Impact of Microstrucutre on Dwell Fatigue in Dual-Phase Titanium Alloys

Michelle Harr,Samantha Daly,Adam L Pilchak

doi:10.1051/matecconf/202032111066

Abstract

Dual phase titanium alloys, such as Ti-6242, experience a significant reduction in fatigue lifetime when the peak load is held at each cycle. This type of sustained peak loading, also known as dwell fatigue, mimics the long periods of high mean stress experienced by titanium fan and compressor components during takeoff and cruise. The reduction in fatigue lifetime is known as the dwell debit, and is attributed to the phenomenon of load shedding. Both local microstructure and temperature are known to impact load shedding and thereby the macroscopic response of Ti-6242 when subject to dwell fatigue, but the underlying mechanisms are still under active investigation. This study utilized electron backscatter diffraction (EBSD) and digital image correlation (DIC) to characterize the role of local microstructure and temperature on load shedding during dwell fatigue. EBSD was used to determine local orientation and texture information, and DIC provided information about the heterogeneity of the strain distribution and plastic strain accumulation. Ex-situ tests were performed to investigate the link between the deformation of local microstructures and macroscopic damage. The resultant strain fields and orientation maps were statistically analyzed to provide quantitative insights into the impact of local microstructure on load shedding during dwell fatigue.

Highlights

Near-alpha titanium alloys are utilized extensively in the aerospace industry for their high strength-to-weight ratio, tailorable mechanical properties, and corrosion resistance
This hold a peak load mimics the long period of high mean stress that the titanium components experience during takeoff and, to a lesser extent, cruise
By aligning the orienta on informa on collected with electron backscatter diffraction (EBSD) with the strain maps from SEM digital image correlation (DIC), the long-range slip traces can be matched to the orienta on of the individual grains involved in the coopera ve slip transfer

Summary

Introduction

Near-alpha titanium alloys are utilized extensively in the aerospace industry for their high strength-to-weight ratio, tailorable mechanical properties, and corrosion resistance. A significant reduction in lifetime occurs when comparing dwell fatigue lifetimes to predicted low cycle fatigue lifetimes This reduction in lifetime can be up to 10 to 20 times for Ti-6242 and is attributed to the phenomenon of load shedding. The present study investigates the mechanisms behind the temperature and microstructural dependence of load shedding to inform modeling and to improve dwell fatigue resistance and lifing of Ti-6242. The slip activity and the distribution of plastic strain within MTRs and grains elucidate how dislocations move and pile up during dwell loading. This information serves to validate and further inform the mechanistic understanding of load shedding

Material and Experiments

Results and Discussion

Conclusions