Large plastic origin and multi-scale effect in iron-based amorphous alloys

Poor plasticity of most amorphous alloys is a major constraint that hinders performance optimization and application promotion. However, in the past two decades, some new bulk amorphous alloy forming systems have overcome this deficiency, with large compression plasticity and relatively high yield strength. The degree of deformation of amorphous alloys is related to the evolution of uniform nucleation and high-density shear bands. However, the intrinsic mechanism by which bulk deformation of the bulk amorphous alloy produces large plasticity is still unclear compared to the same system in which the pre-morphed form has a crystalline or inversely vitrified second phase.

Summary of results

Recently, the Jürgen Eckert team of the Austrian Academy of Sciences/Leoburn University of Mining published a research article entitled "Origin of large plasticity and multiscale effects in iron-ba sed me tallic glasses" on Nature Communication. The author and first author of the article is Dr. Baran Sarac of the Austrian Academy of Sciences. The large plasticity of newly developed bulk amorphous alloys during quasi-static compression has led researchers to question the atomic scale effects. In this paper, high-resolution transmission electron microscopy with spherical aberration correction is used to observe the presence of nanocrystals of the order of 1-1.5 nm in iron-based amorphous alloys. The aggregation of nanocrystals is related to the appearance of hard/soft regions, which are verified by nanoindentation techniques and are related to microscale hardness and elastic modulus. In addition, the authors systematically simulated high-resolution TEM images of different sample thicknesses and established a theoretical model for estimating the size of the shear transition region. The results show that the main mechanism for forming the soft zone is uniformly dispersed nanocrystals, which dominate the start and stop mechanisms of the shear transition zone, and therefore are essential for the improvement of mechanical properties.

Graphic guide

Figure 1: Mechanical characterization of Fe50Ni30P13C7BMG.

(a) The engineering stress-strain curve of the deformed BMG when the quasi-static strain rate is 2×10-4/s;

(b) compressing the deformed surface of the BMG;

(c) BMG large plastically deformed fracture surface.

Figure 2: Thermodynamic properties of Fe50Ni30P13C7BMG.

(a) Tg is the glass transition point, Tx is the crystallization temperature, and ΔHx is the latent heat of crystallization;

(b) Tm is the melting temperature, Tl liquefaction temperature, and ΔHm is the heat of fusion;

(c) Tc is the Curie temperature of BMG.

Figure 3: Microscale inhomogeneity study.

(a) a low power STEM image of the cross section of Fe50Ni30P13C7BMG;

(b) Hardness & reduced modulus diagram of a Fe-based BMG disc sample of 1 mm size;

(c) the relationship between the corresponding indentation load and the depth of the indentation;

(d) Changes in contact depth, modulus reduction, and hardness obtained from nanoindentation experiments.

Figure 4: HRTEM study of nanoscale inhomogeneities.

(a, f) regions 1A and 2A of the nanocrystals in the amorphous matrix (1B and 2B);

(b, d, g, i) In the 8 nm thick sample, the HRTEM image of the different regions and the corresponding fast Fourier transform of (c, e, h, j).

Figure 5: Determination of crystal-like clusters.

(a) a local atomic arrangement map obtained by self-correlation analysis of the HRTEM image in FIG. 4;

(b) an image for volume fraction calculation of nanocrystalline clusters;

(c) an energy loss spectrum of Fe50Ni30P13C7BMG having a thickness of 8 nm;

(d) HRTEM images filtered after auto-correlation analysis, and insets 1-4 represent changes in the fast Fourier transform mode;

(e) X-ray diffraction reveals a broadened diffraction maximum value, and no crystal phase peak is detected;

(f) a low-magnification STEM bright region image showing nanocrystal clusters;

(g) Low-magnification STEM dark-area images show the dispersion of nanocrystalline clusters.

Figure 6: HRTEM simulation of nanocrystals in Fe-based BMG.

(a) atomic model;

(b-d) HRTEM molecular dynamics simulation of sample thickness in the order of 8-, 15-, and 23-nm;

(e-g) The fast Fourier transform obtained from the HRTEM simulation of 8-, 15-, and 23-nm in order of sample thickness shows a close correlation with the fast Fourier transform pattern of the nanocrystalline region in FIG.

Figure 7: Schematic depicting the multi-scale deformation behavior of Fe50Ni30P13C7BMG.

(a) the first deformed portion of the nanocrystals (blue particles) having a lower shear modulus than the BMG (small gray lines in the nanocrystals);

(b) At the microscale, the shear band does not develop into a crack because the distance between the soft zones is less than the critical crack length of the BMG.

summary

In this article, the authors confirmed by aberrant spherical aberration HRTEM that there is a uniform diffused metal nanoscale crystal phase in the Fe-based bulk amorphous alloy. Since the Fe and Ni atoms have a small heat of mixing and a similar atomic radius, the amorphous forming ability of the system decreases as the Ni content increases, prompting the formation of nanocrystals. Significant changes in mechanical properties as measured by nanoindentation experiments, as well as bias on thin samples, demonstrate the presence of two phases of hardness in the bulk amorphous alloy. TEM simulations and experimental findings are well matched, and the only possible lower than 20 nm sheet thickness threshold should be nanocrystalline segregants. In summary, it can be seen that the observation of the significant shearing phenomenon of amorphous alloys under quasi-static compression, as well as the improvement of plasticity by the initiation and hindrance of the shear transition zone, are closely related to the nanocrystals. Controlled nanocrystalline induced plasticity and nanoscale grain size can help researchers better understand the deformation mechanism of other amorphous alloy systems with large plasticity.