# Cryogenic effects on the mechanical behavior of bulk metallic glasses

Laura Mangourny, Bruno Lavisse, Yessine Ayed, Guénaël Germain

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Laura Mangourny, Bruno Lavisse, Yessine Ayed, Guénaël Germain. Cryogenic effects on the mechanical behavior of bulk metallic glasses. ESAFORM, May 2025, Paestum, Italy. pp.1685-1693, 10.21741/9781644903599-181. hal-05343659

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# Cryogenic effects on the mechanical behavior of bulk metallic glasses

Laura MANGOURNY¹,ᵃ*, Bruno LAVISSE¹,ᵇ, Yessine AYED¹,ᶜ and
Guénaël GERMAIN¹,ᵈ

¹Arts et Métiers, LAMPA, 2 Bd du Ronceray, 49035 Angers, France

ᵃlaura.mangourny@ensam.eu, ᵇbruno.lavisse@ensam.eu, ᶜyessine.ayed@ensam.eu,

ᵈguenael.germain@ensam.eu

**Keywords:** Bulk Metallic Glasses, Cryogenic Temperatures, Plastic Strain

**Abstract.** Bulk metallic glasses (BMGs), unlike crystalline alloys, exhibit significantly enhanced plastic deformation when tested at cryogenic temperatures. This enhanced plasticity is primarily characterized by the slowed propagation of shear bands and the formation of multiple shear bands, which play a crucial role in the material's behavior at low temperatures. Due to their amorphous nature, BMGs are prone to catastrophic fractures once shear band nucleation and propagation occur, a behavior distinct from that of crystalline materials. However, the underlying mechanisms of BMG failure and the effect of strain rate remain controversial. This study investigates the mechanical behavior of a Zr-based BMG under cryogenic conditions. Compression tests were conducted at room temperature and -180°C, using liquid nitrogen, across a range of strain rates. The results show that, at cryogenic temperatures, ductility increases, though it remains relatively low, leaving uncertain its impact on machinability. Notably, larger stress drops were observed at ambient temperature, likely linked to shear band formation. Additionally, the study identified two distinct fracture modes during dynamic tests, warranting further investigation. This research provides valuable insights into the behavior of BMGs under cryogenic conditions and their machinability.

**Background**

Amorphous metals, commonly known as BMG, represent an advanced class of materials currently being developed to meet the growing demands of various industries. Characterized by their non-crystalline atomic structure and exceptional yield strength, these materials are of significant interest across sectors such as aerospace, biomedical engineering, and horology.

Despite their advantages, BMGs face several challenges that limit their broader application. Machinability is a primary concern, as their high elastic recovery and considerable hardness lead to rapid tool wear or even tool breakage [1]. Additionally, at elevated temperatures, BMGs are prone to partial recrystallization [2,3], which compromises their amorphous structure and thus their unique mechanical properties. In this regard, cryogenic cooling during machining has emerged as a promising solution, as it can prevent recrystallization. This technique is already widely used for machining heat-resistant materials like titanium, which tend to cause rapid tool degradation due to excessive localized heating. By effectively minimizing local heat generation, cryogenic cooling significantly reduces tool wear and enhances machining efficiency [4,5].

Bulk metallic glasses (BMGs) exhibit distinct mechanical properties due to their amorphous structure. Compared to crystalline alloys, they have a slightly lower density (0.5–2% less) and a reduced bulk modulus, attributed to larger interatomic distances. Their shear modulus and Young's modulus are also approximately 30% lower than those of crystalline alloys [7], which cannot be explained solely by atomic spacing [8]. These properties arise from the presence of atomic transformation zones where plasticity is localized. Under critical stress, these zones generate shear bands that propagate through the material, leading to significant macroscopic deformations [9].

<figure><img src="image_2.png" /></figure>
The mechanical properties of amorphous alloys appear to be sensitive to temperature variations. At room temperature, these alloys exhibit limited shear banding, which is confined near the fracture zone, restricting their plasticity. In contrast, at cryogenic temperatures, plasticity is considerably enhanced, marked by the formation of multiple shear bands and smoother creep curves. Experimental studies [10-13] have demonstrated an increase in the plastic domain (Fig. 1) and in the number of shear bands (Fig. 2), making these materials more ductile under cryogenic conditions [11].

<figure><img src="image_3.png" /><figcaption>Figure 1 – Stress-Strain Compression Curves of Cu57Zr43 at 25°C, -75°C, -100°C, and -196°C [11]</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Figure 2 – Optical Micrographs Recorded from the Lateral Surfaces of Fractured Samples Tested at (a) 25°C, (b) -75°C, and (c) -196°C, and (d) Secondary Electron Micrograph of (c) [11]</figcaption></figure>

However, it should be noted that the literature does not provide unanimous conclusions, and some studies adopt a more cautious stance regarding the effects of cryogenic conditions [14]. This caution is primarily attributed to variations in the composition of amorphous metals, underscoring the need for further research to address the variability in behavior depending on the alloy composition.

Another crucial factor to explore is the influence of strain rate on the mechanical properties of BMGs. Unlike crystalline materials, BMGs exhibit low sensitivity to strain rate at ambient temperature, even up to 10 s⁻¹. At cryogenic temperatures, however, additional research is necessary to determine the precise impact of high strain rates on their mechanical behavior [15].

In this context, our study aims to comprehensively investigate the deformation and failure mechanisms of BMGs, with a particular focus on zirconium-based alloys subjected to compressive conditions at cryogenic temperatures and varying strain rates. This work seeks to enhance the fundamental understanding of BMGs and to identify strategies for optimizing their machinability and performance in extreme environments.

## Experimental procedure

In this study, a series of quasi-static and dynamic compression experiments are conducted on samples made from a Zr-based amorphous alloy to investigate their behavior across a range of strain rates. The cylindrical specimens, with a diameter of 5 mm and a length of 7.6 mm, were produced through thermo-molding processes. The material properties were certified by the manufacturer, ensuring that all specimens were made from the same Zr-based amorphous alloy according to their specifications. Afterward, they were cut to the specified length and polished to ensure that the top and bottom faces are parallel, flat, and orthogonal to the cylinder axis.

The samples are taken from different sections of the same bar, and it is assumed, according to the manufacturer, that the structural properties are uniform along its length. However, the structural state may vary between different castings. To address this, five batches of samples, each
originating from a distinct casting, are used for testing. This approach allowed us to verify that the results are independent of the casting, ensuring that the tests are representative of the material's overall properties. To ensure the reliability and repeatability of the campaign, a minimum of three valid tests were conducted for each condition. The stress-strain curves and results presented in this document represent the average of these tests.

The experiments are carried out at ambient temperature and at cryogenic temperature (-180°C), under atmospheric pressure. The Gleeble thermomechanical machine is used for both quasi-static and dynamic testing without shock. In the quasi-static tests, a deformation rate of $10^{-3} s^{-1}$ is applied, while in the dynamic tests, the strain rate is $1 s^{-1}$. The results from both quasi-static and dynamic tests are used to derive true stress-strain curves, offering insights into the material's behavior under different loading conditions and temperatures.

## Experimental results

Stress-Strain Behavior and Mechanical Response. Fig. 3 and Fig. 4 present the stress-strain curves obtained for the four experimental conditions (quasi-static at room temperature, quasi-static at cryogenic temperature, dynamic at room temperature, and dynamic at cryogenic temperature). Each curve is representative of the results obtained from at least three identical tests, all yielding the same curve.

The tests conducted at room temperature show that the behavior of the Zr-based BMG is consistent with results reported in the literature [15], where the strain rate has little impact on the mechanical properties of the material. Indeed, for the strain rate range used in this study, the fracture stress shows minimal variation with respect to the deformation rate.

In contrast, a distinct behavior was observed under cryogenic conditions. Under quasi-static conditions, a more extensive plastic region was observed, with an average plastic strain of 0.03 true strain, three times greater than that observed under ambient conditions. In dynamic conditions, fracture occurs more abruptly, further confirming the effect of temperature on the deformation and fracture mechanisms of the BMG. The fracture stress reached approximately 1200 MPa, significantly lower than the 1800 MPa observed under all other conditions.

<figure><img src="image_5.png" /><figcaption>Figure 3 – Typical True Stress-Strain Curves from Quasi-Static Compression Experiments at (a) ambient and (b) cryogenic temperatures</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Figure 4 – Typical True Stress-Strain Curves from Dynamic Compression Experiments at (a) ambient and (b) cryogenic Temperatures</figcaption></figure>

Another interesting phenomenon observed under quasi-static conditions is the presence of "notches" in the plastic deformation curve (Fig. 5). These irregularities could be interpreted as "stress drops" related to the propagation of shear bands in the BMG. According to Kyeu-Sang Yoon [6], this notch phenomenon results from the accumulation of stresses in regions where shear bands do not propagate immediately. As the shear bands propagate, the stresses are released, causing a drop in the observed stress, followed by stabilization. Each displacement peak in the curve corresponds to a peak in the propagation rate of the shear bands, followed by a gradual decrease in this rate until it reaches a stable value corresponding to the speed of the test machine's crosshead. We quantified the number of "notches" in a portion of the curve within the plastic region. On average, 12 notches were observed for a deformation range of 0.01, with a standard deviation of 2 at room temperature, and 20 notches with a standard deviation of 3 at cryogenic temperature. The average amplitude of the notches was 37 MPa at room temperature and 12 MPa at cryogenic temperature, representing a threefold reduction.

<figure><img src="image_7.png" /><figcaption>Figure 5 – Visible Stress Drops in the Stress-Strain Curve of a Quasi-Static Test at Ambient Temperature (a) and Cryogenic Temperature (b)</figcaption></figure>

Fracture Surface Analysis Under Quasi-Static and Dynamic Conditions. The fracture surfaces of the amorphous metal specimens subjected to quasi-static compression tests at room temperature show similar macroscopic patterns. Rupture is primarily initiated at a 45° angle to the loading axis (Fig. 6).
<figure><img src="image_8.png" /><figcaption>Figure 6 – Specimens Fractured During Quasi-Static Tests at Room Temperature</figcaption></figure>

Most specimens break into two large fragments after moderate plastic deformation, while some display multi-fragmentation, with two main large pieces and smaller secondary fragments. The fracture surfaces are characterized by a regular appearance (Fig. 7), with irregular initiation zones (i) and smoother propagation zones (ii), indicating ductile deformation. Features such as shear veins, typically associated with shear bands formed during plastic deformation, are observed on the fracture surfaces.

Under quasi-static conditions at cryogenic temperatures, the fracture behavior of the amorphous metal specimens exhibits additional features. While fracture initiation occurs at a 45° angle, multi-fragmentation is more common. As shown in Fig. 8, the fracture surfaces show propagation zones with two main orientations, suggesting the involvement of multiple shear bands. Compared to room temperature tests, the shear vein patterns are less regular, and non-sheared zones become more prominent. The activation of multiple shear bands appears to be a dominant mechanism under cryogenic conditions.

In dynamic tests at room temperature, the fracture surfaces of the amorphous metal specimens show more irregular features, including bulging and hollow zones, as in Fig. 9. These irregularities are attributed to the rapid deformation, which prevents the formation of a dominant shear band, instead favoring the appearance of multiple shear bands. Cracks of varying sizes are observed, indicating that the shear bands were unable to absorb all the deformation energy. The simultaneous formation of cracks and multiple shear bands contributes to rapid specimen fragmentation.

<figure><img src="image_9.png" /><figcaption>Figure 7 – Fracture Surface of a BMG Tested under Quasi-Static Compression at room Temperature</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Figure 8 – Fracture Surface of a BMG Tested under Quasi-Static Compression at cryogenic Temperature</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Figure 9 – Fracture Surface of a BMG Tested under Dynamic Compression at room Temperature</figcaption></figure>

In dynamic tests conducted at cryogenic temperatures, two distinct types of fracture surfaces were observed (Fig. 10). For samples that did not fracture vertically, the fracture surfaces look like those observed in room temperature tests, with similar initiation and propagation zones. However, for samples that fractured along the cylinder axis (vertically), the propagation zone appeared to approach the initiation zone, resulting in a clean fracture characteristic of brittle rupture. These zones exhibited cracks and shear veins, but the fragmentation was more pronounced due to the higher deformation rates, emphasizing the brittle fracture behavior under cryogenic conditions.

<figure><img src="image_12.png" /><figcaption>Figure 10 – Fracture Modes of Specimens During Dynamic Tests</figcaption></figure>

Discussion

Shear Band and Vein Patterns Formation in Amorphous Metals. The origin of the vein patterns, characteristic of metallic glasses, is attributed to Saffman-Taylor instabilities [16]. This instability, or viscous fingering, occurs when a less viscous fluid displaces a more viscous fluid within a thin channel, destabilizing the interface between the two fluids and leading to the formation of
Saffman-Taylor fingers or Hele-Shaw cells, as illustrated in Fig. 11. These cells are formed by two glass plates separated by a small gap relative to the width of the cell. In this setup, the viscous fluid is pushed by a less viscous fluid, destabilizing the interface and causing the formation of finger-like patterns. Key parameters for these fluids include the interfacial tension and the viscosity difference between them. These zones consist of a low-viscosity alloy layer, highly deformable, located between two more rigid solid planes, thus forming a Hele-Shaw cell. This low-viscosity layer remains fluid throughout the deformation process until fracture occurs, as plastic deformation is localized within this low viscous region. The fracture is then associated with the rapid separation of these two surfaces after sliding against each other [16]. This results in vein-like fracture surfaces, with the orientation indicating the direction of shear. The shear band sliding is therefore attributed to the presence of Hele-Shaw cells, which play a role in the plastic domain. The shear vein patterns typically have an average length of 8 µm in the propagation direction, with the largest reaching up to 12 µm.

<figure><img src="image_13.png" /><figcaption>Figure 11 – Schematic view of a Hele-Shaw cell, consisting of two glass plates with a gap b much smaller than the width W of the cell. The interface between the two fluids destabilizes when a less viscous fluid displaces a more viscous fluid within the cell [16]</figcaption></figure>

Fracture Mechanisms Under Quasi-Static and Dynamic Conditions. The observations obtained under quasi-static conditions reveal rupture modes typical of ductile materials, with plastic deformation characterized by the formation of shear bands. At room temperature, amorphous metal samples create a main shear band that moves discontinuously. This allows the material to accommodate energy until the stress becomes too high, leading to fracture. The shear veins and smooth propagation zones observed suggest ductile fracture, as reported in the literature [11-13]. However, the cryogenic results indicate that the formation of multiple shear bands allows for more complex plastic deformation, with the propagation zones splitting into several orientations. This phenomenon is consistent with the idea that, at cryogenic temperatures, materials exhibit higher resistance to deformation, thus activating multiple fracture mechanisms.

The results from dynamic tests show a different type of fracture, more related to the deformation rate. At room temperature, the irregular fracture surfaces and observed cracks confirm that the multiple shear bands do not have time to reorganize properly, leading to premature fragmentation of the material. Furthermore, cracks, typically absent in quasi-static tests, indicate that the material could not accommodate the energy due to the rapid deformation. At cryogenic temperatures, the two distinct fracture surfaces highlight the importance of both the deformation rate and temperature on the fracture mechanics. Samples that fracture along the cylinder axis appear to exhibit brittle behavior, with fractures occurring without prior plastic deformation. This fracture mode suggests that dynamic and cryogenic conditions favor cleaner fractures, where atomic bonds break rapidly without prior shear band formation. This brittle fracture mechanism may be due to the material's inability to accommodate energy due to the rapid increase in stress and local temperature.

Stress Drop Phenomenon. The "stress drop" effect observed in the stress-strain curve is attributed to the propagation of shear bands within the material. When shear bands do not propagate immediately, stresses accumulate in these regions, leading to an increase in the observed stress in the specimen. Superimposed on the stress-strain curve is the variation in shear displacement velocity, showing that each time a stress drop occurs, the shear velocity reaches its maximum before gradually decreasing to a constant value, corresponding to the crosshead speed of the testing machine. As a result, the maximum shear displacement velocity can be interpreted
as the propagation speed of the shear band, while the constant shear displacement velocity corresponds to the crosshead speed. This discontinuous shear band movement explains the presence of the "stress drop" observed in the stress-strain curve.

Additionally, the "stress drop" phenomenon is visible in the plastic deformation region with less frequent occurrences at room temperature. These "stress drops" are less visible at lower temperatures due to their smaller amplitude and higher frequency.

Therefore, shear bands are more numerous and finer, at cryogenic temperatures due to the higher frequency of stress drops, and they move more slowly over shorter distances, as indicated by their smaller amplitudes.

## Conclusion

In this research, the mechanical behavior of zirconium-based bulk metallic glasses (BMGs) under quasi-static and dynamic compressive conditions at both room and cryogenic temperatures was thoroughly investigated. By analyzing stress-strain curves, fracture surfaces, and stress-drop phenomena, key insights into the deformation and failure mechanisms were obtained. The conclusions can be summarized as follows:

1. At cryogenic temperatures, BMGs exhibit enhanced plasticity under quasi-static conditions, characterized by multiple shear bands and smoother deformation curves. However, their ductility remains limited. Under dynamic conditions, cryogenic temperatures lead to a rapid transition to brittle fracture mechanisms, with fracture stress decreasing significantly compared to room temperature, highlighting the combined effects of temperature and strain rate.

2. Quasi-static tests at room temperature revealed ductile fracture surfaces with dominant shear veins and localized deformation. Cryogenic conditions enhanced the formation of multiple shear bands, resulting in more complex fracture surfaces with irregular shear vein patterns. Dynamic tests showed distinct fracture modes. At room temperature, irregular cracks and shear veins dominated, while cryogenic temperatures led to brittle fractures with cleaner propagation zones.

3. The stress-drop effect observed in stress-strain curves is strongly correlated with shear band propagation. At room temperature, stress drops were larger but less frequent, while at cryogenic temperatures, they were smaller in amplitude but occurred more frequently due to shorter and more numerous shear bands propagations.

In future work, additional dynamic tests could be performed at higher strain rates to further understand the interplay between temperature, strain rate, and fracture mechanisms. Moreover, a detailed comparison between the fracture patterns observed after Gleeble tests and chips produced during BMG machining could provide valuable insights into the mechanisms governing material removal and failure in such alloys.

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