On the Prediction of <Emphasis Type="Italic">α</Emphasis>-Martensite Temperatures in Medium Manganese Steels

A new composition-based method for calculating the α-martensite start temperature in medium manganese steel is presented and uses a regular solution model to accurately calculate the chemical driving force for α-martensite formation, \( \Delta G_{\text{Chem}}^{\gamma \to \alpha } \). In addition, a compositional relationship for the strain energy contribution during martensitic transformation was developed using measured Young’s moduli (E) reported in literature and measured values for steels produced during this investigation. An empirical relationship was developed to calculate Young’s modulus using alloy composition and was used where dilatometry literature did not report Young’s moduli. A comparison of the \( \Delta G_{\text{Chem}}^{\gamma \to \alpha } \) normalized by dividing by the product of Young’s modulus, unconstrained lattice misfit squared (δ ²), and molar volume (Ω) with respect to the measured α-martensite start temperatures, \( M_{\text{S}}^{\alpha } \), produced a single linear relationship for 42 alloys exhibiting either lath or plate martensite. A temperature-dependent strain energy term was then formulated as \( \Delta G_{\text{str}}^{\gamma \to \alpha } \left( {{\text{J}}/{\text{mol}}} \right) = E\varOmega \delta^{2} (14.8 - 0.013T) \), which opposed the chemical driving force for α-martensite formation. \( M_{\text{S}}^{\alpha } \) was determined at a temperature where \( \Delta G_{\text{Chem}}^{\gamma \to \alpha } + \Delta G_{\text{str}}^{\gamma \to \alpha } = 0 \). The proposed \( M_{\text{S}}^{\alpha } \) model shows an extended temperature range of prediction from 170 K to 820 K (?103 °C to 547 °C). The model is then shown to corroborate alloy chemistries that exhibit two-stage athermal martensitic transformations and two-stage TRIP behavior in three previously reported medium manganese steels. In addition, the model can be used to predict the retained γ-austenite in twelve alloys, containing ε-martensite, using the difference between the calculated \( M_{\text{S}}^{\varepsilon } \) and \( M_{\text{S}}^{\alpha } \).     

Twin Interactions in Pure Ti Under High Strain Rate Compression

Twin interactions associated with {11\( \overline{2} \)1} (E2) twins in titanium deformed by high strain rate (~2600 s^?1) compression were studied using electron backscatter diffraction technique. Three types of twins, {10\( \overline{1} \)2} (E1), {11\( \overline{2} \)2} (C1), and {11\( \overline{2} \)4} (C3), were observed to interact with the preformed E2 twins in four parent grains. The E1 variants nucleated at twin boundaries of some E2 variants. And the C3 twins were originated from the intersection of C1 and E2. The selection of twin variant was investigated by the Schmid factors (SFs) and the twinning shear displacement gradient tensors (DGTs) calculations. The results show that twin variants that did not follow the Schmid law were more frequently observed under high strain rate deformation than quasi-static deformation. Among these low-SF active variants, 73 pct (8 out of 11) can be interpreted by DGT. Besides, 26 variants that have SF values close to or higher than their active counterparts were absent. Factors that may affect the twin variant selections were discussed.     

Effects of Interfacial Lattice Mismatching on Wetting of Ni-Plated Steel by Magnesium

In this study, wetting has been characterized by measuring the contact angles of AZ92 Mg alloy on Ni-electroplated steel as a function of temperature. Reactions between molten Mg and Ni led to a contact angle of about 86 deg in the temperature range of 891 K to 1023 K (618 °C to 750 °C) (denoted as Mode I) and a dramatic decrease to about 46 deg in the temperature range of 1097 K to 1293 K (824 °C to 1020 °C) (denoted as Mode II). Scanning and transmission electron microscopy (SEM and TEM) indicated that AlNi + Mg₂Ni reaction products were produced between Mg and steel (Mg-AlNi-Mg₂Ni-Ni-Fe) in Mode I, and just AlNi between Mg and steel (Mg-AlNi-Fe) in Mode II. From high resolution TEM analysis, the measured interplanar mismatches for different formed interfaces in Modes I and II were \( 17{\kern 1pt} \;{\text{pct}}_{{\{ 10\overline 11\}_{\text{Mg}} //\{ 110\}_{\text{AlNi}} }} \)-\( 104.3\;{\text{pct}}_{{\{ 110\}_{\text{AlNi}} //\left\{ {10\overline{1}0} \right\}_{{{\text{Mg}}_{ 2} {\text{Ni}}}} }} \)-\( 114\,{\text{pct}}_{{\left\{ {0003} \right\}_{{{\text{Mg}}_{ 2} {\text{Ni}}}} //\{ 111\}_{\text{Ni}} }} \) and \( 18\,{\text{pct}}_{{\{ 10\overline 11\}_{\text{Mg}} //\{ 110\}_{\text{AlNi}} }} \)-\( 5\,{\text{pct}}_{{\left\{ {110} \right\}_{\text{AlNi}} //\{ 110\}_{\text{Fe}} }} \), respectively. An edge-to-edge crystallographic model analysis confirmed that Mg₂Ni produced larger lattice mismatching between interfaces with calculated minimum interplanar mismatches of \( 16.4\,{\text{pct}}_{{{\text{\{ 10}}\overline 1 1 {\text{\} }}_{\text{Mg}} / / {\text{\{ 110\} }}_{\text{AlNi}} }} \)-\( 108.3\,{\text{pct}}_{{{\text{\{ 110\} }}_{\text{AlNi}} / / {\text{\{ 10}}\overline 1 1 {\text{\} }}_{{{\text{Mg}}_{ 2} {\text{Ni}}}} }} \)-\( 17.2\,{\text{pct}}_{{{\text{\{ 10}}\overline 1 1 {\text{\} }}_{{{\text{Mg}}_{ 2} {\text{Ni}}}} / / {\text{\{ 100\} }}_{\text{Ni}} }} \) for Mode I and \( 16.4\,{\text{pct}}_{{{\text{\{ 10}}\overline1 1 {\text{\} }}_{\text{Mg}} / / {\text{\{ 110\} }}_{\text{AlNi}} }} \)-\( 0.6\,{\text{pct}}_{{{\text{\{ 111\} }}_{\text{AlNi}} / / {\text{\{ 111\} }}_{\text{Fe}} }} \) for Mode II. Therefore, it is suggested that the poor wettability in Mode I was caused by the existence of Mg₂Ni since AlNi was the immediate layer contacting molten Mg in both Modes I and II, and the presence of Mg₂Ni increases the interfacial strain energy of the system. This study has clearly demonstrated that the lattice mismatching at the interfaces between reaction product(s) and substrate, which are not in direct contact with the liquid, can greatly influence the wetting of the liquid.     

Investigating the Applicability and Limitations of Glass-Forming Criteria Based on Bond Parameters on Thermal Stability in Mg-Based Multicomponent Bulk Metallic Glasses

In this paper, correlation of thermal stability (\( \Delta {T_x} \)) with the bond parameters such as electronegativity (\( \Delta x \)), atomic radius mismatch (\( \delta \)), and valence electron concentration (\( \Delta {n^{1/3}} \)) for Mg-based multicomponent bulk metallic glasses (BMGs) have been evaluated. A statistical approach of regression analysis has been adopted to investigate correlations among these parameters. Available experimental data have been used for the systematic investigation from ternary to multicomponent Mg-based BMGs. In addition, the applicability of the criteria has been assessed for the systems with and without rare earth (RE) elements. We have found that BMG systems containing RE group elements have significant effect on width of supercooled liquid region. Results obtained from our modified empirical equation have been compared with that of earlier models and have shown better correlations.     

Molecular Dynamics Simulation of Diffusion of Fe in HCP Ti Lattice

Diffusion of a single Fe atom in a defect free hcp Ti lattice was studied using molecular dynamics (MD) simulation. Modified Embedded Atom Method potentials derived by Sa et al. (Scripta Mater 59:595, 2008) were used for carrying out the MD simulations. These potentials were verified by estimating the physical properties of the Fe–Ti system such as cohesive energy, bulk modulus and the shear constants. Fe atom trajectory in Ti lattice was traced in the temperature range of 500–900 °C. Diffusivity of Fe (\( D_{Fe} \)) atom in Ti lattice was obtained from the estimation of mean square displacement for total time of simulation at each temperature. \( D_{Fe} \) at 900 °C was obtained as 7.784?×?10^?15 m²/s. From the Arrhenius analysis of \( \ln D_{Fe} \) versus temperature, the activation energy required for the diffusion of Fe atom in hcp Ti lattice was obtained as 117 kJ/mol.     

Distribution Ratios of Phosphorus Between CaO-FeO-SiO<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Na<Subscript>2</Subscript>O/TiO<Subscript>2</Subscript> Slags and Carbon-Saturated Iron

In order to effectively enhance the efficiency of dephosphorization, the distribution ratios of phosphorus between CaO-FeO-SiO₂-Al₂O₃/Na₂O/TiO₂ slags and carbon-saturated iron (\( L_{\text{P}}^{\text{Fe-C}} \)) were examined through laboratory experiments in this study, along with the effects of different influencing factors such as the temperature and concentrations of the various slag components. Thermodynamic simulations showed that, with the addition of Na₂O and Al₂O₃, the liquid areas of the CaO-FeO-SiO₂ slag are enlarged significantly, with Al₂O₃ and Na₂O acting as fluxes when added to the slag in the appropriate concentrations. The experimental data suggested that \( L_{\text{P}}^{\text{Fe-C}} \) increases with an increase in the binary basicity of the slag, with the basicity having a greater effect than the temperature and FeO content; \( L_{\text{P}}^{\text{Fe-C}} \) increases with an increase in the Na₂O content and decrease in the Al₂O₃ content. In contrast to the case for the dephosphorization of molten steel, for the hot-metal dephosphorization process investigated in this study, the FeO content of the slag had a smaller effect on \( L_{\text{P}}^{\text{Fe-C}} \) than did the other factors such as the temperature and slag basicity. Based on the experimental data, by using regression analysis, \( \log L_{\text{P}}^{\text{Fe-C}} \) could be expressed as a function of the temperature and the slag component concentrations as follows:

$$ \begin{aligned} \log L_{\text{P}}^{\text{Fe-C}} & = 0.059({\text{pct}}\;{\text{CaO}}) + 1.583\log ({\text{TFe}}) - 0.052\left( {{\text{pct}}\;{\text{SiO}}_{2} } \right) - 0.014\left( {{\text{pct}}\;{\text{Al}}_{2} {\text{O}}_{3} } \right) \\ \, & \quad + 0.142\left( {{\text{pct}}\;{\text{Na}}_{2} {\text{O}}} \right) - 0.003\left( {{\text{pct}}\;{\text{TiO}}_{2} } \right) + 0.049\left( {{\text{pct}}\;{\text{P}}_{2} {\text{O}}_{5} } \right) + \frac{13{,}527}{T} - 9.87. \\ \end{aligned} $$

     

Monte Carlo Simulation Study of Diffuse Scattering in PZT,Pb(Zr,Ti)O<Subscript>3</Subscript>

Transverse polarized diffuse streaks have been observed in diffraction patterns of Pb(Zr_1?x Ti _x )O₃ (PZT) ceramics for compositions ranging from x = 0.3 (rhombohedral phase) to x = 0.7 (tetragonal phase) including the important morphotropic phase boundary (MPB) region (x = 0.48). The streaks correspond to diffuse planes of scattering in three dimensions, and these are oriented normal to the (cubic) \( \langle 111\rangle_{c} \) directions. A Monte Carlo (MC) model has been developed that convincingly reproduces the observed diffraction patterns. In this model, the displacements of Pb ions running in chains along each of the \( \langle 111\rangle_{c} \) directions are directed along the chain and are strongly correlated from cell to cell. There is no evidence of lateral correlation. Neighboring chains are essentially independent. At this stage, it is not clear what role the local order revealed by the scattering might play in governing the exceptional piezo-electric properties of the material, but its presence requires the currently accepted models for the average structure to be reassessed.     

Dephosphorization Kinetics between Bloated Metal Droplets and Slag Containing FeO: The Influence of CO Bubbles on the Mass Transfer of Phosphorus in the Metal

Dephosphorization kinetics of bloated metal droplets was investigated in the temperature range from 1813 K to 1913 K (1540 °C to 1640 °C). The experimental results showed that the overall mass transfer coefficient, \( {k_{\text{o}}} \), decreased with increasing temperature because of decreasing phosphorus partition ratio, \( {L_{\text{P}}} \). It was also found that the mass transfer coefficient for phosphorus in the metal, \( {k_{\text{m}}} \), had the highest value at the lowest temperature [i.e., 1813 K (1540 °C)] because the formation of smaller CO bubbles increased the rate of surface renewal, leading to faster mass transport. Meanwhile, metal droplets without carbon were also employed to study the effect of decarburization on dephosphorization. The results show that although decarburization lowers the driving force significantly, \( {k_{\text{m}}} \) (6.2 × 10^?2 cm/s) for a carbon containing droplet is two orders of magnitude higher than that for carbon free droplets (5.3 × 10^?4 cm/s) because of the stirring effect provided by CO bubbles. This stirring offers a faster surface renewal rate, which surpasses the loss of driving force and then leads to a faster dephosphorization rate.     

Characterization and Optimization of Ni-WC Composite Weld Matrix Deposited by Plasma-Transferred Arc Process

This work is dedicated to optimization of carbide particle system in a weld bead deposited by PTAW technique over D2 tool steel with high chromium content. The paper reports partial melting of the original carbide grains of the Ni-based filling powder, and growing of the secondary carbide phase (Cr, Ni)\(_3\)W\(_3\)C in the form of dendrites with wide branches that enhanced mechanical properties of the weld. The optimization of bead parameters was made with design of experiment methodology complemented by a complex sample characterization including SEM, EDXS, XRD, and nanoindentation measurements. It was shown that the preheat of the substrate to a moderate temperature 523 K (250 \(^\circ \)C) establishes linear pattern of metal flow in the weld pool, resulting in the most homogeneous distribution of the primary carbides in the microstructure of weld bead.     

Sulfate Formation and Decomposition of Nickel Concentrates

Nickel sulfide concentrates from two Canadian nickel concentrators were investigated to improve the understanding of SO₂ formation and release during processing. The concentrates were heated in gases of various oxygen concentrations up to 1573 K (1300 °C) in a thermal gravimetric analysis unit to simulate what may take place during calcine collection and processing. The resulting SO₂ gases were also measured. It was determined that during oxidation, there are competing reactions, such as \( 3{\text{FeS}} + 5{\text{O}}_{2} = {\text{Fe}}_{3} {\text{O}}_{4} + 3{\text{SO}}_{2} \) leading to mass loss, or \( 2{\text{FeS}} + 5{\text{O}}_{2} + {\text{SO}}_{2} = {\text{Fe}}_{2} \left( {{\text{SO}}_{4} } \right)_{3} \) causing mass gain. At temperatures up to approximately 973 K (700 °C), sulfates were formed readily, whereas at higher temperatures, they would decompose, evolving SO₂. By lowering the oxygen content in the surrounding gas, the sulfates decomposed more readily. In an argon or hydrogen atmosphere or in vacuum, it is possible to enhance the sulfate decomposition greatly, possibly allowing for reduced SO₂ emissions from the electric furnaces.     

Texture and Crystal Orientation in Ti-6Al-4V Builds Fabricated by Shaped Metal Deposition

The texture and crystal orientation of Ti-6Al-4V components, manufactured by shaped metal deposition (SMD), is investigated. SMD is a novel rapid prototyping tungsten inert gas (TIG) welding technique leading to near-net-shape components. This involves sequential layer by layer deposition with repeated partial melting and heat treatment, which results in epitaxial growth of large elongated prior β grains. This leads to a directionally solidified texture, where the prior β grains exhibit only a small misorientation with each other. The β grains grow in \( \left\langle { 100} \right\rangle \) direction with a second \( \left\langle { 100} \right\rangle \) direction perpendicular to the wall surface. During cooling, the α phase transformation follows the Burgers orientation relationship leading to a Widmanstätten structure, with orientation relations between most of the α lamellae and also of the residual β phase. The directionally solidification and the transformation into the α phase following the Burgers relationship results in a texture, where the hcp pole figures look similar to bcc pole figures.     

Effect of Tungsten on Primary Creep Deformation and Minimum Creep Rate of Reduced Activation Ferritic-Martensitic Steel

Effect of tungsten on transient creep deformation and minimum creep rate of reduced activation ferritic-martensitic (RAFM) steel has been assessed. Tungsten content in the 9Cr-RAFM steel has been varied between 1 and 2 wt pct, and creep tests were carried out over the stress range of 180 and 260 MPa at 823 K (550 °C). The tempered martensitic steel exhibited primary creep followed by tertiary stage of creep deformation with a minimum in creep deformation rate. The primary creep behavior has been assessed based on the Garofalo relationship, \( \varepsilon = \varepsilon_{\text{o}} + \varepsilon_{\text{T}} [1-\exp (-r^{\prime} \cdot t)] + \dot{\varepsilon }_{\text{m}} \cdot t \) , considering minimum creep rate \( \dot{\varepsilon }_{\text{m}} \) instead of steady-state creep rate \( \dot{\varepsilon }_{\text{s}} \) . The relationships between (i) rate of exhaustion of transient creep r′ with minimum creep rate, (ii) rate of exhaustion of transient creep r′ with time to reach minimum creep rate, and (iii) initial creep rate \( \dot{\varepsilon }_{\text{i}} \) with minimum creep rate revealed that the first-order reaction-rate theory has prevailed throughout the transient region of the RAFM steel having different tungsten contents. The rate of exhaustion of transient creep r′ and minimum creep rate \( \dot{\varepsilon }_{\text{m}} \) decreased, whereas the transient strain ? _T increased with increase in tungsten content. A master transient creep curve of the steels has been developed considering the variation of \( \frac{{\left( {\varepsilon - \varepsilon_{\text{o}} } \right)}}{{\varepsilon_{\text{T}} }} \) with \( \frac{{\dot{\varepsilon }_{\text{m}} \cdot t}}{{\varepsilon_{\text{T}} }} \) . The effect of tungsten on the variation of minimum creep rate with applied stress has been rationalized by invoking the back-stress concept.     

Isothermal Reaction of NiO Powder with Undiluted CH<Subscript>4</Subscript> at 1000 K to 1300 K (727 °C to 1027 °C)

In this study, isothermal reaction behavior of loose NiO powder in a flowing undiluted CH₄ atmosphere at the temperature range 1000 K to 1300 K (727 °C to 1027 °C) is investigated. Thermodynamic analyses at this temperature range revealed that single phase Ni forms at the input \( {{n_{{{\text{CH}}_{ 4} }}^{\text{o}} } \mathord{\left/ {\vphantom {{n_{{{\text{CH}}_{ 4} }}^{\text{o}} } {\left( {n_{{{\text{CH}}_{ 4} }}^{\text{o}} + n_{\text{NiO}}^{\text{o}} } \right)}}} \right. \kern-0pt} {\left( {n_{{{\text{CH}}_{ 4} }}^{\text{o}} + n_{\text{NiO}}^{\text{o}} } \right)}} \) mole fractions (\( X_{{{\text{CH}}_{ 4} }} \)) between ~0.2 and 0.5. It was also predicted that free C co-exists with Ni at \( X_{{{\text{CH}}_{ 4} }} \) values higher than ~0.5. The experiments were carried out as a function of temperature, time, and CH₄ flow rate. Mass measurement, XRD and SEM-EDX were used to characterize the products at various stages of the reaction. At 1200 K and 1300 K (927 °C and 1027 °C), the reaction of NiO with undiluted CH₄ essentially consisted of two successive distinct stages: NiO reduction and pyrolytic C deposition on pre-reduced Ni particles. At 1200 K (927 °C), 1100 K (827 °C), and 1000 K (727 °C), complete oxide reduction was observed within ~7.5, ~17.5, and ~45 minutes, respectively. It was suggested that NiO was essentially reduced to Ni by a CH₄ decomposition product, H₂. Possible reactions leading to NiO reduction were suggested. An attempt was made to describe the NiO reduction kinetics using nucleation-growth and geometrical contraction models. It was observed that the extent of NiO reduction and free C deposition increased with the square root of CH₄ flow rate as predicted by a mass transport theory. A mixed controlling mechanism, partly chemical kinetics and partly external gaseous mass transfer, was responsible for the overall reaction rate. The present study demonstrated that the extent of the reduction can be determined quantitatively using the XRD patterns and also using a formula theoretically derived from the basic XRD data.     

Investigating the Dislocation-Driven Micro-mechanical Response Under Non-isothermal Creep Conditions in Single-Crystal Superalloys

The creep responses of the superalloy CMSX-4 under thermal cycling conditions (900 °C to 1050 °C) and constant load (\( \sigma_{0} = 200 MPa \)) were analyzed using TEM dislocation analysis and compared to the modeled evolution of key creep parameters. By studying tests interrupted at different stages of creep, it is argued that the thermal cycling creep rate under these conditions depends on the creation of interfacial dislocation networks and their disintegration by the γ′-shear of dissimilar Burgers vector pairs.     

The Influence of Heat Treatment on Nanoscale Microstructure and Crystal Orientation of 7055 Aluminum Alloy Before and After High-Speed Milling

This paper is intended to examine changes in the microstructure and crystal orientation of 7055 aluminum alloy before and after cutting. Single-factor cutting speed test was designed and implemented to investigate the influence of three heat treatment processes, T6, T87 and T815, on the microstructure and crystal orientation of 7055 aluminum alloy before and after cutting. Results showed that, before cutting, T6-state microstructure had uniform grain size with pinning in θ′ phase; T815-state grains were obviously elongated as a result of predeformation; T87-state grains also displayed some elongation, but their overall elongation was not as long as that of T815-state grains; there was a dislocation in the TEM microstructure after both T87 and T815. After cutting, T6-state initial grains were elongated; their horizontal and longitudinal sizes were 46 and 92 μm, and the low-angle boundary (LAB) and high-angle boundary (HAB) densities of T6, T87 and T815-state grains were \(1. 8 5\times 10^{ - 1}\), \(3. 2 5\times 10^{ - 2}\), \(1. 2\times 10^{ - 1}\), \(2. 2\times 10^{ - 2}\), \(2. 5\times 10^{ - 1}\) and \(4. 3\times 10^{ - 2}\) μm^?1. The crystal structure and orientation relationship of T6-state alloy after aged for 4, 8 and 12 h was θ′′, θ′, and many mixed regions of θ′′ and θ′, were observed along {001}α. After aged for 12 h, the T8-state microstructure along [001]α and [011]α was roughly the same as that after aged for 4 h, except that the share of θ′ particles along [011]α had increased from 55 to 90% while θ′′ particles along [001]α had reduced a little. After aged for 12 h, the precipitated particles of the cutting layer of T815-state alloy along [001]α were all θ′ phase while those along [011]α were composed of θ′ and Ω phases. From the boundary microstructure, before cutting, the grain boundary of T6-state alloy was a continuous one with no obvious non-precipitate zone; the grain boundary of T87-state alloy displayed some discontinuity as a result of predeformation, and quite a lot of the precipitated particles were concentrated on the boundary; the grain boundary of T815-state alloy was a discontinuous one, but the non-precipitate zone on the boundary was not as wide as that of T87-state alloy. After cutting, T6-state alloy had the widest non-precipitate zone of all at about 42 nm. The non-precipitate zone of T6-state alloy was 25 nm wide, and the particles were mainly grown θ′ particles, and θ particles incoherent to the aluminum matrix. The non-precipitate zone of T815-state alloy was the narrowest at approximately 15 nm.     

Effect of Particle Size on the Deformation Behaviour of Sintered Al–TiC Nano Composites

Cold upsetting experiment was impeccably carried out on sintered Al–TiC preform to evaluate their deformation characterization. Effects of TiC content and aspect ratio of the preform on deformation behaviour were completely investigated by using Zinc stearate as a lubricant. Cylindrical preforms with different particle size at 5% (2 μm and ≤200 nm) and different aspect ratios (1.00 and 0.75) were prepared by using suitable die, on a 1.0 MN capacity hydraulic press and sintered in electrical muffle furnace for 1.5 h and followed by cooling the furnace to its room temperature itself. Analysis of the experimental data have proved the power law relationship between fractional theoretical density \(\left( {\frac{{\rho_{f} }}{{\rho_{th} }}} \right)\) and strain factor \(e^{{(\varepsilon_{z} - \varepsilon_{\theta } )}}\). Further, it was found that the preforms with low size of TiC content showed higher value of deformation properties such as axial stress and the Poisson’s ratio than high size of TiC preform, provided that the initial fractional density taken was kept constant.     

Microstructural Evolution in Ti-5111 Friction Stir Welds

The microstructural evolution occurring during friction stir welding of a near-α titanium alloy, Ti-5111, has been examined by backscattered electron imaging and electron backscatter diffraction. The unaffected baseplate (BP) microstructure consists of millimeter-scale prior β grains containing ~100 μm large colonies of aligned α laths, related to each other by a strain-accommodating Burgers orientation relationship. The α laths are separated by fine, 100 to 150-nm-thick, interlath β ribs. A heat-affected zone (HAZ) is observed ~1.5 to 2.5 mm from the tool surface, characterized by a thickening of the β ribs and the formation of secondary α platelets within them closer to the tool. There is a narrow thermomechanically affected zone (TMAZ), comprised of outer and inner regions, observed ~1.0 to 1.5 mm from the tool surface. Deformation is first observed in a ~200-μm-wide outer TMAZ, where the microstructure is refined through an increase in fine secondary (α laths) α laths and the lattice orientations rotate to align the close-packed \(\langle{11\bar{2}0}\rangle\) directions with the shear direction (SD). Continued deformation closer to the tool produces periodic shear bands within a ~300-μm-wide inner TMAZ, resulting in alternating regions of material that are deformed below and above the β transus. Material in the stir zone (SZ) within ~1 mm of the tool surface consists of fine (~10 to 20-μm diameter) equiaxed prior β grains that are delineated by ~500-nm-thick α and contain 150-500-nm thick α laths. The texture exhibits both \(D_1({\bar{1}\bar{1}{2}})[111]\) bcc and \({P_1({1}\bar{1}00)}[11\bar{2}0]\) hcp shear texture components, indicating that this material exceeded the β transus during welding.     

Effect of Multi-Scale Thermoelectric Magnetic Convection on Solidification Microstructure in Directionally Solidified Al-Si Alloys Under a Transverse Magnetic Field

The influence of a transverse magnetic field (B < 1 T) on the solidification structure in directionally solidified Al-Si alloys was investigated. Experimental results indicate that the magnetic field caused macrosegregation, dendrite refinement, and a decrease in the length of the mushy zone in both Al-7 wt pct Si alloy and Al-7 wt pct Si-1 wt pct Fe alloys. Moreover, the application of the magnetic field is capable of separating the Fe-rich intermetallic phases from Al-7 wt pct Si-1 wt pct Fe alloy. Thermoelectric magnetic convection (TEMC) was numerically simulated during the directional solidification of Al-Si alloys. The results reveal that the TEMC increases to a maximum ( \( u_{\rm{max} } \) ) when the magnetic field reaches a critical magnetic field strength ( \( B_{\rm{max} } \) ), and then decreases as the magnetic field strength increases further. The TEMC exhibits the multi-scales effects: the \( u_{\rm{max} } \) and \( B_{\rm{max} } \) values are different at various scales, with \( u_{\rm{max} } \) decreasing and \( B_{\rm{max} } \) increasing as the scale decreases. The modification of the solidification structure under the magnetic field should be attributed to the TEMC on the sample and dendrite scales.     

Development of Low-Fluoride Slag for Electroslag Remelting: Role of Li<Subscript>2</Subscript>O on the Viscosity and Structure of the Slag

The effect of the substitution of CaF₂ with Li₂O on the viscosity and structure of low-fluoride CaF₂-CaO-Al₂O₃-MgO slag was studied with an aim to develop low-fluoride slag for electroslag remelting. Increasing Li₂O addition up to 4.5 mass pct was observed to significantly reduce the slag viscosity monotonically. Increasing temperature significantly lowered the viscosity of slag, whereas this influence is less effective with increasing Li₂O content especially above 3.5 mass pct. The activation energy for viscous flow decreases with increasing Li₂O content. The polymerization degree of aluminate networks decreased with increasing Li₂O content, as demonstrated by Raman analysis. The dominant structural unit in [AlO₄]^5?-tetrahedral network is \( {\text{Q}}_{\text{Al}}^{4} \). The amount of symmetric Al-O⁰ stretching vibrations significantly decreased with increasing Li₂O content. The relative fraction of \( {\text{Q}}_{\text{Al}}^{4} \) in the [AlO₄]^5?-tetrahedral units shows a decreasing trend, whereas \( {\text{Q}}_{\text{Al}}^{2} \) increases with the increase in Li₂O content accordingly. The change in slag viscosity with chemistry variation agrees well with the changes in slag structural units.