Positron lifetime and Doppler broadening study of defects in hydrogen charged Al-Mg alloys
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rong Zhong and Baoyi Wang Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, People’s Republic of China
Hongyan Zhang Department of MIME, University of Toledo, Toledo, Ohio 43606 (Received 28 August 2008; accepted 22 January 2009)
The results of positron lifetime and Doppler broadening spectrum of defects in the hydrogen charged non-heat treatable 5xxx Al alloys are presented in this work. The yield stress of the sample was reduced for about 20 MPa after hydrogen was charged. A similar trend was observed in positron lifetime measurement, as the average lifetime tav descended remarkably to almost the level of Al matrix. The change in coincidence Doppler broadening (CDB) spectroscopy was also significant, exhibited by the characteristic change in CDB radio curves of a sample before and after hydrogen was charged. After hydrogen charging, there is an obvious enhancement in the high momentum region compensating dehancement in the low momentum region. This indicates the existence of hydrogen filling effect. The vacancies around the Mg atoms should be preferential filling sites for hydrogen because Mg has a strong affinity for hydrogen. The formation of an Mg–H bond parallel to a grain boundary is an important factor in weakening the grain boundary cohesion. I. INTRODUCTION
The non-heat treatable 5xxx Al alloys with Mg as the major alloying element have found important industrial applications because of their medium level strength, high ductility, excellent corrosion resistance, and weldability.1,2 Among them AA5754 is a typical non-heattreatable aluminum alloy widely used in automotive applications, and it is often welded using a resistance spot welding (RSW) technique. However, cracking may occur during welding because of the rapid temperature change in the process.3 Cracking in the heat affect zone (HAZ) during spot welding usually doesn’t initiate from the fusion line, rather a wide root of a crack is often found at a certain distance from the fusion line. Experimental observation has shown that there was Mg segregation to the cracks.4 Such segregation is directly related to the formation and migration of vacancies around a crack, similar to a grain boundary, as it is promoted by suitable temperatures and microstructures. It is a direct indicator of the initiation and propagation of cracks. Previous theoretical calculation has pointed out that the
II. EXPERIMENTAL PROCEDURE
Al–3.0%Mg sheets (AA5754) were used in this experiment. The thickness of AA5754 sheet was 1.5 mm. The hydrogen charging was performed in a hydrogen
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0195 J. Mater. Res., Vol. 24, No. 5, May 2009
http://journals.cambridge.org
existence of Mg in the boundary region could reduce the grain boundary cohesion,5 which might promote the formation and growth of cracks during RSW aluminum alloys. In addition to Mg at grain boundary from segregation, the result from a first principle calculation has shown that hydrogen around Mg atoms
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