Excellent creep properties of Mg-Zn-Cu-Gd-based alloy strengthened by quasicrystals and Laves phases
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Kato Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
Kenji Amiya Inoue Superliquid Glass Project, ERATO, Japan Science and Technology Corporation, Sendai 982-0807, Japan
Akihisa Inoue Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan (Received 7 November 2004; accepted 7 February 2005)
A new type of Mg–Zn–Cu–Gd-based alloy strengthened by quasicrystal and Laves phase was developed. This alloy exhibits much better creep properties compared to AE42(Mg–4 wt% Al–2 wt% rare-earth) alloy, which is the benchmark creep-resistant magnesium die-casting alloy under the compressive creep condition of 180 °C and 80 MPa. The new alloy also exhibits high room-temperature mechanical properties close to that of AZ91 alloy. The good mechanical properties are attributed to the special microstructure; the thermally stable icosahedral quasicrystals phase (i-phase) and Laves phase distributed along the grain boundary as a hard skeleton, and some fine ⬘1 precipitates distributed homogenously on the matrix. The dislocation morphology after the creep test was studied, and the strengthening mechanism was proposed.
I. INTRODUCTION
As the lightest metallic structural materials, magnesium and its alloys have received increasing attention in the last decade because of their high potential in automotive and aerospace applications.1 One group of magnesium alloys already finding use in the automotive industry, which is currently limited to a few selected applications such as instrument panels, steering wheels, and valve covers, is based on Mg–Al-based casting alloys such as AZ91 alloy (Mg–9 wt% Al–1 wt% Zn) and AM series [Mg–(2–6)% Al].2 These alloys offer a good combination of room-temperature strength and ductility, good corrosion resistance, and excellent castability. Major growth areas in automotive use for Mg alloys are powertrain applications, such as the transmission case and the engine block. These applications require high elevated temperature performance at service conditions of 150–200 °C and stresses in the range of 50–70 MPa. Commercial AZ and AM types of magnesium alloy do
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0156 1278
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 5, May 2005 Downloaded: 18 Mar 2015
not meet such requirements because they are prone to excessive creep deformation when exposed to moderate levels of load in the temperature range 100–200 °C.3 Current interpretations attribute the poor creep resistance to the occurrence of discontinuous precipitation of the ␥-phase (Mg17Al12, space group I4¯m, a ⳱ 1.054 nm)4 at grain boundaries, which will give rise to the excessive grain boundary sliding. A major development in creep resistant magnesium alloys has been the emergence of rare-earth (RE) containing alloys such as Mg–4Al–2RE (AE42). This alloy system exhibits major improvement in creep resistance due to the complete suppression of the formation of the ␥-phase and the presence, instead, of the
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