Interface Strengthening of α-Mg/C14–Mg2Ca Eutectic Alloy

This study investigates the effect of the α/C14 interface on the creep strength of α-Mg/C14–Mg2Ca eutectic alloy at 473 K under a stress of 40 MPa. The α/C14 interface is composed of terraces and steps, with terraces parallel to the (1101)α pyramidal plane of the α-Mg lamellae and to the (1120)C14 columnar plane of the C14–Mg2Ca lamellae. The creep curves of the alloy exhibit three stages: a normal transient creep stage, a minimum creep rate stage, and an accelerating stage. The minimum creep rate is proportional to the lamellar spacing, indicating that the α/C14 lamellar interface plays a creep-strengthening role. In the high-resolution transmission electron microscopy image captured of the specimen after the creep test, <a> dislocations can be mainly seen within the soft α-Mg lamellae, and they are randomly distributed at the α/C14 interface. In contrast, dislocations are rarely introduced in the hard C14–Mg2Ca lamellae. It is deduced that the α/C14 interface presents a barrier to dislocation gliding within the α-Mg lamellae and does not help rearrange the dislocations.

Effects of Al and Mo on Microstructure and Hardness of As-Cast TNM TiAl Alloys

The effects of Al and Mo elements on the microstructure and hardness of TNM TiAl alloys (Ti-43.5Al-4Nb-1Mo-0.1B) were studied by decreasing 0.5 at.% Mo and/or increasing 1.5 at.% Al. The results showed that the changed composition of the alloy had a slight influence on the morphology, but had important effects on the volume fraction, size, and composition of each phase. All the alloys had nearly full lamellar (NL) microstructures, with a few βo phases at the boundaries of the colony or in the lamellar colony. The lamellar colony size and the lamellar spacing increased with the decrease in Mo and the increase in Al. The reduction in Mo content reduced the content of each phase in proportion, but the increase in Al content in the alloys led to the corresponding increase in Al content in the α2 and γ phases. The hardness of the alloys decreased with the increase in Al content and the decrease in Mo content. This is mainly due to the increase in lamellar spacing caused by the change in composition. Therefore, the increased content of Al and decreased Mo content are unbeneficial for the microstructure. The relationship between the Vickers hardness and the lamellar spacing obeyed the Hall–Petch relationship.

Study on the Influence of Compression Ratio on the Rail Contact Fatigue Resistance Property and Its Mechanism

Contact fatigue resistance properties of four commercial steel rails were systematically studied and compared. The contact fatigue limit cycles, fracture toughness, and fatigue crack growth rate of the corresponding rails with different compression ratios were tested and analyzed, which showed a close relationship between them. Moreover, microstructures of the rails were carefully observed by using the TEM to elucidate the scientific mechanisms. Observations and obtained results indicated that pearlite lamellar spacing further decreased with the rise of compression ratio, which greatly and rightly contributed to the improved fatigue resistance property.

Additive manufacturing of high-strength multiphase nanostructures in the binary Ni–Nb system for the discovery of new types of superalloys

Ni-based superalloys have been studied extensively due to their impressive mechanical properties, including strength and creep resistance at high temperatures. Growing interest surrounding additive manufacturing (AM) methods has led to recent investigations of alloys that are traditionally difficult to process, including Ni-based superalloys. Recent work has shown that AM methods enable high-throughput materials discovery and optimization of difficult- or impractical-to-process alloys, including those with high or even majority refractory element compositions. This work focuses on AM-enabled investigations of composition-dependent mechanical and microstructural properties for Ni–Nb binary alloys. Specifically, we report on the mechanical behavior of compositionally-graded NixNb1−x and uniform composition Ni59.5Nb40.5 specimens made with AM. The AM fabrication process resulted in extraordinarily high strength, attributed to the formation of a dual-phase microstructure consisting of δ-Ni3Nb and µ-Ni6Nb7 intermetallic compounds with nanostructured and multimodal grain size and eutectic lamellar spacing. Graphic Abstract

Improving the Tensile Properties of Additively Manufactured β-Containing TiAl Alloys via Microstructure Control Focusing on Cellular Precipitation Reaction

The effect of a two-step heat treatment on the microstructure and high-temperature tensile properties of β-containing Ti-44Al-4Cr (at%) alloys fabricated by electron beam powder bed fusion were examined by focusing on the morphology of α2/γ lamellar grains and β/γ cells precipitated at the lamellar grain boundaries by a cellular precipitation reaction. The alloys subjected to the first heat treatment step at 1573 K in the α + β two-phase region exhibit a non-equilibrium microstructure consisting of the α2/γ lamellar grains with a fine lamellar spacing and a β/γ duplex structure located at the grain boundaries. In the second step of heat treatment, i.e., aging at 1273 K in the β + γ two-phase region, the β/γ cells are discontinuously precipitated from the lamellar grain boundaries due to excess Cr supersaturation in the lamellae. The volume fraction of the cells and lamellar spacing increase with increasing aging time and affect the tensile properties of the alloys. The aged alloys exhibit higher strength and comparable elongation at 1023 K when compared to the as-built alloys. The strength of these alloys is strongly dependent on the volume fraction and lamellar spacing of the α2/γ lamellae. In addition, the morphology of the β/γ cells is also an important factor controlling the fracture mode and ductility of these alloys.

Molecular Dynamics of Janus Nanodimers Dispersed in Lamellar Phases of a Block Copolymer

We investigate structural and dynamical properties of Janus nanodimers (NDs) dispersed in lamellar phases of a diblock copolymer. By performing molecular dynamics simulations, we show that an accurate tuning of the interactions between NDs and copolymer blocks can lead to a close control of NDs’ space distribution and orientation. In particular, NDs are preferentially found within the lamellae if enthalpy-driven forces offset their entropic counterpart. By contrast, when enthalpy-driven forces are not significant, the distribution of NDs, preferentially observed within the inter-lamellar spacing, is mostly driven by excluded-volume effects. Not only does the degree of affinity between host and guest species drive the NDs’ distribution in the polymer matrix, but it also determines their space orientation. In turn, these key structural properties influence the long-time dynamics and the ability of NDs to diffuse through the polymer matrix.