The Investigation Of Microstructure,Mechanical And Corrosion Properties Of Mg-Y-Zn(Ni) Alloys

With the rapid development of our economy,the amount of oil and gas extraction is increasing year by year,among which shale oil and gas resources are an important part of oil and gas extraction.The fracturing tool is a key component in horizontal fracturing in shale oil and gas exploitation engineering.The tool material requires good bearing capacity and self-cracking ability in well.Magnesium alloy has moderate compression strength and excellent degradation performance,so it can be used as a candidate material for fracturing tools production.There exists a long-period stacking ordered（LPSO）phase in Mg-Y-Zn alloy.The precipitation of the LPSO phase can effectively improve the mechanical properties of the alloy,and a large potential difference between the phase and magnesium matrix can significantly improve the degradation rate of the alloy.It is worth mentioning that the addition of Ni can not only promote the precipitation of the LPSO phase but also accelerate the degradation of the alloy.At the same time,Zn and Ni alloying,hot extrusion and heat treatment are used to optimize the microstructure of the alloy,which can further improve the mechanical properties and degradation properties of the alloy.Revealing the relationship between alloy microstructure and mechanical and degradation properties will also provide a theoretical basis for improving the mechanical and degradation properties of magnesium alloys as well as the design and development of new fractured soluble magnesium alloys.Therefore,in this paper,Mg-2Y-1Zn（MYZ,at.%）as the research object,Mg-2Y-0.5Zn-0.5Ni（MYZN）and Mg-2Y-1Ni（MYN,at.%）alloys were prepared by Ni alloying.The effects of hot extrusion and high-temperature annealing on the microstructure,mechanics and corrosion properties of the alloy were studied.The main research contents are as follows:First,the effect of Ni alloying on the microstructure,mechanical and corrosion properties of cast Mg-2Y-1Zn（at.%）alloys was investigated.The microstructure of as-cast MYZ,MYZN and MYN alloys is mainly composed ofα-Mg,mesh-like distribution of 18R LPSO phase,Mg₂Y phase,Mg₂₄Y₅phase and Y-rich phase.However,with the increase of Ni content,the secondary dendrite spacing of alloys decreases gradually,and the volume fraction of the LPSO phase increases.The average secondary dendrite spacing of the cast MYN alloy was 22.6μm and the volume fraction of the LPSO phase was 33.1%.The alloy also showed the highest mechanical properties with ultimate tensile strength(σ_UTS),yield strength(σ_0.2),ε_fand maximum tensile strength(σ_UCS)of 220 MPa,102 MPa,12.8%and 314 MPa,respectively.In addition,the addition of Ni significantly increased the degradation rate of the alloy.The weight loss rates of as-cast MYZN and MYN alloys were 10mg/cm²·h and 23.75 mg/cm²·h,respectively,which increased by 471%and 1257%compared with as-cast MYZ.The high corrosion of both alloys is mainly due to the galvanic corrosion caused by the high potential difference between the Ni-containing LPSO phase and theα-Mg matrix.Meanwhile,the surface films of the cast MYZN and MYN alloys are loose and porous,providing a channel for Cl^-penetration.As a result,as-cast MYZN and MYN alloys have the potential as fracturing tool materials.Second,the effects of hot extrusion on the microstructure,mechanical and degradation properties of as-cast MYZN alloy were investigated.The results show that dynamic recrystallization（DRX）occurs during the extrusion process,and the grain size of the alloy is reduced from 26.07μm to 1.87μm.The 18R-LPSO phase distribution changes from the continuous network distribution at the grain boundary to the lamellae distribution along the extrusion direction,and the volume fraction decreases from 25.72%to 19.2%.At the same time,a small amount of the 14H-LPSO phase precipitates in the grain.Theσ_UTS,σ_TYS,andε_fare 364 MPa,316 MPa,and 15%,respectively,which are 97.5%,334%,and 38.9%higher than those of the cast alloy,and theσ_UCSvalue of the extruded alloy reaches 389 MPa.The high mechanical properties of the alloy are mainly due to grain refinement,LPSO phase strengthening and texture strengthening.In addition,the degradation rate of the extruded alloy（17.5mg/cm²·h）is much higher than that of the as-cast alloy（10 mg/cm²·h）.This is mainly attributed to the introduction of a large number of defects（grain boundaries,phase interfaces and dislocations）and grain refinement.Finally,the effect of high-temperature annealing on the microstructure,mechanics and corrosion properties of extruded MYN alloy was studied.The microstructure of the alloy changed significantly after high-temperature annealing,and the 18R-LPSO phase changed from a lamellar distribution along the grain boundaries to a disordered rod-like distribution.The volume fraction of the18R-LPSO phase was also reduced from 31.83%to 24%with the precipitation of a large number of parallel 14H-LPSO phases in the grain.The average grain size was1.81μm and 105.5μm for the extruded and annealed alloys,respectively.The mechanical test results show that the values ofσ_UTSandσ_UCSof annealing MYN alloy are still high,268 MPa and 314 MPa,respectively,while the plasticity of the alloy increases from 13.8%to 17.3%,which is mainly due to the texture weakening and the coarsening of the grains in the recrystallization area with high volume fraction.Notably,the annealed alloy showed excellent degradation properties with a degradation rate of up to 45 mg/cm²·h,which is 1.8 times that of the extruded alloy（24 mg/cm²·h）.This is mainly attributed to the combined effect of the increased18R/14H interface,the generation of crystal orientation corrosion,and galvanic coupling corrosion caused by the second phase remaining in the corrosion pits of the annealed alloy.In addition,cracks in the surface film of the annealed alloy provide a channel for the penetration of the corrosion solution.