Preparation of concrete specimen for internal sulfate attack analysis using electron backscatter diffraction

Concrete cores were obtained from houses in eastern Connecticut, USA, that had varying degrees of crumbling foundations and wall cracking. Electron backscatter diffraction (EBSD) was used simultaneously with energy dispersive X-ray spectroscopy to investigate the degradation of these samples. This combination allowed the precise correlation of elemental composition with mineral crystallography phase mapping. EBSD examination showed the presence of pyrrhotite, pyrite, and marcasite phases in some of the samples, whereas internal sulfate attack (ISA) is triggered by the release of sulfates through the oxidation of such iron sulfides. Secondary expansion products from ISA are associated with foundation cracking, wall bulging, and drastically decreased structural stability. The main contribution of this study is therefore an automated procedure for preparation of concrete samples and analysis of aggregates using EBSD

A Review On Alpha Case Formation And Modeling Of Mass Transfer During Investment Casting Of Titanium Alloys

Titanium alloys have excellent corrosion resistance, high temperature strength, low density, and biocompatibility. Therefore, they are increasingly used for aerospace, biomedical, and chemical applications. Investment casting is a well-established process for manufacturing near-net-shape intricate parts for such applications. However, mass transfer arising from metal-mold reactions is still a major problem that drastically impairs the surface and properties of the castings. Although there have been astounding developments over the past 20 years, they remain scattered in various research papers and conference proceedings. This review summarizes the current status of the field, gaps in the scientific understanding, and the research needs for the expansion of efficient casting of titanium alloys. The uniqueness of this paper includes a comprehensive analysis of the interfacial reactions and mass transfer problems. Additionally, momentum and heat transfer are presented where applicable, to offer a holistic understanding of the transport phenomena involved in investment casting. Solutions based on modeling and experimental validation are discussed, highlighting ceramic oxide refractories like zirconia, yttria, calcia, alumina, and novel refractories namely, calcium zirconate and barium zirconate. It was found that while mold material selection is vital, alloy composition should also be carefully considered in mitigating metal-mold reactions and mass transfer

Mechanical Properties, Translucency, And Low Temperature Degradation (LTD) Of Yttria (3–6 Mol%) Stabilized Zirconia

The mechanical properties (hardness and biaxial flexural strength), optical properties (translucency and gloss), and low temperature degradation (LTD) of 3–6 mol% yttria stabilized zirconia have been investigated. The specimens were prepared by conventional solid-state sintering at 1400 °C. The mechanical and optical properties were evaluated for sintered and polished specimens. X-ray diffractometer (XRD) and scanning electron microscope (SEM) were used, respectively, for structural and microstructural investigation. LTD was determined by XRD analysis of the samples aged in a hydrothermal autoclave at 134 °C for 5–108 h. The mechanical and optical properties are found to be microstructure sensitive. The LTD, due to tetragonal to monoclinic phase transformation, decreases with increasing Y2O3 content. The mechanical strength, translucency, and LTD are the lowest for 6 mol% Y2O3 stabilized zirconia

High-Temperature Interactions Between Titanium Alloys And Strontium Zirconate Refractories

We investigated interactions between Ti6Al4V alloys and strontium zirconate (SrZrO3) ceramic to assess its potential as a refractory mold material in investment casting. We developed a robust yet simple procedure to examine both the liquid–solid and solid–solid interactions using pellets in drop casting and diffusion couple methods. Reaction layers were characterized using optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and x-ray diffraction (XRD). The results were compared to alumina (Al2O3) which is still a common refractory ceramic for molds in investment casting. Our findings indicate that Ti6Al4V surfaces in contact with SrZrO3 had no apparent changes in surface chemistry nor microstructure. On the other hand, Ti6Al4V surfaces in contact with Al2O3 developed γ-TiAl and α 2-Ti3Al intermetallics with thicknesses of ~ 100 μm in diffusion couples and ~ 10 μm in drop-casting experiments. Nanoindentation results showed that the surface of Ti6Al4V in contact with Al2O3 was significantly harder compared to SrZrO3, confirming our conclusion. Given the time and costs associated with mechanical and chemical removal of reaction layers on Ti6Al4V castings, SrZrO3 can be a better choice for a mold material in the investment casting of titanium alloys

A First Principles Analysis Of Oxidation In Titanium Alloys With Aluminum And Vanadium

We carry out first principles computations to understand mechanisms of oxygen accommodation on titanium surfaces doped with aluminum and vanadium. Our findings provide explanation in an atomistic level on why titanium alloys such as Ti-6Al-4 V (6 wt. % Al, 4 wt. % V, balance Ti) have a better oxidation resistance than commercially pure titanium. We determine the ground state positions, the related formation (adsorption) energies of oxygen atoms, their migration pathways into the bulk. We also explore the correlation between the concentration-dependent configurational stability of the alloyed surface with the oxygen migration energy. Our results show that oxygen atoms are concentrated on the surface and occupy face centered cubic hollow sites. A relatively high energy barrier for oxygen migration into the titanium lattice is found, indicating that such dopants provide a passivating effect in oxidizing environments

Solid-state Calcination And Synthesis Of Homogeneous Strontium Zirconate By Slip Casting

Solid-state synthesis of strontium zirconate (SrZrO3) at 1200 °C by slip casting is proposed as an alternative to the traditional pellet pressing method to improve accessibility and efficiency in laboratory settings. Powders prepared through both methods were characterized for particle size, morphology, chemical purity, and sintering performance. It was found that the slip casting adaptation produced orthorhombic strontium zirconate with similar particle or microstructural characteristics to pellet pressing. Furthermore, production efficiency in a laboratory setting compared to traditional pellet method was significantly increased. The results reported here indicate that slip casting is a suitable alternative method for solid-state synthesis of ceramic powders, particularly in laboratory settings where access to industrial equipment is limited. Synthesis of SrZrO3 by slip casting allowed 70% reduction in person-hours compared to a pellet press approach in a manual laboratory setting

An overview of ceramic molds for investment casting of nickel superalloys

Accelerating advancements in technological systems have demonstrated a need for alloys with drastically improved thermomechanical and chemical properties, called superalloys. Ceramic molds are typically used in near-net shape investment casting processes of superalloy components due to their chemical inertness and high-temperature capabilities. Ceramic molds, however, often suffer from shortcomings in vital properties including flexural strength, thermal shock resistance, permeability, dimensional stability, corrosion resistance, and leachability, which have restricted their ability to adequately process modern alloy castings. This study analyses these limitations and illustrates how to address them, particularly regarding ceramic mold and slurry design, processing of shells and cores, material selection, and testing and characterization. By utilizing advanced processing methods including additive manufacturing and gel-casting, more dimensionally accurate and preferentially built molds can be formed. Additionally, by varying the mold composition to achieve more chemically inert structures, reactions with the mold can be mitigated to reduce chemically induced defects