Juntao Huo

Mechanical properties and structural features of novel Fe-based bulk metallic glasses with unprecedented plasticity

Fe-based bulk metallic glasses (BMGs) have attracted great attention due to their unique magnetic and mechanical properties, but few applications have been materialized because of their brittleness at room temperature. Here we report a new Fe50Ni30P13C7 BMG which exhibits unprecedented compressive plasticity (>20%) at room temperature without final fracture. The mechanism of unprecedented plasticity for this new Fe-based BMG was also investigated. It was discovered that the ductile Fe50Ni30P13C7 BMG is composed of unique clusters mainly linked by less directional metal-metal bonds which are inclined to accommodate shear strain and absorbed energy in the front of crack tip. This conclusion was further verified by the X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy experiments of Fe80-xNixP13C7 (x = 0, 10, 20, 30) and Fe72-xNixB20Si4Nb4 (x = 0, 7.2, 14.4, 21.6, 28.8) glassy systems. The results also indicate a strong correlation between the p-d hybridization and plasticity, verifying that the transition from brittle to ductile induced by Ni addition is due to the change of bonding characteristics in atomic configurations. Thus, we can design the plasticity of Fe-based BMGs and open up a new possible pathway for manufacturing BMGs with high strength and plasticity.

High-entropy bulk metallic glasses as promising magnetic refrigerants

In this paper, the Ho20Er20Co20Al20RE20 (RE = Gd, Dy, and Tm) high-entropy bulk metallic glasses (HE-BMGs) with good magnetocaloric properties are fabricated successfully. The HE-BMGs exhibit a second-order magnetic phase transition. The peak of magnetic entropy change ( ΔSMpk) and refrigerant capacity (RC) reaches 15.0 J kg−1 K−1 and 627 J kg−1 at 5 T, respectively, which is larger than most rare earth based BMGs. The heterogeneous nature of glasses also contributes to the large ΔSMpk and RC. In addition, the magnetic ordering temperature, ΔSMpk and RC can be widely tuned by alloying different rare earth elements. These results suggest that the HE-BMGs are promising magnetic refrigerant at low temperatures.

Magnetocaloric effect in heavy rare-earth elements doped Fe-based bulk metallic glasses with tunable Curie temperature

The effects of heavy rare earth (RE) additions on the Curie temperature (T-C) and magnetocaloric effect of the Fe-RE-B-Nb (RE-Gd, Dy and Ho) bulk metallic glasses were studied. The type of dopping RE element and its concentration can easily tune T-C in a large temperature range of 120K without significantly decreasing the magnetic entropy change (Delta S-M) and refrigerant capacity (RC) of the alloys. The observed values of Delta S-M and RC of these alloys compare favorably with those of recently reported Fe-based metallic glasses with enhanced RC compared to Gd5Ge1.9Si2Fe0.1. The tunable T-C and large glass-forming ability of these RE doped Fe-based bulk metallic glasses can be used in a wide temperature range with the final required shapes.

Interactions of Shear Bands in a Ductile Metallic Glass

Shear bands play a key role in the plastic deformation of metallic glasses (MGs). Even though there are extensive studies on the initiation and propagation of shear bands, the interactions among them have not been systematically studied yet. The interactions between the primary shear bands (PSBs) and secondary shear bands (SSBs) in a ductile Zr-based MG were studied. The residual stress near PSBs can deflect the propagation direction and reduce the propagation velocity of SSBs, which contributes to the plasticity and toughness of the MG. It was demonstrated that the probability and strength of the interactions between PSBs and SSBs would become stronger for MGs with larger Young s modulus and smaller shear modulus, i. e., larger Poisson s ratio. These results are valuable in understanding the plastic deformation of MGs and may be helpful in designing new MGs with desirable mechanical properties.

Chemical independent relaxation in metallic glasses from the nanoindentation experiments

In this article, we studied the anelastic properties at low-load nanoindentations of different metallic glasses, including Zr-, Pd-, La-, Mg-, and Au-bases. It is verified that the “two-parameter Kelvin model” is suitable for analyzing the anelastic mechanical behavior. Despite the difference in the chemical composition of these alloys, the energy barrier against the local relaxation is almost equal. The energy barrier is much smaller than that of slow β relaxation, which denotes a faster relaxation mechanism. These findings give insights into the heterogeneous nature of mechanical behavior and relaxation characteristics of metallic glasses.

Direct production of hard magnetic ribbons with enhanced magnetic properties by controlling cooling rate of melt

We produced a high-quality hard magnetic Fe81Co2Nb1Nd10B6 alloy by melt spinning without additional treatment. The as-spun ribbons produced at a wheel speed of ∼25 m/s had the best hard magnetic properties: a remanence Br, coercive force Hc, and maximum energy product (BH)max of 0.97 T, 676 kA/m, and 140 kJ/m3, respectively. The Nd2Fe14B/α-Fe nanocomposite phases had grain sizes of ∼10–30 nm. We investigated how the magnetic properties changed with wheel speed, finding that they depended on the as-spun structure and magnetic structure, which changed upon over- and under-quenching from the melt.

Nonlinear fragile-to-strong transition in a magnetic glass system driven by magnetic field

Relaxation dynamics in nonlinear response regime have become an emerging novel tool to study the dynamics and structure of glassy materials. It provides additional insights relative to the standard linear response experiments. However, limited by inherent endurance of the materials to external fields, up to now, almost all the probed nonlinear effects are very weak. Here, strong nonlinear effects are observed in magnetic systems with disordered spins (i.e. magnetic glass). In particular, we report a pronounced fragility transition as driven by the external magnetic field as a result of nonlinear dynamic response. Such model systems provide a new platform to study the glassy dynamics with large and tunable nonlinearity.