Microstructure, mechanical properties and texture evolution of AZ31 alloy containing trace levels of strontium

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Abstract

The effect of low levels of Sr (0.01, 0.03, 0.05 wt.%) on the microstructure, mechanical properties and texture of AZ31 magnesium alloy has been investigated. Thermodynamic modeling has been used to study the effect of Sr on phase precipitation at different temperatures. Cooling curve analysis reveals a decrease in solidification superheat with the addition of 0.03 wt.% Sr to AZ31. The as-cast microstructures of the alloys have been studied using optical microscopy (OM) and electron probe micro analysis (EPMA). Results show the refining effect of Sr on the grain size and on the β-Mg17Al12 precipitates through growth poisoning and inoculation, respectively. To investigate the mechanical properties of the alloys, as-cast samples were compression tested at elevated temperatures. Hot compression peak stress (σmax) and critical strain before recrystallization (εc) initially drop but then increase. σmax first decreases due to the depletion of Al from solid solution and then increases when the amount of the Al–Sr precipitates reach a significant amount. εc drops due to the acceleration of dynamic recrystallization kinetics as a result of grain refinement. It increases with increasing Sr when the concentration Sr in solid solution is increased leading to dislocation pinning and retardation of recrystallization. X-ray texture measurements on the hot compressed and extruded samples show a decrease in maximum intensity of the basal pole figures with increasing Sr as a result of reduced twining and the changes in Al and Sr concentrations in solid solution.

Research Highlights

► Microstructures/precipitates of AZ31+trace levels of Sr were examined by OM and EPMA. ► Thermodynamic calculations & cooling curve analysis were used to predict ppt formation. ► It has been shown that Sr refines the grains and Mg17Al12 precipitates. ► Mechanical properties & texture were studied using hot compression & X-ray diffraction. ► Different micro-mechanisms were responsible for mechanical and texture changes.

Introduction

Strontium is known to be an effective grain refining element in Mg alloys containing high levels of Al [1], [2]. Having a low equilibrium solid solubility, Sr segregates to the surface of Mg grains and slows down the grain growth [3]. It has also been shown that Sr refines the beta (Mg17Al12) precipitates in AZ31 [4]. The effects of Sr in AZ31 are dependent on the concentration of Sr in the alloy and the type of second phase which is precipitated. When added to AZ31, Sr forms Al–Sr, Mg–Al–Sr, Mg–Sr and Sr–Zn precipitates. In the Mg–Al–Zn system Sr prefers to bind with Al rather than the other elements. Because of this high affinity of Sr to Al, all the precipitates in the AZ + Sr system could be classified into two groups: precipitates with Al and precipitates without Al. Sr-containing precipitates without Al (Mg17Sr2 and SrZn5) only form at high concentrations of Sr where the affinity of free Al is decreased when most of it is captured in Al–Sr intermetallics. Sr precipitates with Al (Al4Sr and Al2Sr) are formed when Sr meets a high concentration of Al in the alloy. When the concentration of Al is low or the solidification condition slows down the diffusion of Al, Al–Mg–Sr precipitates form. In different alloying systems, many different stoichiometries for Al–Mg–Sr precipitates have been suggested [5], [6], [7], [8], [9]. Recently Janz et al. [10] have shown that τ-Al38Mg58Sr4 is the only equilibrium ternary precipitate in the Mg–Al–Sr system and all the other reported stoichiometries are non-equlibrium precipitates which depend on the alloy composition and solidification conditions. L'Esperance et al. [9] have shown the decomposition of the non equilibrium Al–Mg–Sr ternary precipitate into Al4Sr and Mg, by rejecting its excess Mg and absorbing Al from the matrix.

The addition of low levels of Sr into Mg–3Al–1Zn will result in a high Al/Sr ratio, in which case, Sr will find a high concentration of Al atoms and a precipitate with high Al/Sr ratio (Al4Sr) will form. Although, the amount of precipitate is very small, it has significant effects of microstructure, mechanical properties and deformation texture [11], [12], [13], [14]. However, research concerning the effect of trace levels of Sr on the most commonly used wrought AZ31 has not been extensively studied. In this work, the formation of new phases resulting from the addition of trace levels of Sr in AZ31 and its effects on mechanical properties and texture were investigated in as-cast and extruded samples by cooling curve analysis, thermodynamic predictions, electron probe micro-analysis (EPMA), hot compression and texture measurement.

Section snippets

Experimental Procedure

AZ31 alloys containing 0.01, 0.03 and 0.05 wt.% Sr were prepared using AZ31 extruded bars supplied by Applied Magnesium (Denver, CO) and Sr–10 wt.%Al master alloy (from Timminco, Haley, ON). AZ31 was melted in a graphite crucible using a high frequency induction furnace (NORAX). The melt was kept at 700 °C for 15 min under a gas mixture of CO2-SF6 to allow for sufficient dissolution and mixing of the alloying additions. The melt was cast at 720 °C into preheated (400 °C) cylindrical steel dies. The

Microstructure

One of the important questions in this study was related to how trace levels of Sr added to AZ31 would be incorporated into the microstructure during solidification. There are four possible scenarios for Sr atoms to be located in different regions and phases. They could (i) dissolve in α-Mg as a solid solution element, (ii) dissolve in the β-Mg17Al12 precipitates, (iii) segregate at the grain boundaries, or (iv) precipitate either as Al–Sr or Al–Mg–Sr second phases. Large atomic radius

Conclusion

Microstructure, mechanical properties and deformation texture of AZ31 and AZ31 alloys containing low levels of Sr (0.01 wt.% to 0.05 wt.%) were compared and the following was concluded:

  • 1-

    Microstructural observations supported by cooling curves and EPMA show grain and precipitate (β-Mg17Al12) refinement by growth poisoning and via nucleant formation, respectively with the addition of low levels of Sr to AZ31.

  • 2-

    Low levels of Sr show a significant effect on the hot deformation behavior of AZ31. The peak

Acknowledgment

This study was carried out under a Strategic project grant from Natural Sciences and Engineering Research Council (NSERC) of Canada and Applied Magnesium (formerly Timminco) providing industrial support, raw materials and master alloy. Thanks are due to Scott Shook of Applied Magnesium for technical support. One of the authors, A. Sadeghi, gratefully acknowledges the financial support of McGill University through MEDA (McGill Engineering Doctoral Award) program.

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