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The incipient plasticity of an amorphous solid represents the onset of shear deformation, which is a stochastic progress closely related to microstructure evolution. It is well known that the cooling rate plays a crucial role in the microstructure evolution of a glass. Nevertheless, how the cooling rate affects the incipient plasticity is still unclear. In this work, the incipient plastic deformation of the amorphous phase in Cu 47.5 Zr 48 Al 4 Nb 0.5 bulk metallic glass composites (BMGCs) cast with different cooling rates is systematically studied. The incipient plasticity of the glassy matrix of the BMGCs is found to occur via a first displacement burst, the occurrence of which is more stochastic as the cooling rate decreases. According to the auto-correlation functions and cooperative shear model, the stochastic behavior is attributed to the effects of the cooling rate on the initial free volume and subsequent activated shear transformation zones (STZs). A higher content of free volume is present in the glassy matrix of the studied samples cooled at a faster rate, which facilitates the operation of STZs. Moreover, the larger incipient burst size is correlated with the higher content of free volume and larger STZs in the glassy matrix. The larger STZs result in more preferable propagation of multiple shear bands, leading to less stochastic deformation and more obvious plastic deformation. This study provides further understanding on the incipient plasticity of glassy matrix in BMGCs. Image 1 • Hardness and elastic modulus of amorphous phase are higher at a lower cooling rate. • The deformation of amorphous phase is more stochastic as cooling rate decreases. • Larger shear transformation zones facilitate the formation of multiple shear bands. • Initial free volume is the dominant role in mechanical behavior of amorphous phase. [ABSTRACT FROM AUTHOR]

The plastic deformation of metallic glasses (MG) is well-known to occur via shear transformation zones (STZs) on the scale of atomic clusters, yet fracture of MG takes place via shear bands of the micron scale. So far, understanding on how the operation of STZs leads to shear localization and fracture remains limited. In this work, tensile tests on Cu/Zr-based MG micro-wires show that both the first-yield and fracture stress exhibit the Weibull distribution, and fractography reveals that shear localization in the form of intense shear bands leads to shear fracture. Molecular dynamics (MD) simulations show that shear bands form via the correlated emergence and operation of discrete STZs close to one another. To describe how the stochastic yet correlated occurrence of the discrete STZs leads to shear localization, a model is constructed to relate the probability of the successive operation of discrete STZs, to their nucleation density. The model predicts that, if nucleation density of the STZs grows along the strain path, as prior shear events triggers the emergence of new STZs, then successive occurrence of discrete shear events speeds up rapidly to an asymptotic state which is exactly the condition of shear localization and shear banding. Furthermore, the MD results suggest an exponential growth law for the occurrence of the STZs along the strain path, which also gives predictions in good agreement with the experimental Weibull distributions of the first-yield stress of Cu/Zr-based MGs. • Tensile tests show that both first-yield and fracture stress of Cu/Zr-based MG micro-wires exhibit Weibull distributions. • MD simulations show that shear bands form via the correlated formation of successive shear zones of atomic sizes. • A statistical model is proposed to describe the survival probability against successive occurrence of discrete shear events. • The model predicts that shear localization is a consequence of acceleration of shear event occurrence, as in MD simulation. • An exponential growth law for the shear event occurrence agrees with both the MD and experimental results. [ABSTRACT FROM AUTHOR]

Eutectic alloys are widely used as solder-joint materials due to their suppressed melting points, but when used in micro-devices with small dimensions, their characteristic lamellar microstructure may lead to an internal length scale that affects strength. Here, we report an unusual ‘smaller-being-weaker’ phenomenon in eutectic Sn/Pb alloys with fine lamellar microstructure, namely, in the specimen-size regime close to and slightly larger than the interphase lamellar spacing, the strength decreases with decreasing size, while above this regime the strength tends towards the bulk value. Theoretical modeling indicates that in a fine lamellar microstructure, high contents of dislocations are retained, so that strength is governed by mutual dislocation interactions, rather than by dislocation starvation. Therefore, in smaller samples, fewer interphase lamellar boundaries are present to block dislocations, thus resulting in a ‘smaller-being-weaker’ behavior. In samples a lot larger than the lamellar spacing, significant strengthening arises from Taylor hardening and mutual dislocation interactions as a result of significant dislocation retention by the interphase boundaries, so that strength does not depend on specimen size anymore. In a course lamellar microstructure, however, even a larger micro-specimen may contain insufficient interphase boundaries to significantly retain dislocations, and strength may be governed by the starvation effect due to significant loss of dislocations at free surfaces. In this case, the size effect of strength may become a lot milder, or even exhibit the conventional “smaller-being-stronger” behavior. The results here supplement conventional knowledge on size effects in micro-scaled crystalline materials, and provide important implications on solder-joint design in micro-devices. [ABSTRACT FROM AUTHOR]

Through the addition of minor alloying elements, we have designed and prepared a new non-equiatomic six-membered Fe 41 Ni 20 Co 20 Cr 10 Al 5 V 4 (at. %) high-entropy alloys (HEAs), with the aim of further improving HEA performance. The effect of annealing temperature on HEA microstructure and mechanical properties was systematically studied by using a variety of characterization methods. The results show that HEA Fe 41 Ni 20 Co 20 Cr 10 Al 5 V 4 is a single-phase face-centered cubic (FCC) solid solution structure. The yield strength HEA Fe 41 Ni 20 Co 20 Cr 10 Al 5 V 4 doped with minor element doped is 15% higher than that of the base alloy Fe 40 Ni 20 Co 20 Cr 20 with a similar grain size while retaining good plasticity (elongation >50%). This can be attributed to the increased lattice distortion induced by the doping elements (i.e., Al and V). In addition, the strengthening mechanisms of Fe 41 Ni 20 Co 20 Cr 10 Al 5 V 4 HEAs were analyzed. HEA strengthening mechanisms mainly include solid-solution strengthening, grain-boundary strengthening, and dislocation strengthening. Among them, in partially recrystallized HEA, fine-sized dislocation cells formed by the intertwining of high-density dislocations significantly increase material strength. Due to the excellent plasticity (∼ 61%) inherent in HEA as-homogenized, further research can be carried to improve its strength with minimal compromise to plasticity in order to achieve a better balance between strength and ductility. • A novel non-equiatomic six-membered high-entropy alloy is investigated. • Minor element doping enhances strength while preserving ductility. • Fine-sized dislocation cells significantly increase material strength. [ABSTRACT FROM AUTHOR]