SCRIPTA Vol. 189, Dec. 2020, P1-6

英文摘要

SCRIPTA Vol. 189, Dec. 2020, P21-24

The potentiality of composition graded AlMgSi wires for optimized combination of electrical conductivity and torsion strength has been investigated. Composition graded wires were obtained by co-drawing commercially pure Al with an AlMgSi alloy followed by diffusion annealing. Diffusion gradients and local hardening response to precipitation treatments were evaluated thanks to nano-indentation measurements. Resulting microstructures with spatial gradients of nanoscaled precipitates were characterized by transmission electron microscopy. Finally, it is shown that such graded structures give rise to an improved combination of electrical conductivity and mechanical strength in torsion as compared to the predictions based on a classical rule of mixture.

SCRIPTA Vol. 189, Dec. 2020, P42-47

A new formation mechanism of micropores whose radii are approximately 1 μm is investigated during solidification of Sn-Ni alloy through the in-situ observation by confocal laser scanning microscope. A series of liquid lines which are closely related to the stability of the growing L/Ni₃Sn₂ interface were formed during solidification, then, these liquid lines decomposed into different rows of liquid droplets by Rayleigh instability in solid Ni₃Sn₂ phase. The relationship between the formation of the micropores and the liquid droplets is confirmed. In addition, the distribution of the radii of the micropores agrees with the prediction of the Rayleigh instability theory.

SCRIPTA Vol. 189, Dec. 2020, P48-52

CuNb composites were processed by high pressure torsion using specially designed disks consisting of a pre-defined number of single sector elements called multi-sector disks. By variation of the applied plastic strain the composite structure could be tuned ranging from relatively coarse multilayer structures to nanostructured laminates. In this first study the technique is introduced, the microstructural and hardness evolution of CuNb with varying strain is presented and a fracture toughness study of the finest composite structure showing good damage tolerance has been performed.

SCRIPTA Vol. 189, Dec. 2020, P53-57

Deformation-induced martensitic transformation (DIMT) has been investigated for many years mainly under monotonic loading conditions. However, the evolution of the DIMT under non-monotonic loading conditions has rarely been studied. The present work experimentally explored the development of DIMT under tension-compression as well as torsion cyclic loading conditions and showed fluctuating behavior within loading cycles in austenitic stainless steels measured by Feritscope, which was not observed under monotonic loadings. The present study showed that the fluctuation of the martensite measurement was related to the dual-phase features and could be explained by the Villari effect in the martensite phase. The martensite contents are assumed to increase monotonically with the effective plastic strain and are distributed along with principal stresses. A unified kinetics model for DIMT was proposed for both monotonic and cyclic loading conditions and verified experimentally. The anisotropic development of DIMT can be described by the model in combining with cyclic plasticity.

SCRIPTA Vol. 189, Dec. 2020, P67-71

Fatigue crack growth rate (FCGR) tests were used to investigate the fatigue property of low-carbon steel in actual natural gas/hydrogen mixtures and carbon dioxide/hydrogen/nitrogen mixtures. It is found for the first time that the acceleration of FCGRs in the mixtures are much faster than that in hydrogen or natural gas. Natural gas/hydrogen mixtures changed the fracture mode from ductile fatigue striation to brittle cleavage fracture, and restricted the deformation activities around the crack. Synergy effect of carbon dioxide and hydrogen on the material is proposed to be the major reason for the abnormal performance of steel in the mixtures.

SCRIPTA Vol. 189, Dec. 2020, P78-83

Enhanced elastocaloric properties in a directionally solidified Ni₅₅Mn₁₈Ga₂₆Ti₁ alloy were achieved by combining composition tuning and microstructural control. Owing to the introduction of Ti and the microstructural feature with coarse columnar grains and strong <001>_A preferred orientation, the thermal hysteresis and the stress hysteresis were successfully reduced to 4K and 16MPa, respectively. Large adiabatic temperature change of -6.2K was obtained on removing a low stress of 120MPa, resulting in the specific adiabatic temperature change of 51.7K/GPa. Moreover, the alloy demonstrated enhanced cyclability of elastocaloric effect up to 497 loading/unloading cycles and high coefficient of performance value of 25.6.

SCRIPTA Vol. 189, Dec. 2020, P106-111

The crystal structure of a four-layered martensite phase in Ni₅₀Mn_37.5Sn_12.5 single crystal following various modes of training procedure was examined by high energy synchrotron radiation and high-resolution transmission electron microscopy. Three distinct types of unit cell i.e. 4O, 4M, and 8M were unveiled, depending on the mechanical history of the specimen. The most stable phase turned out to be the 4M martensite. Analysis of the intensity of modulation reflections allowed for defining a periodic atomic displacement of 0.3497 Å. The results support the concept of a periodic atomic shuffling as opposed to the adaptive nanotwining approach.

SCRIPTA Vol. 189, Dec. 2020, P122-128

Recurring step straining was developed to investigate the environmentally assisted cracking (EAC) initiation of a Ni-30Cr-10Fe alloy (Alloy 690) in simulated pressurized water reactor (PWR) primary water. Thanks to this technique, the roles of corrosion and straining in the EAC initiation of Alloy 690 was decoupled and typical microstructures in stress corrosion cracking, i.e. diffusion induced grain boundary migration and oxidation into the migration zone, were reproduced. Digital image correlation using indigenous surface oxide particles as markers was successfully incorporated into this technique for high-resolution in-plane strain measurement. The preliminary results show that intergranular crack initiation is correlated with the high local strain in both adjacent grains, indicating that slip transfer across the grain boundary promotes crack initiation. Recurring step straining is desirable for the mechanistic study of EAC as it makes the separate roles of chemical and mechanical processes adjustable and traceable.

SCRIPTA Vol. 189, Dec. 2020, P129-134

High-entropy alloys (HEAs) provide a new perspective to design metastable alloys with the stress-induced martensitic transformation (SIMT) for overcoming the strength-ductility trade-off. Here, we report a novel SIMT, orthorhombic to hexagonal close-packed martensite, in a single orthorhombic refractory HEA (Ti₁₆Zr₃₅Hf₃₅Ta₁₄ RHEA), showing a good yield strength-ductility matching. The analysis of the elastic distortion energy (∆E_els) of Ti₁₆Zr₃₅Hf₃₅Ta₁₄ and several other RHEAs reveals that severe lattice distortion is a key factor which causes this SIMT. Combined the “d-electron alloy design” approach with the ∆_Eels, the phase configuration and SIMT path in RHEAs can be well predicted. Our work brings new insights between the lattice distortion and SIMT of RHEAs, benefiting the metastable alloy development.

SCRIPTA Vol. 189, Dec. 2020, P135-139

Coupling of nano-indentation and crystal plasticity finite element (CPFE) simulations is widely used to quantitatively probe the small-scale mechanical behaviour of materials. Earlier studies showed that CPFE can successfully reproduce the load-displacement curves and surface morphology for different crystal orientations. Here, we report the orientation dependence of residual lattice strain patterns and dislocation structures in tungsten. For orientations with one or more Burgers vectors close to parallel to the sample surface, dislocation movement and residual lattice strains are confined to long, narrow channels. CPFE is unable to reproduce this behaviour, and our analysis reveals the responsible underlying mechanisms.

SCRIPTA Vol. 189, Dec. 2020, P140-144

英文摘要

SCRIPTA Vol. 189, Dec. 2020, P145-150

Ferritic/martensitic steels are one of the best candidates for structural materials for high dose applications in next generation nuclear reactors. The composition of structural materials must be optimized for reliable service during irradiation. This study reports the effect of interstitial nitrogen on radiation response in 12Cr ferritic/martensitic HT9 steels having a controlled nitrogen concentration. Results show that a high amount of ‘free’ nitrogen in the matrix stabilizes the interstitial clusters which leads to (i) larger loop sizes (ii) lower loop density and (iii) slightly reduced radiation induced hardening. It also affects diffusion mechanism of Ni and formation of Ni/Si-rich precipitates.