Comportamento mecânico em fadiga da liga de titânio beta metaestável Ti-5553
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Data
2016-10-14Autor
Campanelli, Leonardo Contri
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The microstructure of metastable beta Ti alloys is a decisive factor that
governs their properties and, therefore, the assessment of the relation between
the microstructure and the fatigue response means an advance in the still
incipient knowledge on these alloys. The purpose of this work was to study the
fatigue behavior of Ti-5Al-5Mo-5V-3Cr (Ti-5553) alloy recently developed for
landing gear applications, which was kindly supplied by Boeing for this work.
The material was subjected to different heat treatments (BASCA and double
aging) aiming at to comprehend the influence of the alpha phase precipitation
over the beta matrix, as well as to understand the effect of hydrogen even in
commercially acceptable contents. BASCA microstructure comprised large laths
and fine needles of primary and secondary alpha phase respectively, with a film
of approximately 2 μm of thickness at grain boundaries. A refined and aligned
structure of alpha phase resulted from the double aging treatment, reducing to
0.2 μm the thickness of the grain boundary alpha film. The fatigue limit at 5x106
cycles raised by the standardized staircase method initially showed an
improved result for the harder microstructure, which resulted from the double
aging, for 100 ppm of hydrogen. The fatigue crack nucleation, whether at the
surface or subsurface, was attributed to the thickness of the grain boundary
alpha film. In spite of this film brittleness, crack propagation predominantly
occurred through the grains and exhibited quasi-cleavage facets and dimples.
For the structure arising from the BASCA treatment, the decrease of the
hydrogen content for 25 ppm led to a significant increase of the fatigue
resistance as a consequence of the absence of hydrides and the higher
toughness, overcoming the effect of the coarse alpha phase particles in the
beta phase matrix.