Mechanical And Degradation Properties Of High Strength Soluble Mg-Gd-Ni Alloy For Fracturing

Magnesium alloy has many advantages,such as low density,high strength,good rigidity,good toughness,strong shock absorption,good cutting performance,low melting point,excellent die-casting performance,good heat dissipation,excellent electromagnetic shielding performance,and abundant resources.It is widely used in aerospace,automobile manufacturing,3C,pharmaceutical and chemical industry,rail transit and other fields,and is of great significance to green development.Pure magnesium has poor mechanical properties,but strengthening methods such as alloying,plastic deformation,and heat treatment can effectively improve the overall mechanical properties of the alloy and control the degradation performance of the alloy.The poor corrosion resistance of magnesium alloys greatly limits the application range of magnesium alloys as structural materials,but it brings new research directions to downhole tool materials for fracturing.At the same time,the study found that the introduction of long-period stacking ordered phase(LPSO)can improve the mechanical properties of magnesium alloys while regulating its degradation performance.This is for the development of a new type of fracturing with high strength and controllable degradation performance.Magnesium alloy materials for downhole tools provide a new way of thinking.In this study,a certain amount of Gd and Ni elements were added to magnesium to develop a Mg-Gd-Ni alloy with excellent comprehensive mechanical properties and a controllable degradation rate.A series of magnesium alloys containing nickel-containing LPSO phases were prepared,and the effects of the content,distribution and morphology of these LPSO phases on the mechanical properties and degradation properties of the alloys were studied.Firstly,as cast Mg-Gd₁-Nix(x=0.38,0.75,2.25)was obtained by controlling the atomic percentage of Gd to 1 and changing the content of Ni.The alloy is mainly composed of?-Mg matrix phase,LPSO phase and eutectic phase.The experimental results show that the proper content of Ni containing LPSO phase can improve the comprehensive mechanical properties of the alloy and accelerate its degradation,but,excessive Ni promotes the eutectic reaction,on the contrary,it will reduce its mechanical properties and degradation rate.Among them,Mg-Gd₁-Ni_0.75 alloy shows the best performance,the compressive strength,compressive yield strength,elongation and hardness are 340 MPa,128 MPa,13.9%and 103 HV respectively.The average degradation rate is 2066 mm/y at room temperature(25?)and 8193 mm/y at simulated fracturing environment(93?).Then,on the basis of Mg-Gd₁-Ni_0.75 alloy,Mg-Gd_1.33X-Ni _X series as cast alloys were obtained by fixing the atomic percentage of Gd/Ni to 1.33 and changing the contents of Gd and Ni.With the increase of Gd and Ni content,the content,distribution and morphology of the second phase in each alloy are different.The content of LPSO phase increases gradually,from incomplete continuous network distribution to complete continuous network distribution,and then to a large number of LPSO phase wrapped in the magnesium matrix.At the same time,the grain is also refined.The LPSO phase containing nickel can effectively improve the comprehensive mechanical properties of Mg-Gd_1.33X-Ni _X alloy.However,with the increase of LPSO content,the degradation rate of the alloy decreases gradually.The results show that Mg-Gd_2.66-Ni₂ alloy has the best comprehensive mechanical properties,the compressive strength,compressive yield strength,elongation and hardness are 516 MPa,176 MPa,23.0%and 121 HV respectively.The average degradation rate of Mg-Gd_0.665-Ni_0.5 alloy is 1678 mm/y at room temperature(25?)and 8104 mm/y at simulated fracturing environment(93?).Then,the extrusion deformation experiment of Mg-Gd_1.33X-Ni _X alloys was carried out.The microstructure of Mg-Gd_1.33X-Ni _X was refined after extrusion deformation.However,the LPSO phase in the alloy consists of massive LPSO phase which originally distributed around the grain boundary and layered LPSO phase precipitated.With the increase of Gd and Ni content,the layer LPSO phase precipitated by magnesium matrix increases first and then decreases.The LPSO phase in the alloy after extrusion presents a streamline and a crisscross distribution with magnesium matrix.The mechanical properties of Mg-Gd_1.33X-Ni _Xalloy after extrusion deformation were significantly improved.The comprehensive mechanical properties of Mg-Gd_1.33-Ni₁ alloy are the best,and the compressive strength,compression yield strength,elongation and hardness are615MPa,290MPa,18.9%and 93HV respectively.However,with the increase of the LPSO content,the degradation rate of the alloy gradually decreased.The degradation rate of Mg-Gd_0.665-Ni_0.5 alloy is excellent,the average degradation rate at room temperature(25?)is 1438 mm/y,and the average degradation rate in simulated fracturing environment(93?)is 6337 mm/y,which is slightly lower than that of as cast alloy.In a word,through alloying control and deformation process control,a series of Mg-Gd-Ni alloys with high strength,excellent comprehensive mechanical properties and different degradation rates can be obtained,which can be used as candidate materials for downhole fracturing ball materials.The galvanic corrosion in a series of alloys designed is the dominant part in the whole corrosion process.LPSO phase has a high corrosion potential as the cathode phase of galvanic corrosion,and Mg matrix acts as its anode.The cathode phase can not only form micro electric coupling with magnesium matrix to accelerate corrosion,but also excessive LPSO can be used as a barrier to inhibit corrosion.Corrosion occurs at the junction of LPSO phase and magnesium matrix phase,and further expands to make magnesium matrix block fall off.