Mg-V and Mg-Zr alloys with nominal compositions 1, 6, 17.5, 27 wt% V and 2, 8.6 and 10.6 wt% Zr respectively were produced by PVD. All deposits exhibited compositional inhomogeneity, columnar microstructures and a strong basal texture. The solid solubilities of V and Zr in Mg were extended approximately to 17 wt% V and 10 wt% respectively. Grain refinement occurred with increasing solute content. The solid solution break up temperature decreased as the V and Zr content in the alloys increased. Pure V precipitated when the extended solid solubility of was exceeded. Both c and a lattice parameters, as well as the c/a ratio decreased with increasing V content in the Mg-V alloys. The slight increase of the a-lattice parameter and the decrease of the c one led to a decrease of the c/a ratio with increasing Zr additions in the Mg-Zr alloys. The air-formed oxide on the surfaces of the Mg-V alloys consisted predominantly of hydromagnesite at the outermost surface with Mg(OH)2 in excess of MgO underneath. No evidence of V oxide in the surface film was found. Magnesium oxide was also found between the grains of the deposits. The air-formed oxide on the surfaces of the Mg-Zr alloys consisted of ZrO2, MgO and possibly Zr sub-oxide. The presence of the oxides beween the columnar grains gave rise to graded metal/oxide interfaces. The outermost surfaces of the Mg-Zr alloys were similar to the Mg-V ones. Analysis of changes of the Auger parameters of the Mg-V and Mg-Zr alloys was also undertaken in order to investigate the electronic changes that take place upon alloying Mg with V and Zr. Charge transfer between 0.09 and 0.11 electrons/atom from Mg to V as well as changes in the V d charge were calculated by measuring the Mg and V Auger parameters and using the charge transfer model of Thomas and Weightman. Electron transfer between 0.02 and 0.03 electrons/atom from Mg to Zr was also found to occur upon alloying Mg with Zr. The electron transfer has been related to changes in crystal structure. The Mg-V and Mg-Zr alloys were examined after immersion in 3 wt% NaCl solution for 5 and 15 minutes, 9 hours and 7 days. The dramatic increase in the corrosion rate of the Mg-V alloys was attributed to the precipitation of pure V. The unsatisfactory corrosion performance of the Mg-V alloys was attributed to the absence of compositional uniformity through the thickness of the Mg-V deposits and the low thermodynamic stability of the corrosion products in the saline environment. Hydromagnesite at the outermost surface and Mg(OH)2, MgO and V2O4 in the bulk of the corrosion layer were the corrosion products. MgH2 and areas enriched in metallic V within the bulk of the corrosion products were also detected. The low corrosion rates of the Mg-Zr alloys, the lowest ever reported for Mg alloys, were attributed to the nature of the corrosion products and particularly the Zr contribution. The corrosion products were enriched in Zr, and were non-porous and in many cases well adherent. X-ray and electron diffraction suggested the existence of only Mg(OH)2 and MgO in the corrosion products, indirectly implying the participation of zirconium oxide/hydroxide in an amorphous/nanocrystalline state. Surface analysis indicated that a Zr oxide coexisted with Mg(OH)2 and MgO below a magnesium carbonate overlayer and also suggested the existence of Zr hydrous oxide (hydroxide). The repetition of the substrate pattern, as well as the fact that Zr hydroxide was replaced with ZrO2 and Zr sub-oxide as the metal-oxide interface was approached, implied a corrosion mechanism involving inwards diffusion of the anionic species.

No description available.

Lightweight structural components are important to the automotive and aerospace industries so that better fuel economy can be realized. Magnesium alloys in particular are being examined to fulfill this need due to their attractive stiffness- and strength-to-weight ratios when compared to other materials. However, when introducing a material into new roles, one needs to properly characterize its mechanical properties. Fatigue behavior is especially important considering aerospace and automotive component applications. Therefore, quantifying the structure-property relationships and accurately predicting the fatigue behavior for these materials are vital. This study has two purposes. The first is to quantify the structure-property relationships for the fatigue behavior in an AM30 magnesium alloy. The second is to use the microstructural-based MultiStage Fatigue (MSF) model in order to accurately predict the fatigue behavior of three magnesium alloys: AM30, Elektron 21, and AZ61. While some studies have previously quantified the MSF material constants for several magnesium alloys, detailed research into the fatigue regimes, notably the microstructurally small crack (MSC) region, is lacking. Hence, the contribution of this work is the first of its kind to experimentally quantify the fatigue crack incubation and MSC regimes that are used for the MultiStage Fatigue model. Using a multiaceted experimental approach, these regimes were explored with a replica method that used a dual-stage silicone based compound along with previously published in situ fatigue tests. These observations were used in calibrating the MultiStage Fatigue model.

No description available.

In the present investigation, MMCs have been fabricated using the creep-resistant AE42 magnesium alloy as matrix and reinforcing it with saffil short fibres (essentially δ-Al2O2) and SiC particles in various combinations. These MMCs have been investigated for their creep, wear and corrosion behaviour. The above properties of the matrix AE42 alloy have also been investigated for comparison. Further, laser surface melting has been carried out on a creep-resistant MRI 230D Mg alloy and the corrosion and wear behaviour of this alloy before and after laser surface melting has been investigated. The creep tests on the AE42 alloy were carried out in the temperature range of 1500 to 2400C at the stress levels ranging from 40 to 120 MPa and the composites were tested in the temperature range of 1750C to 3000 at the stress levels ranging from 60 to 140 MPa both in the longitudinal direction (LD) and in the transverse direction (TD). Wear tests were conducted on a pin-on-disc set-up under dry sliding condition at a constant sliding velocity of 0.837 m/s for a constant sliding distance of 2.5 km in the load range of 10 to 40 N for the AE42 alloy and the composites, which were tested both in LD and TD, and for a constant sliding distance of 1km in the load range of 5 to 20 N for the MRI 230D alloy before and after laser melting. All the materials were subjected to electrochemical corrosion tests in a 5 wt.% NaCl solution having ph value 11 for 22 hours. All the composites in both LD and TD exhibit lower creep rate as compared to the AE42 alloy and it is higher in TD than in LD. The creep resistance of the hybrid composites, in which saffil short fibres are partially replaced by SiC particles, is observed to be comparable , i.e., of the same order of magnitude , to that of the composite reinforced with Saffil short fibres alone at all the temperatures and stresses employed in both LD and TD. Wear rate of all the composites in both LD and Td is found to be lower than the alloy at all the loads employed and it is higher in TD than LD, Wear rate progressively decreases with the partial replacement of Saffil short fibres by Sic Particles, and is lowest for the composites reinforced with 10 vol.% Saffil short fibres and 15 vol.% Sic particles in both LD and TD. It is 34% and 35% lower than the 20% Saffil composite at 40 N load in LD and TD, respectively. The Ae42 alloy exhibits the best corrosion resistance and the addition of the Saffil short fibres and/or Sic particles in the AE42 alloy deteriorates its corrosion resitance. The composite reinforced with Saffil short fibres alone exhibits slightly better corrosion resitance than the hybrid composites. However, there is no systematic trend of corrosion resistance with SiC particles content. The laser surface melting is found to improve the corrosion, hardness and wear resistance of the MRI 230D alloy. High temperature climb of dislocation is found to be the dominant creep mechanism in the AE42 alloy in the stress and temperature range employed. Various glide and climb of dislocation are found to be the dominant creep mechanisms for all the composites in both LD and TD in the stress and temperature range employed. The presence of SiC particles in the hybrid composites improves the wear resistance in both LD and TD since these particles remain intact and retain their load bearing capacity even at the highest load employed in the present investigation. They promote the formation of iron-rich transfer layer and they also delay the fracture of Saffil short fibres to higher loads in case of the composites in LD. Under the experimental conditions used in the present investigation, the dominant wear mechanism is found to be abrasion for the AE42 alloy and its composites in both LD and TD. It is accompanied by severe plastic deformation of surface layers in case of the alloy, the fracture of Saffil short fibres as well as the formation of iron-rich transfer layer in case of the composites in Ld, and the fracture and pull-out of the Saffil short fibres in case of the composites in TD. The lower corrosion resistance of all the composites is not caused by the galvanic coupling between reinforcements and matrix, and is related to the microstructural changes, such as, distribution of precipitates and the nature of the film formed at the surface. The improved corrosion resistance following laser surface melting is due to the absence of the Al2Ca phase at the grain boundary, microstructural refinement and increased solid solubility, particularly of Al, owing to rapid solidification; the improved hardness and wear resistance is due to grain refinement and solid solution strengthening. To conclude, the creep resistance of the hybrid composites is comparable, wear resistance is better and corrosion resistance is slightly inferior to the composite reinforced with Saffil short fibres alone. Therefore, from the commercial point of view, the use of the hybrid composites, replacing a part of the expensive Saffil short fibres by cheap SiC particles, is beneficial. The laser surface melting is beneficial for the corrosion and wear resistance of the MRI 230D alloy.

Call number: LD2668 .T4 1948 F74 / Master of Science

published_or_final_version / Orthopaedics and Traumatology / Master / Master of Philosophy

Mg-Zn-Zr (ZK) alloys are widely employed in weight critical applications due to their high specific strength properties. Zinc and zirconium are added into the alloy for the purpose of forming precipitation hardening and grain refinement, respectively. However, both elements are likely to form some intermetallic compounds and may have effect on strengthening of the alloys. The purpose of this work was to explore this interaction to understand how to optimise the composition and processing of ZK alloys. In the present study, ZK60 with composition of 6 wt.%Zn and 0.6 wt.%Zr was heat treated by solutionizing and aging at various temperatures and times. The resultant microstructure was characterised by using optical microscope, X-ray Diffractometer (XRD), Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM) and the age hardening response determined by using Vicker's microhardeness testing. The results show that the interaction between zinc and zirconium causes stable compound formation, including hcp (Zn_2Zr) Laves phase and cubic Zn_2Zr phase. These compounds are distributed unevenly, causing beta'_1 strengthening precipitate to form inhomogeneously during aging. This reduces the strengthening effect of aging, e.g. low age hardening response. It has been shown that the areas containing Zn_2Zr particles are essentially free of strengthening beta'_1 precipitates, an undesirable microstructure. The aged hardening and strengthening of ZK60 alloys accounting for the interaction between Zn and Zr has been explored through a new model for ZK alloys. The simulated age hardening strengthening in ZK60 is similar to the combination of the grain size strengthening and solution strengthening. The prediction has shown that the benefit of adding zirconium, which is grain boundary strengthening improvement by refining grain size, is balanced by a depletion in age hardening response. The total strength is estimated to increase by 15% if the grain size, that is normally obtained by zirconium alloying, could be achieved by another grain refinement method without the loss of zinc solute. The experiments and the models have identified the importance of considering the interaction between Zn and Zr when optimising the processing and composition of ZK magnesium alloys. That had never been emphasised and very well presented. In particular, when ZK alloys are used in wrought applications, control of grain size by recrystallisation rather than Zr may be beneficial in enabling a greater age hardening response.

Microstructural evolution during heat treatment and the precipitation hardening response of Mg-Ca-Zn alloys were investigated. The binary Mg-2.5Ca alloy was chosen as the base alloy and the effects of adding one and two wt% zinc on the microstructural characteristics and precipitation hardening of the alloy system were studied. The as-cast microstructure of all three alloys showed dendritic solidification of α-Mg and the formation of the eutectic mixtures and/or multiple phases within the interdendritic regions. Homogenization heat treatment of the binary alloy removed the dendritic structure of α-Mg and spherodized the lamellar eutectic of α-Mg+Mg2Ca. Homogenization heat treatment in the ternary Mg-Ca-Zn alloys resulted in the formation of α-Mg grains with Mg2Ca and zinc-containing particles mainly dispersed along the grain boundaries. The EDS analysis suggested that zinc is incorporated in Mg2Ca particles. To study the precipitation hardening response of the alloys, homogenized alloys were aged at 175°C, 200°C and 220°C. At all three isothermal aging temperatures, the binary alloy showed only a slight increase in hardness, i.e. from 50 VHN in the homogenized state to approximately 53 VHN when peak aged. On the other hand, adding zinc was found to promote the age hardening response of the ternary alloys and caused the hardness to increase up to 70 VHN at the peak-aged condition. To further study the precipitation hardening behavior of the alloys, DSC and IC studies were conducted on the homogenized, as–quenched, alloys. The DSC result of the binary alloy showed only one exothermic heat effect, while the ternary alloys showed multiple exothermic peaks. Analysis of the DSC and IC traces, along with the evaluation of the slight increase in microhardness, suggested that non-coherent equilibrium precipitates formed in the aged binary alloy. In addition, it was suggested that the formation of coherent precipitates during aging can be the cause of the pronounced increase in hardness in the aged ternary alloys. The IC traces of the alloys were used to evaluate the kinetics of precipitation in the ternary alloys. It was concluded that increasing aging temperature from 175°C to 220°C greatly increased the precipitation rate. Finally the JMAK model was fit to the experimentally analyzed data for the evolution of the relative volume fraction of precipitates. It was found that the experimentally analyzed data was reasonably well described by the JMAK model. The corresponding JMAK kinetic parameters k and n were relatively close for the two ternary alloys at any aging temperature. The temperature dependence of k was modeled using the Arrhenius-type rate relationship. This analysis resulted in a smaller value for the apparent activation energy in the ternary alloy containing the higher zinc level, i.e. in Mg-2Ca-2Zn alloy.

Microstructural evolution during heat treatment and the precipitation hardening response of Mg-Ca-Zn alloys were investigated. The binary Mg-2.5Ca alloy was chosen as the base alloy and the effects of adding one and two wt% zinc on the microstructural characteristics and precipitation hardening of the alloy system were studied. The as-cast microstructure of all three alloys showed dendritic solidification of α-Mg and the formation of the eutectic mixtures and/or multiple phases within the interdendritic regions. Homogenization heat treatment of the binary alloy removed the dendritic structure of α-Mg and spherodized the lamellar eutectic of α-Mg+Mg2Ca. Homogenization heat treatment in the ternary Mg-Ca-Zn alloys resulted in the formation of α-Mg grains with Mg2Ca and zinc-containing particles mainly dispersed along the grain boundaries. The EDS analysis suggested that zinc is incorporated in Mg2Ca particles. To study the precipitation hardening response of the alloys, homogenized alloys were aged at 175°C, 200°C and 220°C. At all three isothermal aging temperatures, the binary alloy showed only a slight increase in hardness, i.e. from 50 VHN in the homogenized state to approximately 53 VHN when peak aged. On the other hand, adding zinc was found to promote the age hardening response of the ternary alloys and caused the hardness to increase up to 70 VHN at the peak-aged condition. To further study the precipitation hardening behavior of the alloys, DSC and IC studies were conducted on the homogenized, as–quenched, alloys. The DSC result of the binary alloy showed only one exothermic heat effect, while the ternary alloys showed multiple exothermic peaks. Analysis of the DSC and IC traces, along with the evaluation of the slight increase in microhardness, suggested that non-coherent equilibrium precipitates formed in the aged binary alloy. In addition, it was suggested that the formation of coherent precipitates during aging can be the cause of the pronounced increase in hardness in the aged ternary alloys. The IC traces of the alloys were used to evaluate the kinetics of precipitation in the ternary alloys. It was concluded that increasing aging temperature from 175°C to 220°C greatly increased the precipitation rate. Finally the JMAK model was fit to the experimentally analyzed data for the evolution of the relative volume fraction of precipitates. It was found that the experimentally analyzed data was reasonably well described by the JMAK model. The corresponding JMAK kinetic parameters k and n were relatively close for the two ternary alloys at any aging temperature. The temperature dependence of k was modeled using the Arrhenius-type rate relationship. This analysis resulted in a smaller value for the apparent activation energy in the ternary alloy containing the higher zinc level, i.e. in Mg-2Ca-2Zn alloy.