Elsevier

Materials & Design

Volume 63, November 2014, Pages 439-445
Materials & Design

Effect of Ca and Sr on the compressive creep behavior of Mg–4Al–RE based magnesium alloys

https://doi.org/10.1016/j.matdes.2014.06.027Get rights and content

Highlights

  • Steady state creep rate of AE41 decreases with increasing Ca and Sr concentration.

  • Steady state creep rate of AE41 is 11.4 × 10−7 mm/s, and decreased to 1.4 × 10−7 mm/s of AECJ411202.

  • The precipitation of AECJ411202 consists of Al2Nd, (Mg, Al)2Ca and Al4Sr.

  • Creep resistance is proved by hindering dislocation climbing and grain boundary sliding.

Abstract

The compressive creep behavior of Mg–4Al–RE–1.2Ca (AEC4112) alloy and Mg–4Al–RE–1.2Ca–0.2Sr (AECJ411202) alloy is investigated on a special apparatus. The stress exponent n under different stresses and the creep activation energy Qc under different temperatures of AEC4112 alloy and AECJ411202 alloy are estimated by an empirical equation. The results reveal that the compressive creep resistance of Mg–4Al–RE (AE41) alloy is improved significantly by a small amount of Ca and Sr. The steady state compressive creep rates of AEC4112 alloy and AECJ411202 alloy are increased with increasing temperature and applied stress. The steady state compressive creep rates of AEC4112 alloy and AECJ411202 alloy are 2.2817 × 10−7 mm/s and 1.3041 × 10−7 mm/s under 150 °C and 100 MPa, respectively. The compressive creep resistance is improved by intermetallic compounds Al2Ca, Al2RE and Al4Sr inhibiting the climbing of the dislocation, the sliding and diffusion of the grain boundary. The compressive creep constitutive equations of AEC4112 alloy and AECJ411202 alloy are written as ε̇s1 = 2.0276 × 10−10 × σ5.662exp(−47447/RT) and ε̇s2 = 5.8208 × 10−11 × σ6.123exp(−68252/RT) respectively. The compressive creep mechanism of AEC4112 alloy and AECJ411202 alloy is the combined effects of grain boundary diffusion and grain boundary sliding determined by dislocation climbing.

Introduction

Magnesium alloys such as AZ91D, AZ31 and AM60B are widely applied in the automotive and aerospace industries due to low density, good corrosion resistance, superior die castibility, good balance of ductility and strength at room temperature, high specific strength and specific stiffness [1], [2], [3], [4]. However, the commercial utilization of magnesium alloy is limited by the poor creep resistance and poor mechanical strength at temperature about 150 °C [5], [6], [7]. The poor creep properties of Mg–Al based alloy are caused by the softening of β-Mg17Al12 phase at elevated temperature, and the grain boundary sliding and grain deformation of the magnesium alloy are not hindered by the coarsening β-Mg17Al12 phase.

Attempts were made to improve the creep resistance of magnesium alloys by adding a small amount of alloying elements for the formation of strengthening phases. Zhang et al. [8], [9] concluded that Al11RE3 with relatively thermally stable precipitated along the grain boundary provided a considerable deformation when the alloys underwent high stress and low temperature. The A111RE3 was decomposed to Al2RE with high thermally stable at elevated temperature, and the grain was refined by Al2RE acting as the nucleation site for α-Mg phase [10], [11], and the grain deformation was restrained [12], [13]. Nayyeri and Mahmudi [14], [15] found that the thermally stable second phase particle Mg3Sb2 (1280 °C) occurred in the interior parts of the dendrites and at the grain boundary regions, and it was benefit from both strengthening of the grains and grain boundaries during creep deformation at high temperatures. Zhang et al. [16], [17] revealed that the grain size and polygonal morphology of magnesium alloy was improved by the Mg2Si (1085 °C), and then the mechanical properties of magnesium alloy were increased by precipitation hardening by Mg2Si. Celikin et al. [18], [19] demonstrated that the high thermal stability intermetallic Al–Sr phase in the interdendritic regions exerted a strengthening effect in the primary creep stage, and the growth of grains was hindered by Al4Sr. The alkaline-earth elements (Ca and Sr) and the rare earth elements (La/Pr/Ce or Nd/Pr) are considered as outstanding options to improve the creep resistance of Mg–4Al–RE system alloys. Amounts of effort are made to estimate the creep resistance of Mg–4Al–RE system alloys [20], [21], but the compressive stress method is seldom reported.

In the paper, the compressive creep property of a quaternary Mg–4Al–RE–1.2Ca alloy and a quinary Mg–4Al–RE–1.2Ca–0.2Sr alloy was investigated. The creep parameters and creep constitutive equations of AEC4112 alloy and AECJ411202 alloy were estimated. The effect of Ca and Sr on the compressive creep behavior of AEC4112 alloy and AECJ411202 alloy was discussed.

Section snippets

Experimental details

Three AE41 magnesium alloys were prepared for the compressive creep test and the chemical compositions were tested by inductive coupled plasma (ICP) and listed in Table 1. The alkaline-earth elements and the rare earth elements in the magnesium alloys were conducted into magnesium ingot by adding master alloys of Mg–30Ca, Al–10Sr and Mg–10RE (RE = (Nd, Pr), Nd/Pr weight ratio is about 9:1) alloys. The amount of pure magnesium ingot, aluminum ingot and each master alloy was determined by the

Results

Fig. 2 depicts the compressive creep curves of three AE41 magnesium alloys under the temperature 150 °C and applied stress 100 MPa for 100 h. The compressive creep curve consists of decelerating creep stage and steady state creep stage, and the decelerating creep stages of three alloys are sustained for about 5 h and followed by the steady state creep stage with the characteristic of linear creep curve. The creep resistance of magnesium alloys is evaluated by the creep rate that is defined as the

Creep parameters

The relationship between steady state creep rate ε̇s, stress exponent n and creep activation energy Qc of the polycrystalline material can be expressed by an empirical equation [22]:ε̇s=Aσnexp(-Qc/RT)where A is the structure-dependent constant, σ is the nominal normal stress, n is the stress exponent determined by deformation mechanism, and Qc is the creep activation energy, R is the gas constant and T is the absolute temperature. Eq. (1) is converted to Eq. (2) by taking the logarithm:lnε̇s=lnA

Conclusions

The compressive creep resistance and the constitutive equations of AE41 based magnesium alloys are estimated, and the creep mechanism of the magnesium alloys is analyzed. Following conclusions are drawn:

  • (1)

    The compressive creep resistance of AEC4112 alloy and AECJ411202 alloy is superior to AE41 alloy. The steady state compressive creep rates are increased with increasing temperature and applied stress.

  • (2)

    The creep constitutive equations of AEC4112 alloy and AECJ411202 alloy are written as ε̇s1 = 2.0276

Acknowledgments

This research was financially supported by the funding of the National Natural Science Foundation of China (Grant No. 51275060), the Science and Technology Program of Jiangsu Province of China (Grant No. BK20141228) and the Science and Technology Program of Suzhou (Grant Nos. SYG201421, SYG201348, SYG201251 and 13KJB430001).

References (26)

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