Effect of microstructure on oxygen rich layer evolution and its impact on fatigue life during high-temperature application of α/β titanium

Abstract

The near alpha titanium alloy, Ti-6424S, is utilized in manycritical high-temperature aerospace components due to its unique properties. However, oxygen ingress during elevated-temperature exposure induces formation of a subsurface brittle oxygen-rich layer (ORL), resulting in a deterioration of mechanical performance. This paper, for the first time, establishes the effect of the underlying microstructure on the formation and evolution of the ORL inα/β titanium alloys.In addition, models were developed to predict (i) the evolution of ORL as a function of the material microstructure, (ii) the effect of ORL on the critical strain for in-service crack initiation, and (iii) estimates of fatigue life of components made from a specific microstructure during in-service high temperature exposure and formation of ORL. In particular, five different microstructures wereproducedby tailored heat-treatments and thermally exposed at 650°C up to 420 hrs. The base metal and the ORL were quantified using microhardness indentations, optical microscopy, and scanning electron microscopy (electron backscatter diffraction (EBSD), backscattered electron (BSE), and secondary electron (SE) imaging). The effective diffusion coefficients (D eff) for each microstructure were calculated and then integrated into a critical strain model to predict crack initiation strain as a function of exposure time. The predicted ORL thickness was used to estimate fatigue life using experimentally measured crack growth data.The largest D eff coefficient was observed in a colony microstructure, whilea basketweave microstructure showed the smallestD eff. For several bimodal microstructures, D eff was noted to increase with increasing area fraction of secondary alpha colonies.

Bio-corrosion behaviour of oxygen diffusion layer on Ti-6Al-4V during tribocorrosion

“…However, as the diffusion time increased from 3 to 6 hours, zone I extended deeper followed by zone II. The formation of alpha-case (zone I) and oxygen-rich layer in ( + )-Ti (zone II) by thermal oxidation were also reported by Satko [28].…”

“…Therefore, with a high amount of oxygen near surface ( Fig. 4) during heat treatment, -Ti could be promptly saturated by oxygen and the transformation of oxygen-saturated -Ti to an unsaturated -Ti also called alpha casing [28] subsequently occurred in zone I. Figure 3 also shows that the thermal oxidation treatment caused a growth of alpha phase in a deeper area, i.e., zone II, as a result of to transformation.…”

High‐performance infrared emissivity of micro‐arc oxidation coatings formed on titanium alloy for aerospace applications

“…For structure applications in hypersonic vehicles, surfaces made of Ti alloys, however, suffers from severe aerodynamic heating during hypersonic flight. The friction caused by the aerodynamic drag generates a dramatic increase in the surface temperature and therefore tends to deteriorate the overall thermo‐mechanical performance of the materials . As a consequence, there is a strong demand in the industry for a coating targeted at Ti alloys that can radiate a large amount of heat in the surrounding space .…”

“…The friction caused by the aerodynamic drag generates a dramatic increase in the surface temperature and therefore tends to deteriorate the overall thermo-mechanical performance of the materials. 3,4 As a consequence, there is a strong demand in the industry for a coating targeted at Ti alloys that can radiate a large amount of heat in the surrounding space. 5 A conventional technique to fabricate high-emissivity coating is based on the use of brushes.…”

Microstructure-Informed Cloud Computing for Interoperability of Materials Databases and Computational Models: Microtextured Regions in Ti Alloys

“…The third module includes a collection of materials behavior models such as Taylor-type crystal plasticity models [23], fast Fourier transform-based elastoviscoplastic model [24,25], and an oxygen ingress in titanium model [26]. The goal of this module is to provide easy access to essential but complicated material behavior models that often require significant time and computational skills to code and compile.…”