Design And Research On Microstructure And Property Of Marine Seawater Piping Titanium Alloys

The long-term practice has proved that titanium alloys are materials with excellentperformance in marine vessel piping. Howerve, compared with common steels, the highcost of titanium alloys limits its wide application. The thesis takes the application of themarine seawater pipe matetials as the background. With the Thermo-Cacl software, thecomposition of titanium alloys was designed on the base of the principles and methods ofalloy design. Next the designed titanium alloys were prepared by vacuum unconsumableelectrode arc furnace.And the part of designed titanium alloys were to be annealled. Thechemical composition, the microstucture and the phase structure of designed titanium alloyshave been studied using the metallugical microscope, SEM,XRD and EDX.The mechanicalproperties and the anti-corrosion performance of designed titanium alloys have been studiedby tensile measurment, microhardness measurement and electrochemical measurement.Combined the theory with practice, the comprehensive performance of designed titaniumalloys were given in the last.The study shows that all designed alloys have inhomogeneous distribution behaviors ofalloying elements. The grian structure of all designed titanium alloys is coarsening. And themicrostructure at room temperature is widmanstatten structure which mianly has α phase withflat-shaped or basket-shaped as the mian morpholoy. Besides, the widmanstatten structurecontains with net-like boundary α. The change of the type and the content of alloyingelements does not cause the significant change of the microstrucure morphology at roomtemperature, but the width of lamellar α is slightly different.The phase of all designed alloysmianly is α-phase with a bit of β phase. And the part of designed alloys may precipitatemetallic compound Al3Ti.For Ti-Al-Fe-Mo-Zr alloys, the tensile strength of them can meet the requirements ofthe desired design, but their plasticity is low. The tensile strength and the microhardness ofdesigned titanium alloys rises with the increase of alloying element content. This is due tothe solution strength effect of alloying elements. Wherein the solution strengthening effect ofalloying element Al is the strongest compared with the other alloying elements. The tensilestrength of designed titanium alloys increases by about80MPa as the content of alloyingelement Al increases by1%(mass fraction). However, the increase of Al content also reduce the plastic of the designed alloys. When the mass fraction of designed alloys is2%and3%, plastic deformation may occurs in the tensile deformation process of the alloy.When the mass fraction of designed alloys is3.5%, brittle fracture occurs in the tensiledeformation process of the alloy. It may be relevant with the segregation of metalliccompound Al3Ti.For the Ti-2Al-1Fe-1Mo-1Zr and Ti-2Al-1.5Fe-1Mo-1Zr alloy, over-passivationphenomenon or the breakdown of the alloy passivation layer has not been seen in the testingprocess of potentiodynamic polarization curve. Thus it can be concluded that the testingtitanium alloy has good corrosion resistance in3.5%NaCl solution. By the analysis of thefitting equivalent circuit, the polarization resisitance of the alloys is high. It also indicatesthat the testing titanium alloy has good corrosion resistance in3.5%NaCl solution. Bycomparing the electrochemical measurement related parameters of the two kinds testingalloys, it finds that Ti-2Al-1Fe-1Mo-1Zr alloy has a better corrosion resistance comparedwith another alloy. And it also indicates that the alloying element Fe can reduce the corrosionresistance property of the titanium alloys.