Experimental and finite element study of mechanical behavior of titanium-titanium monoboride composites and functionally graded materials based on them.

摘要

This research is focused on the mechanical characterizations of functionally graded materials (FGMs) and composites based on Ti-TiB (titanium-titanium monoboride), through experiments and modeling.; The FGM tile considered in this research consisted of Ti and in-situ formed TiB whiskers, as well as mixtures of both phases in the intermediate graded layers. The Ti-rich composites contained randomly arranged pristine high-aspectratio TiB whiskers in a Ti matrix, while the TiB-rich composites were mainly comprised of a complex network of coarse interconnected TiB whisker. Composites containing intermediate volume fractions of TiB revealed TiB whiskers that became thinner and shorter and more agglomerated as the volume fraction of TiB increased.; The elastic properties of the Ti-TiB composites were obtained through a resonant frequency technique and tensile testing. The elastic and shear moduli of all the composites were found to increase with an increase in the volume fraction of TiB. The Poisson ratio decreased with increasing vol. % of TiB. The probable values of the elastic modulus, the shear modulus, and the Poisson's ratio of the TiB phase were estimated using theories based on two-phase composites.; The linear coefficient of thermal expansion (CTE) values as a function of temperature were also obtained. It has been found that the CTE of the composites decreased with an increase in the amount of TiB except for the composite with 54 vol. % of TiB (Ti-54M). The Ti-69TiB and Ti-83M-7T1B₂ composites additionally exhibited large amounts of expansion near the terminal measurement temperature, and this was found to be due to the growth of a reaction layer between the specimen and the alumina push rods of the dilatometer.; The Ti-TiB FGMs showed different cracking patterns, depending on gradation, and it was necessary to determine how the cracking occurred. To analyze the thermal stresses in the Ti-TiB FGM, 3D finite element models (FEMs) were constructed using the ADINA finite element code. Four models were formulated and were based on the two FGM tile gradation schemes, linear and parabolic, and two different sets of boundary conditions were applied to each gradation scheme. It was found that the thermal stresses in the freely contracting model were smaller than the tensile fracture stress levels of the layers and should allow the in-situ manufacturing of the FGM tile, without cracking. On the contrary, the FGM tiles cracked during the cool-down after fabrication. It was found that the magnitude of tensile thermal residual stress was amplified by the mechanical constraints induced by the flash that formed during the processing, leading to the catastrophic cracking of the tiles during cooling. The residual stresses in the two gradation models considered and the cracking patterns of the corresponding FGM tiles were found to be in reasonable correlation with each other.