The influence of the boriding conditions on the boride layers was examined by boriding Ti−{1}Al−2Zr−1Mo−1V {2}loy in the temperature range of 920−1120℃.The experiment{2} results show that the boride layers were composed of a continuous thin outer layer of TiB_(2) and a thick inner layer of TiB with whiskers or needle-like morphologies that extended into the substrate.Thick and compact boride layers were obtained when the boriding temperatures were 1000−1080℃,and the treatment time exceeded 8 h.The boride layer depth increased with the boriding temperature and time,and the growth kinetics of the boride layers was characterized by a parabolic curve.The growth kinetics of the boride layers,including both TiB_(2) and TiB layers,were predicted by establishing a diffusion model,which presented satisfactory consistency with the experiment{2} data.As a result,the activation energies of boron in the TiB_(2) and TiB layers were estimated to be 223.1 and 24{1}.9 kJ/mol,respectively.
De-lai OUYANGSheng-wei HUCheng TAOXia CUIZhi-shou ZHUShi-qiang LU
The hot deformation behavior,microstructure evolution and fracture characteristics of bimodal microstructured Ti-6Al-2Zr-1Mo-1Valloy were investigated by isothermal tensile tests.Results reveal that flow softening is caused by dynamic globularization of the bimodal microstructure,which also results in a relatively high stress exponent and thermal activation energy.The corresponding SEM,EBSD and TEM observations indicate that the dynamic globularization at750and800℃is accomplished by the formation ofα/αsub-grain boundary and penetration of theβphase.However,dynamic recrystallization(DRX)is the main globularization mechanism at850℃,which was proved by the generation of fine grains with a necklace-like character due to the transformation of low-angle boundaries(LABs)into high-angle boundaries(HABs).With an increase in the deformation temperature or a decrease in the strain rate,the fracture mechanism changes from microvoid coalescence to intergranular fracture.
Abstract： The dynamic spheroidization kinetics behavior of Ti-{2}.5Al-2Zr-1Mo-1Valloy with a lamellar initial microstructure was studied by isothermal hot compression tests in the temperature range of 750-950℃ and strain rates of 0.001-10 s^-1. The results show that the spheroidized fraction of alpha lamellae increases with the increase of temperature and the decrease of strain rate. The spheroidization kinetics curves predicted by JMAK equation agree well with experimental ones. The corresponding SEM and TEM observations indicate that the dynamic spheroidization process can be divided into two stages. The primary stage is boundary splitting formed by two competing mechanisms which are dynamic recrystallization and mechanical twin. In the second stage, the penetration of beta phase into the alpha/alpha grain boundaries is dominant, which is controlled in nature by diffusion of the chemical elements such as Al, Mo and V.
The dynamic recrystalliza{1}on（DRX） behavior of Ti-{2}Al-2Zr-1Mo-1Valloy was inves{1}gated at deforma{1}on temperature of 1000-1100 °C and strain rate of 10-3-1.0 s-1 by using compression test.The results show that discon{1}nuous dynamic recrystalliza{1}on（DDRX） is the predominant recrystalliza{1}on mechanism at temperature higher than 1050 °C and strain rate lower than 0.01 s-1.Meanwhile,con{1}nuous dynamic recrystalliza{1}on is the main mechanism observed at temperature below 1050 °C and strain rate above 0.01 s-1,mixed with a few DDRX grains.In addi{1}on,decreasing strain rate and increasing deforma{1}on temperature are found to facilitate the progress of DRX and refinement of grains in the Ti alloy in β forging process.
