The Widmansite structure of titanium alloy is characterized by a continuous α network distributed on the grain boundaries of the original coarse β grains, flaky α bundle domains distributed within the β grains, and lamellar β between the flaky α Mutually.
Since the α phase and β in the Widmanstatten structure maintain a strict crystallographic phase relationship, the structure has a tenacious "heritability", and it is difficult to improve its structure through conventional heat treatment and cyclic heat treatment under non-quenching conditions. The α β transformation in titanium alloy is a process of nucleation and nucleus growth. When the specific volume difference between the two allotropes is large, the volume effect will be caused during the phase transformation process and internal stress will be generated, forming Dislocations or vacancies promote recrystallization, resulting in changes in the α-sheet structure and even shortening and spheroidization. The researchers effectively improved the Widmanstatten structure through β zone quenching and α+β zone cyclic heat treatment.
The test material is a Φ280mm TC4 disc provided by a certain research institute. The phase transformation point was measured by metallographic method to be (985±5)°C, and the samples taken were all on 1/2 of the circumference of the disk. In order to obtain the Widmanstatten structure required for the test, the obtained sample was first annealed in the β zone (1020°C × 1h, AC), then the sample was quenched in the β zone (1020°C × 30min, WQ), and then in the α zone In the +β zone, 960℃×10min and 750℃×10min were cycled once, 3 times, 5 times, 7 times and 9 times respectively. After the cycle, all samples were double annealed (940℃×30min, AC+700℃×4h, AC), and the tensile test was performed after double annealing; the tensile fracture was observed and analyzed using a GSM6460 scanning electron microscope. After the metallographic samples were etched with HF+HNO3+H2O (volume ratio 1:2:5) etching solution, the microstructure was observed under an OLYMPUSPMG3 optical microscope.
Research result:
(1) During cyclic heat treatment in the range of 960℃×10min and 750℃×10min, as the number of cycles increases, the strength of TC4 alloy slowly decreases and the plasticity increases. After 9 cycles, the area shrinkage increases significantly.
(2) As the number of thermal cycles increases, the aspect ratio of the strip α gradually decreases, and partial spheroidization occurs. After 9 cycles, secondary flaky α phase precipitated in the flaky β matrix. Under the same number of cycles, after double annealing, the strip α is obviously coarsened and the structure is more stable.
(3) SEM observation and analysis of the tensile port shows that the fracture form of the tensile fracture changes from cleavage fracture before cycling → cleavage + dimple fracture → dimple fracture.
