Improvement of mechanical properties of extruded AZX912 magnesium alloy using high-temperature solution treatment
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Structural Materials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Nagoya, Aichi 463-8560, Japan Technical Headquarter Fuji Light Metal Co., Ltd., Tamana, Kumamoto 869-0912, Japan Technical center Tobata Seisakusho Co., Ltd., Kitakyushu, Fukuoka 800-0211, Japan a) Address all correspondence to this author. e-mail: [email protected] b) Present Address: Technical Headquarter Fuji Light Metal Co., Ltd., Tamana, Kumamoto 869-0912, Japan. 2 3
Received: 12 June 2019; accepted: 26 August 2019
For achieving flame-retardant AZX912 magnesium alloy with superior mechanical properties, cast ingots were solution-treated at different temperatures of 420–525 °C prior to extrusion at 280 °C. With increasing solution treatment temperature, brittle Al2Ca intermetallic compound changed from a network-like morphology to a spheroidized shape, with an increase in hardness and became unbroken during extrusion. As the solution treatment temperature increased, cracking of Al2Ca particles during tensile deformation tended to be restricted due to hardening and spheroidizing behaviors, and tensile elongation of extruded alloys significantly enhanced from 11.2 to 19.2%. High mechanical strength was maintained with an improvement in ductility when increasing the solution treatment temperature up to 510 °C. The extruded alloy solution-treated at 510 °C exhibited a superior balance between mechanical strength and ductility, with a high ultimate tensile strength of 367 MPa and a good elongation of 16.8%.
Introduction Magnesium (Mg) alloys are attractive as lightweight materials due to their low density, high specific strength, good castability, and so on [1]. However, flammability of Mg alloys causes safety concerns [2]. For expanding the use of Mg alloys, development of flame-retardant Mg alloys with superior mechanical properties is required. Ignition temperature can be significantly increased by adding Ca (.1 wt%) into Mg alloys [2, 3, 4]. For example, adding 2 wt% Ca into AZ91 (Mg–9Al–1Zn in wt%) alloy (i.e., becomes AZX912 alloy) may remarkably increase ignition temperature from 427–527 to 667–977 °C [5], which is higher than the liquidus temperature (598 °C) of AZ91 alloy. This means that burning can be restricted even after melting into a liquid state. Consequently, it is possible to safely cast those flame-retardant alloys avoiding the use of protective gases such as SF6, which has a quite high global warming potential. On the other hand, flame-retardant AZX912 alloy contains a large amount of second-phase particles, which are mostly in
ª Materials Research Society 2019
the form of Mg17Al12 and Al2Ca compounds [6, 7, 8]. The existence of second-phase particles induces damage initiation, and thus, remarkably affects the failure of metals. Particle cracking or void formation due to particle–matrix decohesion may occur during deformation depending on the hardnesses of second-phase particle and matrix [9, 10]. For AZX (Mg–Al– Zn–Ca) and AMX (Mg–Al–Ca) series Mg alloys, brittle Al2Ca particles tend
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