determining-the-mechanical-strength-of-ultra-fine-grained-metals

Determining the Mechanical Strength of Ultra-Fine-Grained Metals

The mechanical strengthening of metals is the long-standing challenge and popular topic of materials science in industries and academia. The size ...

Center for High Pressure Science and Technology Advanced Research

We report the superconductivity enhancement of ZrTe on compression up to 33 GPa. The superconducting transition occurs above 4.1 GPa and the superconducting temperature (T ) increases with pressure in further compression, reaching a maximum of 7.1 K at ~28 GPa. An anomalous change of superconducting temperature is seen in the compression above 21 GPa. No structural phase transition is observed in the whole compression up to 36 GPa, but a subtle change in structural parameter is seen between 17-19 GPa, which seems relevant to the anomalous increase in the superconducting temperature. First-principle calculations reveal that the density of states at the Fermi level increases with pressure, which explains the enhancement of T  in ZrTe under compression.

Pressure-induced enhancement in the superconductivity of ZrTe.

The Hall-Petch relationship, according to which the strength of a metal increases as the grain size decreases, has been reported to break down at a critical grain size of around 10 to 15 nanometres. As the grain size decreases beyond this point, the dominant mechanism of deformation switches from a dislocation-mediated process to grain boundary sliding, leading to material softening. In one previous approach, stabilization of grain boundaries through relaxation and molybdenum segregation was used to prevent this softening effect in nickel-molybdenum alloys with grain sizes below 10 nanometres. Here we track in situ the yield stress and deformation texturing of pure nickel samples of various average grain sizes using a diamond anvil cell coupled with radial X-ray diffraction. Our high-pressure experiments reveal continuous strengthening in samples with grain sizes from 200 nanometres down to 3 nanometres, with the strengthening enhanced (rather than reduced) at grain sizes smaller than 20 nanometres. We achieve a yield strength of approximately 4.2 gigapascals in our 3-nanometre-grain-size samples, ten times stronger than that of a commercial nickel material. A maximum flow stress of 10.2 gigapascals is obtained in nickel of grain size 3 nanometres for the pressure range studied here. We see similar patterns of compression strengthening in gold and palladium samples down to the smallest grain sizes. Simulations and transmission electron microscopy reveal that the high strength observed in nickel of grain size 3 nanometres is caused by the superposition of strengthening mechanisms: both partial and full dislocation hardening plus suppression of grain boundary plasticity. These insights contribute to the ongoing search for ultrastrong metals via materials engineering.

High-pressure strengthening in ultrafine-grained metals.

Transition metal dichalcogenides (TMDs) have attracted wide attention due to their quasi-two-dimensional layered structure and exotic properties. Plenty of efforts have been done to modulate the interlayer stacking manner for novel states. However, as an equally important element in shaping the unique properties of TMDs, the effect of intralayer interaction is rarely revealed. Here, we report a particular case of pressure-tuned re-arrangement of intralayer atoms in distorted 1T-NbTe, which was demonstrated to be a new type of structural phase transition in TMDs. The structural transition occurs in the pressure range of 16-20 GPa, resulting in a transformation of Nb atomic arrangement from the trimeric to dimeric structure, accompanied by a dramatic collapse of unit cell volume and lattice parameters. Simultaneously, a charge density wave (CDW) was also found to collapse during the phase transition. The strong increase in the critical fluctuations of CDW induces a significant decline in the electronic correlation and a change of charge carrier type from hole to electron in NbTe. Our finding reveals a new mechanism of structure evolution and expands the field of pressure-induced phase transition.

Yanju Wang

Center for High Pressure Science and Technology Advanced Research

Determining the Mechanical Strength of Ultra-Fine-Grained Metals