>Magister Scientiae - MSc / Architecturally enhanced electrode materials for lithium ion batteries (LIB) with permeable morphologies have received broad research interests over the past years for their promising properties. However, literature based on modified porous nanoparticles of lithium manganese phosphate (LiMnPO₄) is meagre. The goal of this project is to explore lithium manganese phosphate (LiMnPO₄) nanoparticles and enhance its energy and power density through surface treatment with transition metal nanoparticles. Nanostructured materials offer advantages of a large surface to volume ratio, efficient electron conducting pathways and facile strain relaxation. The material can store lithium ions but have large structure change and volume expansion during charge/discharge processes, which can cause mechanical failure. LiMnPO₄ is a promising, low cost and high energy density (700 Wh/kg) cathode material with high theoretical capacity and high operating voltage of 4.1 V vs. Ag/AgCl which falls within the electrochemical stability window of conventional electrolyte solutions. LiMnPO₄ has safety features due to the presence of a strong P–O covalent bond. The LiMnPO₄ nanoparticles were synthesized via a sol-gel method followed by coating with gold nanoparticles to enhance conductivity. A magnesium oxide (MgO) nanowire was then coated onto the LiMnPO₄/Au, in order to form a support for gold nanoparticles which will then form a thin film on top of LiMnPO₄ nanoparticles crystals. The formed products will be LiMnPO₄/Mg-Au composite. MgO has good electrical and thermal conductivity with improved corrosion resistance. Thus the electronic and optical properties of MgO nanowires were sufficient for the increase in the lithium ion diffusion. The pristine LiMnPO₄ and LiMnPO₄/Mg-Au composite were examined using a combination of spectroscopic and microscopic techniques along with cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Microscopic results revealed that the LiMnPO₄/Mg-Au composite contains well crystallized particles and regular morphological structures with narrow size distributions. The composite cathode exhibits better reversibility and kinetics than the pristine LiMnPO₄ due to the presence of the conductive additives in the LiMnPO₄/Mg-Au composite. This is demonstrated in the values of the diffusion coefficient (D) and the values of charge and discharge capacities determined through cyclic voltammetry. For the composite cathode, D= 2.0 x 10⁻⁹ cm²/s while for pristine LiMnPO₄ D = 4.81 x 10⁻¹⁰ cm2/s. The charge capacity and the discharge capacity for LiMnPO₄/Mg-Au composite were 259.9 mAh/g and 157.6 mAh/g, respectively, at 10 mV/s. The corresponding values for pristine LiMnPO₄ were 115 mAh/g and 44.75 mAh/g, respectively. A similar trend was observed in the results obtained from EIS measurements. These results indicate that LiMnPO₄/Mg-Au composite has better conductivity and will facilitate faster electron transfer and therefore better electrochemical performance than pristine LiMnPO₄. The composite cathode material (LiMnPO₄/Mg-Au) with improved electronic conductivity holds great promise for enhancing electrochemical performances, discharge capacity, cycle performance and the suppression of the reductive decomposition of the electrolyte solution on the LiMnPO₄ surface. This study proposes an easy to scale-up and cost-effective technique for producing novel high-performance nanostructured LiMnPO₄ nanopowder cathode material.

Master of Science / Department of Chemistry / Kenneth J. Klabunde / In this study, low molecular weight nanocomposites of L-lactic acid were synthesized with Commercial, Nanoactive®, and Nanoactive Magnesium Oxide Plus®, each of which differs in both surface area and shape. Synthesis of the composites was carried out by refluxing the nanoparticles in a solvent suspension. Both methanol and 1-propanol were used during this work. Heating was necessary in order to achieve adequate dispersion of the particles before adding L-lactic acid. Upon addition of the lactic acid monomer, the reactants were refluxed for a total of 3 hours, followed by evaporation of the excess solvent. The products were characterized via DSC, TGA, FTIR, [superscript]1H and [superscript]13C NMR, UV-Vis, XRD, and TEM. Additionally, titrations were performed with the reactants to ensure the particles were not being consumed by the acid regardless of their size. The results of this study indicate that condensation reactions are the primary polymerization route of lactic acid and polymerization appears to initiate on the surface of the magnesium oxide particles, resulting in physically unique composites of lactic acid and magnesium oxide.

The continued interest in the use of magnesium alloys for new applications demand the successful production of high quality wrought alloys. Magnesium Elektron seek to reliably produce high quality alloy billets by the DC casting method combined with ultrasonic inspection. The main objectives of this study are to characterize the defects which are currently found in the material and to understand the ability of the ultrasonic inspection technique currently employed to detect the defects. This study began by locating defects using the ultrasonic inspection method which were then characterised using X-ray Computed Tomography (XCT) 3D imaging technique. Attempts were then made to understand and simulate the mechanisms by which the defects form during the casting process. The simulations were used to investigate the flow patterns during casting and the growth kinetics of the intermetallic phase. The initial phase of this research established that the defects found comprised of an entrained oxide film entangled with an abundance of intermetallic phase particles. These defects were found to be present in the size range of 0.5 – 5 mm, and were deleterious to the materials mechanical properties. Greater understanding of the ultrasonic inspection process was achieved and informed improvements to assisting the production of high quality feedstock. Simulation of the formation of the defects indicated that there was a region in which the oxide films could form and be free to enter into the final cast product. Simulation of the growth of the intermetallic particles demonstrated that precipitation from the liquid occurs in the mould during which particles are carried by the melt flow and experiences a complex thermal history. The combination of the two phases was established to be due to entanglement of the oxide and particles which when combined will settle out of the melt as a single defect. Improved filtering and melt handling methods were recommended to eliminate the defects and reliably produce high quality alloys.

This Thesis describes the synthesis and characterisation of new Group 2, Group 3 and lanthanide amide, alkyl, halide, borohydride and alkoxide complexes, and their uses as catalysts for the living ROP and immortal ring-opening polymerisation (iROP) of rac-, L, D- and meso-lactide. <strong>Chapter One</strong> introduces cyclic esters and possible mechanistic pathways leading to polyesters by ROP. Living and immortal ROP, including their kinetic characteristics are discussed. An overview of ROP from an industrial perspective and a literature review are also given. <strong>Chapter Two</strong> describes the synthesis and characterisation of a new series of magnesium and zinc amide, alkyl, halide, borohydride and alkoxide complexes supported by a carbazole-bis(dimethyloxazoline) ligand. Their activities towards the ROP of rac-, L- and meso-lactide are presented. Detailed mechanistic studies using spectroscopic techniques are discussed and a new mechanism is proposed. <strong>Chapter Three</strong> describes the synthesis and characterisation of a new series of calcium, strontium, yttrium, lanthanum and samarium amide, alkyl, halide, borohydride and alkoxide complexes supported by a carbazole-bis(dimethyloxazoline) ligand. Their activities towards the ROP of rac-, L- and meso-lactide are presented. Detailed mechanistic studies using spectroscopic techniques are discussed. <strong>Chapter Four</strong> describes the synthesis and characterisation of a new series of magnesium, calcium, strontium, yttrium, lanthanum and samarium amide, halide and borohydride complexes supported by a chiral carbazole-bis(isopropyloxazoline) ligand. Their activities towards the ROP of rac-, L-, D- and meso-lactide are presented. <strong>Chapter Five</strong> contains experimental details and characterising data for the new complexes reported in this Thesis. <strong>CD Appendix</strong> contains .CIF files for all the new crystallographically-characterised complexes.

Biomaterials have been used for more than a century in the human body to improve body functions and replace damaged tissues. Currently approved and commonly used metallic biomaterials such as, stainless steel, titanium, cobalt chromium and other alloys have been found to have adverse effects leading in some cases, to mechanical failure and rejection of the implant. The physical or chemical nature of the degradation products of some implants initiates an adverse foreign body reaction in the tissue. Some metallic implants remain as permanent fixtures, whereas others such as plates, screws and pins used to secure serious fractures are removed by a second surgical procedure after the tissue has healed sufficiently. However, repeat surgical procedures increase the cost of health care and the possibility of patient morbidity. This study focuses on the development of magnesium based biodegradable alloys/metal matrix composites (MMCs) for orthopedic and cardiovascular applications. The Mg alloys/MMCs possessed good mechanical properties and biocompatible properties. Nine different compositions of Mg alloys/MMCs were manufactured and surface treated. Their degradation behavior, ion leaching, wettability, morphology, cytotoxicity and mechanical properties were determined. Alloying with Zn, Ca, HA and Gd and surface treatment resulted in improved mechanical properties, corrosion resistance, reduced cytotoxicity, lower pH and hydrogen evolution. Anodization resulted in the formation of a distinct oxide layer (thickness 5-10 μm) as compared with that produced on mechanically polished samples (~20-50 nm) under ambient conditions. It is envisaged that the findings of this research will introduce a new class of Mg based biodegradable alloys/MMCs and the emergence of innovative cardiovascular and orthopedic implant devices.

Mg-alloys, despite being the lightest structural metallic materials, find limited applications due to their poor workability, which is due to the hcp structure that does not provide sufficient number of independent slip systems for compatible deformation. Workability improves with the increase in the deformation temperature, when non-basal slip starts playing a larger role in deformation. Efforts were made to improve the workability through control of texture, grain refinement and alloying. Alloying activates non-basal slip by decreasing the critical resolved shear stress (CRSS) on non-basal planes or by promoting cross slip through an increase in the stacking fault energy (SFE) on basal planes. In this thesis, the effect of Li addition to the most widely used wrought Mg-alloy AZ31 on its workability is examined. Plastic deformation behaviour of a series of AZ31-Li alloys with temperature, T, and strain rates, ε , as variables was studied, so as to identify the optimum Li content that results in highly workable alloy. The T and ε combinations that are best suited for hot deformation of these alloys were also identified through processing maps and microstructural analysis. First, deformation behaviour of the base AZ31 is examined in detail. Compression tests were carried out, with T ranging between 150 and 400 °C and at ranging from 10-3 to 102 s-1, covering entire hot working range of the alloy. The results suggest that the deformation behaviour of AZ31 could be partitioned into three temperature regimes. In low T regime, twinning played an important role. It changes the orientation and increases hardening rate, θ (given by dσ/dε where σ and ε are true stress and strain respectively); material exhibits macroscopic flow localization and cracking along twin boundaries. The onset of twinning was examined in detail by examining the local maxima before ϵpeak strain in plot between d2σ/dε2 vs. ε. Twinning was found to occur at all the deformation conditions. Dynamic recrystallization (DRX) was observed at temperatures above 250 °C whereas deformation at low T (< 250 °C) led to extensive twinning at all . ε . At intermediate T of 250-300 °C, plastic strains tend to localize near grain/twin boundaries, confining DRX only to these regions. Increase in T promotes non-basal slip, which, in turn, leads to uniform deformation; DRX too becomes uniform. The dependence of critical stress (σc) for the onset of DRX and peak flow stress (σp) on Zener-Hollomon parameter (Z) indicates that these stresses increase with Z. Activation energy (Q) for the deformation of AZ31 was estimated at peak stress and steady state conditions. High values of Q (150-200 kJ/mol) indicate cross slip as the rate controlling mechanism, at the peak, in the stress-strain responses. For steady state, Q corresponds to lattice/grain boundary diffusion (90-150 kJ/mol). Next, the effect of Li on deformation behaviour of AZ31 was examined. In addition to AZ31 without any Li (0Li), three alloys 1 (1Li), 3 (3Li) and 5 (5Li) wt% Li were prepared with the aid of a specially designed set-up for melting and casting of Li containing alloys. Experimental results on homogenized alloys show that 1Li alloy’s overall response is similar to that of 0Li alloy, but 3Li and 5Li alloys exhibit distinctly different deformation behaviour. Li addition facilitates cross slip by increasing SFE on basal planes, thus leading to change in the deformation mechanism of the alloy. Increased softening due to cross slip decreases θ and also the twin density at low ϵ (<10-2 s-1). During deformation at low ϵ and low T, high Li alloys reveal cavities along the grain boundaries in contrast to cracking along twin boundaries that was observed in AZ31. In the intermediate T range, high Li alloys reveal the presence of a small mantle, which can be attributed to the increased cross slip with increasing Li. In fact, Li addition was found to restrict DRX and promote dynamic recovery (DRY). As ϵ increases in this T regime deformation becomes more homogeneous and twinning occurs extensively in high Li alloys. This results in remarkable increase in dσ/dε (θ) in these alloys and DRX was predominantly seen at twinned regions. At high ϵ -T regime, where non-basal slip and twinning occur uniformly, DRX is observed throughout the samples. On the basis of d2σ/dε2 – ε plots, it was found that twinning occurs at almost all -T combinations examined in present study for 0Li and 1Li alloys. In high Li alloys, twinning activity was found to be insignificant at low ε , resulting in low twin density than low Li alloys. Twinning occurs at very early stages of deformation. In the low T and high ε regime, extensive twinning in high Li alloys is noted. In high T regime, presence of twins was not prominent due to the preferential occurrence of DRX at twin boundaries. Estimated values of Q in high Li alloys were found to be very low and correspond to lattice/grain boundary diffusion of Li in Mg, indicating that cross slip is no longer the rate controlling mechanism. Instead, unpinning of kinks from Li atoms appears to control the deformation. Cross slip is promoted by Li through increase in SFE at basal planes. Onset of the DRX was predicted and it was observed that high Li alloys posses lower σc at low ε , but at high ε , σc was either comparable to or higher than low Li alloys. Processing maps were generated for all the alloys using Prasad's as well as Murty's models. Instability predictions of Prasad’s and Murty’s models are similar, except that isoefficiency contours in the latter are slightly shifted to higher ε . These maps indicate to an increase in the workability with the addition of Li to AZ31. Instability predicted by processing maps in the low ε regime in high Li alloys is attributed to underestimation of stress values due to spline interpolation. High sensitivity observed for high Li alloy at intermediate ε (10-1 – 100 s-1) is attributed to the change in the deformation mode i.e. from slip to twinning. Deformation at high T leads to dissolution of Li containing precipitates, which in turn increases the solid solution strengthening in the alloy. Hence, increase in flow stress is observed with increase in T in high Li alloys. This structural change too causes instability predictions in the high -T regime. The 0 Li alloy exhibits peak efficiency of 45% in T = 250-400 °C and ε = 10-1.25 - 100.25 s-1 regime. DRX is observed in this regime and optimum conditions for deformation predicted for this alloys are T = 350 °C and ε = 10-1 s-1. These alloys can be worked at low ε regime too (T = 250-400 °C and ε = 10-2.5 – 10-1 s-1) where the softening mechanism is DRY. Accordingly, it is concluded that the intrinsic workability of AZ31Mg-alloy increases with the addition of 3% and 5% Li.

Magnesium alloys have poor formability at room temperature, due to a limited number of slip systems owing to the hexagonal closed packed structure of magnesium. One possibility to increase the formability of magnesium alloys is to refine the grain size. A fine grain magnesium alloy shows high strength and high ductility at room temperature, hence an improved formability. In addition to grain refinement, the formability of Mg alloys can be improved by controlling crystallographic texture. Severe plastic deformation (SPD) processes namely, equal channel angular pressing (ECAP) and multi-axial forging (MAF) have led to improvement in room temperature mechanical property of magnesium alloys. Further, it has been reported that by adding rare earth elements, room temperature ductility is enhanced to nearly 30%. The increase in property is attributed to crystallographic texture. Many rare earth elements have been added to magnesium alloys and new alloy systems have been developed. Amongst these elements, Ce addition has been shown to enhance the tensile ductility in rolled sheets at room temperature by causing homogeneous deformation. It has been observed that processing of rare-earth containing alloys below 300°C is difficult. Processing at higher temperatures leads to grain growth which ultimately leads to low strength at room temperature. The present thesis is an attempt to combine the effect SPD and rare earth addition, and to examine the overall effect on microstructure and texture, hence on room temperature mechanical properties. In this thesis, Mg-0.2%Ce alloy has been studied with regard to the two SPD processes, namely, ECAP and MAF. The thesis has been divided into six chapters. Chapter 1 is dedicated to introduction and literature review pertaining to different severe plastic deformation processes as applied to different Mg alloys. Chapter 2 includes the details of experimental techniques and characterization procedures, which are commonly employed for the entire work. Chapter 3 addresses the effect of ECAP on the evolution of texture and microstructure in Mg-0.2%Ce alloy. ECAP has been carried out on two different initial microstructure and texture in the starting condition, namely forged and extruded. ECAP has been successfully carried out for the forged billets at 250°C while cracks get developed in the extruded billet when ECAP was done at 250°C. The difference in the deformation behaviour of the two alloys has been explained on the basis of the crystallographic texture of the initial materials. The microstructure of the ECAP materials indicates the occurrence of recrystallization. The recrystallization mechanism is identified as “continuous dynamic recovery and recrystallization” (CDRR) and is characterized by a rotation of the deformed grains by ~30⁰ along c-axis. The yield strengths and ductility of the two ECAP materials have been found quite close. However, there is a difference in the yield strength as well as ductility values when the materials were tested under compression. The extruded billet has the tension compression asymmetry ~1.7 while the forged material has the asymmetry as ~2.2. After ECAP, the yield asymmetry reduces to ~1 for initially extruded billet, while for the initially forged billet the yield asymmetry value reduces to ~1.9. In chapter 4, the evolution of microstructure and texture was examined using another severe plastic deformation technique, namely multi axial forging (MAF). In this process, the material was plastically deformed by plane strain compression subsequently along all three axes. In this case also two different initial microstructures and texture were studied, namely the material in as cast condition and the extruded material. The choice of initial materials in this case was done in order to examine the effect of different initial grain size in addition to different textures. By this method, the alloy Mg-0.2%Ce could be deformed without fracture at a minimum temperature of 350⁰C leading to fine grain size (~3.5 µm) and a weak texture. Grain refinement was more in the initial cast billets compared to the initial extruded billet after processing. The mechanism of grain refinement has been identified as twin assisted dynamic recrystallization (TDRX) and CDRR type. The mechanical properties under tension as well as under compression were also evaluated in the present case. The initially extruded billet has shown low tension compression asymmetry (~1.2) than cast billet (~1.9), after MAF. Chapter 5 addresses the exclusive effect of texture on room temperature tensile properties of the alloy. Different textures were the outcomes of ECAP and MAF processes. In this case, in order to obtain an exact role of texture, a third of deformation mode, rolling, was also introduced. All the processed materials were annealed to obtain similar grain size but different texture. A similar strength and ductility for ECAP and MAF, where the textures were qualitatively very different, was attributed to the fact that texture of both the ECAP and MAF processed materials, was away from the ideal end orientation for tensile tests. In chapter 7, the final outcomes of the thesis have been summarized and scope for the future work has been presented.

The present study concerns the application of LSM using an excimer laser to enhance the corrosion resistance of rare-earth Elektron 21 magnesium alloy. The alloy has been treated by an excimer laser to produce a highly homogeneous and refined microstructure for improvement of corrosion resistance. The laser surface treatment was applied on two different prepared surfaces of the alloy; i) a ground surface up to 1200 SiC grit; ii) a chemically cleaned surface using CrO3 +AgNO3 boiling solution. The intermetallic phases within the α-matrix that are believed to initiate corrosion have been dissolved by two methods. The first is by the excimer laser, where they were dissolved in the melted layers. The second is by a chemical dissolution prior LSM. Variation of the laser parameters such as changed laser influence (low, medium and high) and increased number of pulses, resulted in formation of thicker melted layers, but promoted the formation of porosity and micro-cracks particularly at overlap regions. The initial stage of this study was aimed at optimising the laser conditions for production of a uniform microstructure, with an increase in the corrosion resistance of the alloy being determined by potentiodynamic polarization measurements in sodium chloride solution. A laser fluence of 6 and 7 J/cm2 with 10, 20, 25, 40 and 50 pulses with a different overlap ratio of 7%, 20% and 50% were subsequently selected as the optimum condition to treat the surface of the alloy. After laser treatment, the top surfaces and the cross-sections of the alloy showed a relatively homogenous melted layer and a significant reduction in the number of large intergranular Mg-Zn-RE phase was achieved resulting in a significant improvement of the corrosion resistance of the alloy. This work also investigated the mechanism of corrosion and the interaction between the intergranular Mg-Zn-RE phase, the Zr-rich regions within the grains and the bulk Mg-rich matrix. The results obtained by scanning electron microscopy (SEM) / energy-dispersive X-ray (EDX) and scanning Kelvin prop forced microscopy (SKPFM) potential map measurements as well as transmission electron microscopy (TEM) / energy-dispersive X-ray (EDX) have shown the importance of the microstructure in the initiation of corrosion in 3.5 wt% NaCl solution, where the Zr-rich regions played a distinct role in the early stages of corrosion in this alloy. However, the obtained results have demonstrated that such laser melted layers improved the corrosion resistance of the alloy, but further work is still needed to obtain the fully understanding of such behaviour which can better the research results, particularly the selectively chemical dissolution of the second phases prior LSM.

Friction joining techniques, such as Friction Stir Spot Welding (FSSW) and high power Ultrasonic Welding (USW), could offer a solution for joining dissimilar materials combinations, such as aluminium (Al) to magnesium (Mg), where high intermetallic reaction rates make the use of conventional joining techniques problematic. Ultrasonic welds have been produced between 1 mm gauge Al 6111-T4 and Mg AZ31-H24 sheets, and the interfacial reaction has been studied as a function of welding time. For this welding system, the mechanical properties of the joints were optimised when a double reed welding system was employed to join materials that had been prepared using 800 grit SiC paper under a clamping force of 1.9 kN, and when the materials were oriented with the rolling direction parallel to the vibration direction. Welds produced between Al and Mg achieved similar peak lap shear strengths to those produced between Mg and Mg at welding times of 0.4 s, but the failure energy of the Al-Mg welds was less than half that of the parent material. In addition, the Al-Mg welds always failed at the interface between the sheets, rather than the desirable, and more energy intensive, pullout mechanism. The inferior mechanical properties were attributed to the rapid formation of a brittle intermetallic layer that initially formed as islands of the γ-Al12Mg17 phase. These islands rapidly spread and became continuous within 0.3 s of welding time, at which point a second sublayer of the β-Al3Mg2 phase began to form on the Al side of the intermetallic reaction layer. The combined layers reached a total thickness of 20 µm within 0.9 s of welding time, with the β-Al3Mg2 sublayer becoming the thicker of the two by this point. At longer welding times, interface liquation was observed at temperatures below the recognised lowest temperature eutectic reaction in the Al-Mg binary phase diagram. This was the result of the alloying elements present in the system and there was no depression in the melting point as a result of the high strain rate associated with this process, as has been proposed elsewhere. The rate of growth of the intermetallic layer during welding was higher than in static heat treatments, which was most likely due to the deformation causing microcracking in the brittle intermetallic layer, allowing short circuit diffusion to occur, and enhancing the growth rate by a factor of approximately 2. Finally, attempts were made to limit the rate of intermetallic compound (IMC) formation by applying coatings to the Mg sheet. The effect of the coatings was to reduce the overall IMC layer thickness by 50 %.

Magnesium is the lightest structural metal and magnesium alloys are therefore obvious candidates in weight critical applications. The environmental imperative to reduce vehicle emissions has recently led to intensified research interest in magnesium, since weight reduction is one of the most effective ways of improving fuel efficiency. The hexagonal close-packed structure of magnesium results in poor room temperature formability. However, on heating, several magnesium alloys show superplastic properties, with the ability to deform to very high strains (up to 3000%). This opens up the possibility of forming complex components directly by superplastic forming (SPF). As a result, SPF of magnesium is a highly active research topic. The most widely used class of magnesium alloys contain aluminium as the major alloying addition, which has a relatively high solubility in magnesium, and manganese, which has a less solubility. The effect of these elements on the deformation behaviour and failure mechanisms operating in the superplastic regime is not yet well understood. The objective of this work was to gain fundamental insights into the role of these elements. To do this, alloys with different aluminium content (AZ31 and AZ61) and manganese levels have been studied in-depth.After casting, all alloys were subject to a hot rolling procedure that produced a similar fine grain size and texture in each material. Hot uniaxial testing was performed at temperatures between 300 to 450 degC and at two strain rates to investigate the material flow behaviour, elongation to failure and failure mechanism. All of the alloys exhibited flow curves characterised by an initial hardening and extensive flow softening region. Dynamic recrystallization did not occur, and the flow softening was attributed to grain growth and cavity formation. Increasing the level of aluminium in solution was observed to increase the grain growth rate, and also reduce the strain rate sensitivity. The elongation to failure, however, depended strongly on the manganese level but not on the aluminium content. This attributed to the role of manganese in forming coarse particles that act as sites for cavitation.To study cavity formation and growth, and its effect on failure, a series of tests were conducted to different strain levels followed by investigation of cavitation in 3-dimensions using X-ray tomography. New methods were developed to quantify the correlation between cavities and coarse particles using X-ray tomography data and it was shown that over 90% of cavities are associated with particles. Cavity nucleation occurred continuously during straining, with progressively smaller particles forming cavities as strain increased. The mechanism of cavity formation and growth was identified, and it has been demonstrated that particle agglomerates are effective sites for cavity formation even when the individual particles in the agglomerates are below the critical size predicted by theory for cavity nucleation sites. These results suggest that to improve the ductility of magnesium alloys in the superplasticity regime, it is most critical to minimise the occurrence of particle agglomerates in the microstructure.