Recent Insights into Mechanical Properties of Metallic Alloys

1. Introduction

Engineering reliable metallic components across transportation, energy, manufacturing, and cryogenic technologies hinges on controlling how processing routes determine microstructure and how microstructure governs strength, toughness, and durability. Over recent years, metal additive manufacturing was an intriguing challenge in materials science and engineering, distinguished by its ability to produce novel geometries that conventional metallurgical processes cannot replicate. In additive manufacturing, advances in heat-flow control, defect mitigation, and in situ monitoring now allow us to control the microstructure and lack of fusion, keyhole and porosity defects, and their effects on mechanical performance [1,2,3,4,5]. In parallel, superalloy metallurgy and modern design frameworks continue to set benchmarks for elevated temperature integrity and damage tolerance [6], while comprehensive reviews on Inconel 718 clarify how constituent phases and thermal histories dictate fatigue and thermomechanical fatigue response [7,8].

Problems arising in service are equally central. Hydrogen technologies have renewed focus on embrittlement and trapping, with protocols that expose the differential impact of hydrogen on ductility and fracture toughness versus the near-invariance of strength in many pressure windows [9,10,11]. At low temperature, cryogenic and sub-zero treatments in martensitic/tool steels offer insights into how to tune retained austenite stability and carbide precipitation with measurable gains in wear and cutting performance [12,13]. For structural systems, rail integrity remains a canonical case where microstructure, contact mechanics, and service spectra intersect, and damage tolerance and contact fatigue perspectives guide how local geometries and defects evolve into cracks under real loading [14]. Recent insights into the brittle–ductile transition in BCC systems analyze screw-dislocation mobility, suggesting strategies to shift the ductile–brittle transition window through alloying and heat treatments [15]. Finally, another interesting field is the study of transition-metal nitrides, which are high-hardness materials used for cutting and wear-resistant tools, forming dies and protective coatings.

In light of these advances, this Special Issue of Metals assembles nine open access contributions that collectively advance mechanism-aware testing, microstructure-targeted processing, and modelling routes that close the gap between laboratory protocols and service conditions. Together, these works provide relevant progress toward robust integrity assessments across steels, Cu- and Ni-based alloys, and engineered structural systems.

2. Highlights of the Contributions

The different contributions tackle engineering problems related to the microstructure and mechanical properties of different metallic alloys.

2.1. Ductile–Brittle Transition (DBT) in BCC Metals: Mechanisms and Dilute-Solution Softening

Zhang, Zhao, Hou, Li, and Wu synthesize how the DBTT in BCC alloys is governed by the relative mobility of screw dislocations. The review consolidates theory and simulation on double-kink nucleation and migration in screw dislocations and rationalizes the low-temperature dilute-solution softening observed in refractory and BCC systems. The addition of specific alloying elements, such as Ce, Re, and Ni, to BCC metals can favour the dilute solid solution softening effect, reduce the DBTT, and promote low-temperature toughness. Further microscopic investigations are required to substantiate the screw dislocation-controlled DBT mechanism and explain the effects of dilute solid solution softening [C1].

2.2. Mechanical Testing Methods for Studying Hydrogen Embrittlement (HE) for Pipeline Steels

Paterlini, Re, Curia, Ormellese, and Bolzoni map tensile, fracture toughness, and fatigue methodologies in high-pressure hydrogen, with emphasis on drivers such as hydrogen pressure and strain rate. Across the examined test methods, hydrogen mainly reduces ductility and fracture-toughness, while strength levels remain essentially unchanged over the pressure range relevant to hydrogen transport. This pattern indicates that hydrogen primarily promotes crack initiation and growth rather than altering the bulk elastic–plastic response [C2].

2.3. Cold-Work Tool Steel X160CrMoV12: Secondary Hardening, Retained Austenite (RA) Control, and Wear

Bendikiene and Kavaliauskiene quantify the roles of hardening temperature (1100–1200 °C), tempering (600 °C), and Bridgman-anvil deformation on RA, carbide precipitation, and tribological performance. RA peaks (~69%) at 1200 °C and drops at a factor of about 7; plastic deformation introduces a hardened surface layer ~0.08 mm deep. Abrasive wear tests carried out on this steel identify 1100 °C as an optimal hardening temperature for wear resistance [C3].

2.4. Coalesced Bainite to Mitigate Hydrogen Embrittlement in Tempered Martensitic Steels

Shin, Kim, and Hwang show that slower cooling promotes coalesced bainite, which hinders hydrogen accumulation at prior-austenite grain boundaries, shifting fracture from intergranular to transgranular. Slow strain rate tests on smooth and notched specimens, together with fractography, corroborate the microstructure-controlled improvement in hydrogen embrittlement resistance. Exploring the role of coalesced bainite offers valuable insights for improving the performance of tempered martensitic steels in hydrogen-rich environments [C4].

2.5. Wrought Inconel 718 Under Variable-Amplitude Thermal Cycles with Mechanical Load

Pan, Xu, and Yu combine pulsed-laser thermal cycling with constant load experiments and calibrated simulations to study crack initiation and growth under realistic temperature gradients. This work describes experimental and numerical perspectives on the damage process occurring under thermal cycles of different amplitudes combined with mechanical loading. This is of fundamental importance for conducting failure analyses and for designing components working at high temperature [C5].

2.6. 50Cr15MoV Martensitic Stainless Steel: Residual Austenite Stability and Cutting Performance

Guo et al. show that hardness and edge sharpness arise from the interplay between carbides and residual austenite stability. Cryogenic processing increases hardness by 3.89 HRC and boosts initial sharpness by 15.3% and durability by 18.8%. The residual austenite in the knives was found to be unstable and easy to transform during high-rate deformation processes. Beyond residual austenite fraction, stability under load governs tool performance [C6].

2.7. PBF-LB GRCop-42: Thermomechanical Properties at Low Temperature and Thermal Conductivity

Cortis et al. report the elastic modulus and Poisson’s ratio from low temperature to room temperature and correlate these with thermal conductivity in as-built vs. heat-treated states. Heat treatment redistributes strengthening precipitates, improving thermal transport while retaining strength and ductility at low T. This is useful for cryogenic components in physics infrastructure [C7].

2.8. Rails with Lubrication Holes: Experiments and Finite Element Modelling

Sainz-Aja et al. validate finite-element predictions against full scale three-point-bending fatigue tests on rail coupons. The finite element model developed showed excellent agreement with the experimental S–N curve; moreover, simulation results highlighted the influence of geometric and load parameters on crack initiation near the hole. The results yield actionable guidelines for rail maintenance and design [C8].

2.9. Elastic Origins of Hardness in Quenchable High-Pressure Metal Nitrides

Zhang et al. review bulk transition-metal nitrides synthesized at high pressure–high temperature and recoverable under ambient conditions. They established that hardness shows a strong association with the shear modulus G and highlighted that elasticity-based models cannot be used for making absolute predictions, especially when defect-rich phases are present. On the basis of the results, they suggested that future research should prioritize the study of the link between elasticity, bonding, and hardness [C9].