etd@IISc Collection:

Microstructure And Mechanical Properties Of Consolidated Magnesium Chips

Wed, 03 Apr 2013 18:30:00 GMT

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

Tue, 19 Feb 2013 18:30:00 GMT

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

Wed, 27 Feb 2013 18:30:00 GMT

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.

Some Critical Issues Pertaining To Deformation Texture In Close-Packed Metals And Alloys : The Effect Of Grain Size, Strain Rate And Second Phase

Sun, 19 May 2013 18:30:00 GMT

Title: Some Critical Issues Pertaining To Deformation Texture In Close-Packed Metals And Alloys : The Effect Of Grain Size, Strain Rate And Second Phase Authors: Prakash, Gurao Nilesh Abstract: Crystallographic texture in polycrystalline materials are known to play an important role in tailoring suitable properties for various technological applications. In addition, the evolution of texture provides a profound basis to develop scientific understanding of physical processes occurring in the material during deformation and annealing. Between the two, the understanding of deformation texture is much broader. However, certain issues pertaining to the evolution of deformation texture evolution are yet to be explored or not uniquely agreed upon. A few notable examples are the effects of extreme grain sizes and strain rates. Moreover, most of the studies are pertaining to single phase metals and alloys. While many engineering alloys consist of two phase microstructures, the effect of second phase in the microstructure on the evolution of texture in the individual phases has not been studied in a comprehensive manner. The present thesis is an attempt to addresses these issues in a more generic manner. The studies have been specifically aimed at examining the aforesaid issues in the close packed Face Centre Cubic (FCC) and Hexagonal Close Packed (HCP) metals and alloys. In brief, this thesis addresses the following problems pertaining to deformation texture: (i) the effect of extreme grain sizes, (ii) the effect of extreme strain rates and (iii) the effect of a second ductile phase. Chapter 1 of the thesis gives a detailed survey of literature pertaining to the evolution of deformation textures in different metals and alloys, while chapter 2 includes the details of the experimental techniques and simulation procedures, which are mostly common for the entire work. The issue of grain size is addressed in chapter 3. In the present investigation, the evolution of deformation texture in nickel (FCC) and titanium (HCP) with the extreme grain sizes (nanometre and millimetre) has been studied. Nanocrystalline nickel with the grain size ~ 20 nm was obtained by pulse electro-deposition while the other extreme of the grain size in nickel was obtained by annealing of a cold rolled sheet at 1373 K. The rolling texture in nanocrystalline nickel had a higher volume fraction of Brass component than in nickel with normal grain size. These results have been explained on the basis of inhibition of cross slip in small grain sizes and the operation of planar slip. This has been validated by viscoplastic self-consistent simulations. The texture of coarse grain nickel samples (typified as oligocrystalline, owing to the lesser number of grains in the thickness direction) also had higher Brass component like the nanocrystalline sample. A detailed analysis was performed by examining misorientation development in the grain interior and in the vicinity of the grain boundaries. The similarity at the two extreme length scales has been explained on the basis of lower “Grain Boundary Affected Zone” at the extreme length scales. To examine the effect of grain size in the case of HCP materials, commercially pure titanium with ultra-fine (500 nm) and normal grain size (~50 μm), was investigated. A monotonic evolution of texture was observed in the former, which has been attributed to the absence of twinning, a situation that could arise due to the lack of coordinated movement of twinning partials in the sub-micron grain size regime. Thus, a reasonable understanding of the evolution of deformation texture in hitherto unexplored regime of grain sizes was developed for the two materials. The chapter 4 of the thesis is dedicated to the study of strain rate effects in both FCC and HCP materials. The issue of strain rate has been addressed by two ways: (a) deforming the materials at extreme strain rates, namely 10-3 s-1 to 10+3 s-1 under compression up to a reasonable strain, and (b) deforming the materials under torsion within a reasonable range of strain rates, but up to large strains. In this case, in addition to nickel, copper was also investigated owing to the different strain hardening behaviour of the two materials. The compression texture in nickel and copper was characterized by the presence of <101> component at low strain rates. At high strain rate, ~10+3 s-1, there was a decrease in the intensity of the <101> component for nickel but it strengthened for copper. This has been explained on the basis of continuous dynamic recrystallization in copper. The torsion texture evolution in nickel and copper was similar at low strain rate (10-3 s-1) and was characterized by the presence of important shear texture components. At high strain rate (1 s-1), texture weakened for nickel, while for copper a rotated cube component was observed which has been attributed to dynamic recrystallization. The effect of strain rate was studied more comprehensively in hexagonal titanium by adding one more variable, that is, the initial texture. Extreme strain rates were imparted using static and dynamic compression tests. It was found that different initial textures led to different mechanical response in terms of yield strength and strain hardening as well as microstructural response in terms of twin fractions. The samples deformed at high strain rate showed increased twinning that led to some scatter in the texture components compared to low strain rate deformed samples. VPSC simulations were able to successfully capture the evolution of texture as well as microstructural evolution in terms of twin activity in the deformed samples at the extreme strain rates. Torsion tests on titanium at different strain rates indicated evolution of inhomogeneous nature of fibre texture components with increase in strain rate. Thus, weakening of texture was observed irrespective of the strain path (compression or torsion) and crystal structure (FCC or HCP) unless additional restoration mechanism like recrystallization (continuous or discontinuous) intervened. In chapter 5, the evolution of rolling texture in two phase FCC + BCC (Ni-Fe-Cr alloys) and HCP + BCC (Ti-13Nb-13Zr ) alloys has been studied. This study was aimed at examining the effect of second deforming phase on the texture evolution in the primary phase. The effect of various parameters like volume fraction and morphology of the second phase on deformation texture evolution was studied experimentally as well as by VPSC simulations. A reduction in the Brass component of texture was observed in the austenite phase due to the presence of harder ferrite phase while a characteristic rolling texture evolved in the ferrite phase. It has been established that the softer austenite phase carried maximum strain at low volume fractions of ferrite while the harder ferrite phase carried the maximum strain at higher volume fractions of ferrite. In case of the two phase HCP+BCC alloy Ti-13Nb-13Zr, both the hexagonal α and the cubic β phases showed a characteristic rolling texture irrespective of two different morphologies. For both the equiaxed and colony microstructures, the softer β phase carried the maximum strain. VPSC simulations were able to model the deformation texture evolution as well as microstructural parameters like strain partitioning and twin fraction satisfactorily for both the microstructural conditions. It was found that the deformation mechanism in one phase could be affected by the presence of the second phase and that a characteristic change in deformation texture could be produced in the presence of the second phase. Thus, a comprehensive perspective has been developed pertaining to the evolution of texture in FCC and HCP phases in the presence of a second ductile phase. The overall findings of the three investigations carried out for the thesis are summarised in chapter 6.