Deformation Behavior of Ti₄₈Zr₁₈V₁₂Cu₅Be₁₇ Semi-Solid Amorphous Composites

Abstract

In the light of increasing research into amorphous composites and their applications, as-cast specimens of multicomponent Ti₄₈Zr₁₈V₁₂Cu₅Be₁₇ amorphous composites were prepared via water-cooled copper mold suction casting. Subsequently, the as-cast specimens were subjected to semi-solid isothermal treatment to obtain semi-solid specimens. Taking the semi-solid specimens as the research object, room temperature compressive deformation behavior was investigated by analyzing the shear band characteristics on the side surfaces of the compressed specimens. The evolution of shear bands at various stages of plastic deformation was investigated via scanning electron microscopy (SEM). Additionally, significant work hardening was observed after yielding. Surface deformation morphologies indicate that the work-hardening behavior is associated with plastic deformation, interactions between shear bands, and interactions between shear bands and β-Ti crystals. Experiments have demonstrated that at a specific deformation extent, shear bands preferentially initiate at the crystal–amorphous matrix interface. In the final stage of plastic deformation, shear bands propagate through work-hardened β-Ti crystals into the amorphous matrix, with their propagation retarded by the β-Ti crystals. When shear bands in the amorphous matrix are obstructed by β-Ti crystals and can no longer propagate, some evolve into cracks. These cracks then propagate exponentially, leading to eventual fracturing of the specimens and termination of plastic deformation. The research findings provide a theoretical basis for analyzing the deformation capacities of various amorphous composites.

1. Introduction

Bulk amorphous alloys have been extensively studied owing to their unique structures, which endow them with advantages such as high elastic limits, superior strength [1], excellent corrosion resistance [2], and high electrical and magnetic conductivity [3]. They hold significant value for applications in the electronics industry, healthcare, and aerospace fields. However, under applied loads, amorphous alloys tend to undergo catastrophic brittle fracturing [4]. This is attributed to their highly localized shear behavior, which leads to extremely low plasticity and toughness [5]. This critical disadvantage significantly restricts their engineering applications. To address this challenge, amorphous composites have been developed by introducing in situ-formed ductile crystalline phases into the amorphous matrix to enhance plasticity [6]. These composites are advanced materials derived from amorphous alloys, consisting of both crystalline and amorphous phases. As such, they not only exhibit considerable plasticity, but also retain the beneficial properties inherent in amorphous alloys.

Zr₅₃Cu₃₀Ni₉Al₈ amorphous composites have been obtained by incorporating carbon nanoparticles, with their plasticity reaching 6.81%—nine times higher than that of the amorphous alloy [7]. Additionally, amorphous composites prepared by adding tungsten particles to the Zr₅₅Cu₃₀Al₁₀Ni₅ amorphous matrix show an increase in compressive deformation from 1.5% to 3.4% [8]. For Zr₇₄Al₇Ni₁₇Cu₂ amorphous composites containing in situ-formed ductile β-Zr and ω-Zr crystalline phases, the plastic strain increased from 1.3% to 3.9% [9]. Zr-based amorphous composites with Ta-precipitated reinforcing phases fabricated via selective laser melting (SLM) have exhibited plasticity enhanced from nearly 0 to 2.15% [10]. Wang et al. used laser powder bed fusion to prepare Cu₄₇Zr₄₇Al₆ amorphous composites toughened by Mo particles, which achieved a fracture strain of 2.6% (compared to 0 for the original alloy) [11]. Zou et al. developed Cu particle-reinforced FeCrMoCB amorphous composites with a plastic strain of 8.69% [12]. These studies collectively demonstrate that introducing second-phase particles to prepare amorphous composites is an effective approach to improving plasticity.

Although the plasticity of amorphous composites has improved compared to amorphous alloys, it remains at a relatively low level. To further enhance plasticity, a semi-solid isothermal treatment is employed for modification [13]. This technique provides an effective means to optimize plastic properties through microstructural regulation [14]. The treatment can effectively control solute diffusion, liquid–solid interface morphology, and structural homogenization [15]. Additionally, this method helps reduce solidification shrinkage and internal stress. In recent years, such treatment processes have been applied to Zr-, Ti- and Cu-based amorphous composites to regulate microstructures and improve plasticity [16,17]. For instance, they have effectively regulated the microstructure of Zr–Ti–Nb amorphous composites, significantly enhancing their mechanical properties [18]. Similarly, for Ti–Zr–V system composites, these processes optimize microstructures to improve mechanical performance [19]. Thus, such treatments serve as key approaches for enhancing the mechanical behavior of these composite systems [20].

This treatment process involves heating specimens to a specific temperature within the solid–liquid range, holding them for a certain duration, and then rapidly water-quenching them. For Ti₄₈Zr₁₈V₁₂Cu₅Be₁₇ amorphous composites, after such treatment (held at 900 °C for 10 min), their plasticity increased from 4.2% to 18.0% [21]. Similarly, Ti_44.3Zr_35.2V_11.8Cu_8.1Be_2.6 amorphous composites exhibited a plasticity improvement from less than 1% to 20% after treatment [22]. For Zr_56.2Ti_13.8Nb_5.0Cu_6.9Ni_5.6Be_12.5 amorphous composites, the treatment resulted in a yield strength of 1325 MPa and plasticity of 12.0%, representing increases of 13% and 20%, respectively, compared to the untreated counterparts [23]. Likewise, the plasticity of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 amorphous composites increased from 9% to 44% post-treatment [24]. Semi-solid isothermal treatment thus enables a significant enhancement in compressive plastic strain, offering an effective approach for amorphous composites. However, the fundamental mechanism underlying such remarkable performance improvements remains unelucidated by previous studies. This research aims to investigate it from the perspective of deformation behavior.

Our research group have previously investigated the deformation characteristics and microstructural evolution of this type of composite after semi-solid isothermal treatment [25,26]. However, the deformation behavior at different stages remains unexplored. This study focuses on the performance enhancement mechanism of Ti₄₈Zr₁₈V₁₂Cu₅Be₁₇ amorphous composites. By preparing semi-solid processed specimens, it systematically investigates the plastic deformation behavior and shear band evolution of semi-solid specimens under different deformation degrees. Emphasis is placed on analyzing the fundamental reasons for the significant improvement in material ductility induced by semi-solid isothermal treatment from the perspective of the deformation behavior of in situ ductile crystalline phases. This work aims to compensate for the insufficient exploration of the deformation behavior and performance-enhancement mechanisms of such composites at different stages in existing research.

2. Experimental

Five pure metals were selected: Ti (99.96%), Zr (99.95%), V (99.94%), Cu (99.96%), and Be (99.93%). These metals were uniformly mixed and melted in a high-vacuum (10⁻⁴ Pa) arc melting furnace (Model: DHL-500 II) under a high-purity Ar atmosphere to prepare Ti₄₈Zr₁₈V₁₂Cu₅Be₁₇ alloy ingots with the target molar ratio. The ingots were remelted 5–6 times to ensure compositional uniformity, then cast via water-cooled copper mold suction casting into cylindrical specimens with lengths of 70 mm and diameters of 4 mm. Thus, the as-cast specimens of Ti₄₈Zr₁₈V₁₂Cu₅Be₁₇ amorphous composites were prepared.

Semi-solid amorphous composite specimens were prepared through three steps: First, the as-cast cylindrical amorphous composite specimens were encapsulated in high-vacuum quartz tubes to prevent high-temperature oxidation. Second, an electric furnace (model: YFFG40/13G-YC) was preheated to 900 °C, after which the encapsulated specimens were placed inside and held at 900 °C for 10 min. Third, the encapsulated specimens were removed from the furnace and immediately water-quenched to obtain semi-solid specimens. A schematic of the semi-solid isothermal treatment process is shown in Figure 1.

The phase compositions of specimens were analyzed via X-ray diffraction (XRD, Cu Kα) using a diffractometer (model: Bruker D8 ADVANCE). Mechanical test specimens were machined to diameters of 4 mm and heights of 8 mm. Their strength and plasticity were evaluated via room-temperature quasi-static compression tests using a mechanical testing machine (model: E45.105) at a strain rate of 5 × 10⁻⁵ s⁻¹. Five sets of semi-solid specimens were prepared for compression tests, categorized by deformation extent: (A) deformed to ε₁ = 4% then unloaded; (B) deformed to ε₂ = 7% then unloaded; (C) deformed to ε₃ = 18% then unloaded; (D) deformed to ε₄ = 28% then unloaded, and (E) compressed until fracture. The side-surface deformation morphologies of specimens with different deformation extents and fractured specimens were observed via scanning electron microscopy (SEM, model: JSM6701). The fracture morphology of the fractured specimens was also observed by SEM and the schematic diagram is shown in Figure 2.

3. Results

3.1. Phases Composition Analysis

The phase composition of semi-solid specimens was analyzed via XRD, as shown in Figure 3. The XRD pattern exhibits typical characteristics of amorphous composites, consisting of broad humps and sharp high-intensity peaks which correspond to diffuse scattering and crystalline diffraction, respectively. Analysis indicates that the broad humps are attributed to the amorphous matrix, while the sharp diffraction peaks correspond to the β-Ti crystalline phase [27].

Studies have shown that deformation can induce phase transformations [28,29]. To determine whether strain-induced phase transitions occur during compressive deformation, the phase compositions of specimens before and after compression were compared via XRD analysis of the deformed specimens. This allows verification of whether phase transformations contribute to plasticity improvement. XRD patterns of specimens deformed to ε₃ = 18% and the fractured specimens are presented in Figure 4. Notably, no significant differences were observed between the XRD patterns of the undeformed and the deformed specimens. It is therefore inferred that no phase transformations occurred during deformation, including in specimens with other plastic strain magnitudes.

3.2. Microstructure Observation

The microstructures of semi-solid specimens were observed by SEM. After semi-solid isothermal treatment, the β-Ti crystals in Figure 5 exhibited rich and diverse morphologies. The majority of the crystals were approximately spherical or ellipsoidal, characterized by regular morphologies and relatively smooth surfaces. Statistical analysis revealed that their equivalent diameters (defined as the diameter of a sphere with the same volume as the irregular particle) were primarily concentrated in the range of 5–20 μm, with some smaller crystals approaching 3 μm and a few larger ones reaching approximately 25 μm. These crystals are typical nucleation-growth products formed during semi-solid isothermal treatment.

Additionally, irregular, elongated, and branched crystal structures were observed, formed through the fusion and interactive growth of crystals, with some crystals interconnected. These irregular elongated and branched crystals showed large variation in length, with their long axes ranging from several tens to tens of micrometers. Consequently, the overall crystalline morphology of the sample presents diverse features, including complex geometric configurations.

Both regularly shaped crystals (such as spherical or ellipsoidal ones) and irregularly shaped crystals (such as elongated or branched ones) were uniformly embedded in the amorphous matrix. This indicates that the distribution of crystal nucleation sites is uniform during semi-solid isothermal treatment.

3.3. Mechanical Performance Testing and Analysis

The mechanical properties of semi-solid specimens were obtained via compression tests, and the stress–strain curve up to fracture is presented in Figure 6. Analysis of the curve reveals that after yielding, the strength increased with increasing strain and, thus, with strain rate. There was no post-yield softening under this strain rate, and the material exhibited significant strain hardening. The results for the semi-solid specimens are consistent with the previous findings reported in Ref. [30]. Curve analysis indicates a total strain of 29.6%, with an elastic strain (εₑ) of 5.1% and a plastic strain (ε) of 24.5%. Additionally, the strength parameters are as follows: yield strength σ_0.2 = 1249 MPa, and maximum strength σ_max = 1896 MPa (with a true fracture strength of 1721 MPa). It can also be observed that the specimens exhibited significant work hardening after yielding.

The fracture surface morphologies of fractured specimens were observed via SEM, as shown in Figure 7. When amorphous composites were loaded, shear bands first initiated, then propagated, bifurcated, and expanded. Their movement trajectories interacted with the plastic flow of the material, facilitating the nucleation and growth of internal micropores [31]. These micropores aggregate along the paths of shear bands, ultimately forming vein patterns on the fracture surface [32]. Such patterns intuitively record the progressive fracture process mediated by shear bands and serve as a typical microscopic hallmark of ductile fractures [33]. Thus, morphological characteristics reflect whether fracturing is brittle or ductile. A typical feature of ductile fracturing is the presence of numerous dimples distributed across the fracture surface [34]. In contrast, a brittle fracture is marked by molten droplets and smooth regions on its surface [35]. As shown in Figure 7, the fracture morphology of specimens is dominated by vein patterns, with some well-developed veins. This indicates that the specimens underwent ductile fracturing and experienced significant plastic deformation prior to fracturing, demonstrating good plasticity.

3.4. Observation of Side-Surface Morphology of Compressed Specimens

To investigate the deformation mechanism of amorphous composites, some specimens were compressed to various deformation extents and then unloaded, while another set was compressed until fracture. The corresponding compressive stress–strain curves are presented in Figure 8, with the following designations: (A) specimens undergoing only elastic deformation (plastic deformation ε₁ = 0); (B) specimens deformed to 7% (plastic deformation ε₂ = 1.9% with 5.1% elastic deformation); (C) specimens deformed to 18% (plastic deformation ε₃ = 12.9% with 5.1% elastic deformation); (D) specimens deformed to 28% (plastic deformation ε₄ = 22.9% with 5.1% elastic deformation); and (E) fracture specimens deformed to 29.6% (plastic deformation ε₅ = 24.5% with 5.1% elastic deformation). The side-surface morphologies of specimens with specific compressive strains (non-fractured) and fractured specimens were observed via SEM, as shown in Figure 9.

When the plastic deformation is ε₁ = 0%, (corresponding to Figure 9a), the morphology is identical to that of the uncompressed specimen. Since this plastic deformation (ε₁ = 0%) lies within the elastic range, no morphological changes occur, as elastic deformation is fully recovered upon unloading. For the specimen with a plastic deformation of ε₂ = 1.9% (undergoing both elastic strain and a small amount of plastic deformation), the side surface morphology is shown in Figure 9b. Further observation of Figure 9b reveals that shear bands are absent in the amorphous matrix and are only distributed and confined within the crystalline phases, with these shear bands being approximately parallel. This indicates that in the early stage of plastic deformation, shear bands initiate within the β-Ti crystals rather than within the amorphous matrix. Given that no shear bands are observed in the purely elastically deformed specimens, it can be concluded that shear bands are the primary mechanism for achieving plastic deformation. To quantify the density of shear bands, parallel spacing was introduced as an indicator of plastic deformation magnitude. For the specimen with 1.9% plastic deformation, the average spacing of parallel shear bands was approximately 2.5 μm. Additionally, from the edges of the β-Ti crystals, these shear bands had not yet induced crystal slip. The side surface of the specimen with 12.9% plastic deformation is depicted in Figure 9c, showing distinct characteristics compared to the specimen with 1.9% plastic deformation. In addition to shear bands within the β-Ti crystals, a set of shear bands was also present in the amorphous matrix, with the parallel shear band spacing reduced to approximately 1.4 μm. Furthermore, crystal slips are observed at the edges of β-Ti crystals—a feature not seen in the 1.9% plastic deformation specimen. The number and density of shear bands in the amorphous matrix remained significantly lower than those in crystals. Moreover, shear bands in the amorphous matrix originated from the extension of shear bands within the crystals. Both the crystalline and amorphous phases, containing abundant shear bands, exhibited extensive wrinkling. Additionally, a stepped morphology was observed at the crystal–amorphous matrix interface, indicating that shear band density increases rapidly as plastic strain exceeds 12.9%. Figure 9d shows the side surface morphology of the specimen with ε₄ = 22.9% plastic deformation. Here, highly dense shear bands were present not only within β-Ti crystals and the amorphous matrix, but also formed along other directions. The average spacing of parallel shear bands was approximately 1.1 μm. Shear bands in different directions propagated and intersected, with triangular morphologies often observed around the intersection regions of two sets of shear bands within dendrites. When amorphous composites were subjected to stress, multi-directional shear bands were initiated and intersected. Each shear band slid along its specific orientation and height differences (defined as steps) were generated at intersections due to the superposition of sliding displacements. These steps interlock and overlap, resulting in a rough texture analogous to a pineapple surface. This “pineapple-like” surface morphology can thus be attributed to both the directional extension of shear bands and the staggered protrusions formed by their multi-directional intersections. Additionally, a microcrack can be observed within the amorphous matrix, as indicated by the red arrow in Figure 9d. For the fractured specimen with ε₅ = 24.5% plastic deformation, the side-surface morphology is shown in Figure 9e. Cracks propagated through both crystalline and amorphous phases indefinitely, ultimately leading to fracturing. To clarify the shear band characteristics near the fracture, the red dashed box in Figure 9e was selected for local magnification, as shown in Figure 9f. This reveals numerous intersecting shear bands in both the crystalline and amorphous phases near the fracture, with high density and an average parallel shear band spacing of approximately 0.7 μm. Similarly, pineapple-like surfaces can also be observed in the crystals.

4. Discussion

From the perspective of phase composition, no strain-induced phase transformation was observed during deformation. By excluding the contribution of phase transitions to plasticity, the core mechanism underlying performance enhancement was identified as the synergistic effect between crystalline microstructure and shear band behavior.

The phased evolution of shear bands enables a detailed analysis of deformation mechanisms and behaviors:

① Early stage of plastic deformation (ε = 7%): Shear bands initiate exclusively within β-Ti crystals, exhibiting a parallel distribution with an average spacing of 2.5 μm, without extending into the amorphous matrix. This phenomenon indicates that β-Ti crystals, as ductile phases, preferentially undergo plastic deformation and act as “carriers” for shear band nucleation, thereby preventing premature localized shearing in the amorphous matrix. This contrasts sharply with the behavior of traditional amorphous alloys, where shear bands initiate directly in the matrix, leading to brittle fracture, thus revealing the regulatory role of crystalline phases in “prioritizing deformation to protect the matrix.”

② Intermediate stage (ε = 18%): Shear bands propagate from the crystals into the amorphous matrix, with a significant increase in density (spacing reduced to 1.4 μm). Crystal edge slip and stepped morphologies at the interfaces are observed. This stage highlights the critical role of “crystal-amorphous interfaces”. These interfaces not only serve as channels for shear band propagation but also promote increased shear band density through stress concentration effects. Concurrently, crystal slip further distributes the deformation load, and the synergistic induction of work hardening by these mechanisms (evidenced by continuous post-yield strength enhancement in the stress–strain curve) provides a mechanical foundation for plastic strain accumulation [36].

③ Late stage (ε = 28%~29.6%): Multidirectional shear bands intersect to form a “pineapple-like” surface (spacing 0.7~1.1 μm), accompanied by microcrack initiation. Mechanically, the intersection and interaction of multidirectional shear bands effectively disperse stress concentrations, preventing the rapid penetration of individual shear bands. Unlike the “unidirectional shear band propagation leading to low plasticity” observed in other systems, this indicates that the diverse morphologies (spherical and dendritic) and uniform distribution of β-Ti crystals enable more efficient blocking and dispersion of shear bands, which is the core reason for the significant plasticity improvement.

This study makes the following three key contributions: first, it is the first to systematically quantify the correlation between shear band spacing and deformation (from 2.5 μm to 0.7 μm), providing a microscopic indicator for evaluating plastic potential; second, it clarifies the “dual role” of β-Ti crystals—as both nucleation carriers and propagation barriers for shear bands—with their intrinsic plastic deformation further enhancing deformation coordination; and third, the vein patterns on the fracture surface confirm ductile fracture, directly validating the uniform deformation mode mediated by shear bands and challenging the traditional perception of “brittle fracture tendency” in amorphous matrix-based materials [37].

In conclusion, this study advances the theory of shear band evolution in amorphous composites and provides experimental and theoretical support for the design of high-performance amorphous matrix-based structural materials from the perspective of microstructure–property relationships, holding significant academic and engineering value.

5. Conclusions

① The Ti₄₈Zr₁₈V₁₂Cu₅Be₁₇ semi-solid amorphous composites exhibit work hardening after yielding, with a total strain of 29.6% (comprising 5.1% elastic strain and 24.5% plastic strain). Their yield strength and maximum strength are 1249 MPa and 1896 MPa, respectively.

② Once shear bands initiate, stresses drive their evolutionary behaviors, including propagation, intersection, multiplication, and branching. These behaviors contribute to plasticity enhancement and work hardening.

③ The compressive deformation of this composite primarily proceeds in three stages: in the early stage, shear bands are confined to forming and propagating within β-Ti crystals; in the middle stage, they extend into the amorphous matrix; and in the late stage, multidirectional shear bands interweave into a “pineapple-like” morphology, accompanied by microcracks, with significant work hardening and ductile fracturing characteristics evident.

6. Outlook

Structural components manufactured from amorphous composites generally require long-term serviceability. Nevertheless, this study neglected to examine the relationship between fatigue performance and shear band stability under long-term service conditions. Furthermore, the quantitative correlation between β-Ti crystal size distribution and material properties awaits further exploration.