Improvement of reliability of aluminum alloys by controlling the microstructure of grain-boundary precipitates

2019 Fiscal Year Final Research Report

• Project/Area Number: 17K06854
• Research Category: Grant-in-Aid for Scientific Research (C)
• Allocation Type: Multi-year Fund
• Section: 一般
• Research Field: Material processing/Microstructural control engineering
• Research Institution: Ibaraki University
• Principal Investigator: ITOH Goroh 茨城大学, 理工学研究科(工学野), 教授 (80158758)
• Co-Investigator(Kenkyū-buntansha): 岩本 知広 茨城大学, 理工学研究科(工学野), 教授 (60311635) ; 倉本 繁 茨城大学, 理工学研究科(工学野), 教授 (10292773) ; 小林 純也 茨城大学, 理工学研究科(工学野), 助教 (20735104)
• Project Period (FY): 2017-04-01 – 2020-03-31
• Keywords: 7000系アルミニウム合金 / 時効析出 / 粒界 / 粒内 / 水素脆性 / 強度 / 応力腐食割れ

Outline of Final Research Achievements

It is known that Al-Zn-Mg alloys, having highest strength among the aluminum alloys arising from precipitation strengthening, suffer from stress corrosion cracking (SCC) and hydrogen embrittlement (HE), and that HE can be alleviated by coarsening the microstructure of grain-boundary precipitates (GBPs). However, coarsening by means of over-aging that has been commercially used so far causes a decrease in strength by up to 15%. The goal of this study is to realize a combination of coarsened GBPs and strengthening precipitates inside grains, which leads to a best combination of strength and resistance to SCC/HE. For this goal, an Al-Zn-Mg alloy was step-quenched at temperatures from 160 to 300 degrees centigrade between solution treatment and water-quenching and then artificially aged in the same way as in peak aging. It was concluded that the material step-quenched at 180 degrees centigrade has resistance to SCC/HE as high as over-aged material, with no strength decrease.

Free Research Field

金属材料組織制御学

Academic Significance and Societal Importance of the Research Achievements

これまで長年にわたり粒内の析出組織の解析にのみ注目が集まり、粒界析出組織は、好ましくないものの粒内析出に伴い不可避的に決まるものとして、その制御がほとんど諦められてきた。しかし本研究では析出相の核生成に関する理論に基づき、析出組織を粒界と粒内別々に制御できることを示した点に、大きな学術的意義がある。一方、これまで水素脆性が懸念され最高強度状態での使用が制限されてきたAl-Zn-Mg系合金において、強度と耐水素脆化性の最善の状態での使用が可能となることを見出した社会的意義も大きい。