstructural evolution and properties are investigated systematically.After LSP,the surface grains of the Ti80 alloy are significantly refined,and many defects such as dislocation tangles,stacking faults,and deformation twins are formed.The strengthening effect of crystal defects combined with fine grain strengthening increase the surface microhardness by 21.7%.However,the large compressive residual stress is formed,resulting in enhanced strength and elongation,and the overall mechanical properties of the Ti80 alloy become better.Grain refinement provides more nucleation sites for the formation of passivation film of the Ti80 alloy,while the compressive residual stress makes the passivation film denser and less susceptible to destruction,resulting in a positive shift of the corrosion potential of the Ti80 alloy in the 5mol/L HCL solution by 119mV.The corrosion current density is reduced by 0.52μA·cm^(-2) and the corrosion rate is reduced by 0.16mm·a^(-1),therefore the anticorrosion property is improved.