Fatigue crack growth rate investigation of cold rolled and aged Al-Mg-Zn alloy

Abstract

This research explored the fatigue crack growth rate (FCGR) characteristics correlating with microstructure of Solution heat treated (SHT), SHT + peak aged (PA), SHT + 45 % cold rolled (CR), SHT + 60 % warm rolled (WR) and SHT + PA + 90 % CR Al-Mg-Zn alloy. The findings indicate that artificial aging and cold rolling process decelerate fatigue crack initiation because of precipitation hardening and work hardening as well as grain boundary strengthening, respectively. Utilizing the Paris model, fatigue crack growth rates during the linear extension stage were analyzed. Analysis of crack paths via scanning electron microscope (SEM) techniques revealed ductile fracture(dimples) in case of SHT and SHT + 60 % WR sample, while ductile and brittle mix fracture (dimples and facets) in case of SHT + PA and SHT + 45 % CR sample, whereas brittle fracture (river pattern, facets) in case of SHT + PA + 90 % CR sample were observed. The broadening of precipitate peaks in the X-ray diffraction (XRD) graph of SHT + PA + 90 % CR indicates a rise in dislocation density which is 11.1 $\times {10}^{14}{m}^{-2}$. The microstructural evolution is characterized using optical −microscopy, EBSD and transmission electron microscopy (TEM) techniques. Rod like shape η″ precipitates were observed in TEM images in the case of SHT + PA + 90 % CR sample. Through the partition of IPF image it was observed that higher volume fraction of recrystallized grains was formed in SHT + 60 % WR sample, whereas nano-meter to micrometer size sub grains were formed in the case of SHT + PA + 90 % CR sample. It was observed through orientation distribution function that SHT + 45 % CR is showing strong brass ({110} < 112 > ) texture, whereas SHT + 60 % WR sample is showing strong rotated cube({001} < 110 > ) texture, while 90 % CR sample is showing strong brass({110} < 112 > ), strong Cu({112} < 111 > ) and strong S({123} < 634 > ) texture. Mechanical properties are assessed through tensile, hardness, and fracture tests. The highest values for Vickers hardness (226 HV), tensile strength (526 MPa), and conditional elastic–plastic fracture toughness (J_Q) (344.54 kJ/m²) were obtained after SHT (470 °C) for a duration of 8 h), PA (140 °C for a duration of 21 h) and 90 % CR.