Hot deformation and microstructural characteristics of Al and Si containing Mg-3Sn-2Ca (TX32) alloys : correlation with processing maps

含 Mg-3Sn-2Ca (TX32) 合金的鋁與硅之熱加工微型結構特性 : 加工效果圖相互關係研究

Student thesis: Doctoral Thesis

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Author(s)

  • Dharmendra CHALASANI

Detail(s)

Awarding Institution
Supervisors/Advisors
Award date2 Oct 2013

Abstract

The worldwide need to reduce energy consumption has pushed the emergence of lightweighting technologies and, among them, research towards developing new alloys of Mg - the lightest of all structural metals - is of great interest for structural applications. Mg and Mg alloys suffer from poor plasticity due to the hexagonal close packed (HCP) crystal structure, which results in limited number of individual slip systems for active deformation. Due to this reason, most Mg alloys develop strong textures during thermomechanical processes such as rolling and extrusion, resulting in pronounced anisotropy. Formability can be improved by randomizing or weakening the texture either by modifying the existing alloys with minor additions of other elements or developing new alloy systems. The response of the intrinsic nature of material to the imposed processing parameters, namely temperature, strain rate and strain, is of significant importance to the workability. The knowledge of interrelation between process parameters, microstructure and mechanical properties will help in achieving reliable wrought products for longer service. From this view point, the development of a 'processing map' is of great significance, which defines 'safe' window(s) to process a material within certain temperature and strain rate range(s). Among several Mg-Sn-Ca alloys, TX32 alloy (Mg-3Sn-2Ca) is found to be the best compromise between corrosion resistance and creep strength due to Sn and Ca, respectively, through the formation of CaMgSn and Mg2Ca intermetallic phases. For further improvement of strength and/or weakening of texture, additions of aluminum (0.4 and 1 wt.%) and silicon (0.2 - 0.8 wt.%) are made to develop a set of six cast alloys. Very limited information is available in the literature on hot workability studies of these alloys to achieve an irreversible change in the microstructure which is essential for processing. Metallurgical phenomena are complex and metallic alloys are ratesensitive during high temperature deformation, which necessitates metal forming processes to be carried out within correct ranges of parameters. The technique of processing map, which is based on dynamic materials model (DMM) involving irreversible thermodynamics, has proved to be highly successful in accurately identifying 'safe' processing windows. This approach has been adopted by several researchers in obtaining critical information towards optimizing hot workability and achieving microstructural control for bulk forming of several metallic materials. The main aim of the present investigation is to study the hot deformation behavior of the selected set of TX32 series cast alloys through the development of their processing maps utilizing the interrelationships between flow stress and a wide range of process parameters (temperature and strain rate). The emphasis of this study is to establish the effect of Al and Si additions on the features of processing map of TX32 base alloy with respect to domains of dynamic recrystallization (DRX), optimum deformation conditions, flow instability and cracking regimes. Another aim of the study is to identify the dominant mechanisms of hot deformation through kinetic approach and to establish an interrelation between the process parameters, microstructure and evolving texture during compression. The effect of process parameters on the activation of important slip systems along the compression direction needs to be analyzed in terms of their relative orientations. Cylindrical specimens of 10 mm diameter and 15 mm height were machined from the as-cast billets and uni-axial compression tests were performed in the temperature range 300 °C to 500 °C at constant true strain rates in the range 0.0003 s-1 to 10 s-1 using computer controlled servo-hydraulic test system. The microstructure and microtexture characterizations after deformation were carried out using optical microscopy (OM) and scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS) and an electron back scattered diffraction (EBSD) facility. Pole figures and Schmid factors are used to analyse the activity of individual slip systems at various deformation conditions. Transmission electron microscopy (TEM) was used for select specimens to supplement the microstructural features. Tensile tests were carried out in the case of TX32-1Al alloy to correlate the characteristics of different domains by observing the fracture surfaces of the tested samples in SEM. The major conclusions drawn from the present study are listed below. (i) The processing map of cast TX32 alloy exhibits two domains of DRX in the temperature and strain rate ranges in hot deformation: (a) 300 °C to 350 °C and 0.0003 s-1 to 0.001 s-1 (Domain 1) and (b) 390 °C to 500 °C and 0.005 s-1 to 0.6 s-1 (Domain 2). Texture evolution as characterized by EBSD analysis indicates that specimens deformed under conditions in Domain 1 exhibit a basal texture with a maximum intensity of basal poles located at about 35° to 45° with the compression direction. At temperatures higher than 400 °C (Domain 2), texture was randomized due to increase in the activity of second-order pyramidal slip. While CaMgSn particles in the matrix contribute to significant back stress to dislocation moment, the grain boundary phase Mg2Ca reduces the grain boundary sliding. (ii) The homogenization treatment of cast TX32 alloy has negligible influence on its hot deformation behavior and texture evolution, implying that homogenization step may be eliminated in hot deformation schedules. Compared with the most widely used AZ31 magnesium alloy, TX32 alloy can be hot worked over a broad temperature and strain rate range. (iii) With the addition of 0.4 wt.% Al, the ultimate compressive strength (UCS) of TX32 alloy has improved in the testing temperature range of 25 °C to 250 °C, which may be attributed to the effect of solid solution strengthening. However, the addition of Al did not significantly change its hot working behavior (300 °C to 500 °C) as the basic features of the processing map remain unchanged. At low temperatures, the alloy exhibited flow instability in the form of flow localization at intermediate strain rates and adiabatic shear bands at high strain rates. (iv) The addition of 1 wt.% Al promoted prismatic slip at intermediate temperatures (between 350 °C to 400 °C), causing changes to the features of processing map compared to TX32 alloy. The processing map revealed three workable DRX domains and a fourth domain related to grain boundary sliding at high temperature and low strain rate range (430 °C to 500 °C and 0.0003 s-1 to 0.002 s-1) which is unsuitable for processing. The specimens deformed in lower temperature and strain rate (Domain 1) exhibited basal textures whereas second-order pyramidal slip randomized the texture in specimens deformed in high temperature and intermediate strain rate range (Domain 3). (v) The alloy with 0.4 wt.% Al and 0.2 wt.% Si has exhibited UCS closer to that of the TX32 alloy between 25 °C to 250 °C. However, increased additions of Si (from 0.4 to 0.8 wt.%) significantly decreased UCS at higher temperatures (100 °C to 250 °C), likely due to the differences in intermetallic phases formed (CaMgSi and Ca2Sn in Si containing alloys vs. CaMgSn and Mg2Ca in TX32) and the increase of their volume fraction. Moreover, both CaMgSi and Ca2Sn are distributed in the matrix compared to the presence of Mg2Ca at the grain boundaries. All the Si-containing alloys have exhibited pronounced ductility at 250 °C indicating the beginning of hot workability temperature range. (vi) The processing map of TX32-0.4Al-0.4Si alloy showed a shift of DRX domains to high temperatures and reduced flow instability regime particularly at high temperatures as compared to TX32. The addition of 0.4% Si is favorable for enhancing the hot workability since it widens the processing windows (domains). The basal poles are spread out from the compression axis and the (0001) <11 2 0> slip dominated as DRX grains have high Schmid factors for basal slip at low temperatures (300 and 350 °C) and low strain rates (0.0003 and 0.001 s-1). The texture got randomized at ≥450 °C at intermediate strain rates in Domain 2. (vii) The apparent activation energy values obtained through kinetic analyses for these alloys indicate that the deformation in the low strain rate DRX domain is controlled by climb and recovery process, whereas the deformation in the high strain rate DRX domain is attributed to cross-slip since the stacking fault energy on the pyramidal slip systems is high. (viii) For 0.6% Si addition to TX32-0.4Al alloy, an additional DRX domain (Domain 3) occurs at high temperatures and high strain rates. Domain 1 is characterized as cracking domain, whereas in Domains 2 and 3, DRX is occurring predominantly by basal slip with climb as a recovery process. (ix) With further increase in Si (TX32 with 0.4 wt.% Al and 0.8 wt.% Si), the first DRX domain at low strain rates has shifted further towards high temperature and the second DRX domain at high temperature shifted to high strain rates. Deformation is basal slip dominated and the recovery is by climb in both the domains. (x) When the volume fraction of intermetallic particles increased steeply (in 0.6 and 0.8% Si-containing alloys), the back stress increases significantly and thus, the activation of basal slip required considerably high temperatures for its extensive participation in plastic flow.

    Research areas

  • Microstructure, Hot working, Magnesium alloys, Deformations (Mechanics)