David R. Johnson

Directional solidification of TiAl-base alloys

Abstract Mechanical properties of the lamellar microstructure of TiAl-base alloys are extremely anisotropic with respect to the lamellar orientation. However, if the lamellar microstructure can be aligned parallel to the growth direction, the resulting material should possess a good combination of strength and ductility. Unfortunately, simple casting operations often lead to a solidification texture with the lamellar boundaries all perpendicular to the heat flow direction. This difficulty can be overcome by directionally solidifying TiAl-base alloys. We have been performing directional solidification experiments with and without using a seeding technique. The current status of directional solidification of TiAl-base alloys is reviewed.

A composition window in the TiAl-Mo-Si system suitable for lamellar structure control through seeding and directional solidification

Abstract The thermal stability of the lamellar microstructure of cast Ti–46Al–1.5Mo–(0–3)Si (at.%) alloys was investigated to find suitable seed compositions for growing ingots of these alloys with the aligned lamellar microstructure through seeding and directional solidification. In order for the seeding to be made successfully, the original orientation of the lamellar microstructure must be restored upon heating to and cooling from the melting temperature. In this study, the lamellar stability was determined by examining whether or not the lamellar structure in the as-cast alloys is preserved after quickly heating to just below melting temperature, holding, and then cooling to room temperature. This requirement was found to be fulfilled for the Si contents larger than 1.0 at.%. Ti–46Al–1.5Mo–1Si and Ti–46Al–1.5Mo–1.5Si (at.%) alloys are typical examples of alloys which can be used as seed materials. Directional solidification experiments of Ti–46Al–1.5Mo–1Si alloy were performed using Ti–46Al–1.5Mo–1.5Si alloy as a seed material and ingots with the aligned lamellar microstructure were obtained.

Directional solidification of TiAl-based alloys and properties of directionally solidified ingots

The mechanical properties of TiAl-based alloys with lamellar microstructure are extremely anisotropic. However, if the lamellar microstructure can be aligned parallel to the growth direction, the resulting material should possess a good combination of mechanical properties. Unfortunately, simple casting operations often lead to a solidification texture with the lamellar boundaries perpendicular to the heat flow direction. This difficulty can be overcome by directionally solidifying TiAl-based alloys. We have been performing directional solidification experiments with and without using a seeding technique. The current status of directional solidification of TiAl-based alloys is reviewed.

Effects of refractory metals on microstructure and mechanical properties of directionally-solidified TiAl alloys

Abstract By using an appropriately oriented seed from the TiAl–Si system, the TiAl/Ti 3 Al lamellar structure was aligned parallel to the growth direction for ingots having composition of Ti–46Al–0.5Si–0.5X (X=Re, W and Mo) and Ti–47.5Al–0.5Re (at.%). The seeded and directionally solidified quaternary alloys containing either Re, W or Mo exhibit a nice balance of low- and high-temperature mechanical properties. Tensile elongation more than 20% is noted for the W- and Mo-containing alloys. The creep resistance for the three quaternary alloys is more than an order of magnitude better than other relevant TiAl alloys produced by conventional ingot metallurgy methods, with the best creep properties being obtained for the Re-containing alloy. The method to predict alloy compositions appropriate for aligning the lamellar structure of two-phase TiAl alloys of multi-component by directional solidification is proposed based on the assignment of ‘Al-equivalent’ for each of the alloying elements.

Effect of microstructure and strain on the degradation behavior of novel bioresorbable iron–manganese alloy implants

Advancing the understanding of microstructural effects and deformation on the degradability of Fe-Mn bioresorbable alloys (specifically, Fe-33%Mn) will help address the current problems associated with designing degradable fracture fixation implants for hard tissues. Potentiostatic polarization tests were conducted on a wide variety of metal samples to examine how different deformation processes affect the instantaneous rate of degradation of Fe-Mn alloys. Large-strain machining (LSM), a novel severe plastic deformation (SPD) technique was utilized during these experiments to modify the degradation properties of the proposed Fe-Mn alloy. It was discovered that Fe-33%Mn after LSM with a rake angle of 0° (effective strain = 2.85) showed the most promising increase in degradation rate compared to as-cast, annealed, and additional deformation conditions (rolled and other LSM parameters) for the same alloy. The mechanisms for enhancement of the corrosion rate are discussed.

Modeling defects in castings using a volume of fluid method

Purpose – A single-domain multi-phase model is developed for macrosegregation and shrinkage pipe formation in castings, as functions of buoyancy- and shrinkage-induced flow. The paper aims to discuss these issues. Design/methodology/approach – Using a volume of fluid (VOF) method, both the air/liquid and air/solid interfaces are tracked during shrinkage pipe formation. A set of mixture advection-diffusion equations are derived and solved for velocity, temperature, composition, and phase field evolution. The fluid mechanics of the model are verified using a transient ditch drainage problem. Findings – Results showing the interaction of macrosegregation and pipe formation are presented for two alloys under faster and slower cooling conditions. Originality/value – This model provides a comprehensive tool to investigate relationships between the developing composition distribution and shrinkage pipe formation.

Nanoporous metals for biodegradable implants: Initial bone mesenchymal stem cell adhesion and degradation behavior

Nanostructured Fe-Mn and Fe-Mn-Zn metal scaffolds were generated through a well-controlled selective leaching process in order to fulfill the growing demand for adjustable degradation rates and improved cellular response of resorbable materials. Mouse bone marrow mesenchymal stem cells (D1 ORL UVA) were seeded onto eleven, carefully chosen nanoporous surfaces for 24 h in vitro. Using a combination of fluorescence microscopy, scanning electron microscopy (SEM), and an MTS assay, it was discovered that scaffolds with nanoscale roughened surfaces had increased cell attachment by up to 123% compared to polished smooth Fe-Mn surfaces. Significant cell spreading and construction of cell multilayers were also apparent after 24 h, suggesting better adhesion. Additionally, static electrochemical polarization experiments revealed an improvement of up to 26% in the actual rate of biodegradation for Fe-Mn surface-modified materials. However, any residual concentration of zinc after leaching was shown to slightly increase corrosion resistance. The results demonstrate that selectively leached, nanostructured Fe-Mn surfaces have the potential of being tailored to a diverse set of transient implant scenarios, while also effectively boosting overall biocompatibility, initial cell attachment, and degradation rate.

Influence of riser design on macrosegregation in static castings

Abstract The flow between a casting and riser and the consequent macrosegregation development in an aluminium alloy is examined. The riser is designed using Campbell’s rules and solidification is simulated. The influence of cooling conditions and riser shape on macrosegregation patterns is studied. The relative importance of buoyancy and shrinkage induced flows is also simulated. For castings with a fixed riser shape, the average composition in the cast part increases due to the exchange of interdendritic fluid between the riser and the part. This exchange is smaller at higher solidification rates and when the bottom of the mould is chilled. Although the buoyancy induced flow does move some interdendritic liquid from the riser to the casting, shrinkage is more significant in increasing its average composition. A wider riser provides a barrier to flow, slowing the introduction of heavier interdendritic liquid from the riser into the casting, keeping it closer to the nominal composition.

Diffusional analysis of a multiphase oxide scale formed on a Mo-Mo3Si-Mo5-SiB2 alloy

Diffusional analyses were performed to understand the oxidation at 1300 °C of a multiphase Mo-13.2Si-13.2B (at.%) alloy. During oxidation, a protective glass scale formed with an intermediate layer of (Mo+glass) between the base alloy and external glass scale. Compositional profiles across the (Mo+glass) layer and the external glass scale were determined, and interdiffusion fluxes and effective interdiffusion coefficients for the various components were determined by using “MultiDiFlux” software. The motion of the (alloy/Mo+glass) and (Mo+glass/glass) interphase boundaries after passivation was examined. Additionally, vapor-solid diffusion experiments at 1300 °C were carried out with single-phase Mo3Si and T2 specimens in addition to a multiphase Mo-10Si-10B (at.%) alloy. These specimens were exposed to vacuum to induce silicon loss resulting in the formation of a Mo layer. An average effective interdiffusion coefficient of Si in Mo at 1300 °C was estimated from the Mo3Si-vapor couple to be in the order of 8×10−17 m2/s.

Precipitation of Al3Zr Dispersoids during Homogenization of Al-Zn-Cu-Mg-Zr Alloys

Dispersoids of Al3Zr in Al-Zn-Cu-Mg-Zr alloys are important as they pin grain boundaries and inhibit recrystallization during extrusion and solution heat treatment. A high volume fraction, \({V_{A{l_3}Zr}}\) , and low dispersoid radius, \({r_{A{l_3}Zr}}\), give \({V_{A{l_3}Zr}}/{r_{A{l_3}Zr}}\) values above a critical value which are essential to prevent recrystallization. Precipitation of Al3Zr during homogenization is studied for different homogenization schedules. A 1D finite difference model based on classical nucleation and growth theory for a multicomponent system is developed to simulate precipitation of Al3Zr dispersoids. The 1D domain represents one half of a secondary dendrite arm spacing with initial concentration gradients based on Scheil type solidification. Single-step homogenization, slow heating to homogenization temperatures, and two-step homogenization are evaluated for maximum dispersoid number density, maximum volume fraction and minimum dispersoid radius. V/r ratios along the length of the SDAS are compared. Two step homogenization (10hrs at 420°C and more than 10hrs at 475°C) provides better V/r ratios across the grain and provides the maximum recrystallization resistance.