Study On Microstructure And Properties Of High-Strength Dissolvable Mg-Gd-Y-Zn-Cu(-Ni) Alloys For Oil And Gas Drilling

Nowadays,multi-stage fracturing is becoming an effective technology to exploit unconventional oil and gas resources(e.g.shale gas),and plugging tools are the core equipment of the technology.Due to its low density,high specific strength and high corrosion rate,dissolvable magnesium alloys are the good candidate applied as plugging tools.However,the existing dissolvable magnesium alloys exhibit low strength and incompatible mechanical properties with corrosion rates,which cannot meet the requirements.Therefore,it is urgent to develop novel dissolvable magnesium alloys with high-strength and rapid corrosion rates working in extreme extraction in the field of oil and gas drilling in China.In this work,based on high-strength dissolvable Mg-Gd-Y-Zn-Cu(-Ni)alloy,the microstructure evolution of the alloy and its impact on performance are systematically studied through composition design and process control,revealing the strengthening.toughening,and corrosion mechanisms of the alloy,demonstrating different factors affecting the corrosion behaviors of the alloy,and providing the theoretical basis for the preparation and application of deformed high-strength soluble magnesium alloys.The effect of Cu element(0～0.8 wt.%)on the microstructure,mechanical and corrosion properties of the Mg-9.5Gd-2.7Y-0.9Zn alloy is studied.The results indicate that with the increase of Cu content,the eutectic phase of Mg3RE and MgsRE in the cast alloy decreases,and the fraction of LPSO phase increases.The block LPSO phase in the extruded alloy promotes the dynamic recrystallized(DRXed)process through the particle-stimulated nucleation(PSN)mechanism.However,the migration of the sub-structure is inhibited caused by the plate LPSO phase.Furthermore,the LPSO phases with different morphologies can effectively pin the grain boundaries of DRXed grains,thereby hindering the formation and growth of DRXed grains.As a result,the grain size and the fraction of DRXed grains decreased with increasing Cu content.With the addition of Cu content,the contents of rare earth elements decrease in the alloy,which leads to a decrease in solid solution strengthening and precipitation strengthening.On the other hand,the formation of a higher LPSO phase leads to a significant increase in second phase strengthening and dislocation strengthening.Ultimately,the alloy exhibits higher yield strength.Meanwhile,under the combined effect of grain refinement,increased block LPSO phases,and kinking and refining of plate LPSO phases,the plasticity of the alloy is effectively improved.In terms of corrosion,the addition of Cu not only increases the potential difference between the LPSO phase and matrix,but also increases the fraction of LPSO phase,resulting in more micro-galvanic effects.Therefore,the alloy containing 0.8 wt.%Cu exhibits the best strength with a tensile strength of 494 MPa and yield strength of 435 MPa,elongation of 4.0%and corrosion rate of 15.6 mg·cm-2·h-1 at 93℃ in 3 wt.%KCl solution.Based on Mg-9.5Gd-2.7Y-0.9Zn-0.8Cu alloy,the influence of different Ni contents on the microstructure and properties of alloys was studied.The results show that with the addition of Ni content(0～1.2 wt.%),the plasticity of the alloy is significantly improved due to the increased fraction of LPSO phases,decreased c/a axis ratio,and the increased average Schmidt factor of pyramidal<c+a>slip.But the solid solution strengthening and precipitation strengthening decrease.Finally,the comprehensive properties of the alloy containing 0.4 wt.%Ni is the best,with a tensile strength of 458 MPa,yield strength of 400 MPa,and an elongation of 6.1%.As for the corrosion,the addition of Ni increases the potential difference between LPSO phases with different morphologies and the matrix,and promotes the formations of Ni-containing LPSO phases.Eventually,the alloy containing 1.2 wt.%Ni obtain an excellent corrosion rate of 78.6 mg·cm-2·h-1(3 wt.%KCl solution at 93℃).In addition,the corrosion mechanism of the Mg-Gd-Y-Zn-Cu(-Ni)alloy is also studied:under the galvanic corrosion with LPSO phase and α-Mg,the Mg matrix preferentially corrodes and the MgO,Mg(OH)2,Gd2O3,and Y2O3 products on the alloy surface are formed.Subsequently,Cl-destroyed the protective product film and decreases the protective effect.As the corrosion time prolongs,the surface of the alloy substrate peels off until overall corrosion occurs.The effect of different solution treatments on the microstructure,mechanical and corrosion properties of the selected Mg-9.5Gd-2.7Y-0.9Zn-0.8Cu-0.8Ni alloy.The results show that the alloys with different fractions(20%,30%,40%)LPSO phase are obtained by adjusting the solution treatment.The DRXed mechanism of the extruded alloy includes the PSN induced by block LPSO phases,CDRX,and DDRX.Meanwhile,the dense basal precipitates inhibit the transformation of the sub-structure to DRXed grains,together resulting in the formation of bimodal structures.The extruded alloy exhibits good strength and plasticity under the synergistic effect of basal slip of randomly oriented DRXed grains in the early stage of tensile deformation and non-basal slip of deformed grains in the later stage.The solution treatment alloy containing 30%LPSO phase has the highest yield strength(342 MPa)after extrusion,which be attributed to a more obvious bimodal structure,fine DRXed grains and strong texture strengthening effect.The corrosion results indicate that the solution treatment alloy containing 20%LPSO phase has the highest corrosion rate after extrusion.The main reasons are as follows:the alloy forms many plate LPSO phases during hot extrusion,and the abundant "interlayer"structure with plate LPSO phase/Mg matrix/plate LPSO phase can provide more favorable locations for galvanic corrosion.In addition,there are partially dispersed block phases in the alloy and there is no formed continuous network structure to promote corrosion.Last but not least,the average grain size is small,and the grain boundary density is high,which improves the surface activity of the alloy and promotes corrosion.Based on the optimizing solution treatment,the Mg-9.5Gd2.7Y-0.9Zn-0.8Cu-0.8Ni alloy exhibits excellent comprehensive properties,with a yield strength of 372 MPa and a corrosion rate of 63.4 mg·cm-2·h-1 in 3 wt.%KCl solution at 93℃.This provides an excellent material selection for the use of high strength dissolvable magnesium alloys in oil and gas drilling.