Processing and characterization of In-situ magnesium metal matrix composites containing SiCNO particles

Abstract

Polymer derived metal matrix composites (P-MMCs) represent a new paradigm shift in the processing of in-situ metal matrix composites. The polymer precursor can be infused into molten metal by a stir-casting method. The in-situ conversion of polymer into ceramic phases takes place within molten metal by the release of volatile compounds and hydrogen gases in a temperature range of 700 – 1000 oC. Both the oxides and non-oxide ceramic particles are derived from polymer precursors. In these polymer precursors, all the constituents of the ceramic phase are present within the organic molecules itself. Among different polymer precursors, Si-based polymer precursors are generally inflammable, can be stored in air. Further they do not contain heavy metals and most often the polymers are environmentally benign. The direct injection of liquid polysilazane precursor into molten metal and the subsequent casting process seems to be most advantageous and viable route of producing P-MMCs. The diversity of polymer precursor selection and processing strategies can spur further innovation. Magnesium metal matrix composites have a significant potential in automotive structural application owing to the combination of high specific strength and better fuel efficiency. If ceramic particles of size (dp) are dispersed in dilute volume fraction, (Vf ) then assuming cuboid shaped particles dispersed uniformly in a cube-like lattice, the yield strength of metal matrix composites is given by GmVf 1/3(b / dp), where Gm is the shear modulus, b is the Burgers vector for the dislocations. It should be noticed that finer the ceramic particles, greater the yield strength of MMCs. However, achieving uniform dispersion of fine scale ceramic particles in the metal matrix is often challenging task in the processing of MMCs owing to particle agglomeration by Van Der-waals force of attraction. Most of the technical challenges in solidification processing of MMCs can be greatly reduced by adopting an insitu composite approach by which ceramic particles are generated within the molten state via chemical reaction between the added precursor and the host metal. The main objective of present investigation was to fabricate polymer derived magnesium insitu magnesium metal matrix composites (P-MMCs) by a liquid stir-casting process and to characterize their microstructures and strength properties. Pure Mg (99.9%) was used as matrix material and poly(ureamethylvinyl)silazanes or ceraset as polymer precursor to generate sub-micron/nano-sized amorphous silicon oxy-carbonitride (SiCNO) ceramic particles in the magnesium matrix. Ceraset was introduced into the magnesium melt in two different ways either as cross-linked micron sized powder (1-100 μm), or as-received liquid ceraset. Four different types of composites were fabricated via in-situ pyrolysis at process temperatures of 700 oC and 800 oC. The nomenclature of these composites was designated as PP700, PP800, PL700, and PL800. Here, PP refers to polymer derived composites made using cross-linked powder, and PL indicates polymer derived composites made using asreceived liquid ceraset. The last 3 digits refer to process or pyrolysis temperature. In addition, Mg-based in-situ metal matrix composites with three different matrix materials (including pure Mg, AZ91 and AE44 Mg-alloys) were also fabricated by injecting cross-linked polymer directly into, and having it converted to the SiCNO ceramic phase, within the molten Mg/Mgalloys at 900 oC. Thermo gravimetric analysis (TGA) was performed on both cross-linked polymer powders and as-received liquid ceraset in the temperature range of 25oC to 900 oC at heating rate of 10 oC min-1 in order to determine the temperature range of pyrolysis. Raman spectroscopy was used to identify the existence of peaks associated with conversion of polysilazane precursor into ceramic phases in the matrix material. Optical, scanning and transmission electron microscopes were utilized to characterize the microstructures of the fabricated composites. The phase identification of fabricated P-MMMCs was obtained from X-ray diffraction (XRD) analysis. Room temperature strength properties such as Vickers micro-hardness, nano-indentation hardness and compression properties of all the as-cast P-MMMCs were characterized. Hot deformation mechanisms and resultant texture evolution of as-cast PL700 specimen were also investigated using Dartec Mechanical Testing Instrument at strain rate ranging from 1 x 10-3 - 1 s-1, and in the temperature ranges of 150 - 350 oC under uni-axial compression. The existence of „D‟ and „G‟ peaks in Raman spectra of the as-cast Mg matrix composites confirmed the conversion of sp3 carbon in the polymeric state into sp2 carbon in the ceramic state for the reinforced particles during in-situ pyrolysis. Results indicated that the overall relative intensities of these two peaks decrease with reduction in pyrolysis temperature from 800 to 700 oC. Microstructural analysis revealed that most of the polymer derived SiCNO particles were pushed by the solidification front and as a result segregated at the grain boundaries of as-cast composites (mean grain size in range of 50-65m) during subsequent solidification process. XRD studies verified that the intensity in the formation of Mg2Si intermetallics could be minimized by reducing the pyrolysis temperature from 800 to 700 oC. Single pass friction stir processing (FSP) of as-cast PL800 and PL700 magnesium matrix composites led to improved homogeneity in the SiCNO particle distribution, particle refinement (mean particle size of about 200-300 nm) and grain refinement (mean grain size of 2.5-3.5 m). Mechanical properties (hardness, compressive yield strength, ultimate compressive stress, strain hardening exponent and ductility) of the FS processed magnesium matrix composites were enhanced significantly compared to their as-cast counterparts.Strengthening mechanism and numerical models are also being evoked to explain the observed yield strength in these two-stage processed composites. The deformed PL700 magnesium matrix composites follow a power-law creep having a stress exponent n = 8 and activation energy of 149 kJ.mol-1, which suggests that the deformation mechanism is controlled by lattice self-diffusion for constant microstructure creep. At 150 oC, twinning induced shear band formation occurs and propagates along the direction of maximum shear stress at all the strain rates. It was found experimentally that if the ratio of rate of work softening/hardening approaches 0.80, shear bands can be induced within the as-cast Mg matrix composites. When temperature approaches 350 oC, the plastic flow is dominated by slip producing deformation bands along with the emergence of elongated grains. Analysis of the Zener-Hollomon parameter (Z) revealed that the transition from twinning into slip dominated deformation occurs at 1013 s-1 < Z < 1015 s-1. Twinning induced shear band formation occurs at higher Z values, and slip dominated plastic flow progresses by non-basal planes under lower Z values. Microstructural/XRD analysis showed that in-situ chemical reaction takes place within the molten slurry tending to produce 42 and 18 vol% Mg2Si crystals in Mg and AE44 matrix composites, respectively. However; it did not occur in AZ91 matrix composite. The experimental observations revealed that size and morphologies of Mg2Si crystals vary with the matrix material. Microstructural evolution of these in-situ formed Mg2Si crystals was discussed on the basis of availability of heterogeneous nucleation sites and amount of Alatoms in the molten slurry. The strengthening behavior of these composites was also investigated. The hardness of Mg and AE44 matrix composites is enhanced by several orders of magnitude compared to their unreinforced counterparts. Taylor strengthening was found to be the predominant hardening/strengthening mechanism due to thermal mismatch effect between the dispersed particles and the matrix material. In summary, the present investigation confirmed that friction stir processed P-MMMCs possess better dispersion of polymer derived ceramic particles and superior mechanical properties. Therefore, the fabricated in-situ magnesium matrix composites via polymer precursor approach (P-MMMCs) have a significant potential to be employed in automobile structural applications.