Microstructure and Mechanical Behavior of Structural Materials: 2nd Edition

Metallic materials are the workhorses of structural applications and remain central to modern engineering across energy, transportation, process equipment, and extreme-environment service. Their mechanical performance is governed by microstructure, defined by the spatial distribution of phases, defects, and interfaces that arise from alloy chemistry, thermomechanical history, and joining and surface treatments. Because these features are path-dependent, modest changes in processing or environment can reconfigure phase fractions, grain morphology, residual stress, and interfacial chemistries, with first-order effects on strength, toughness, fatigue resistance, creep, and corrosion. Meeting performance targets under tighter carbon, cost, and time constraints requires treating microstructure as a design variable with explicitly quantified uncertainty. This in turn demands explicit processing pedigrees, multiscale characterization tied to full stress–strain and fracture evidence, and mechanistic or interpretable data-driven models that enable inverse design and reproducible mechanical property windows. As a continuation of our first edition [1], this Special Issue emphasizes understanding and engineering the microstructure–property nexus to deliver reliable and efficient components across scales and sectors.

This second edition of our Special Issue gathers 10 original research articles and 1 review, spanning ferrous alloys, medium-entropy and TiAl systems, joining and surface engineering, corrosion, high-temperature behavior, process modeling, and data-driven design. Together, the papers illustrate how experiments, simulations, and machine learning are converging to accelerate microstructure control and property optimization.

Within steels, phase transformation pathways and tailored processing windows are used to balance strength and toughness. Guo et al. [2] mapped static and dynamic continuous cooling transformation (CCT) curves for a Ce-micro alloyed 30MnNbRE steel and showed that dynamic cooling at 50 °C/s after hot sizing increases hardness by 56.7 HV5 and the martensite fraction, while lowering the deformation temperature broadens the bainitic region, indicating a practical route to tougher oil-well tubing. Liang et al. [3] examine chrysanthemum-like pearlite formed during the aging of 100Mn13 high-C–Mn steel, identifying M₇C₃/ferrite lamellae with a specific orientation relationship and proposing a growth model to rationalize the emanative colony morphology, useful for tuning wear resistance under moderate impact. Cheng et al. [4] couple flow, thermal, and stress simulations with plant observations to re-engineer roll cooling: an optimized channel design reduces the roll-surface T_max from 810 K to 591 K, cuts the circumferential ΔT by 38%, and lowers the peak equivalent stress from 791 MPa to 558 MPa, mitigating thermal-fatigue damage routes. In sour service environments, Sun et al. [5] investigate pitting at the weld heat-affected zone (HAZ) of Incoloy825/X65 bimetal in suspended sulfur solutions. They report a 7–13 µm sulfide-rich film (NiS, FeS, Cr₂S₃), time-dependent changes in pitting resistance, and a film-mediated barrier effect on anion transport, evidence that local welding metallurgy dominates corrosion initiation. Refill friction stir spot welding (RFSSW) appears in two complementary contributions. For pure copper, Ge et al. [6] show that, in pure-copper RFSSW, tool speed governs defects, hook shape, micro-hardness across S- and P-zone/HAZ, and the force–torque trace, defining a process–microstructure–property map. In AA6061, Jiang et al. [7] combine stop-action metallography with finite-element simulation to show that the dwell stage controls peak temperature and refill, identify center-slow and rim-fast metal flow, and propose a ‘spiral suction–refill injection layer-stacking’ mechanism akin to complete friction plug riveting. For turbine-class Ni-base materials, Cheng et al. [8] assess K417G superalloy joints fabricated by wide-gap brazing. Above 1000 °C, the brazed region exhibits superior oxidation resistance to the base metal, with parabolic kinetics. Mechanistically, they track oxide-scale stratigraphy and NiO spallation in coarse-grained base metal versus more stable Cr₂O₃/Al₂O₃ (and TiO₂) layers, implicating grain-coarsening-induced scale destabilization at high temperature. Zhang et al. [9] examine a CoCr_0.4NiSi_0.3 medium-entropy alloy from room temperature to −150 °C. At −150 °C, they report YS 618 MPa, UTS 1055 MPa, and elongation 21%, tied to metastable phase evolution (tetragonal/orthorhombic precipitates, coherent hcp, L1₂ and LPSO structures) and Shockley partials that form nanometric APBs within the L1₂ phase, rarely documented in the cryogenic deformation of metals. Ishtiaq et al. [10] build an ANN predictor for 5Cr–0.5Mo steels across service temperatures, achieving low errors (3.5% YS, 0.97% UTS, 1.9% elongation) and providing a graphical user interface (GUI) to back-calculate the composition for target properties, an example of experiment-efficient alloy design. Complementing this, Liu and Liang [11] use SHapley Additive exPlanations (SHAP) analysis over random-forest regression to clarify elemental main effects and interactions governing room-temperature tensile properties in cast TiAl: B, C, and Nb predominantly raise YS/UTS, while Cr, Mn, and Al tend to improve elongation, an interpretable blueprint for composition navigation.

Finally, Meng et al. [12] review wear-resistant coatings on copper and Cu-alloys, comparing one-step routes (laser cladding, electroplating, thermal/cold spraying, electro-spark deposition) and two-step hybrids (e.g., electrodeposition + laser cladding). They synthesize mechanism, microstructure, and performance links and chart future directions for friction and wear mitigation on high-conductivity substrates. Overall, the studies in this Special Issue provide valuable insights into how microstructural modifications impact the mechanical properties of structural materials, supporting advancements in material design and process optimization.