Creep Behavior of ZK60 Alloy and ZK60/SiC w Composite After Extrusion and Precipitation Hardening
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Creep Behavior of ZK60 Alloy and ZK60/SiCw Composite After Extrusion and Precipitation Hardening Morteza Tayebi1 · Hamidreza Najafi1 · Said Nategh1 · Alireza Khodabandeh1 Received: 24 June 2020 / Accepted: 2 September 2020 © The Korean Institute of Metals and Materials 2020
Abstract Creep properties of ZK60 alloy and ZK60/SiCw composite have been investigated after extrusion and precipitation hardening by accelerated creep test. Creep tests were conducted at 150 °C in the stress range of 10–120 MPa. At low stresses, the stress exponents of 1.93 and 1.75 were obtained for the unreinforced alloy and the composite, respectively. Stress exponents of the unreinforced alloy and composite sample were 5.82 and 7.07, respectively, at high stresses. The creep mechanism changed by increasing the stress from grain boundary sliding (GBS) to dislocation creep due to the fact that the average true creep activation energy changed from 55 to 95.06 kJ/mol. Based on the microstructural observations, at low stresses, the grain refinement induced by twinning caused the GBS mechanism. However, at high stresses, slip changed from basal planes to the pyramidal secondary slip system which was associated with increase in twin density. Examination of the fracture surfaces revealed that cavity nucleation in the grain corners and around the precipitates was the main reason for creep failure. Keywords ZK60/SiCw composite · Accelerated creep test · Dynamic precipitation · Double twinning · Grain boundary sliding · Dislocation creep
1 Introduction Concerns about greenhouse gases and depletion of nonrenewable fuel resources encourage engineers to develop advanced lightweight materials. Since automotive industries require lightweight parts with a working temperature of ~ 150 °C, low-density magnesium alloys are considered as an attractive choice. However, low strength of magnesium alloys is the major restriction. Despite the formation of thermally-stable precipitates, Mg–Al alloys exhibit poor creep properties due to the formation of low-temperature eutectic phases in the interdendritic and intergranular regions as a result of solute segregation [1]. ZK60 alloy (a * Hamidreza Najafi [email protected] Morteza Tayebi [email protected] Said Nategh [email protected] Alireza Khodabandeh [email protected] 1
Department of Materials Engineering, Science and Research Branch, Islamic Azad University, Tehran 14515‑755, Iran
magnesium-base alloy containing Zn and Zr) has been developed to overcome this limitation by replacing Al with Zn which prevents the formation of low-temperature eutectic phases. The addition of Zn and Zr to magnesium improves the creep strength in different ways. Zn limits the cross-slip of screw extended dislocations by decreasing the stacking fault energy (SFE) [2]. Large atomic size difference between Zr and Mg leads to a considerable solid solution strengthening and slowing down the diffusion of Zr in Mg [3–6]. Moreover, it is possible to benefit from precipitation hardening in Mg–Zn–Zr alloy system by
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