Iranian Journal of Materials Science and Engineering

Thermomechanical Analysis and Optimization of Residual Stresses in SS316L Components Fabricated by Directed Energy Deposition

Farnoosh Turki

The Directed Energy Deposition (DED) process is very sensitive to thermal environments, tends to produce residual stresses and geometric distortion. It is important to understand the influence of processing parameters on these effects in order to enhance build quality. The objective of this study is to control residual stress and distortion in the DED process by investigating baseplate thickness, number of layers deposited before laser interruption, dwell time, and laser power by a full factorial experiment design. The numerical results were validated by experimental measurement of residual stress using the X-ray diffraction (XRD) technique. The optimized processing conditions resulted in a 43% reduction in residual stress and a 33% decrease in dimensional distortion compared to the baseline setup. Among the four factors, baseplate thickness had the most significant effect, whereas dwell time had the least impact. To the best of the authors’ knowledge, the combined effect of baseplate thickness, dwell time, number of layers deposited before laser interruption, and laser power on residual stress and distortion in DED has not been previously investigated. The findings of this study provide a mathematical basis for future research aimed at optimizing process and material parameters in DED process.

Electrochemical properties for BaSr1-xGdxCo2O5+δ and Ba0.5Sr0.5-xGdxCoO3-δ as a cathode for intermediate-temperature solid oxide fuel cells

Sara Tafaroji

The structural and electrochemical properties of Gd-doped perovskite oxides were investigated to improve the performance of solid oxide fuel cell (SOFC) cathodes. Ba_0.5Sr_0.5-xGd_xCoO_3-δ and BaSr_1-xGd_xCo₂O_5+δ (BSGC) compounds were synthesized via a sol–gel thermolysis method to elucidate the effects of Gd incorporation on crystal structure, microstructure, and electrochemical activity. X-ray diffraction (XRD) and scanning electron microscopy (SEM) confirmed the coexistence of simple and double perovskite phases, with Gd substitution leading to finer grains (down to 0.4 ± 0.14 μm) and improved phase homogeneity. Area-specific resistance (ASR) and conductivity measurements revealed a strong structure–performance relationship. The optimal composition, Ba_0.5Sr_0.3Gd_0.2CoO_3-δ, exhibited an exceptionally low ASR of 0.12 Ω cm² at 700 °C which further decreased to a minimum of 0.04 Ω cm² at 850 °C, significantly outperforming GdBaCo₂O_5+δ (GBCO), (1.76 Ω cm²). These findings demonstrate that rational structural design through rare-earth doping effectively enhances oxygen transport and electrochemical activity, providing a promising pathway for high-performance intermediate-temperature SOFC cathodes.

Design and Optimisation of Corrosion Behaviour of Mg-Zn-Ca Alloy System Using Thermodynamic and Statistical Simulation

M. Reza Aboutalebi

In recent years, Mg- based alloys have been considered as a biodegradable biomaterial for implant applications. However, the high corrosion rate and hydrogen gas evolution in an aqueous environment are the most important challenges for these alloys. This study has focused on optimizing the biocorrosion properties of bioalloys in Mg-Zn-Ca system, using a combined approach of thermodynamic simulations, and response surface methodology (RSM). In, Mg₂Ca and Ca_xMg_yZn_z precipitates have a significant effect on corrosion mechanisms. It is believed that, in the Mg-Zn-Ca alloy system, the Mg₂Ca secondary phase usually enhances corrosion rate with microgalvanic coupling mechanisms. The ternary phases Ca_xMg_yZn_z have less detrimental effects on corrosion resistance. To minimize harmful phases, CompuTherm’s PANDAT software was utilized for phase evolution prediction, with outputs directly validated against experimental measurements. Potentiodynamic tafel polarization experiments and long-term immersion tests were conducted to evaluate the corrosion rate. In order to design an alloy with the least detrimental phase, a statistical model (RSM) was developed based on predicted results from thermodynamic model. The results showed that the ZX31 (Mg-2.6Zn-1Ca) exhibited the highest corrosion potential (-1.6336 V_Ag/AgCl) and the lowest corrosion current density (216 µA/cm²), showing higher biocorrosion resistance compared to ZX24 (Mg-2.3Zn-4.0Ca) and ZX15 (Mg-1.3Zn-4.8Ca).

Effect of Temperature-Time on simultaneous hydrogen reduction of molybdenum and tungsten oxides

Hossein Aghajani

The Mo-30 wt% W solid solution is a well-established molybdenum-based alloy. A recent approach to alloying Mo and W involves the simultaneous reduction of their oxides by hydrogen. Several studies have reported the production of Mo-W alloys via the reduction of mixed oxides. But the alloying mechanism and the effects of key reduction parameters on alloy formation remain uninvestigated. In the present study, the simultaneous hydrogen reduction of MoO₃–WO₃ mixed oxides was investigated as a potential route for producing pre-alloyed Mo–W powders. In this study pre-alloyed Mo-30 wt% W is. The effects of reduction temperature and holding time on phase evolution, morphology, and alloy formation were systematically examined. The chosen temperature range is 600-1050°C. The influence of time on phase formation and morphology during the hydrogen reduction of molybdenum and tungsten oxides is examined over a 30-360 minute period. Other parameters such as heating rate (20℃min ), bed height (10mm), H₂ flow rate (500mlmin ), and material weight (14.3g) were kept constant. The reduced samples are analyzed using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The effects of temperature and time on the reduction processes are closely linked. Results indicate the formation of MoO₂ and WO₂ phases at 600°C after 60 minutes of hydrogen reduction. No Oxide phases were detected in XRD analysis of the sample that was reduced at 800°C for 180 minutes. Therefore, tungsten and molybdenum coexist in all particles of the samples. Mass transfer between Mo and W takes place during the reduction of oxides to the metallic phase. But isothermal reduction does not yield a homogeneous alloy. In these studies, for T > 800°C, increasing time has minimal effect on alloying these elements. At T=800°C, a significant change in morphology occurs at t>180 min. At T=800°C, a notable morphological change occurs after t>180 min. These changes are caused by the CVT mechanism. The optimal conditions for achieving chemical homogeneity in the alloy powder particles after reduction were identified as: 600°C for 60 min, 600 to 1050 for 240 min, and 180 min at 1050°C. This cycle leads to the formation of homogeneous alloy powder particles. The achieved homogeneity is attributed to providing sufficient time for mass transport between molybdenum and tungsten oxide particles through gaseous chemical species formed within this temperature range.

Effect of Aging Duration on Mineral Composition, Microstructure, and Texture in a Malachite based Co precipitate and its Composite (Metallic) Oxidic Derivatives

Hajar Ghanbari

This study deals with the dependence of mineral, microstructural, and textural properties on the duration of the indispensable yet easily controlled step of aging. For this purpose, a set of Cu/Zn/Al precursors were synthesized using a conventional constant‑pH method. The precipitates were aged for three different durations of zero, two, and four hours. The obtained precipitates, after several washing cycles and overnight drying, were then calcined at 623 K. Precursor and calcined samples were studied in terms of elemental composition, mineral composition, microstructure, texture, porosity, and metallic copper surface area. The results suggest that the dependence of precursor surface area on aging is realized by its dependence on phase composition. In the precipitate aged for two hours, the relative dominance of malachite leads to the highest pre‑calcination surface area of 57 m²/g. The individual thermal behaviors of the precursor phases and their pre‑calcination surface area values seem to conjointly determine the post‑calcination surface area of the samples. Such that a combination of ripened particle structure and susceptibility to calcination induced surface deterioration yields the lowest post‑calcination surface area after aging for four hours. The calcines derived from the unaged and the two‑hour‑aged precursors yield surface area values of approximately 57–58 m²/g, sensibly higher than that of the four‑hour‑aged sample. The Cu⁰ surface area values of the samples depend on aging duration through the effect it imparts on the Cu/Zn‑interdispersion of the precursor malachite phase. The highest Cu/Zn‑interdispersion is achieved in the two‑hour‑aged sample that finally exhibited a maximum Cu⁰ surface of 112 m²/g_Cu.

Effect of Twin Boundary Density on Mechanical Response and Dislocation Evolution in a Nickel-Based Superalloy: A Molecular Dynamics Study

mojtaba zolfaghari

Twin-boundary engineering provides an effective approach for tailoring the mechanical response of nickel-based superalloys; however, the density-dependent atomistic mechanisms remain insufficiently understood in explicit dual-phase γ/γ′ microstructures. Molecular dynamics simulations were performed on a single-crystal model and models containing one, two, and eight twin boundaries oriented perpendicular to the loading direction. The tensile response was analyzed in conjunction with dislocation-density evolution, microstructural changes, and Constructed-surface-mesh analysis to characterize crack initiation and growth. The TB1 model exhibited higher yield stress and strain than the single-crystal model because the isolated twin boundary impeded dislocation motion. In contrast, TB2 and TB8 yielded at lower stresses and strains because the increased twin-boundary density introduced additional preferential nucleation sites at the twin boundaries and twin-boundary/γ–γ′ interface intersections. Despite its earlier yielding, TB2 exhibited the highest ultimate stress among all models and the highest ultimate strain among the twinned models. This behavior was attributed to deformation partitioning between two comparatively stable twin boundaries, which promoted distributed precipitate shearing, dislocation storage, and sustained strain hardening. TB8 exhibited the highest initial dislocation density, followed by a decrease during plastic deformation associated with twin-boundary migration, defect rearrangement, and strain localization. Constructed-surface-mesh analysis further showed that crack initiation was delayed to a strain of approximately 0.07356 in TB2, compared with approximately 0.066 in TB1 and TB8. Crack propagation occurred predominantly along the twin boundaries and γ/γ′ phase interfaces. These findings reveal a non-monotonic, density-dependent transition from barrier-controlled strengthening in TB1 to deformation-partitioning-assisted hardening in TB2 and boundary-migration-assisted, localization-dominated softening in TB8.

Enhancing Dielectric and Ferroelectric Properties in PFN–PT Ceramics via (Ca,Sr)ZrO3 Modification for Advanced Electronic Applications

marjaneh jafari fesharaki

The structural, dielectric, and ferroelectric properties of (1−x)PFN–PT/x(Ca,Sr)ZrO₃ ceramics (x = 0–0.08) were systematically investigated. X-ray diffraction analysis confirmed the formation of a single-phase perovskite structure, while Rietveld refinement revealed a gradual reduction in the monoclinic lattice distortion with increasing CZ and SZ contents, indicating structural evolution toward a pseudocubic-like state near the morphotropic phase boundary (MPB). Quantitative FE-SEM analysis showed dense microstructures with progressive grain refinement at higher modifier concentrations. Temperature-dependent dielectric measurements exhibited the highest dielectric constant for the x = 0.04 compositions, whereas the modified Curie–Weiss analysis confirmed diffuse phase transition and relaxor behavior, with γ values of 1.94 and 1.68 for PFN–PT:0.04CZ and PFN–PT:0.04SZ, respectively. Room-temperature ferroelectric measurements yielded maximum remanent polarization (P_r) values of 24.32 and 34.26 μC cm⁻² for the CZ- and SZ-modified ceramics at x = 0.04. The enhanced dielectric and ferroelectric properties are closely correlated with the reduced monoclinic distortion near the MPB, highlighting the crucial role of structural optimization in improving the functional performance of PFN–PT-based ceramics.