Vol. 205 目录

SCRIPTA

Phase boundaries in multiphase alloys govern defect interaction and chemical intermixing across different phases during plastic deformation. Dynamic interaction of defects with phase boundaries in multiphase alloys, especially for immiscible alloys, has generated more research interest in recent years. Here, we describe a novel approach for carrying out in-situ TEM shear deformation to directly observe interfacial microstructural evolution of a Cu-Nb alloy. A unique double shear specimen geometry is microfabricated by a focused ion beam technique to apply shear deformation upon push loading inside the TEM. From the real-time observation, we discover that the phase boundary with a zigzag morphology effectively blocks stacking faults nucleated in a Cu grain from slipping into a Nb grain. Meanwhile, the Cu phase bears the most plastic deformation through slip or twinning mechanisms. This work sheds light on understanding the shear deformation and the behavior of phase boundaries in multiphase alloys during shear deformation.

SCRIPTA

The occurrence of multiple generations of annealing twins (MGAT) in low stacking fault energy (SFE) materials leads to significant texture weakening with the evolution of some new prominent texture components. However, the exact correlation between MGAT and texture evolution is a long-standing enigma. In the current research, the presence of MGAT was confirmed by transmission Kikuchi diffraction (TKD) and high resolution electron backscattered diffraction (EBSD) in partially (700°C/5min) and fully (700°C/1h) recrystallized specimens of equiatomic CoCrFeMnNi high entropy alloy, respectively. Further, the twin clusters in the vicinity of the potential nucleation sites (Brass, Goss and S orientations) were investigated to gain insights on the contribution of MGAT on texture development. The newly developed (φ₁ = 90°, Φ=30°, φ₂ = 45°) orientation showed its roots originating from 2nd generation twin of the Brass orientation while Cube orientation was evolved from the 1st and 5th generation twins of the S orientation.

SCRIPTA

Certain cold-worked martensitic Ti-alloys exhibit tunable linear thermal expansion along a specific sample direction. These intriguing properties are related to the anisotropic thermal expansion of the orthorhombic αʺ martensite. However, the tailored expansion obtained in this way is limited to the single-phase martensitic temperature field, below the reverse transformation of martensite into austenite. Here, we demonstrate that the orthorhombic αʺ_iso formed by diffusion can also regulate the thermal expansion using a prototypical Ti22Nb alloy. More importantly, the temperature range in which the tailored linear thermal expansion occurs is enormously broadened relative to the cold-worked martensitic alloy. We clarify the formation path of nano-sized αʺ_iso during thermal cycling and resolve the crystallographic relationship with the parent phase. These findings provide new routes for designing low/negative thermal expansion Ti-alloys with large operating temperature ranges.

SCRIPTA

The liquid-liquid phase separation (LLPS) behavior was investigated in Cu-Fe alloys that consisted of 60 wt % or 80 wt % Cu and balance iron, without or with addition of 1 wt% Zr. Both Zr-free alloys clearly showed LLPS at metastable spinodal temperatures prior to γ-Fe nucleation, whereas Zr-added showed γ-Fe phase evolution without LLPS. Addition of Zr was highly effective to activate γ-Fe nucleation by decreasing the mixing enthalpy and also accelerating heterogeneous nucleation. A combination of thermal instrumental analysis and classical nucleation theory yielded an effective assessment made of LLPS behavior below the liquidus temperature of Cu-Fe alloys. Furthermore, it was possible to control the solidification mechanism of Cu-Fe alloy by zirconium addition.

SCRIPTA

We develop high-strength of Al-Zn-Mg-Cu-based alloys (7xxx series) by optimizing the amount of Mg/Cu with a trace addition of Sc. The highest mechanical properties are achieved with σ_UTS=672 MPa (σ_y=644 MPa) and ε=8.6%. Based on quantitative symmetry measurements, the improved mechanical strength originates from nanoscopic structural distortion caused by the excessive solutes of Mg and Cu as the amount of Mg/Cu increases. In addition, Sc microalloying induces grain refinement, thus enhancing the mechanical properties.