Effect of silicon content on hot working, processing maps, and microstructural evolution of cast TX32–0.4Al magnesium alloy

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Abstract

The effect of silicon (0.2–0.8 wt%) addition on the hot working behavior and deformation mechanisms of the Mg–3Sn–2Ca–0.4Al (TX32–0.4Al) alloy has been evaluated by generating processing maps in the temperature and strain rate ranges of 300–500 °C and 0.0003–10 s−1. The processing map for the base TX32–0.4Al alloy exhibited two dynamic recrystallization (DRX) domains in the ranges (1) 300–360 °C and 0.0003–0.001 s−1 and (2) 400–500 °C and 0.003–0.7 s−1. While 0.2% Si addition did not result in any significant change in the processing map of the base TX32–0.4Al alloy, 0.4% Si addition has enhanced hot workability by widening the processing window(s) and by reducing flow instability. The rate controlling mechanism in Domain 1 is identified as climb, whereas it is cross-slip in Domain 2. When the Si content is increased to 0.6 and 0.8%, the volume fraction of hard intermetallic particles has increased nearly two fold. The processing map for the alloy with 0.6% Si addition exhibited an additional Domain 3 at higher temperatures and high strain rates (475–500 °C and 0.01–10 s−1). However, cracking has occurred in Domain 1 due to void formation at hard particles. In Domains 2 and 3, DRX occurred predominantly by basal slip with climb as a recovery process, as confirmed by the resulting basal texture and tilt type sub-boundary structure. This is attributed to the large back stress generated by the increased volume fraction of intermetallic particles due to which the extensive activation of basal slip required considerably high temperatures. Increase in the volume fraction of hard particles due to higher Si content reduces the flow instability by generating a high rate of entropy production through increasing the nucleation sites for power dissipation and enhances the occurrence of void formation and/or ductile fracture.

Introduction

Due to its light weight, high strength to weight ratio and high castability, Mg based alloys have been attractive for components in aerospace and automotive applications. While the use of Mg alloys is mostly limited to the cast condition, Mg wrought alloys with good mechanical properties have high potential especially as structural materials. The major limitation for the usage of wrought magnesium is its poor formability at room temperature as only two independent basal slip systems can get activated, leading to poor ductility [1]. Thus, bulk forming processes such as rolling, forging, and extrusion are usually done at elevated temperatures, where the non-basal slip systems would be activated [2], [3], [4], [5], [6], [7], [8], [9]. In addition, creep and corrosion resistance are also important to employ the magnesium alloys for structural applications. To improve these properties, a new series of Mg–Sn–Ca (TX) alloys are being developed and from the studies, TX32 (Mg–3Sn–2Ca) is found to be an optimum composition to strike a balance between creep resistance and corrosion resistance due to the presence of Mg2Ca and CaMgSn intermetallic particles [10], [11], [12], [13], [14]. The addition of alloying at micro-alloying levels is considered to further broaden the properties of this alloy. Aluminum improves room temperature mechanical properties of Mg by solid solution strengthening [15] due to large atomic size difference (16%) while Si forms intermetallic particles which may enhance creep strength.

Due to their hexagonal crystal structure and limited slip systems, Mg wrought alloys develop strong textures during deformation processing. As an example, when extruded, Mg–3Al–1Zn (AZ31) alloy exhibits basal textures such that 〈101̄0〉 is aligned with the extrusion direction [16], [17], [18], [19], [20]. During rolling, a texture develops such that the basal planes are parallel to the rolling plane and 〈101̄0〉 is along the rolling direction [21], [22], [23]. So, the texture control is important during hot working of magnesium wrought alloys. The aim of the present study is to evaluate the effect of Si addition on the hot workability of TX32–0.4Al alloy with emphasis on the study of the evolution of microstructure and texture and to correlate these with the deformation mechanisms. For this purpose, four alloys with Si additions of 0.2, 0.4, 0.6 and 0.8 wt% have been chosen. The hot working behavior has been characterized with the help of processing maps, and the microstructure and texture development in the deformed specimens have been characterized using optical microscopy, transmission electron microscopy (TEM), and scanning electron microscopy (SEM) with an electron back scatter diffraction (EBSD) facility.

Processing maps are based on the Dynamic Materials Model [24], the principles and basis for it were described in the literature [25], [26]. A processing map reveals the explicit response of the material to the imposed processing parameters in terms of microstructure mechanisms. Such a map is constructed by a superimposition of power dissipation map and an instability map. Power dissipation map represents the pattern in a frame of temperature and strain rate, in which power dissipates by the material through microstructural changes. Efficiency of power dissipation (η) is the parameter which measures the rate of such microstructural changes, which relates to the strain rate sensitivity (m) of the stress flow [η=2m/(m+1)]. Further, the extremum principles of irreversible thermodynamics as applied to continuum mechanics of large plastic flow [27] are explored to define a criterion for the onset of flow instability given by the equation for the instability parameter ξ(ε̇):ξ(ε̇)=ln[m/(m+1)]lnε̇+m0

The variation of the instability parameter as a function of temperature and strain rate represents an instability map which delineates regimes of instability where ξ(ε̇) is negative. In simple words, the meaning of Eq. (1) is that if the material is unable to generate entropy at a rate that at least matches the imposed rate, then plastic flow gets localized and causes flow instability. Processing maps help in identifying temperature–strain rate windows (domains), in which workability of a material is maximal due to softening mechanisms such as dynamic recrystallization (DRX); and regimes of flow instabilities, such as shear bands and flow localization.

Section snippets

Experimental work

Alloys of TX32–0.4Al base and with ‘Si’ additions were prepared using elemental metals with purity of 99.99% Mg, 99.96% Sn, 98.5% Ca, 99.9% Al and Si. The alloying additions were made to the magnesium melt which was held at about 720 °C. The molten alloy was kept under the shielding of Argon+3% SF6 mixed cover gas, followed by gravity casting of the melt in pre-heated permanent steel molds (200 °C) to obtain cylindrical billets of 100 mm diameter and 350 mm length.

The X-ray diffraction (XRD) was

Initial material characterization of silicon containing TX32–0.4Al alloys

The optical microstructures of the starting alloys for the TX32–0.4Al base alloy, and those of alloys containing 0.2%, 0.4%, 0.6%, and 0.8% Si are shown in Fig. 1(a)–(e), respectively. The microstructures exhibit large grain sizes ranging from 500 µm to 800 µm and typical as-cast material dendritic features. In the TX32–0.4Al base alloy (Fig. 1(a)), several intermetallic particles have been observed, the major ones being Mg2Ca appearing mainly at the grain boundaries and CaMgSn forming primarily

Conclusions

Four TX32–0.4Al alloys with 0.2–0.8 wt% Si additions were processed by uni-axial compression tests in the temperature range of 300–500 °C at constant true strain rates in the range of 0.0003–10 s−1. Processing maps were developed for each alloy, and interrelation among the process parameters, microstructure, and evolving texture was established. Domains of the processing maps are interpreted in terms of the associated deformation mechanisms and texture characteristics. Based on the results

Acknowledgment

This work was supported by a grant (Project #115108) from the Research Grants Council of the Hong Kong Special Administrative Region, China.

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