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Microstructure And Mechanical Properties Of Consolidated Magnesium Chips

2013-04-03T18:30:00Z

Title: Microstructure And Mechanical Properties Of Consolidated Magnesium Chips Authors: Anil Chandra, A R Abstract: Development of sustainable manufacturing and conservation of primary materials are the key challenges to environmental degradation and climate change. Recycling of primary materials is one of the approaches suggested for sustainable green manufacturing. In the present study, an attempt has been made to encompass both these concepts, i.e. recycling of waste machined chips of magnesium and development of sustainable manufacturing process. Chips generated during machining operations are of significant importance; they dissipate the heat from the work-piece and control the quality of the finished products. In recent years researchers have shown that by controlled machining it is possible to tailor size, shape and microstructure of chips and this has added new dimensions to the utility of these machined chips. Chips in the form of thin strips, rods, very fine powders with varying aspect ratio have been successfully machined with grain structure having nano size (~80nm) to submicron size. Consolidation of such machined chips and subsequent fabrication of products is of great interest from the point of view of sustainable manufacturing. Consolidation of machined chips by cold compaction followed by hot extrusion was proposed and has been termed as solid state recycling (SSR). This alternative method of manufacturing using machined chips circumvents melting and casting. Although several materials have been tried by this route, magnesium appears to be the most investigated material. Being lightest among the structural materials, magnesium and its alloys have wide ranging applications especially in automotive industry. Further, magnesium melting is cumbersome and environmentally hazardous which necessitates researchers to explore methods of overcoming the melting route. In this pursuit, SSR appears to be a choice for a soft material like magnesium whose products are fabricated by conventional processing techniques which include cold compaction followed by hot extrusion. Most of the work in literature with regard to SSR of magnesium has been centered around development of new alloys and their characterisation at room and elevated temperatures. Effect of oxide contaminants has also been widely studied. However, studies on microstructural evolution during processing (i.e. microstructure prior to and after extrusion) have not been reported. Further, such studies with pure metal is important since it is possible to separate the effect of secondary phases including precipitates which are otherwise present in alloys of Mg. Hence, commercial grade pure magnesium is the material of interest in the present work. Process of consolidation includes room temperature compaction followed by hot extrusion. The aim of the present work includes: Consolidation of machined chips of magnesium into billets by cold compaction at room temperature followed by hot extrusion, Microstructural characterisation of these cold compacted billets prior to and after extrusion, Evaluation of mechanical properties after extrusion at different temperatures. Correlating the mechanical properties with microstructure. In the present study mechanical properties evaluated include: strength properties (hardness, tensile and compressive properties), and damping properties As-cast billet of pure magnesium was turned in a lathe to produce chips at ambient conditions. The chips were cold compacted into billets of 28 mm diameter at a pressure of 350 MPa and held for 30 minutes. The billets of compacted chips (referred here as CC) were later extruded at four different temperatures, viz. 250, 300, 350 and 400°C, with an extrusion ratio of 49:1. Prior to extrusion, the CC was soaked at the desired extrusion temperature for 1 hour. Here, extrusions of compacted chips are designated as CCE (chip compacted and extruded). For comparison, the as-cast billet was extruded under similar conditions and is designated as AE (as-cast and extruded). The extruded rods had a diameter of 4 mm. Microstructural characterisation was done prior to and after extrusion, which forms the first part of the thesis. The extruded rods were characterised for their room temperature strength properties in the second part of the thesis. In the third and last part, damping properties were characterised as a function of time and temperature. Microstructural changes at the end of temperature sweep tests were also examined. Optical microscopy did not reveal the grain structure of CC due to the intense strains associated with chip formation and subsequent cold compaction. However, chip boundaries were found randomly oriented and tri-junctions were found to be porous. The CC showed a relative density of 95.4% and this happens to be the highest amongst the values reported in literature for SSR machined chips. TEM images of CC revealed an average grain size of 0.75µm. Synopsis CCs were soaked at extrusion temperature and quenched to unravel the microstructure that exists prior to extrusion. Grain size and hardness measurements indicate that the material was recrystallised prior to extrusion. Bulk texture estimated from X-ray diffraction, showed weak crystallographic textures. The CC had a typical texture with c-axis aligned along the compaction direction which subsequently got randomised during soaking (pre-heating at extrusion temperature). After extrusion, the 250°C extruded AE had slightly stronger texture than CCE: with clear preference for < 1010 > and < 1120 > plane normals. High working temperatures removed such preference and made the textures randomised for both AE and CCE. In-grain misorientations and the relative presence of the twins, estimated from EBSD scans show a clear pattern for higher in-grain misorientations in CCE compared to AE. The values for AE at higher extrusion temperatures approached that of fully recrystallised magnesium. Higher twin fraction in AE was attributed to its relatively larger grain size compared to CCE. The chip boundaries that were randomly oriented before extrusion appeared aligned along the extrusion direction after extrusion. On the contrary AE had an equiaxed structure. Both longitudinal and transverse section micrographs showed pronounced chip boundaries in the 250°C extruded CCE while it was no so pronounced in the case of 400°C extruded material. Density measurements showed 98.6% relative density for 250°C extruded CCE as compared to 99.9% densification achieved in 400°C extruded CCE. Dislocation density estimated using Variance method from the peaks of the X-ray diffraction data showed higher values for CCE compared to AE. Dislocation density reduced with increase in extrusion temperature. For comparison extruded rods were annealed at 250°C for 2 hours and their dislocation density was estimated. Vickers hardness indentations were done at low load (25g) and higher load (200g). Both showed decreasing values with increase in extrusion temperature. Grain size dependent hardness variation followed the Hall-Petch relationship. CCE showed higher hardness compared to AE. Room temperature tensile test showed higher 0.2% tensile proof stress (TPS) in CCE material and obeyed the grain size dependent Hall-Petch relationship, though the strain to failure was poor. CCE extruded at 250°C showed fibrous fracture surface and was different from the rest of the CCEs with evidence of shearing at chip boundaries before fracture. Synopsis The rest of the CCEs had a typical fracture surface which was similar to AE material. Strain hardening behaviour, measured in terms of hardening exponent (n), hardening capacity (Hc) and hardening rate (θ) was quiet different for CCE compared to AE. Room temperature compression test showed different kind of failure for 250°C extruded CCE with longitudinal splitting (de-bonding at chip boundaries) and shearing at an angle to loading direction. The rest of the CCEs failed in a typical manner similar to AE material. The 0.2% compressive proof stress (CPS) as a function of grain size obeyed the Hall-Petch relationship for AE while the fit was not so good for CCE. Moreover, except 400°C extruded CCE (CPS was higher by ~22%) the rest of the CCEs had lower CPS compared to AE despite having finer grain size. This was contrary to the TPS and hardness findings wherein CCE was consistently higher compared to AE owing to grain refinement. Density measurements showed presence of 1.4%, 0.8% and 0.5% porosity in 250°, 300° and 350°C extruded CCE samples respectively. Prompted by density, hardness and TPS findings, the CPS values were back-calculated using the Hall-Petch relationship of AE. The back-calculated CPS values of CCE were higher than corresponding AE. Strength asymmetry, measured as a ratio of compressive proof stress to tensile proof stress was higher in CCE compared to AE. Damping capacity (tanφ) and dynamic modulus were determined as a function of time (tested upto 30 minutes) and temperature (from RT to 300°C) at a constant frequency (5 Hz). CCE material displayed higher tanφ during time and temperature sweep tests (by 10-15%) with CCE extruded at 250° showing the highest values. Dynamic modulus was comparable for both the materials (with less than 5% difference) though, modulus was higher in materials extruded at higher temperature. Microstructural changes were examined at the end of temperature sweep test, both at the point of loading and away from the point of loading. A significant grain growth was observed in region under the loading point (in a 3-point bending set-up) and was insignificant at regions away from the loading point. Coarsening was low in CCE material on account of suppression at chip boundaries. Microstructure of CCE and AE specimens subjected to similar heating conditions but without loading showed no such coarsening.

Studies On Thermodynamics And Phase Equilibria Of Selected Oxide Systems

2013-02-19T18:30:00Z

Title: Studies On Thermodynamics And Phase Equilibria Of Selected Oxide Systems Authors: Shekhar, Chander Abstract: The availability of high quality thermodynamic data on solid solutions and compounds present in multicomponent systems assists in optimizing processing parameters for synthesis, and in evaluating stability domains and materials compatibility under different conditions. Several oxide systems of technological interest, for which thermodynamic data was either not available or is inconsistent were selected for study. Thermodynamic properties of phases present in the binary systems Nb-O and Ta-O were measured in the temperature range from 1000 to 1300 K using solid state electrochemical cells based on (Y2O3) ThO2 as the electrolyte. Based on these measurements and more recent data on heat capacity and phase transitions reported in the literature, Gibbs energy of formation for NbO, NbO2, NbO2.422, Nb2O5-x and Ta2O5 were reassessed. Significant improvements in the data for NbO2, Nb2O5 and Ta2O5 are suggested. The pseudo binary system MoO2-TiO2 was investigated because of the inconsistency between the phase diagram and thermodynamic properties of the solid solution reported in the literature. Based on new electrochemical measurements, a new improved phase diagram for the system MoO2-TiO2, incorporating recently discovered monoclinic to tetragonal phase transition in MoO2 at 1533 K, is presented. Isothermal section of the phase diagram for the ternary systems Cr-Rh-O and Ta-Rh-O and thermodynamic properties of ternary oxides CrRhO3 and TaRhO4 were measured for the first time in the temperature range from 900 to 1300 K. Phase relations for these systems have been computed as a function of oxygen potential at fixed temperature and as a function of temperature at selected oxygen partial pressures. Metal-spinel-corundum three-phase equilibrium in the Ni-Al-Cr-O system at 1373 K has been explored because of its relevance to high temperature corrosion of super alloys. The Gibbs energy of mixing of spinel solid solution was derived from the tie-line data and is compared with the values calculated from cation distribution models. An oxygen potential diagram is developed for the decomposition of spinel solid solution to nickel and corundum solid solution at 1373 K under reducing conditions. The high temperature thermodynamic properties of the phases present in quaternary systems Ca-Co-Al-O and Ca-Cu-Ti-O have been measured by solid state electrochemical cells based on stabilized zirconia. Gibbs energies of formation of the quaternary oxides Ca3CoAl4O10 in the temperature range from 1150 to1500 K and CaCu3Ti4O12 in the range from 900 to 1350 K are presented. Chemical potential diagrams have been computed for the system Al2O3-CaO-CoO at 1500 K. The oxygen potential corresponding to the decomposition of the complex perovskite CaCu3Ti4O12 (CCTO) has been calculated as a function of temperature from the emf of the cell. The effect of the oxygen partial pressure on the phase relations in the pseudo-ternary system CaO-(CuO/Cu2O)-TiO2 at 1273 K has been evaluated. The phase diagrams are useful for the control of the secondary phases that form during synthesis of CCTO, a material exhibiting colossal dielectric response.

Diffusion Studies On Systems Related to Nickel Based Superalloys

2013-02-27T18:30:00Z

Title: Diffusion Studies On Systems Related to Nickel Based Superalloys Authors: Divya, V D Abstract: Superalloys offer high temperature strength, excellent creep, corrosion and oxidation resistances, microstructural stability and good fatigue life at elevated temperatures. The composition of the superalloys has been modified continuously to improve the properties. The addition of Pt improves oxidation resistance without compromising the mechanical properties of the superalloys. To further enhance the performance of the superalloy components, various coatings are applied on them. The-(NiPt)Al intermetallic compound bond coats, which are presently utilized, have certain drawbacks. Diffusion of Al from the bond coat to superalloy during service leads to accumulation of stress near the bond coat. The refractory elements present in superalloy precipitate as topological close packed (TCP) phases in the interdiffusion zone. Consequently, a Pt enriched γ(Ni) + γ’(Ni3Al) phase mixture has been proposed as a possible alternative since TCP phases do not form in the interdiffusion zone. In this thesis, diffusion studies are performed on several binary and ternary systems with the primary purpose of understanding the effect of Pt in Ni based superalloys and also in γ + γ’ phase mixture bond coats. Further, a detailed interdiffusion study is conducted in Mo- and W- based binary and ternary systems to understand the growth of the TCP phases. By performing bulk and multifoil diffusion couple experiments, different diffusion parameters like, inter, intrinsic, tracer, impurity diffusion coefficients and activation energy that are necessary to understand the diffusion mechanism are determined. Additionally using the nanoindentation technique on diffusion couples, variation of mechanical properties such as, hardness and modulus with composition is studied. First, interdiffusion in Ni-Pt, Co-Pt, Co-Ni, Ni-Fe and Co-Fe binary systems is examined. In Ni-Pt and Co-Pt, experimental results show that Pt is the slower diffusing species at all compositions. In both the systems, driving force is found to be the reason for higher values of intrinsic diffusion coefficients observed in the range of 40-60 at. % Pt. Contribution of vacancy wind effect on diffusion parameters is found to be negligible. It is found from the multifoil diffusion couple experiments that Ni is the faster diffusing species in the Co-Ni system. Bulk diffusion couple experiments are conducted in the Co-Ni-Pt and Co-Ni-Fe systems, by coupling binary alloys with the third element. Uphill diffusion is observed for Co and Ni in Pt rich corner of the Co-Ni-Pt system. Main and cross interdiffusion coefficients are calculated at the compositions where two diffusion profiles intersect. In both the systems, the main interdiffusion coefficients are positive over the whole composition range and the cross diffusion coefficients show both positive and negative values at different regions. Hardness measured by performing the nanoindentations on diffusion couples of both the systems, shows the higher values at intermediate compositions. The effect of Pt in and’ phases of Ni-Al system are examined by conducting interdiffusion experiments between Ni(xPt) alloys and (NixPt)40Al alloy of β phase, so that both and’ phases grow in the interdiffusion zone. The interdiffusion coefficients in Ni-Al binary system increases with the Al content in the -phase, and they do not vary significantly with composition in the ’ phase. The average effective interdiffusion coefficients of Ni and Al in the and ’ phases increase with the addition of Pt. Nanoindentation studies on diffusion couples show that the hardness of both and ’ phase increases with the addition of Pt. In the +’ phase mixture bond coats, effect of Pt on interdiffusion of major alloying elements of CMSX4 superalloys are discussed. A phase mixture of and ’ with increasing Pt content is coupled with CMSX4 superalloy. The addition of Pt to the +’ phase mixture increases the diffusion rate of Ni, while the diffusion rate of Al, decreases with the addition of 5% Pt, and increases with further addition of Pt. No significant change in the diffusion rates of Co or Cr is observed. The growth kinetics and diffusion in systems (both binary and ternary) with TCP phases are examined. Interdiffusion studies performed in Co-Mo system show significant volume change because of the growth of the phase. Intrinsic diffusion coefficient of Mo is found to be higher than that of Co. Diffusion studies conducted in Ni-Mo system show reasonably low activation energy in the phase, indicating the grain boundary controlled diffusion process. The Co-Ni-Mo and Co-Ni-W ternary phase diagrams are revisited and the phase boundary composition of the TCP phases are found to be different from those reported earlier. Following, the average effective interdiffusion coefficients are calculated and compared with the data calculated in the binary systems to examine the role of the third element. It is noticed that the average effective interdiffusion coefficients in the Co(Ni,Mo) and Co(Ni,W) solid solution increases with the addition of Ni. On the other hand, these diffusion coefficients decrease with the addition of Ni in thephase in both the systems. The role of the driving force for diffusion and possible change in defect concentrations on different sublattices are discussed.

Evolution Of Texture And MIcrostructure During Processing Of Pure Magnesium And The Magnesium Alloy AM30

2011-06-30T18:30:00Z

Title: Evolution Of Texture And MIcrostructure During Processing Of Pure Magnesium And The Magnesium Alloy AM30 Authors: Biswas, Somjeet Abstract: Magnesium is the lightest metal that can be used for structural applications. For the reasons of weight saving, there has been an increasing demand for magnesium from the automotive industry. However, poor formability at room temperature, due to a limited number of slip systems available owing to its hexagonal close packed crystal structure, imposes severe limitations on the application of Mg and its alloys in the wrought form. One possibility for improving formability is to form the components superplastically. For this, it is necessary to refine the grain structure. A fine-grained material is also stronger than its coarse grain counterpart because of grain size strengthening. Moreover, fine-grained magnesium alloys have better ductility as well as a low ductile to brittle transition temperature, thus their formability at room temperature could be improved. In addition to grain refinement, the issues pertaining to poor formability or limited ductility of Mg alloys can be addressed by controlling the crystallographic texture. Recently, it has been shown that warm equal channel angular extrusion (ECAE) of magnesium led to reduction in average grain size and shear texture formation, by virtue of which subsequent room temperature rolling was possible. Based on the literature, it was also certain that, in order to make magnesium alloys amenable for processing, grain refinement needs to be carried out and the role of shear texture needs to be explored. Since processing at higher temperature would lead to relatively coarser grain size, large strain deformation at lower temperatures is desirable. The present thesis is an attempt to address these issues. The thesis has been divided in to eight chapters. The chapters 1 and 2 are dedicated to introduction and literature review on the subject that provides the foundation and motivation to the present work. Subsequent chapters deal with the research methodology, experimental and simulation results, discussion, summary and conclusion. In the present investigation, two single phase alloys were chosen, the commercially pure magnesium and the magnesium alloy AM30. These materials were subjected to suitable processing techniques, detailed posteriori. A systematic analysis of microstructure and texture for each of the as-processed materials was performed by electron backscattered diffraction (EBSD) using a field emission gun scanning electron microscope (FEG-SEM). Bulk texture measurement by X-ray diffraction, neutron diffraction and local texture measurement by synchrotron X-rays were also carried out. In addition, dislocation density was measured using X-Ray diffraction line profile analysis (XRDLPA). The experimental textures were validated by using Visco-Plastic Self Consistent (VPSC) simulation. The details of experimental as well simulation techniques used in the present investigation is described in chapter 3. To understand the philosophy of large strain deformation by shear in magnesium and its alloy, free end torsion tests could provide a guide line. Based on the understanding developed from these tests, further processing strategy could be planned. Therefore, a rigorous study of deformation behaviour under torsion was carried out. In chapter 4, the results of free end torsion tests carried out at different temperatures, 250⁰C, 200⁰C and 150⁰C and strain rates, 0.01 rad.s-1, 0.1 rad.s-1, 1 rad.s-1 are presented for both the alloys. In addition to the analysis of stress-strain behaviour, a thorough microstructural characterization including texture analyses pertaining to deformation and dynamic recrystallization was performed. Both pure Mg and the AM30 alloy exhibit similar ductility under the same deformation condition, while the strength of AM30 was more. The strain hardening rate decreased with temperature and increased with strain rate for both the materials. However, the strain hardening rate was always higher in case of the alloy AM30. Large amount of dynamic recrystallization (DRX) was observed for both the alloys. The initial texture had an influence on the deformation behaviour under torsion and the resulting final texture. The initial non-axisymmetric texture of pure Mg samples led to nonaxisymmetric deformation producing ear and faces along the axial direction, and the final texture was also non-axisymmetric. An examination of the texture heterogeneity was carried out in one of the pure Mg torsion tested samples by subjecting it to EBSD examination at different locations of the surface along the axial direction. The strain induced on the ear portion was maximum, and in the face was lower. This has been attributed to the orientation of basal planes in the two regions. The axisymmetric initial texture in case of the alloy AM30 led to the formation of axisymmetric texture with no change in the shape of the material. Owing to this simplicity, the occurrence of dynamic recrystallization (DRX) was studied in more detail for this alloy. The mechanism of texture development due to deformation as well as dynamic recrystallization could be tracked at every stage of deformation. A typical shear texture was observed with respect to the strain in each case. Very low fraction of twins was observed for all the cases indicating slip dominated deformation, which was validated by VPSC simulation. It was found that with the increase in strain during torsion, the fraction of dynamically recrystallized grains increased. The recrystallization mechanism was classified as “continuous dynamic recovery and recrystallization” (CDRR) and is characterized by a rotation of the deformed grains by ~30⁰ along c-axis. After developing an understanding of large strain deformation behaviour of pure Mg and the alloy AM30 through torsion tests, the possibility of low temperature severe plastic deformation for both the materials by equal channel angular extrusion (ECAE) was explored. The outcome of this investigation has been presented in chapter 5. At first, ECAE of pure magnesium was conducted at 250⁰C up to 4 passes and then the temperature was reduced by 50⁰C in each subsequent pass. In this way, ECAE could be carried out successfully up to 8th pass with the last pass at room temperature. A grain size ~250 nm and characteristic ECAE texture with the fibres B and C2 were achieved. The AM30 alloy subjected to similar processing schedule as pure Mg, however, could be deformed only up to 6th pass (TECAE=150⁰C) without fracture. An average grain size ~ 420 nm and a texture similar to ECAE processed pure Mg was observed for this alloy. The difference in the deformation behaviour of the two alloys has been explained on the basis of the anisotropy in the stacking fault energy (SFE) in the case of pure Mg. Neutron diffraction was carried out to confirm and validate the microtexture results obtained from the EBSD data, while the local texture measurement by synchrotron radiation was carried out at different locations of the ECAE samples to give a proper account of the heterogeneity in texture. The effect of grain refinement was examined, deconvoluting the effect of shear in improving the strength and ductility using another severe plastic deformation technique, namely multi axial forging (MAF). In this process, the material was plastically deformed by a combination of uniaxial compression and plane strain compression subsequently along all the three axes. The details of this investigation has been presented in chapter 6. By this method, the alloy AM30 could be deformed without fracture up to a minimum temperature of 150⁰C leading to ultra-fine grain size (~400 nm) with very weak texture. A room temperature ductility ~55% was observed for this material. Finally, a comparison of room temperature mechanical properties of the alloy AM30 was carried out for the ECAE and MAF processed conditions having similar grain size in order to observe the effect of texture formed during both the processes. A similar strength and ductility for both the cases was attributed to the orientation obtained from both the ECAE and MAF, which is away from the ideal end orientation for tensile tests. The final outcomes of the thesis has been summarized in chapter 7.