| Ultra-high strength beta titanium alloy is one of good structural material, with good strong toughness match, which is widely used in the aerospace, ships and weapons manufacturing. In the process of heat treatment, phase transition of ultra-high strength beta titanium alloy is complex, different heat treatment process can cause the big differences of performance. Therefore, The article studied phase precipitation features of new type ultra-high strength beta titanium alloy in the process of isothermal transformation by using optical microscope, scanning electron microscope,transmission electron microscope and X-ray diffraction analysis methods, and it’s beneficial to improve the alloy potential performance further.The microstructure evolution of isothermal phase transformation processing have been studied between 300℃ and 600℃in new type ultra-high strength beta titanium alloy. In the isothermal transformation at low temperature(300~400 ℃), 2# alloy metastable β phase decomposition method for β'β+α, without transition phase in the process; α phase precipitation process needs very long time in aging stage, at about 300℃ to 50 h. About 4# alloy metastable β phase decomposition method forβ'β+ω'β+α, in the process appeared intermediate transiting phase of omega phase.The nanoscale particle omega phase precipitated of 4# alloy at 300℃aging 4h, as the aging time prolonged, omega phase began to grow larger; Secondary alpha phase precipitation at 400 ℃ isothermal treatment process, the part with the aid of middle transition phase omega(β'β+ω'β+ω+α), another part directly by the beta phase precipitate(β'β+α).In isothermal transformation at medium temperature(450~500℃),2# and 4# alloy phase precipitation rule is similar. A large amount of secondary alphaphase precipitation first appears on the grain boundary, and intracrystalline dot dispersive precipitation; With the increase of aging time, alpha began to grow up,amount of precipitation increase, intracrystalline acicular alpha interconnecting distributed evenly, each other is 45°、60°or 90° angle, the precipitation morphology is similar to the "V" word, and the α phase growth inβ grain always maintained a directional orientation relationship;After reaching the precipitation amount peak value,continue to aging, acicular alpha phase transition for short rod, lamella spacing also began to increase. In isothermal transformation at high temperature(550~600 ℃),secondary alpha phase morphology gradually transformed into a layered of 2# and 4#alloy, first within the grain nucleation, rather than the grain boundary; as the aging time prolonged, alpha fine structure has been transformed into precipitation has short rod-shaped morphology.The new beta titanium alloy age hardening characteristics was researched,through analyzes the relations between hardness and the aging temperature, aging time,get two kinds of alloy of age hardening curve and isothermal transformation curve. In the experiment of isothermal temperature, the alloy hardness in general higher with the increase of aging time, after the peak hardness decreased, leveling off in the end. 2#alloy at 450℃aging 24 h, alloy has a maximum hardness value 373.7HV, then 4# alloy at 400 ℃ aging 8h has the most hardness 425.8HV. The new beta alloy isothermal transformation curve show "C" type, the TTT diagram of 2# titanium alloy at 540℃around "tip", 4 # titanium alloy "nose temperature" about 500℃.The new beta titanium alloy phase transformation kinetics of isothermal transformation process have been researched, by using JMAK model and image-tool software, obtained isothermal kinetics curve of 2# titanium alloy between 450℃ and600 ℃. With different aging temperature, kinetics curve is shown as "S" type and parabolic type two kinds; 2# new beta alloy isothermal kinetics of phase transition Avrami exponent n, temperature constant k, the JMAK temperature isothermal kinetics equation and phase transformation activation energy E(about 336.8 KJ/mol) in the process of isothermal transformation have been acquired. |
PDF Full Text Request