Cathode Calcination In article number 2207076, Sugeun Jo, Keeyoung Jung, Jongwoo Lim, and co-workers show how solid-state reaction heterogeneity affects the high-temperature calcination of Li-ion battery particles. Various parallel and serial combinations of the thermal (an)aerobic decomposition and Li2–O insertion/diffusion are chemically imaged during the calcination reaction. The cover image features the calcination intermediates that facilitate the surface Li2–O incorporation, which is impeded by the anaerobically decomposed phase in the particle core.

Cathode Calcination In article number 2207076, Sugeun Jo, Keeyoung Jung, Jongwoo Lim, and co-workers show how solid-state reaction heterogeneity affects the high-temperature calcination of Li-ion battery particles. Various parallel and serial combinations of the thermal (an)aerobic decomposition and Li2–O insertion/diffusion are chemically imaged during the calcination reaction. The cover image features the calcination intermediates that facilitate the surface Li2–O incorporation, which is impeded by the anaerobically decomposed phase in the particle core.

During solid-state calcination, with increasing temperature, materials undergo complex phase transitions with heterogeneous solid-state reactions and mass transport. Precise control of the calcination chemistry is therefore crucial for synthesizing state-of-the-art Ni-rich layered oxides (LiNi_1-x-y Co_x Mn_y O₂ , NRNCM) as cathode materials for lithium-ion batteries. Although the battery performance depends on the chemical heterogeneity during NRNCM calcination, it has not yet been elucidated. Herein, through synchrotron-based X-ray, mass spectrometry microscopy, and structural analyses, it is revealed that the temperature-dependent reaction kinetics, the diffusivity of solid-state lithium sources, and the ambient oxygen control the local chemical compositions of the reaction intermediates within a calcined particle. Additionally, it is found that the variations in the reducing power of the transition metals (i.e., Ni, Co, and Mn) determine the local structures at the nanoscale. The investigation of the reaction mechanism via imaging analysis provides valuable information for tuning the calcination chemistry and developing high-energy/power density lithium-ion batteries.

• δ-to-γ phase transformation model for laser powder bed fusion was proposed. • Lower Cr eq /Ni eq and higher laser power accelerate δ-to-γ transformation. • Heat affected zone shows more phase transformation due to reheating by laser scans. • The model predicts printed microstructure with high accuracy (R 2 = 0.9405). In laser powder bed fusion (LPBF), complex and spatially varying thermal histories emerge due to repetitive laser scanning and localized heat accumulation, which complicate the prediction of microstructure evolution. These thermal histories result in heterogeneous microstructures composed of δ-ferrite, α’-martensite, and retained austenite in martensitic stainless steels fabricated by the LPBF. In particular, the volume fractions of α’-martensite and δ-ferrite are influenced by the δ-ferrite to γ-austenite transformation, which is affected by powder chemistry and local cooling rates. This study proposes a modeling framework to predict the δ-ferrite to γ-austenite transformation in a representative alloy. The numerical model integrates thermodynamic data and thermal simulations to perform time-dependent thermodynamic calculations based on classical nucleation and growth theories along simulated thermal histories. It successfully captures the phase transformation under ultrafast cooling conditions, showing strong agreement with experimental observations (R 2 = 0.9405). These findings offer a novel framework for understanding and controlling phase evolution in additively manufactured martensitic stainless steels, enabling site-specific microstructure control to optimize mechanical performance.

In this study, bimetallic structures composed of pure copper and stainless steel 630 (STS 630) were fabricated using powder bed fusion (PBF) to evaluate deposition quality, solidification cracking mechanisms, and mechanical properties. Optimal processing conditions were determined by analyzing mixed zone depth and interfacial characteristics under various parameters. Stable deposition without defects was achieved at low energy densities (≤0.15 J/mm). Intermediate energy densities (0.17–0.44 J/mm) resulted in deposition cracks without edge cracking, while high energy densities (≥0.5 J/mm) caused deposition cracks and edge cracks due to residual stress. Rapid cooling within the Fe–Cu mixed zone promoted liquid-phase separation into Fe-rich and Cu-rich phases via binodal and spinodal decomposition mechanisms. Deposition cracks were observed in compositions with 13–34 wt% Cu, driven by δ-Fe to γ-Fe transformation, solidification temperature ranges, and insufficient residual liquid. However, compositions with sufficient residual liquid mitigated crack susceptibility through an effective back-fill effect. Nanoindentation tests revealed gradually decreasing hardness distributions with increasing mixed zone depth; however, localized phase separation and cracking led to significant local hardness variations. Tensile tests indicated that fractures predominantly occurred in the pure copper region, whereas specimens with excessively deep mixed zones fractured prematurely at the interface or within the mixed zone. Furthermore, extensive phase separation and cracking markedly reduced elongation. This study underscores the critical importance of optimizing processing parameters, controlling liquid-phase separation and cracking, and carefully managing mixed zone depth to enhance the mechanical performance of bimetallic structures.