Formation Of Oxygen Rich α Layer On Surface Of Titanium Alloys And Its Effect On Machanical Properties

When exposed or heat treated in air or oxidizing atmosphere, titanium alloy surface will absorb numerous oxygen and form oxide film, further absorption causes oxygen rich layer. Due to its hard and brittle nature, oxygen rich layer always causes tiny cracks on the alloy surface and is responsible to failure of certain parts.Disciplines between thickness of oxygen rich layer on surface of titanium alloy and different heat treatment temperature and holding time of TA2, TA15, TC4 alloys have been researched in this thesis. Metallographic and Micro-hardness methods have been used evaluating thickness of oxygen rich layer. Applicability of two methods has been discussed based on comparison and analysis of two methods. Research of forming and microstructure evolution of oxygen rich layer leads to the building of forming dynamic of α layer. Relations of α layer thickness and holding temperature and time have been built. Affection of α layer to tensile mechanical properties of titanium alloy has been researched.Both metallographic and micro-hardness method can evaluate thickness of oxygen rich layer. The results indicate thickness evaluated by metallographic method is less than micro-hardness method. Thickness evaluated by micro-hardness is better related to diffusion of oxygen in surface of titanium alloy. Critical errors metallographic method exist when metallographic method is used in the condition of simple microstructure or light oxygen rich. Micro-hardness method can’t be used or larger errors existing in evaluating α layer without an enough thickness. Proper method should be applied based on real situation in the products producing of SAC.Research of forming of α layer and evolution of microstructures of these three types titanium alloys shows that for TA2 alloy, this process appears to be roughening and fusion of single α phase. The α layer forming process of TA15 alloy is roughening, spheroidizing and fusion of lamellar α phase. Original microstructure of TC4 alloy is duplex microstructure. The forming appears to be roughening, spheroidizing of lamellar α phase and fusion with equiaxed α phase.Thicken of α layer follows parabola discipline. Based on test data, Equations of α layer thickness and holding temperature and time are established. Calculation of Oxygen diffusion coefficients of three types titanium alloys under different holding temperatures have been calculated. Arrhenius equation is used to calculate thermal activation energy of α layer forming. The results of TA2 alloy is 101.6 k J/mol, while thermal activation energy of TA15 and TC4 alloy are respectively 146.5k J/mol and 161.04 k J/mol.Existence of α layer severely affects mechanical properties of titanium alloy. Forming of α layer brings in tension stress, which shear stress component would causes earlier yielding of titanium alloys. Presence of α layer also causes a major loss of harmonious plastic deformability in titanium alloy with non-single microstructure pattern. Without homogeneous plastic deformation, plasticity would have a severe drop. SEM observation of fracture indicate clearly changing of fracture behavior caused by α layer.