The compression behavior of a newly developed near eutectic Al-Si based cast alloy with three different microstructures has been investigated in the present work. Microstructures with modified and unmodified Si particles and matrix with different tempers are investigated. The main objective of this work is to understand the effect of heat treatment and modification on the fracture behavior of the alloy under compression. This alloy is subjected to compressive loading at different strain rates and temperatures during the operation of the engines. Hence, the effect of strain rates and temperatures is also considered. The compression tests are carried out at different strain rates from quasi-static to dynamic viz., 3*10-4 to 102/s and three different temperatures RT, 100°C and 200°C. Microstructure of the alloys studied predominantly consists of eutectic colonies of α-Al and Si with a few interspersed α-Al dendrites. Modified alloy has more globular Si particles than unmodified alloy. Heat treated alloys are found to have hardening precipitates S’ & Al7Cu4Ni and 3-7 atomic layer thick zones, which may be precursors to S’ phase. A variety of large intermetallics, viz., plate like particles Al4.5FeSi, Chinese script like particles Al19Fe4MnSi2 and bulky phase Al3NiCu are also observed in the alloys. Mechanical behavior of the alloys is found to be different for different microstructures. Modification improves strength and ductility. Heat treatment improves strength of the alloy at the expense of ductility. A transition in mechanical behavior is observed after a particular strain rate for all the alloys studied. This transition strain rate is dependent on heat treatment, Si particle size and temperature. This transition can be explained on the basis of dislocation-precipitate and dislocation-Si particle interactions. Work hardening behavior of the alloys depends on the matrix microstructure in the unmodified alloys, and both matrix and particles play a role in the modified alloy. A statistically robust quantitative micro structural analysis has been carried out after compressing the samples at various strain rates and temperatures. The unique contribution of this work is the understanding of combined effect of strain rate and temperature on Si particle fracture characteristics in the alloy with different microstructures. From the fracture characteristics of Si particles, it is concluded that both dislocation pile-up mechanism and fibre loading are responsible for particle fracture in the modified alloy, whereas the fibre loading mechanism alone is sufficient to explain the particle fracture characteristics in the unmodified alloy. Si particles in the modified condition are found to cleave along the lowest surface energy planes {112} & {110} and the particles with orientations {112} & {111} are more prone to fracture. In addition to Si particle fracture, elongated Fe rich intermetallic particles are also seen to show peculiar fracture behavior. The Al4.5FeSi intermetallics with (100) as the plane of the plate cleave along (100) planes. This is a novel finding in this work and could have immense implications on the role of Fe impurities in the fracture behavior of these alloys. Moreover, since these cleavage fractures are seen to be more than 200 microns in size (which implies that the real penny shaped crack would be even larger) their role cannot be assumed to be negligible, as was previously thought. The load sharing between the Al matrix and eutectic Si particles are simulated by microstructure based finite element modeling. The program OOF (Object-Oriented Finite element analysis) is used to generate the finite element meshes for real microstructures with different Si morphology. The experimentally obtained stress – strain properties of the alloy is given as an input to describe the plastic behavior of the Al matrix, in the finite element simulation. This analysis helps to understand the effect of particle size, shape, orientation & clustering and matrix temper on the stress transferred to the Si particles. Combination of Electron Back-Scattered Diffraction (EBSD) and frequency shift, polarized micro-Raman technique is applied to validate the stress states in Si particles with {111} orientations. The stress at fracture of Si particles is also estimated from Raman technique. Even though the alloys with different microstructures show different mechanical behavior, the sequence of fracture mechanisms is found to be same for all the alloys. The failure occurs in three stages: cracking of Si particles at low strains, micro-crack formation along the fractured particles, micro-crack coalescence and propagation leading to final failure. Thus, the proposed analysis links various deformation mechanisms ranging from nano precipitate-dislocation interactions to micro short-fiber theory of load sharing by eutectic silicon along with coupled effect of strain rate and temperature. In addition, negative strain rate sensitivity is also observed in the lower strain rate regimes (3*10-4, 10-3& 102/s) at RT and 100°C for all the three alloys, and serrated flow is also observed in the same strain rate and temperature regimes. Some of the features of serrated flow can be explained by the dynamic strain aging model and some other features by precipitate shearing.

The need to produce lighter components due to environmental aspects and the development of electrical vehicles represents an opportunity for cast aluminium-silicon alloys. With high specific strength, good castability, high corrosion resistance and recyclability, these alloys offer an attractive combination of properties as an alternative to steel, cast iron and titanium-based components in certain applications. To take advantage of such a combination of properties, there is a need to ensure that they can be reliably achieved. In other words, high performing components need to be produced. For that, the production cycle, from alloy selection and melt preparation, to the casting and heat treatment of the component must be understood and controlled as a whole. The different steps in the production cycle will affect the microstructure of the components and hence the resulting mechanical properties. Understanding the relation between the different steps in the production cycle, its consequences on the microstructural features and on the mechanical properties constitutes the aim of this thesis. Experiments applying state-of-the-art knowledge regarding effect of casting process, alloying system and post-process variables were performed aimed at achieving properties similar to those of high pressure die casting (HPDC) components. Different melt quality determination tools were evaluated on three different EN AC-46000 melt qualities. The influence of modification, grain refinement and both treatments together was assessed on an Al-10Si alloy solidified under different cooling rates. The tensile behaviour and the impact of features such as secondary dendrite arm spacing (SDAS) or grain sizes was quantified. It was corroborated that by appropriate selection and control of such alloying system, process and post-process variables it is possible to achieve HPDC EN AC-46000 tensile and fatigue properties through a T5 treated sand cast EN AC-42100 alloy. On the other hand, the available techniques for melt quality assessment are inadequate, requiring further analysis to successfully identify the melt quality. Additionally, it was observed that decreasing the melt quality by additions of 25 wt.% of machining chips did not significantly decrease the tensile properties but slightly increased the variation in them. In relation to the modification and grain refinement of Al-10Si alloys it was concluded that with the slowest cooling rate tested, additions of only grain refiner did not successfully produce equiaxed grains. For cooling rates corresponding to dendrite arm spacings of 15 μm and slower, combined additions of grain refiner and modifier can lead to higher tensile properties compared to the corresponding separate additions. SDAS was observed to describe flow stress through the Hall-Petch equation but grain size did not show a physically meaningful relationship. Furthermore, beginning of cracking was detected in the plastic deformation region at dendrite/eutectic boundaries and propagated in a trans-granular fashion.

The introduction of gas-driven Otto engine and the corresponding usage of bio-fuels in heavy-duty engines will render the exhaust atmosphere more corrosive and bring a higher working temperature to exhaust manifolds. The current service material, a ferritic ductile cast iron called SiMo51, will soon meet its upper temperature limit set by the ferrite-austenite transformation at 860ºC. Three alternative materials, as well as SiMo51 serving as reference, are investigated in the present thesis emphasizing on high-temperature corrosion fatigue.  The first aim of this study is to obtain material data and give a quantitative ranking of the materials’ performance. Low-cycle fatigue (LCF) tests at 800ºC in a synthetic exhaust gas (5%O2-10%CO2-5%H2O-1ppmSO2-N2 bal.) are conducted to evaluate the materials’ performance in simulated real working scenarios, where high-temperature, corrosive atmosphere and fatigue conditions during testings are similar to the conditions experienced by the exhaust manifolds. To evaluate the individual effect from high-temperature fatigue and isolate the impact from corrosion, the materials are tested under the same settings but in an argon atmosphere. To evaluate the individual effect from high-temperature corrosion and isolate the impact from mechanical deformation, oxidation tests are carried out at 800ºC in the same synthetic exhaust gas. The second aim is to identify and understand different oxidation behavior and failure mechanisms in the materials, realized by considerable characterizations of the tested specimens. From the fatigue tests, it is found that the austenitic stainless steel HK30 has the highest fatigue resistance, followed by the austenitic cast iron Ni-resist D5S, and the ferritic ductile cast irons SiMo1000 and SiMo51, a ranking valid in both atmospheres. In the exhaust atmosphere, for instance, the improvement in fatigue strength at 15,000 cycles relative to SiMo51 are 260%, 194% and 26%, respectively. Different crack initiation and propagation mechanisms are found for the various combinations of materials and atmospheres. In the exhaust atmosphere, for instance, crack initiation is assisted by oxide intrusion in SiMo51 and crack propagation is affected by crack branching in HK30, mechanisms not observed in argon. By comparing the S-N fatigue curves in the two atmospheres, the influence of oxidation on fatigue life is evaluated. The fatigue life of the cast irons are surprisingly found to be higher in the exhaust atmosphere. Several explanations are suggested for this, considering their very different oxidation behaviors.  This study provides accurate test data that can be used to help industry avoid over-dimensioned design. The investigation of the failure mechanisms promotes better understanding of the correlation between microstructure and mechanical properties. Moreover, the combination of fatigue tests in argon, fatigue tests in exhaust and oxidation tests in exhaust, shows how corrosion and fatigue individually and synergistically affect the materials’ performance at high temperature. / <p>QC 20180917</p>

<p>Al-Si cast alloys have been in the fore-front of commercial casting application for more than a century. Iron containing intermetallic phases that evolve during the solidification of these alloys play a major role in the resultant mechanical properties and performance of the cast products. Changes in alloy composition and casting parameters significantly alter the evolution of the Al-Si-Fe intermetallic phases. There was a lack of clear understanding of the complex relationships between the solidification parameters and nature intermetallic phases in these alloys. Current thermodynamic model predictions for the nature of these intermetallic phases in the Al corner of the Al-Si-Fe ternary system are strikingly different from the experimental results in this project. Trace levels of Sr (about 0.02wt%) are typically added to the Al-Si commercial alloys to effect a morphological modification of the eutectic phases to improve the properties and performance of the cast products.</p> <p>The nature of the Fe containing intermetallic phases have been characterized as a function of alloy composition (Si, Fe and Sr) and cooling rates during solidification. There was an anomalous evolution of the t₅-Al₈Fe₂Si phase which transformed into the t₆-Al₉Fe₂Si₂ phase during solidification at lower cooling rates and higher Fe concentration in the alloy, alike. Further, Sr addition to these alloys prevented the evolution of the t₅ phase and promoted the evolution of an unidentified Al₅Fe₂Si₃ phase which was noted as k in this dissertation; the k phase also transformed into the t₆ phase at lower cooling rates and higher Fe concentration in the alloy, alike.</p> / Doctor of Philosophy (PhD)

Digital image correlation techniques (DICT), a non-contact deformation measuring technique based on gray value digital images, have become increasingly used over the last years. By using the DIC technique during a tensile test, the deformation behavior of different engineering material under an applied load can be determined and analyzed. Digital images, acquired from a tensile test, can be correlated by using DICT software and from that the local or global mechanical properties can be calculated. The local or global mechanical properties determination of a flat test specimens are based on the displacements or changes in a previous stochastic sprayed or natural pattern. The used material for this purpose is cast silicon (Si) based aluminium (Al) component, designated as AlSi7Mg0.3 (Anticorodal-78 dv). The hypoeutectic Al- Si alloy is widely applicable for engine constructions, vehicle and aerospace constructions, shipbuilding, electrical engineering and constructions for food industry. There are many microstructural parameters in a binary system Al- Si alloys, which the mechanical properties can be depended on, for instance phase distribution, Secondary Dendrite Arm Spacing (SDAS), morphology of Si particles (Roundness) and microscopic defects or pores. All these parameters can contribute to enhance the proper mechanical performance (e.g. Strength and ductility) in the Al-Si cast components.