Journal of Materials Research and Technology Journal of Materials Research and Technology
Original Article
Effects of similar-element-substitution on the glass-forming ability and mechanical behaviors of Ti-Cu-Zr-Pd bulk metallic glasses
Haoling Jiaa,b, Xie Xieb, Lei Zhaoa, Jianfeng Wangc, Yanfei Gaob,d, Karin A. Dahmene, Weidong Lib,, , Peter K. Liawb,, , Chaoli Maa,,
a Department of Materials Science and Engineering, Beihang University, Beijing 100083, China
b Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996, USA
c School of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450001, China
d Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
e Department of Physics, University of Illinois at Urbana Champaign, Urbana, IL 61801, USA
Received 27 June 2017, Accepted 19 August 2017
Abstract

The Ti41Cu31Zr10Pd13 (at.%) metallic glasses are promising for bone-implantation applications due to their exceptional bio-compatibility. However, Pd, as a noble element, keeps the fabrication cost high and prevents the industrial sale production of these alloys. Searching for replacements with comparable glass-forming ability and ductility but lower cost turns out to be imperative. In this article, we used similar but less expensive elements to substitute Pd for such a goal. Specifically, 1–4at.% Ni and Pt are incrementally used to replace Pd in the base alloy. Careful characterizations of the glass-forming ability and the compressive ductility suggest that the Ti41Cu36Zr10Pd10Ni3 metallic glass retains both the glass-forming ability and the ductility, but cuts down the alloy cost by ∼22.66%. The Ti41Cu36Zr10Pd12Pt1 metallic glass, despite no substantial trimming in the alloy cost, doubles the ductility and fairly maintains the glass-forming ability. The serrated flow is observed on the plastic flow of most metallic glasses investigated and is quantitatively studied in the framework of the self-organized criticality. Our work provides important insights on defining appropriate commercialization routes of Ti-based bulk metallic glasses.

Keywords
Glass forming ability, Plasticity, Serrated flow, Self-organized criticality
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1Introduction

Metallic glasses, particularly following the appearance in bulk forms, have been extensively studied over the past few decades, given their exceptional functional and structural properties, such as good electrical conductivity, high strength, large elastic limit, and excellent corrosion and oxidation resistance [1–4]. Though metallic glasses in bulk forms have been reported in numerous systems, such as Zr-, Pd-, Ti-, Fe-, Ni-, and Co-based alloys [5–10], the glass-forming ability and the limited ductility at low temperatures remain two long-standing hurdles, considerably limiting their large-scale engineering applications as a class of advanced materials [2,3]. The glass-forming ability literally refrains metallic glasses from being made into components of large sizes, and the limited ductility virtually downgrades their load-bearing capability, both placing bulk metallic glasses into a dilemma for further advancements.

In terms of glass-forming ability, Inoue's empirical rules [11] are widely utilized as guidelines of searching for bulk metallic glasses with high glass-forming ability. The empirical rules states that an alloy would favor a glassy state when (1) it is composed of more than three types of elements, (2) atoms of major constituent elements are substantially different in size (>12%), and (3) the heat of elemental mixing presents negative values. More recent research results suggest that the glass-forming ability of bulk metallic glasses could be altered as well by partially replacing a given constituent element with similar ones. Similar elements herein refer to the neighboring elements in the periodic table. Examples in line with this principle can be found in Zr55Al10Ni5Cu30 metallic glasses [12], whose critical casting diameter is brought up to 16mm from 5mm (the Zr50Al10Cu40 metallic glasses) after the partial substitution of Cu with Ni, and (La0.5Ce0.5)65Al10Cu25 alloys [13], whose critical size is doubled, following the equal replacement of La with Ce in La65Al10Cu25.

Limited ductility in bulk metallic glasses is a direct macroscopic manifestation of their amorphous microstructure. Owing to the absence of cooperative plasticity-mediation mechanisms like dislocations in crystalline counterparts, shear bands tend to develop into cracks swiftly through void nucleation and coalescence [14], followed by catastrophic propagation throughout the entire sample. Such an express shear-band-to-crack transition essentially leads bulk metallic glasses to fail in a brittle manner in most circumstances. Extrinsically, intentionally-designed geometric constraints can help defer shear-band propagation and improve ductility through prompting the formation of denser shear bands. Preparing metallic-glass composites with nano to micro sized crystalline inclusions [9] and adopting surface modifications, such as affixing thin-film coatings [2,15], or introducing residual stresses [16,17] are two examples of many to stop shear-band propagation and improve ductility. Intrinsically, early research activities empirically advocate that the ductility of metallic glasses is correlated with their Poisson's ratio (or equivalently the shear-to-bulk modulus ratio) [18]. After some exceptions found [19], recent works focus more on identifying a physically-meaningful microstructural parameter. Among copious concepts, the structural heterogeneity concept [3,20] is the one mostly studied, and it indeed offers a multitude of valuable insights for interpreting ductility vs. brittleness in bulk metallic glasses.

The Ti-based metallic glasses are a family of alloys of particular interest since they have promising applications in medical implants as a result of lightweight and bio-compatibility attributes. However, as many other metallic glasses, the Ti-based metallic glasses, which is the focus of the present work, suffer from the same plights aforementioned. Their glass-forming ability is comparatively low with the critical size for the majority of prepared samples, limited to below 5mm. Whilst Ti-Zr-Be-Cu-Ni [21] and Ti-Zr-Cu-Pd-Sn [10] alloys can be fabricated in a size of greater than 8 and 10mm, respectively, both alloys are unrealistic for clinical applications. Ti-Zr-Be-Cu-Ni metallic glass contains the toxic Be; Ti-Zr-Cu-Pd-Sn metallic glasses carry the noble Pd element, which is unfavorable in the spirit of controlling cost. It is therefore necessary to explore the synthesis of low-cost and bio-friendly Ti-based metallic glasses in order to adapt for medical needs. Certainly, sufficient ductility is also a requisite for these metallic glasses to ensure a reasonably long service life. The present work attempts to use Ni and Pt to substitute the Pd element in Ti-Cu-Zr-Pd alloys for the purpose of cutting down the alloy cost by partially removing the noble element. Effects of elemental substitutions on the glass-forming ability and mechanical properties are thoroughly investigated.

2Experimental

Ti41Cu36Zr10Pd13−xNix at.% (x=0, 1, 2, and 3) and Ti41Cu36Zr10Pd13−yPty at.% (y=1, 2, 3, and 4) alloy ingots were prepared by arc-melting constituent elements (purity>99.9%) in argon atmosphere. Rod specimens with a diameter less than 4mm were remelted in the quartz tube and injected into the copper mold, while those having larger diameters were remelted in the quartz cup using a tilting-induction furnace, and then poured into the copper mold. The microstructure of the prepared specimens was examined using the Bruker AXS D8 X-ray diffractometer with CuKα radiation at a scanning rate of 3°/min, and their thermal stability was characterized by a NETZSCH DSC 404C Differential Scanning Calorimeter (DSC) at a heating rate of 0.33K/s. Compressive tests were conducted on the Sans testing machine at a strain rate of 2×10−4m/s, with the specimen size of 2mm in diameter and 4mm in length. The fracture morphology and outer surface of the fractured specimens were examined with scanning-electron microscopy (SEM).

3Results and discussion3.1Glass-forming ability

Fig. 1 gives the X-ray diffraction patterns of the base metallic glass (Ti41Cu36Zr10Pd13) and the Ni-substituted and Pt-substituted counterparts. A broad diffraction peak, a sign of an amorphous microstructure, is characteristic of all alloys but Ti41Cu36Zr10Pd10 Pt3 and Ti41Cu36Zr10Pd9 Pt4. The glass-forming ability of metallic glasses is herein characterized by two out of many proposed criteria – the critical sample size that can retain an amorphous state and the width of the supercooled liquid region, ΔTx=TxTg, where Tx and Tg are the onset temperature of crystallization and the glass-transition temperature. Table 1 lists Tx, Tg, ΔTx, and the critical diameter of rod specimens, Dcr, of all the investigated alloys; the thermal stability data are extracted from the DSC endothermic traces in Fig. 2. The base metallic glass, Ti41Cu36Zr10Pd13, is found to have the best glass-forming ability, with a ΔTx of 48K and the critical sample size up to 6mm in diameter. Substituting the Pd element with 1–3at.% Ni tends to bring down the glass-forming ability slightly, as indicated by the mild reduction in ΔTx and Dcr. But given the fact that the reduction in Dcr is fairly small, and the unit price of the Pd element is nearly 2000 times more expensive than the Ni element, the Ni substitution of Pd can still be regarded as a beneficial act to cut down the alloy cost while sufficiently maintaining the glass-forming ability of the Ti-Cu-Zr-Pd metallic glasses. Using Pt to partially replace Pd, however, degrades the glass-forming ability substantially in most cases. As 2–4at.% Pt is doped, the critical sample size sharply runs below 2mm. Substituting Pd with 1at.% Pt will lead to declined glass-forming ability too but very slightly, Dcr reducing from 6mm to 4mm. It is therefore concluded that substituting Pd with up to 3at.% Ni or 1at.% Pt will not impair the glass-forming ability of the Ti-Cu-Zr-Pd metallic glasses appreciably, but lower the alloy fabrication cost significantly.

Fig. 1.
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XRD patterns of the rod specimens of (a) the Ti41Cu36Zr10Pd13−xNix metallic glasses and (b) the Ti41Cu36Zr10Pd13−yPty metallic glasses at various diameters.

Table 1.

Thermal properties and glass-forming ability information of the base metallic glass, Ti41Cu36Zr10Pd13, and the Ni- and Pt-substituted metallic glasses.

Alloy  x/y  Tg (K)  Tx (K)  ΔTx (K)  Dcr (mm) 
Ti41Cu36Zr10Pd13−xNix696  744  48 
665  708  43 
666  708  42 
694  736  42 
Ti41Cu36Zr10Pd13−yPty662  702  40 
672  704  32 
672  704  32  <2 
670  703  33  <2 
Fig. 2.
(0.18MB).

DSC traces of (a) the Ti41Cu36Zr10Pd13−xNix metallic glasses and (b) the Ti41Cu36Zr10Pd13−yPty metallic glasses with the onset temperature of crystallization (Tx) and the glass transition temperature (Tg) marked.

Since Ni, Pt, and Pd appear in the same group of the periodic table and have the comparable atom sizes, similar valence electronic structures, and identical face-centered cubic structures in their crystalline states, the variation of the glass-forming ability as a consequence of the elemental substitution is mostly likely to be associated with the short- or medium-range order prevailing in amorphous solids [22]. It is known that the superior glass-forming and retaining ability in metallic glasses is governed by the density of icosahedra-like atomic clusters [23,24] or local crystal-like orders [25,26]. Both of these two medium-range ordered features in amorphous solids can effectively amplify the energy barrier for crystallization. The more they exist in an amorphous material, the better glass-forming ability the material possesses. Therefore, the diminishment of the glass-forming ability in our Ni- and Pt-substituted Ti-Cu-Zr-Pd alloys could be attributed to the disturbance of the icosahedra-like atomic clusters profusely present in the base metallic glass by the intrusion of Ni and Pt atoms.

3.2Strength and ductility

The engineering stress–strain data of the Ni-substituted metallic glasses and the Pt-substituted metallic glasses are plotted in Fig. 3(a) and (b) alongside the base metallic glass, Ti41Cu36Zr10Pd13. Note that the partially crystallized Ti41Cu36Zr10Pd13−yPty (Fig. 1(b)) alloys are excluded from the drawing. From Fig. 3(a), it is seen that the substitution of the Pd element with 1at.% Ni element drastically drops the yield strength of Ti-based alloys (∼21%) and transforms the glass from moderately ductile failure to completely brittle failure. A 2at.% Ni substitution results in a minimal deduction in yield strength but the sample still fails as soon as it overpasses yielding. Further growing the Ni content to 3at.%, however, fairly maintains both the strength and ductility, as found in the base metallic glass. In contrast, comparing with the Ni substitution, the Pt substitution plays a remarkably disparate role, as suggested by Fig. 3(b), i.e., the plastic strain is extended twice without the deterioration of strength at Pt=1at.%, and both the strength and ductility are comparable to the base alloy when Pt equals 2at.%.

Fig. 3.
(0.17MB).

The compressive stress–strain curves of (a) the Ti41Cu36Zr10Pd13−xNix metallic glasses and (b) the Ti41Cu36Zr10Pd13−yPty metallic glasses, tested on the specimens of 2mm in diameter.

Two representative metallic glasses, Ti41Cu36Zr10Pd12Ni1 rupturing in brittle manner and Ti41Cu36Zr10Pd12Pt1 failing plastically, are selected to inspect fracture characteristics for the purpose of unveiling mechanisms governing the ductility or brittle failure. The outer surfaces of the fractured specimens of these two metallic glasses are presented in Fig. 4(a) and (b), respectively. As in many other metallic glasses, quasi-static compression leads to failure in an inclined angle of 40–46° with respect to the loading direction, hinting the co-function of the shear stress and the normal stress [27,28]. Another important feature noticed is that in the Ti41Cu36Zr10Pd12Ni1 metallic glass, the outer surface is nearly featureless with very few shear bands detected, but the Ti41Cu36Zr10Pd12Pt1 metallic glass, following the ductile fracture, exhibits an abundance of shear bands interweaving with each other. Some of them are primary while others are secondary or even lower-order branched from the upper-level ones. The magnified SEM image of the fracture surface along the major shear band in Fig. 4(b) is shown in Fig. 4(c), in which the failure features with typical vein-like patterns oriented toward the shearing direction. Moreover, many liquid-like balls are observed on the fractograph, suggesting the occurrence of local melting owing to brisk plastic dissipation [29].

Fig. 4.
(0.35MB).

The fracture surfaces of (a) the Ti41Cu36Zr10Pd12Ni1 metallic glass and (b) the Ti41Cu36Zr10Pd12Pt1 metallic glass, along with (c) the high magnification view of the shearing surface in (b) showing typical vein patterns and melting balls.

It is now widely accepted that the degree of plasticity of a metallic glass is proportional to the shear band density eventually presented. Many monolithic metallic glasses fail catastrophically because all plastic strains are concentrated on a dominant shear band. Since the amount of shear strains that can be sustained by each individual shear band is limited, redistributing the plastic strain to as many shear bands as possible becomes an effective strategy for the ductility enhancement [30,31]. Likewise, the brittleness and the improved ductility in Ti41Cu36Zr10Pd12Ni1 and Ti41Cu36Zr10Pd12Pt1 metallic glasses of the current work relates as well to the denseness of shear bands. At the microscopic level, shear-banding behavior and the related ductile or brittle fracture in bulk metallic glasses is correlated with the dynamical structural heterogeneity, a concept that has ever been probed and verified by molecular dynamic simulations [32] and various experimental techniques, including high energy X-ray diffraction and anisotropic pair-density function analysis [33], statistical atomic force microscopy [34,35], dynamical micropillar compression tests [36], and instrumented indentation [3,20,37]. The conceptual model thinks of a metallic glass composed of liquid-like regions and solid-like regions. Liquid-like regions possess lower packing densities, lower moduli/hardnesses, and higher energy dissipation rates, serving as the source of structural heterogeneities. Mechanistically, liquid-like regions respond in such a way as Newtonian flows, whereas, solid-like regions follow the classical Hooke's law; consequently, the overall constitutive behavior of a metallic glass is represented by a Kelvin-type viscoelastic solid in the form of

where χ and η are the volume fraction and the viscosity of liquid-like regions [36]. The elastic spring term in Eq. (1) embodies solid-like regions, and the viscous dashpot part stands for liquid-like regions. Within this theoretical framework, W. Li et al. [3] successfully established an empirical relationship between the fracture energy density (a quantitative measure of the brittleness or the ductility of a material) and the density of structural heterogeneities, in the exponential form of
where E is the fracture energy density, ρdef is the density of structural heterogeneities, and c1 and c2 are empirical constants.

In light of this theory, the varied brittle and ductile mechanical behaviors of the Ti-based metallic glasses after substituting Pd with 1–3at.% Ni or Pt is most likely to be attributable to the fluctuation in the density of structural heterogeneities. The change in the quantity of structural heterogeneities, on the other hand, roots in the alternation of icosahedral clusters. As discussed in Section 3.1, the variously reduced glass-forming ability in Ni- and Pt-substituted Ti-based metallic glasses is presumably caused by the declining density of icosahedra-like atomic clusters due to the intrusion of Ni and Pt elements [23–25]. Besides, icosahedra-like clusters also serve as the atom-level source of structural heterogeneities [32], and the reduction in their density by the disturbance of Ni and Pt elements induces metallic glasses to transit from being modest ductile to utterly brittle. This rationale, however, finds the exception in the Ti41Cu36Zr10Pd12Pt1 alloy, whose glass-forming ability is slightly reduced but ductility is enhanced compellingly. This trend simply implies that the ductility of metallic glasses might be mandated by factors other than icosahedra clusters. Further investigation on this trend is need.

3.3Serrated plastic flow

The plastic flow of metallic glasses subjected to uniaxial loading universally exhibits serrated behavior, i.e., the stress raises and drops in a cyclic and discontinuous manner. Likewise, the serrated flow is observed for most Ti-based metallic glasses studied presently, as shown in Fig. 3. The detailed examination of the serrated plastic flow for three representative metallic glasses, Ti41Cu36Zr10Pd13, Ti41Cu36Zr10Pd12Pt1, and Ti41Cu36Zr10Pd11Pt2, is provided in Fig. 5, in which each plot is an enlarged view of the enclosed box on the corresponding curve in Fig. 3. Dissecting Figs. 3 and 5 jointly suggests the subdivision of the serrated flow into two regions: in the region I, each cycle starts with an abrupt stress shock of high magnitudes, followed by several aftershocks of smaller magnitudes; in the region II, smaller aftershocks become absent and each cycle of stress shock only contains the major one and some slight vibration originating from the testing fixture [38]. The two-region feature in the serrated flow is reminiscent of the self-organized criticality (SOC), a property commonly existing in complex dynamical systems [39,40]. The smaller serration events in the region I signifies an apparent SOC behavior of metallic glasses in response to dynamically fluctuated loads. These multiple stress undulations tend to drive the system to dynamically self-organize to a new critical state. The gradual absence of the smaller serrations as the flow enters the region II, on the other hand, is an implication of the decaying self-organization behavior.

Fig. 5.
(0.34MB).

The enlarged view of the serrated flow in (a) the Ti41Cu36Zr10Pd13 base metallic glass, (b) the Ti41Cu36Zr10Pd12Pt1 metallic glass, and (c) the Ti41Cu36Zr10Pd11Pt2 metallic glass at the corresponding enclosed box in Fig. 3.

With an intention of withdrawing more useful information from the serrated plastic flow, stress drops in all metallic glass samples but the brittle Ti41Cu36Zr10Pd12Ni1 are extracted and plotted in Fig. 6 against time. The complementary cumulative probability of the stress drops shown in Fig. 7 is fitted using a power-law distribution accompanied by a squared exponential decay function [38]

where A is the normalization constant, β is the scaling exponent, and Sc is the cut-off drop stress. Since it is widely recognized that serration events observed in metallic glasses are a manifestation of the shear-banding events going on at the micro-scale, the fitting parameters in Table 2 obtained through Eq. (3) are expected to offer useful fingerprints on dynamics of shear-banding events. The β values for five metallic glasses vary only in a very narrow range, quantitatively confirming the SOC behavior of the serrated flow, in agreement with Wang et al. [38]. The SOC behavior features systems having a character (β here) independent of chemical composition, plastic strain, yield stress, and so forth [38]. Sc is the cut-off drop stress, beyond which the squared exponential decay term comes into play. Its values for each metallic glass withdrawn from Eq. (3) fitting is marked in the corresponding graph of Fig. 7 and plotted against the plastic strain in Fig. 8. Generally, Sc decreases with the increasing plasticity in a nonlinear way, as opposed to the cut-off elastic energy density studied in the same way in Ref. [38]. Since the elastic strain energy is a multiplication of the stress and strain, the current finding is indicative that the cut-off burst strain positively relates with the plasticity, whereas, the cut-off drop stress negatively correlates with the plasticity in metallic glasses.

Fig. 6.
(0.23MB).

The stress drop as a function of time for (a) Ti41Cu36Zr10Pd13, (b) Ti41Cu36Zr10Pd11Ni2, (c) Ti41Cu36Zr10Pd10Ni3, (e) Ti41Cu36Zr10Pd12Pt1, and (e) Ti41Cu36Zr10Pd11Pt2.

Fig. 7.
(0.23MB).

The complementary cumulative distribution of the stress drop for (a) Ti41Cu36Zr10Pd13, (b) Ti41Cu36Zr10Pd11Ni2, (c) Ti41Cu36Zr10Pd10Ni3, (e) Ti41Cu36Zr10Pd12Pt1, and (e) Ti41Cu36Zr10Pd11Pt2. Red dashed lines indicate the cut-off drop stress, Sc, attained from Eq. (3) fitting.

Table 2.

The plastic strain and fitting parameters in Eq. (3) for the base metallic glass, Ti41Cu36Zr10Pd13, and the Ni- and Pt-substituted metallic glasses.

Alloy  x/y  Plastic strain (%)  A  β  Sc (MPa) 
Ti41Cu36Zr10Pd13−xNix2.0  0.29  69.0 
5.2  0.60  137.9 
4.2  0.51  91.1 
Ti41Cu36Zr10Pd13−yPty12  6.3  0.66  85.5 
2.7  0.35  70.7 
Fig. 8.
(0.07MB).

The cut-off drop stress extracted from fitting Eq. (3) to Fig. 7 as a function of the compressive plastic strain. The trend line shown is for visual guide.

4Summary

In summary, our exploration of using similar but less or comparatively expensive elements to partially substitute the noble element Pd in the Ti41Cu36Zr10Pd13 metallic glass yields the findings as follows.

  • (i)

    Substituting Pd with 3at.% Ni retains both the glass-forming ability and the compressive ductility found in the base metallic glass, but cuts down the alloy fabrication cost by ∼22.66%. The 1–2at.% Ni substitution, on the other hand, yields embrittlement in metallic glasses though the comparative glass-forming ability can be preserved.

  • (ii)

    Pt and Pd have comparable unit prices. Substituting Pd with 1at.% Pt doubles the compressive ductility and also fairly maintains the glass-forming ability relative to the base alloy. The 2at.% Pt substitution leads to both the significantly degraded glass-forming ability and ductility. As the Pt substitution is in excess of 2at.%, apparent crystallization begins precipitating.

  • (iii)

    Considering the fabrication cost, glass-forming ability, and ductility, the Ti41Cu36Zr10Pd10Ni3 metallic glass will be the first priority replacement of the Ti41Cu36Zr10Pd13 metallic glass followed by Ti41Cu36Zr10Pd12Pt1. The Ti41Cu36Zr10Pd10Ni3 metallic glasses can be considered for potential large-scale production, while Ti41Cu36Zr10Pd12Pt1 can meet some special needs in which superior plasticity is required.

  • (iv)

    Statistical analysis of the serrated flow in the investigated metallic glasses suggests the self-organized criticality behavior. The cut-off drop stress is positively correlated with the plasticity, whereas, the cut-off burst strain is negatively correlated.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

The present work was financially supported by the State 863 Project (Grant No. 2007 AA03Z520). We would like to greatly acknowledge the support of the Department of Energy (DOE), Office of Fossil Energy, National Energy Technology Laboratory (DE-FE-0008855, DE-FE-0024054, and DE-FE-0011194), with Mr. V. Cedro, Mr. R. Dunst, Dr. P. Rawls, and Dr. J. Mullen as program managers. We would also very much appreciate the support of the U.S. Army Research Office project (W911NF-13-1-0438) with the program manager, Dr. D.M. Stepp and the support from the National Science Foundation (DMR-1611180) with the program director, Dr. D. Farkas. YFG was sponsored by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division.

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Corresponding author. (Chaoli Ma clma2001@gmail.com)
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