Effect of nano-alumina and calcium additions on microstructure and texture evolution during thermo-mechanical working of magnesium alloy AZ31 : study using processing maps

納米氧化鋁與鈣添加物對鎂合金 AZ31 熱機械加工過程中晶體微結構演變的影響 : 運用加工圖技術方法的研究

Student thesis: Doctoral Thesis

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Author(s)

  • Tao ZHONG

Detail(s)

Awarding Institution
Supervisors/Advisors
Award date2 Oct 2013

Abstract

In order to achieve recycling of resources, enhancing of fuel efficiency and reduction of carbon dioxide emission, development of new materials and relevant technologies is necessary. Magnesium (Mg) is the lightest structural metal and its use in various products leads to reduced energy consumption. Magnesium alloys have many advantages over steel, such as high strength to weight ratio, electromagnetic immunity, recyclability, and absorption of vibrations. Thus, they are regarded as important next generation materials for automobiles, electronic products, aerospace, defense equipment, and so on. Wrought magnesium alloys have many advantages over cast ones, such as low level of porosity, higher mechanical strength, smaller grain size, etc. However, magnesium alloys have been typically used in their cast form before 2000. This is because they suffer from poor plasticity due to their hexagonal close packed (HCP) crystal structure, which offers limited number of slip systems for deformation. Activation of additional slip systems requires high temperature for deformation, but undesirable deficiencies, such as grain coarsening and flow localization, need to be overcome. Complex metallurgical phenomena occur during high temperature deformation, necessitating metal forming processes that should be carried out within proper ranges of parameters. The technique of processing map, which is based on the dynamic materials model (DMM), has been proven to be accurate in identifying "safe" processing windows (called "domains" in this research). Processing map is an explicit representation of the response of a material to the imposed processing conditions (i.e., temperature, strain rate, and strain,) which are critical to its workability. This technique has been adopted by several researchers in obtaining important information on microstructure and deformation mechanisms towards optimizing hot workability of metallic materials for bulk forming processes. However, limited researches exist on processing maps' relationship with texture (crystals orientation) characteristics, especially during high temperature processing. Texture is an important factor, which influences properties of materials; with texture in cast materials being nearly random, while it may be anisotropic in wrought materials. Texture can be controlled by adjusting processing parameters or adding alloying elements so that it can properly serve various application purposes. Microstructural refinement is an effective way of improving strength and ductility of alloys. Therefore, microstructure control, especially at high temperature, is critical to obtaining the desired properties in final products. It has been reported that alloying elements and highly stable ceramic particulates can improve the properties magnesium alloys. Some researchers found that calcium is very effective in enhancing high temperature strength and creep resistance as well as in controlling grain coarsening. Among ceramic particulates, nano-alumina particulates exhibit an excellent combination of specific stiffness, high temperature mechanical properties, oxidation resistance, and so on. However, limited research has been conducted so far on the effects of alloying elements and reinforcing particles on the texture of magnesium alloys. Thus, in the current research 1.5 vol.% nano-alumina and 1 wt.% calcium are added respectively to the base magnesium alloy AZ31-DMD (a synthesis technique namely Disintegrated Melt Deposition) in an attempt to understand their effect on high temperature deformation, texture evolution, and mechanical properties. The DMD process is a modification of the more conventional dispersion and spray processes, while avoiding the disadvantages associated with the latter processes. The following three magnesium alloys are selected for this research study: (1) AZ31-DMD, (2) AZ31 with 1.5 vol.% nano-alumina (AZ31-NAL), and (3) AZ31 with 1 wt.% calcium (AZ31-Ca). All these materials are synthesized by Disintegrated Melt Deposition (DMD) followed by extrusion using a heated die maintained at 350°C. The aim of this research is to investigate the hot deformation characteristics of the three selected materials over a broad range of process parameters, with the aim of specifically addressing the following main objectives: 1. to find out the microstructure evolution and deformation mechanisms as a function of temperature and strain rate; 2. to develop the processing maps of the alloys so as to optimize the hot deformation process parameters; and 3. to study the texture development in the alloys deformed in the safe processing domains. Cylindrical specimens of 10 mm diameter and 15 mm height are machined from the extruded rods for isothermal uniaxial compression along the direction of extrusion. Conditions for the isothermal uniaxial compression are as follows: strain rates in the range of 0.0003 s-1–10 s-1 and temperatures in the range of 250°C–500°C. The specimens are deformed up to a true strain of about 1 and then quenched in water. The deformed specimens are sectioned in the center parallel to the compression axis, after which the cut surface is mounted, polished. The relevant equipment utilized in this research include: computer-controlled servo-hydraulic testing machine, optical microscope (OM), scanning electron microscope (SEM), electron back-scattered diffraction (EBSD) and energy-dispersive x-ray spectroscopy (EDX). The test data obtained from compression and tension tests using servo-hydraulic machine are used to obtain strain-stress curves in order to analyze the strength properties. OM and SEM are used to reveal two-dimensional information on microstructure at low and high magnification, respectively. SEM along with EBSD is used to analyze the texture of materials and EDX is used to obtain elemental analysis or chemical characterization of materials. Major conclusions from this research are listed below: 1. For all the three materials: (i) the processing maps exhibit three domains of high efficiency, although there are some differences in the domain boundaries among the materials; (ii) Dynamic recrystallization (DRX) occurs in domain #1 (low temperature and low strain rate) and domain #3 (medium temperature and high strain rate), with grain size variation with Zener-Holloman parameter following linear relationship; (iii) In domain #2 (high temperature and low strain rate), grain boundary sliding occurs leading to wedge cracking in compression and intercrystalline cracking in tension. 2. Elongation of AZ31-Ca at conditions of peak efficiency in domain #1 is 100%, the highest value among the three materials, which may be attributed to the smallest grain size and uniform grain structure. Although calcium-containing particles significantly improve the mechanical properties of the alloy at high temperatures (250°C-450°C), the elongation of AZ31-Ca drops sharply to 3% for the peak dissipation efficiency condition in domain #2 (500°C/0.0003s-1), possibly due to the formation of brittle CaO-MgO. 3. The addition of calcium and nano-alumina successfully enhance the compression strength and control grain coarsening of base material AZ31-DMD for most of the tested conditions. Compared with the nano-particles, calcium-containing particles are more effective in controlling grain size of the material for all the tested conditions. However, calcium-containing particles are less effective than nano-particles in accelerating DRX. 4. The extruded AZ31-DMD has a strong texture, but the extruded AZ31-NAL has a very weak texture, since nano-alumina hamper basal slip. Meanwhile, the extruded AZ31-Ca has the strongest texture because Al is pulled out of the matrix by Ca in forming the (Mg, Al)2Ca and Al2Ca particles. Therefore its strengthening effect is reduced when the critical resolved shear stress (CRSS) for basal and prismatic slip systems is lowered, thus increasing their occurrence in deformation and leading to the strong texture. 5. In the domain #1, stronger initial <01-10> texture would promote prismatic slip and therefore make the deformation easier. In turn, this increases the efficiency of power dissipation in the processing map. In the domain #2, the two types of additions are unable to impose significant influence on the processing map of AZ31-DMD, except for increasing the temperature of peak efficiency due to their back stress influence during grain boundary sliding. The two types of additions have moved domain #3 at the processing map of AZ31-DMD to lower temperatures, which may be attributed to the grain refinement. 6. Both at the domains #1 and #3, higher initial <01-10> texture intensity of the extruded materials promotes prismatic slip during the following compression at the two domains. This process hampers basal slip to some extent, making texture rotation limited. At domain #2, neither the nano-alumina nor the Ca-containing particles are able to impose significant effect on texture evolution during the following compression. 7. Temperature rise can activate additional slip systems and accelerate DRX. Furthermore, activation of additional slip systems can limit basal slip and therefore hamper texture rotation, thus reducing anisotropy; DRX can also reduce anisotropy by generating randomly textured new grains.

    Research areas

  • Magnesium alloys, Hot working, Microstructure