The intriguing phenomenon of metal superelasticity relies on stress-induced martensitic transformation (SIMT), which is well-known to be governed by developing cooperative strain accommodation at multiple length scales. It is therefore scientifically interesting to see what happens when this natural length scale hierarchy is disrupted. One method is producing pillars that confine the sample volume to micrometer length scale. Here we apply yet another intervention, helium nanobubbles injection, which produces porosity on the order of several nanometers. While the pillar confinement suppresses superelasticity, we found the dispersion of 5-10 nm helium nanobubbles do the opposite of promoting superelasticity in a NiFeGa shape memory alloy. The role of helium nanobubbles in modulating the competition between ordinary dislocation slip plasticity and SIMT is discussed.
Researches have shown that the addition of trace amounts of rare earth element lanthanum (La) to the alloys and composites can significantly improve their microstructure and properties. In this work, Cu/Ti 3 SiC 2 /C composites with 0.05wt%, 0.1wt% and 0.3wt% La were prepared by powder metallurgy methodthrough mechanical alloying, hot-pressing (HP) and hot isostatic pressing (HIP). The effects of different La contents on the microstructure and mechanical properties of Cu/Ti 3 SiC 2 /C composites were investigated. The results show that La has a refinement effect on the grain of the matrix, and with the increase of La content, the size of the matrix of the composite becomes smaller. As the content of La increases, the performance of the composite exhibits a tendency to increase first and then decrease. The composite of 0.1wt% La exhibited the best performance with a hardness of 97.8 MPa, a tensile strength of 174.9 MPa, and a compressive strength and shear strength of 461.1 MPa and 102.1 MPa, respectively. Since the dimple is observed, the tensile fracture indicates that the fracture mode is a ductile fracture. The enhancement mechanism of La mainly includes dispersion strengthening and fine grain strengthening.
The corrosion behavior of copper matrix composites with different Al 2 O 3 whisker content in NaCl solution (3.6 wt%) at different temperatures was studied by electrochemical test and electron microscope. The results showed that with the increase of Al 2 O 3 whisker content, the corrosion tendency of the composites first increases and then decreases. The charge-transfer resistance (Rct) value and corrosion current density tend to decrease first and then rise slowly. The whiskers can cause huge differences in performance in different directions so that it is easy to agglomerate and entangle in the matrix, which can increase the heterogeneity of materials in copper matrix alloys, hence, the addition of whiskers will cause the increase of corrosion phenomenon. As the whisker content continues to increase, a large number of reinforcing phases will form a natural protective layer when exposed on the metal surface, which will slow down the corrosion situation. As the temperature increases, the corrosion resistance decreases first and then increases. When the temperature increases from 25°C to 50°C, the electrochemical reaction is accelerated, the free electrons between the metal atoms obtain external energy, the ion migration rate is accelerated, and the corrosion reaction is more likely to occur. When the temperature increases from 50°C to 75°C, the corrosion reaction rate is mainly controlled by the diffusion of oxygen. As the temperature increases, the solubility of oxygen decreases and the cathode reaction requires oxygen to participate, consequently, the corrosion rate decreases. For the microstructure of the corrosion specimen, corrosion mainly occurred at or around the reinforcement phase/matrix interface. Intermetallic compound particles often appear near corrosion initiation points along the reinforcement phase/matrix interface, and the corrosion behavior of composites is mainly determined by copper. Copper is used as the anode of the battery, and the corrosion products are mainly copper chloride salts.
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