Cryogenic-Mechanical Properties of Amorphous Alloys: History
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Research on the mechanical properties of bulk-amorphous alloys at low temperatures has mostly focused on tension and compression studies and there are relatively few studies on their low-temperature impact properties. In order to expand the application of bulk amorphous alloys in low-temperature extreme environments, it is necessary to continue to explore other properties of bulk-amorphous alloys and to reveal the response mechanism of their microstructures and properties at low temperatures.

  • high-entropy alloys
  • amorphous alloys
  • cryogenic-mechanical properties
  • applications

1. Cryogenic-Mechanical Properties of BMGs

The mechanical behavior of amorphous alloys is closely related to the temperature, strain rate and loading mode. Amorphous alloys exhibit different deformation characteristics at various temperatures and strain rates. At high temperatures or low strain rates, amorphous alloys exhibit macroscopic uniform plastic deformation (viscous flow), while at low temperatures or high strain rates the deformation of amorphous alloys is limited to the deformation region of nano-sized shear bands and the plastic deformation is non-uniform and highly localized [4,5].
At low temperature, the mechanism of the strength and plasticity of amorphous alloys are also studied. For example, Li [6] of Dalian University of Technology investigated the effect of low temperature on the mechanical behavior of a Zr60Ni25Al15 BMG with high glass-forming ability through uniaxial compression and tensile tests. As the temperature decreased from room temperature (293 K) to liquid-nitrogen temperature (77 K), the compressive yield stress increased from 1791 MPa to 2217 MPa, the compressive plastic strain increased from 3.5% to 16.7%, the tensile fracture stress increased from 1743 MPa to 2036 MPa and the tensile plastic strain increased from 0 to 0.14%. Enhanced atomic bonding and shortened effective atomic distances at low temperatures lead to increased strength, while the inhibition of free-volume migration and aggregation leads to enhanced plasticity.
Li et al. [7] reported the temperature-dependent mechanical property of Zr-based metallic glasses. Compression tests were conducted on four Zr-based BMGs with different Nb contents of 1–4% (atomic percent). The results show that adding a small number of elements has no obvious effect on the mechanical properties. The phenomenon is quite different from that observed in crystalline materials. In crystal materials, adding a small number of alloying elements can significantly improve the strength. At 77 K, the strength increases significantly without embrittlement. The normalized intensity (strength/E, E: Young’s modules) is linear with the normalized temperature(T/Tg). At 300 K, strength is insensitive to strain rate. At low and high temperatures, BMGs exhibit strain-rate sensitivity. The strength depends on the strain rate at high and low temperatures.
Yao Jian [8] of Northwestern Polytechnical University also studied the ductile-to-brittle transition of amorphous alloy from room temperature to liquid nitrogen temperature. They mainly studied the quasi-static compressive mechanical behavior of Ti40Zr25Ni8Cu8Be18 bulk-amorphous alloy at low temperature, and compared the room temperature and low temperature mechanical properties of the alloy at different strain rates. At room temperature, the plastic strain of Ti-based amorphous alloy is 1.8% and the plastic strain decreases with the increase of strain rate. The mechanical properties of Ti40Zr25Ni8Cu8Be18 bulk amorphous alloy under a compression load are strongly dependent on ambient temperature and strain rate. The ductile-brittle-transition temperature of the alloy is above 77 K. Sun et al. [9] studied the dynamic compression performance of the alloy at cryogenic temperatures. During dynamic compression at room temperature, the compressive strength does not change significantly with the increase of the strain rate. Under dynamic compression at 77 K, the compressive strength increases significantly with the increase of strain rate, which means that the strain-rate-hardening effect exists. At room temperature, Ti40Zr25Ni8Cu8Be18 bulk amorphous alloy shows obvious yield after elastic deformation and the alloy shows typical brittle fracture characteristics. The dynamic compressive strength at 77 K is obviously higher than that at room temperature.
Li et al. [10] studied the mechanical behavior of an Mg61Cu20.3Ag8.7Er10 amorphous alloy. At these two temperatures, amorphous alloys show brittle fracture to a large extent. When the strain rate is 1 × 10−4 s−1, the amorphous alloy has the highest compressive fracture strength of 970 MPa. With the increase of strain rate, the yield strength of the amorphous alloy gradually decreases. However, the amorphous alloy has no compressive plasticity under different loading rates. At lower temperatures, the compressive strength of the amorphous alloy not only slightly increases to 1046 MPa, but also shows a small amount of plastic deformation (about 0.6%). The effect of loading rate on the strength and ductility of amorphous alloys is not as pronounced at room temperature.
The formation of highly localized shear bands during room-temperature deformation results in the fracture of a metallic glass with almost no macroscopic plastic deformation. In recent years, scholars have proposed a deep cryogenic cycle treatment (DCT), which can effectively improve the room-temperature plasticity and toughness of amorphous alloys without reducing strength and hardness. DCT refers to a heat-treatment method that circulates between room temperature and low temperature. There is a large temperature difference in the process of deep cryogenic cycle treatment. The process of temperature change induces internal stress, which causes atomic rearrangement in the alloy, and the amorphous alloy may rejuvenate. The rejuvenation gives the amorphous alloy a higher energy state, so it has higher plasticity [11].

2. Cryogenic-Mechanical Properties of BMG Matrix Composites

The brittleness of BMGs at room temperature seriously limits their wide application as structural engineering materials. In order to improve the macroscopic plasticity of BMGs at room temperature and their performance, how to effectively prevent a rapid expansion of shear bands and promote the proliferation of shear bands is very important. For this reason, people draw lessons from the concept of introducing second-phase strengthening and toughening into crystalline materials, introducing the second phase into the BMG in situ or ex situ, and developing BMG matrix composites in various alloy systems, so as to achieve the purpose of improving plastic properties [12]. Regardless of an in-situ or ex-situ method, the addition of the second phase improves the plastic-deformation ability of the BMG to varying degrees, especially in the state of compression deformation. In comparison, it is more common to prepare BMG-matrix composites by in-situ methods, and the prepared composites also have better plastic-deformation ability [13].
Li et al. [14] first found that in-situ dendrite-reinforced BMG composites exhibited excellent cryogenic-mechanical properties. The fracture strength of the Ti48Zr20Nb12Cu5Be15 BMG composite at 77 K is 2760 MPa and the plastic strain is 18.4%. The effective interaction between the dendrites and shear bands results in good low-temperature plasticity of dendrites, which determines the low-temperature plasticity of composites. The increase of the low-temperature yield strength of dendrites leads to a corresponding increase in low-temperature yield strengths of composites. A continuous matrix rather than the dendrite is considered to be the cause of fracture behavior.
The low-temperature mechanical properties of bulk amorphous-alloy composites reinforced in situ by plastic particles were studied. As the test temperature decreased from 298 K to 77 K, the yield strength of the W-containing bulk-amorphous alloy composites increased from 930 MPa to 1300 MPa and the plastic strain decreased from 58% to 20%. However, the yield strengths of Ta-containing bulk-amorphous alloy composites increased from 1760 MPa to 2040 MPa, the maximum strength increased from 1830 MPa to 2020 MPa, and the plastic strain remained unchanged. The difference in plastic particles leads to different changes in the plastic behavior of the two composites. W is the BCC structural metal and the ductile-to-brittle transition occurs at 253~263 K, resulting in a significant decrease in the low-temperature plasticity of tungsten-containing composites. Although Ta is a BCC structure, the ductile-to-brittle transition does not occur when it is reduced to 30 K [15].
Amorphous-matrix composites, with a Zr-based amorphous alloy as the matrix and tungsten continuous fibers or porous foams as the reinforced phase, were prepared by the liquid-pressing process and their dynamic compressive properties were evaluated. Tungsten fibers or foams at about 65 to 69 volume percent were uniformly distributed in the amorphous matrix, while defects, such as pores, were eliminated. According to the dynamic-compressive test results of amorphous-matrix composites, tungsten fibers can permit large loads to be carried when working, while the amorphous matrix can bear the load of bent or bucked fibers, so that the maximum strength reaches 3328 MPa and a plastic strain of 2.6%. In amorphous-matrix composites, the compressive stress continued to increase according to the work-hardening degree after the yielding, resulting in a maximum strength of 3458 MPa and a plastic strain of 20.6%. The significant increase in the maximum strength and plastic strain was attributed to the uniform deformation at the tungsten foams and amorphous matrix at the same time, because tungsten foams did not show anisotropy, and the tungsten/matrix interfaces were excellent [16].
In practical applications, cryogenic-wear behaviors are closely related to the safety and durability of mechanical components. A series of cryogenic-wear tests were conducted on metastable BMG composites at different temperatures. The Ti47.2Zr33.9Cu5.9Be13 (at.%) BMG composite has better tribological properties at low temperatures. For example, the wear resistance at 113 K was 48% higher than at 233 K. At 233 K, there was an obvious martensitic-transformation (β-Ti→α”-Ti)-coordinated deformation under the worn surface, while at 113 K, the martensitic transformation was significantly suppressed. This temperature-dependent structural evolution is clarified by artificially inducing a pre-notch by FIB cutting on a β-Ti crystal, demonstrating a strain-dominated martensitic transformation in the BMG composite. The improved strength and hardness of the metallic-glass matrix at cryogenic temperatures contributes to the strong limit of the martensitic transformation and the improvement of wear resistance [17].

This entry is adapted from the peer-reviewed paper 10.3390/met12122075

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