Amit Shyam

High frequency fatigue crack propagation behavior of a nickel-base turbine disk alloy

Abstract Fatigue crack propagation tests were conducted on the powder metallurgy nickel-base superalloy KM4 at room temperature. Two different heat treatments were investigated, one which produced a relatively coarse grain size around 55 μm, and another which produced a very fine grain size around 6 μm. Tests were conducted at 50 Hz and 1000 Hz in an advanced servohydraulic testing machine at R -ratios between 0.4 and 0.7. There was no effect of frequency on the fatigue behavior at room temperature, which is expected in this type of alloy, and this result yields confidence in the reliability of the servohydraulic fatigue testing system. The threshold stress intensity for fatigue crack propagation decreased with decreasing grain size and with increasing R -ratio, again as expected. With increasing grain size, the crack path tortuosity and the crystallographic facet size on the fracture surface both increased substantially, leading to increases in roughness-induced closure and a higher apparent threshold. The threshold ΔK values measured at 10 −10 m/cycle corresponded to essentially infinite lifetimes, as very small decreases in Δ K from the threshold values resulted in complete crack arrest, and led to difficulty in restarting the crack growth at higher Δ K levels. Finally, comparisons of the observed thresholds with existing models revealed significant discrepancies between the predicted and measured values.

Comparative Evaluation of Cast Aluminum Alloys for Automotive Cylinder Heads: Part II-Mechanical and Thermal Properties

The first part of this study documented the as-aged microstructure of five cast aluminum alloys namely, 206, 319, 356, A356, and A356+0.5Cu, that are used for manufacturing automotive cylinder heads (Roy et al. in Metall Mater Trans A, 2016). In the present part, we report the mechanical response of these alloys after they have been subjected to various levels of thermal exposure. In addition, the thermophysical properties of these alloys are also reported over a wide temperature range. The hardness variation due to extended thermal exposure is related to the evolution of the nano-scale strengthening precipitates for different alloy systems (Al-Cu, Al-Si-Cu, and Al-Si). The effect of strengthening precipitates (size and number density) on the mechanical response is most obvious in the as-aged condition, which is quantitatively demonstrated by implementing a strength model. Significant coarsening of precipitates from long-term heat treatment removes the strengthening efficiency of the nano-scale precipitates for all these alloys systems. Thermal conductivity of the alloys evolve in an inverse manner with precipitate coarsening compared to the strength, and the implications of the same for the durability of cylinder heads are noted.

Growth of recrystallising grains in boron doped Ni76 Al24

AbstractCold rolled boron doped Ni76 Al24 was partially recrystallised at 850, 875, and 900°C. The recrystallised volume fraction and the interfacial area between recrystallised and unrecrystallised parts were determined as functions of time. The analysis of results showed the growth rates of recrystallised grains to be dependent on temperature, but independent of time. The activation energy for the transformation was 168 kJ mol-1 and that for growth was 87 kJ mol-1. The activation energy for growth was much less than that for diffusion and was dependent on the degree of cold working. This has been attributed to the strong interaction of boron with the structural defects produced by cold working.

Structure-Property Investigations via SEM In-Situ Micromechanical Testing

Small-scale in-situ testing within the SEM can be used to study deformation behavior in materials, providing some of the most compelling insights into mechanisms that govern mechanical properties. In-situ microand nanomechanical tests conducted on well-shaped specimens provide information about a full suite of properties of interest (e.g. yield phenomena, strength, ductility, modulus, work hardening, etc.). This occurs while simultaneously affording a glimpse into the dynamic structural changes that reflect underlying dislocation processes associated with those properties (e.g. local versus global slip activity, twinning, grain boundary interactions, slip about second phases, necking, etc.). The microstructural evidence can be in the form of secondary and backscattered electron images, ion images or in orientation determination from EBSD analyses. The power of the SEM in-situ microscale test methodology is that it encompasses analysis of the entire body being deformed with the resolution of key features responsible for the structure-property correlation [1]. And further, it inherently provides the ability to study time-dependent development of microstructure during deformation. This has profound implications for deformation modeling where simulation over the entire test volume has now become a tractable endeavor.

Hot-Tearing of Multicomponent Al-Cu Alloys Based on Casting Load Measurements in a Constrained Permanent Mold

Hot-tearing is a major casting defect that is often difficult to characterize, especially for multicomponent Al alloys used for cylinder head castings. The susceptibility of multicomponent Al-Cu alloys to hot-tearing during permanent mold casting were investigated using a constrained permanent mold in which the load and displacement were measured. The experimental results for hot tearing susceptibility are compared with those obtained from a hot-tearing criterion based on temperature range evaluated at fraction solids of 0.87 and 0.94. The Cu composition was varied from approximately 5–8 pct. (weight). Casting experiments were conducted without grain refining. The measured load during casting can be used to indicate the severity of hot tearing. However, when small hot-tears are present, the load variation cannot be used to detect and assess hot-tearing susceptibility.

Process Simulation Role in the Development of New Alloys Based on an Integrated Computational Materials Engineering Approach

To accelerate the introduction of new materials and components, the development of metal casting processes requires the teaming between different disciplines, as multi-physical phenomena have to be considered simultaneously for the process design and optimization for mechanical properties. The required models for physical phenomena as well as their validation status for metal casting are reviewed. The data on materials properties, model validation, and relevant microstructure for materials properties are highlighted. One vehicle to accelerate the development of new materials is through combined experimental-computational efforts. Integrated computational/experimental practices are reviewed; strengths and weaknesses are identified with respect to metal casting processes. Specifically, the examples are given for the knowledge base established at Oak Ridge National Laboratory and computer models for predicting casting defects and microstructure distribution in aluminum alloy components.© 2014 ASME

Constrained Thermal Fatigue Performance of Several Cast Ferrous Alloys

An experimental setup that was utilized to evaluate the constrained thermal fatigue (CTF) behavior of several cast ferrous alloys is described. The tests performed allowed the assessment of the relative performance of different materials in CTF loading. The stable hysteresis loop of the individual CTF tests further allowed the development of a unified parameter that determined the level of inelastic (plastic and creep) deformation and the CTF life under those conditions. A CTF life prediction methodology for cast ferrous alloys is outlined.

Feasibility Study of Making Metallic Hybrid Materials Using Additive Manufacturing

A metallic hybrid structure, consisting of an Inconel-718 matrix and a Co-Cr internal structure, was successfully manufactured using laser direct energy deposition process. Characterizations were performed using energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), neutron-computed tomography (nCT), and electron back-scatter diffraction (EBSD) to verify the interfaces between Co-rich and Ni-rich phases. nCT revealed the internal structures to be continuous without cracking or significant intermixing due to inter-diffusion of Co and Ni (i.e., dissolved boundaries between the two structures). Minor porosity was detected. EBSD confirmed a good bond at the granular level. No precipitate phases were detected with XRD. EDS revealed dilution/intermixing between the Co and Ni interfaces presumably due to melt-pool overlay between the matrix and the internal structures.

Petascale supercomputing to accelerate the design of high-temperature alloys

Abstract Recent progress in high-performance computing and data informatics has opened up numerous opportunities to aid the design of advanced materials. Herein, we demonstrate a computational workflow that includes rapid population of high-fidelity materials datasets via petascale computing and subsequent analyses with modern data science techniques. We use a first-principles approach based on density functional theory to derive the segregation energies of 34 microalloying elements at the coherent and semi-coherent interfaces between the aluminium matrix and the θ′-Al2Cu precipitate, which requires several hundred supercell calculations. We also perform extensive correlation analyses to identify materials descriptors that affect the segregation behaviour of solutes at the interfaces. Finally, we show an example of leveraging machine learning techniques to predict segregation energies without performing computationally expensive physics-based simulations. The approach demonstrated in the present work can be applied to any high-temperature alloy system for which key materials data can be obtained using high-performance computing.

DPF Durability ORNL-04-0692

A substantial amount of testing and analysis was performed over the 13-year-duration of this Cooperative Research and Development Agreement (CRADA) between Cummins Inc. and the Oak Ridge National Laboratory (ORNL). ORNLs work focused on developing fundamental understanding of the mechanical and physical properties and their relationship to the microstructure of Diesel Particulate Filter (DPF) materials (i.e., cordierite, aluminum titanate, Si-SiC). These materials exhibit significant strain tolerance owing to their highly porous and mircocracked microstructures. That is, while very weak, the porous and compliant microstructure could withstand large strains without failing catastrophically. Substantial knowledge was also gained with respect to preparation of test specimens for evaluation of mechanical properties and the care required. Once the role of the microstructure of the wall material on its mechanical response was understood, the DPF honeycombs themselves were re-examined, and the influence of the honeycomb structure was considered. Cummins s work focused mainly on the apparent strength of the DPF honeycomb structure and using the material properties generated at ORNL for input to their regeneration and performance-predictive models.