简介概要

Fatigue fracture of high-strength Al-Zn-Mg-Cu alloy

来源期刊:中国有色金属学报(英文版)2009年第5期

论文作者:蹇海根 姜锋 文康 蒋龙 黄宏锋 韦莉莉

文章页码:1031 - 1036

Key words:Al-Zn-Mg-Cu alloy; fatigue fracture; fatigue crack propagation

Abstract: X-ray diffractometry(XRD), optical microscopy(OM), scanning electron microscopy(SEM) were used to study the fatigue fracture of the T7451 Al-Zn-Mg-Cu alloy (470 ℃, 60 min+115 ℃, 8 h+165 ℃, 16 h). The study reveals mainly the microscopic structure of the alloy in the process of crack formation and crack growth. The fatigue fracture is characterized by three zones:fatigue crack source zone, fatigue crack propagation zone and fatigue fracture zone. The fatigue damage preferably incubates at the fractured inclusion particles at or near (about 25 μm) the specimen free surfaces, and these brittle Fe-rich intermetallic inclusion particles are (7-10) μm×(11-14) μm in size. Some features such as “feather-like”, “river and range” and boundary extrusions can be observed in the fatigue propagation zone, and in the fatigue fracture zone the surface is rough and uneven.

基金信息:the Key International Science and Technology Cooperation Program of China



详情信息展示

­­Fatigue fracture of high-strength Al-Zn-Mg-Cu alloy

JIAN Hai-gen(蹇海根), JIANG Feng(姜 锋), WEN Kang(文 康),

JIANG Long(蒋 龙), HUANG Hong-feng(黄宏锋), WEI Li-li(韦莉莉)

School of Materials Science and Engineering, Central South University, Changsha 410083, China

Received 24 June 2008; accepted 2 March 2009

                                                                                                

Abstract: X-ray diffractometry(XRD), optical microscopy(OM), scanning electron microscopy(SEM) were used to study the fatigue fracture of the T7451 Al-Zn-Mg-Cu alloy (470 ℃, 60 min+115 ℃, 8 h+165 ℃, 16 h). The study reveals mainly the microscopic structure of the alloy in the process of crack formation and crack growth. The fatigue fracture is characterized by three zones: fatigue crack source zone, fatigue crack propagation zone and fatigue fracture zone. The fatigue damage preferably incubates at the fractured inclusion particles at or near (about 25 μm) the specimen free surfaces, and these brittle Fe-rich intermetallic inclusion particles are (7-10) μm×(11-14) μm in size. Some features such as “feather-like”, “river and range” and boundary extrusions can be observed in the fatigue propagation zone, and in the fatigue fracture zone the surface is rough and uneven.

Key words: Al-Zn-Mg-Cu alloy; fatigue fracture; fatigue crack propagation

                                                                                                           

1 Introduction

The Al-Zn-Mg-Cu alloy has properties such as low density, high strength, high hardness and good processing performances. This kind of alloy has been widely used in the aerospace field and civil industry, and is one of the most important structural materials in the aerospace industry[1-3]. Before 1960s, people usually adopted the peak-aging treatment on this kind of alloy in order to get the highest strength value. Under this condition, the strengthening phases inside the grains are G.P. zone and the η′ phases; meanwhile the grain boundary is a continuous precipitate chain which has a high susceptibility to stress corrosion cracking(SCC) and relatively low fracture toughness. To give a consideration to both the strength and stress corrosion cracking resistance of the alloy, some other treatments like T76, T736(T74) and RRA had been developed. Nowadays, these methods have been widely used in the heat treatments of some structural components such as airframe, supporting parts of the landing gears, wing cover, hull and rivet[4-6].

Being the main structural materials in the aircraft industry, these components are usually applied with a cyclic loading, so that it requires a higher fatigue resistance. In the last few decades, large number of research on the fatigue properties of the alloy had been carried out. Through the GOODMAN experiment people set up a fatigue life diagram considering mean stress and provided us some basic data for fatigue design[7]. MANSON and COFFIN proposed the low cycle fatigue fracture laws for material evaluation, i.e. the ?ε—N curve[8]. The fracture mechanics was established in the 1960s, and then it was used to study the fatigue crack propagation characteristic of the material. PARIS and ERDOGAN[9] proposed the general relationship between the crack propagation rate and the stress intensity factor range (?K). In this work, we focused on the microstructure of the fatigue crack propagation zone, because different microstructures relate to different growth threshold and propagation rate of the crack, then these would affect the fatigue properties of the alloy.

2 Experimental

The alloy plates used in this experiment were supplied by the Northeast Light Alloy Corporation, China. After smelting, casting, and homogenization treatment, the plates were hot-rolled into plates with the thickness of 30 mm. The chemical composition of the alloy is shown in Table 1. The alloy first had a solution treatment of 470 ℃, 60 min and a rapid quenching, after that there was a 2% pre-strain on it to release the quenched residual stress. Then, the step of 115 ℃, 8 h+165 ℃, 16 h double aging treatment was followed.

Table 1 Chemical composition of Al-Zn-Mg-Cu alloy (mass fraction, %)

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