Effect Of Heat Treatment On The Microstructure And Mechanical Properties Of Cold-rolled TB8 Alloy Sheet

There are advantages of high strength,good corrosion resistance and high temperature oxidation resistance in high strength metastable ? titanium alloy.It is often used in the manufacture of aircraft structures and engines.With the developing of aircraft,the properties of titanium alloy is highly demanded.Therefore,it is particularly important to study the relationship between its microstructure and properties.A metastable ? titanium alloy TB8(Ti-15Mo-3Al-2.7Nb-0.25Si)as the research goal,these were studied that the microstructure change after cold rolling and annealing treatment,the microstructure and properties after multiple aging treatments,and the influence of different microstructure states on the fracture behavior of the alloy.So as to optimize the thermal treatment process in the actual production and processing.It was studied that the recrystallization grain growth behavior of TB8 titanium alloy cold rolled sheet during high temperature annealing.After high temperature annealing,the recrystallized grains tend to be equiaxed.At the same annealing temperature,the grain size increases with the increase of holding time.In the early stage of annealing,tthe grain growth rate is relatively fast,but in the later stage,the grain size changes little and the growth rate is slow.With the same annealing time,the recrystallized grain size increases with the increase of annealing temperature.The average grain size D and holding time t were fitted linearly.The logarithm of grain size changed with the logarithm of time at different temperatures.The final calculation of the alloy's grain growth index(n)with temperature changes in the range of 0.236-0.607.Fitted the average grain size D with the absolute temperature T,the activation energy of grain surface Q is 625 k J/mol,which is much larger than that of common titanium alloys.These were studied that the effects of different aging treatments on the microstructure and properties of TB8 titanium alloy cold rolled sheet.After aging treatment(590?-680?),the secondary stripe-shaped ? phase precipitates was emerged on the matrix.The volume fraction and size of stripe-shaped ? phase increasing gradually with aging time raising.The number of secondary stripe-shaped was decreased in the alloy with the increase of temperature and the size increasing when high temperature pre-aging treatment is the same.After passing the low temperature re-aging treatment,the small-siezed dot-shaped or needle-shaped ? phases nucleate and grow in the precipitate free zones in the alloy.The needle-shaped ? phase will precipitate from the matrix in a short time when the low temperature re-aging temperature was high.The dot-shaped ?phase will precipitate after a longer holding time when the temperature is low.With the time increase,the dot-shaped ? phase gradually grows into needle-shaped ? phase.The strength of the alloy after 650?/4h high temperature pre-aging treatment was 790 MPa,and the elongation was 15.6%.The strength of the alloy increases significantly with the increase of the low temperature re-aging temperature and the extension of the time,the elongation decreases slightly,the maximum strength reached 1140 MPa,and the elongation decreased to 11.8%.The alloy has been strengthened to obtain good mechanical properties.The secondary ? phases of different sizes precipitated by the low temperature re-aging treatment of the alloy will greatly increase the strength of the alloy and the loss of plasticity a little.By testing the Vickers hardness of the alloy,compared with the high temperature re-aging treatment,the hardness value of the alloy after lowtemperature re-aging is significantly increased,but the change of the low temperature re-aging temperature has no obvious effect on the degree of hardening of the alloy.It was studied that the influence of different morphologies and amounts of ? on the fracture behavior of the alloy.The strengthening effect on the alloy was weak due to the small amount of primary ? phase precipitation when the alloy is an annealed structure.In the process of stretching,the number of slip lines keeps increasing as the stress increases.The crack propagates along a 45° angle of the horizontal stress and slip direction,the fracture is an equiaxed dimple,and the alloy fails by means of transgranular fracture.Because the stripe-shaped secondary ? phase was precipitated in the matrix in large quantities,microcracks appear in the densely packed ? phases when the alloy is an incompletely aged structure.The number of slip lines is greatly reduced,the fracture is dimples and cleavage steps,and the alloy fractures in a mixed way of transgranular and intergranular compared with the annealed structure.When the alloy is a fully-aged structure,with the continuous increase of the tensile stress,the dense ? phase hinders the slip of the crystal grains,the slip line does not appear in the matrix,and a smooth platform appears in the fracture,which breaks through the grain and along the grain.