Since the findings of new multicomponent glassy alloys exhibiting high thermal stability of supercooled liquid associated with the formation of bulk glassy alloys around 1990[1–3], many efforts have been made to develop bulk glassy alloys as a new type of metallic material with glassy structure in a bulk form in conjunction with the clarification of fundamental properties and the searches for novel application fields[4–7]. It is known that the syntheses of bulk glassy alloys since 1989 have been made in the order of La-, Mg- and then Zr-based alloy systems[4–7] and their maximum diameters for glass formation exceed largely 10 mm[5–7]. For Zr-based bulk glassy alloys which are important for engineering applica-tions, their alloy systems can be roughly classified to the following three types: Zr-Al-TM (TM=Co, Ni, Cu)[2], Zr-Ti-Be-Ni-Cu[3] and Zr-Cu-Al-Ag[8] types. The first and the second type bulk glassy alloys have been used as practical materials in machinery, optical, pin-spring, casing, sporting goods and medical instrument parts. For instance, the application of pin-type spring glassy alloy used in personal computer, mobile phones and smart phones based on Zr-Al-Ni-Cu system becomes significant year by year[7]. Considering the recently increasing engineering importance of bulk glassy alloys for Zr-Al-Ni-Cu system, it is important to examine the stability to structural relaxation of the Zr-based glassy alloys subjected to annealing at the temperatures below glass transition temper-ature (Tg)[9–11]. It has been reported that a typical Zr55Al10Ni5Cu30 bulk glassy alloy has rather high stability to annealing-induced structural relaxation at the temperature well below Tg and exhibits a single-stage change in the annealing-induced enthalpy structural relaxation as a function of annealing temperature[12]. The similar single-stage enthalpy relaxation behavior has also been reported for other typical bulk glassy alloys with optimum alloy composition in each alloy system such as La55Al25Ni10Cu10[13], Pd42.5Cu30Ni7.5P20[14], Ni40Pd40P16B4[15] and Ni60Pd20P16B4[15].
For engineering Zr-Al-Ni-Cu bulk glassy alloys, it has been reported that the enrichment of Zr ranging from 55% to 70%, namely, from Zr55Al10Ni5Cu30 (55Zr) to Zr70Ni16Cu6Al8, causes distinct increase in Poisson's ratio, plastic strain, fracture toughness, corrosion resistance and stress-corrosion cracking resistance, though the yield strength, Young's modulus and hardness decrease slightly[16]. The 70Zr bulk glassy alloy has still high glass-forming ability with a large diameter of 10 mm and hence can be regarded as a more interesting engineering structural material. In addition, the enrichment of Zr content from 55% to 70% is presumed to increase the annealing-induced structural relaxation owing to the increase in the number of Zr-Zr atomic pairs with weak bonding nature as compared with 55Zr alloy. Recently, there has been a rapidly increasing interest in high entropy type and pseudo high entropy type bulk glassy alloys. However, little is known about the structural relaxation behavior of their multicom-ponent glassy alloys upon annealing. The clarification is essential for future extension as engineering materials. Here, multicomponent (Zr, Hf)65Al7.5Ni10Cu17.5 glassy alloys were selected to examine the change in the enthalpy and hardness relaxation stabilities of glassy phase by the addition of Hf which has nearly zero heat of mixing to Zr. There are few reports on the annealing-induced enthalpy relaxation behavior of Zr-rich 70Zr and (Zr, Hf)65Al7.5Ni10Cu17.5 bulk glass-type alloys which are expected to exhibit improved characteristics.
This research aims to examine enthalpy relaxation behavior of 70Zr and (Zr0.75Hf0.25)65Al7.5Ni10Cu17.5 (65Zr0.75Hf0.25) bulk glass-type alloys subjected to annealing in a wide temperature range below Tg in comparison with 55Zr alloy, and to clarify the change in the annealing-induced enthalpy relaxation with the shift to Zr-rich composition and the significant replacement of Zr by Hf for Zr-Al-Ni-Cu base bulk glassy alloys. In addition to the enthalpy relaxation, we have also examined, for comparison, the change in Vickers hardness with Ta for 70Zr, 55Zr, 65Zr0.75Hf0.25 and (Zr0.5Hf0.5)65Al7.5Ni10-Cu17.5(65Zr0.5Hf0.5) alloys. Based on the structure and hardness relaxation data, we further investigated the feature of annealing-induced structural and hardness relaxations for the high-order multicomponent bulk glass-type alloys.
Bulk glass-type alloys with nominal atomic composition of Zr70Al7.5Ni8Cu14.5 (70Zr), Zr55Al10Ni5Cu30 (55Zr), (Zr0.75Hf0.25)65-Al7.5Ni10Cu17.5 (65Zr0.75Hf0.25) and (Zr0.5Hf0.5)65Al7.5Ni10Cu17.5 (65Zr0.5Hf0.5) were chosen because they have a large supercooled liquid region and high glass-forming ability with large diameters of centimeter-class in spite of significant difference in the major host metal contents[17–19]. Besides, hypoeutectic bulk glassy alloy with Zr content as high as 70% exhibits excellent mechanical and chemical properties[19]. The specimens used for the present structure and hardness relaxation studies were in a ribbon form with about 25 μm in thickness and 1.2 mm in width, and prepared by the single roller melt spinning method. The amorphous structure was identified by X-ray diffraction (XRD) and ordinary DSC measurement. The measurement of temperature dependence of apparent specific heat was conducted by DSC equipment (PerkinElmer DSC8000 type) at a scanning rate of 0.67 K/s in a flowing argon atmosphere. The annealing treatments at 473‒553 K for 1‒12 h were carried out in a flowing argon atmosphere. Vickers hardness as a function of annealing temperature was also measured at room temperature by a Vickers hardness tester under a load of 0.49 N.
Fig.1 shows apparent specific heat curves of the as-spun 70Zr, 55Zr, 65Zr0.75Hf0.25 and 65Zr0.5Hf0.5 glassy alloys. These four alloys exhibit distinct glass transition, followed by a large supercooled liquid region and crystallization. The glass transition temperature (Tg), onset temperature of crystallization (Tx) and supercooled liquid region (ΔTx) are 604, 710 and 106 K for 70Zr alloy, respectively; 665, 780 and 104 K for 55Zr alloy, respectively; 676 , 757 and 92 K for 65Zr0.75Hf0.25 alloy, respectively; 690, 770 and 80 K for 65Zr0.5Hf0.5 alloy, respectively. The increase in Zr content as well as the replacement amount of Zr by Hf increase Tg and Tx, while the ΔTx decreases with increasing the replacement amount of Zr by Hf through the rapid increase in Tg.
Fig.1 Apparent specific heat curves of as-spun glassy alloys
Fig.2 shows the temperature dependence of apparent specific heats Cp for 70Zr alloy in as-quenched state and annealed at the temperatures between 423 and 623 K for 1, 3, 6 and 12 h. The Cp,q, Cp,s and Cp,a represent the apparent specific heats of the as-quenched sample, the reference sample heated at a heating rate of 0.67 K/s up to 653 K followed by holding at 653 K for 60 s, and then cooled to room temperature at 0.67 K/s, and the samples annealed at different temperatures of 423‒623 K for 1, 3, 6 and 12 h, respectively. The Cp,q shows an irreversible exothermic reaction with an onset temperature of about 395 K. The Cp,a starts to deviate from the Cp,s curve at each annealing temperature (Ta), which shows an excess endothermic reaction, followed by a maximum peak, and then merges with the Cp,q curve. The endothermic reaction occurs reversibly and its peak, temperature and endothermic enthalpy quantity are dependent on annealing condition. The irreversible reaction is due to the change from as-quenched disordered atomic configurations to relaxed disordered configurations upon heating, while the reversible endothermic peak is due to the change of annealing-induced relaxed atomic configurations to disordered configu-rations which are in an internal equilibrium state at higher heating temperatures.
Fig.2 Temperature dependence of apparent specific heats of Zr70Al7.5Ni8Cu14.5 glassy alloy in the as-quenched state (Cp,q), heated to 653 K for 60 s (Cp,s), and annealed at 423‒623 K (Cp,a) for different annealing time (ta): (a) 1 h, (b) 3 h, (c) 6 h, and (d) 12 h
Fig.3 shows the temperature dependence of Cp,q, Cp,s and Cp,a of 55Zr alloy in as-quenched and annealed states at the temperatures between 423 and 673 K for 1, 3, 6 and 12 h. The Cp,s was obtained for the sample heated at a heating rate of 0.67 K/s to 725 K followed by holding for 60 s, and then cooled at 0.67 K/s to room temperature. The Cp,a presents the apparent specific heat of the samples annealed for 1, 3, 6 and 12 h at temperatures of 423‒673 K. The features of the temperature dependence of Cp,q, Cp,s and Cp,a are nearly the same as those (Fig.2) obtained for 70Zr alloy.
Fig.3 Temperature dependence of apparent specific heats of Zr55Al10Ni5Cu30 glassy alloy in the as-quenched state (Cp,q), heated to 725 K for 60 s (Cp,s), and annealed at 423‒673 K (Cp,a) for different annealing time (ta): (a) 1 h, (b) 3 h, (c) 6 h, and (d) 12 h
Fig.4 shows the temperature dependence of Cp,q, Cp,s and Cp,a of 65Zr0.75Hf0.25 alloy in as-quenched and annealed states at the temperatures between 473 and 648 K for 1, 3, 6 and 12 h. The Cp,s was obtained for the sample heated at a heating rate of 0.67 K/s to 718 K followed by holding for 60 s, and then cooled at 0.67 K/s to room temperature. The Cp,a also presents the apparent specific heat of the samples annealed at different temperatures of 473‒648 K for 1‒12 h. The features of the temperature dependence of Cp,q and Cp,s are nearly the same as those for 70Zr (Fig.2) and 55Zr (Fig.3) alloys, while the Cp,a shows a distinct difference in the endothermic peak behavior with annealing temperature (Ta). The endothermic amount of Cp,a curve shows a maximum at Ta=523 K, then decreases with increasing Ta and then increases again rapidly at Ta near Tg. It is thus noticed that the change in Cp,a with Ta for 65Zr0.75Hf0.25 alloy is obviously different from that for 70Zr and 55Zr alloys.
Fig.4 Temperature dependence of apparent specific heats of (Zr0.75Hf0.25)65Al7.5Ni10Cu17.5 glassy alloy in the as-quenched state (Cp,q), heated to 718 K for 60 s (Cp,s), and annealed at 473‒648 K (Cp,a) for different annealing time (ta): (a) 1 h, (b) 3 h, (c) 6 h, and (d) 12 h
Fig.5 shows the temperature dependence of ΔCp,a (ΔCp,a= Cp,a–Cp, s) of 70Zr, 55Zr and 65Zr0.75Hf0.25 alloys on the basis of the data shown in Fig.2–Fig.4. The ΔCp,a of the former two alloys increases gradually with increasing Ta and then increases rapidly at Ta=573 K. The change in ΔCp,a as a function of Ta is monotonous and no distinct sub-peak is recognized. In addition, 65Zr0.75Hf0.25 alloy shows a distinctly different ΔCp,a behavior compared with 70Zr and 55Zr alloys. The ΔCp,a value shows the maximum at Ta=523 K, followed by the decrease to the minimum at Ta=573 K and a rapid increase at Ta near Tg. The change in ΔCp,a as a function of Ta does not occur in a monotonous mode, indicating the separable two maximum peaks for annealing-induced enthalpy relaxation.
Fig.5 Temperature dependence of endothermic peak ΔCp,a for Zr70Al7.5Ni8Cu14.5 (a) and Zr55Al10Ni5Cu30 (b) glassy alloys annealed at 423‒623 K for 1‒12 h, and for (Zr0.75Hf0.25)65Al7.5Ni10Cu17.5 glassy alloy annealed at 473‒648 K for 1‒12 h (c)
To show more clearly the Ta dependence of annealing-induced enthalpy relaxation, the maximum value of ΔCp,a and the calculated ΔHa=∫(Cp,a–Cp,s)dT value of 70Zr, 55Zr and 65Zr0.75Hf0.25 alloys obtained from Fig.5 are plotted as a function of Ta in Fig.6. The former two alloys exhibit a monotonous increase in the values of ΔCp,a,max and ΔHa and show the largest values at Ta=573 K near Tg, indicating the rapid increase due to the easy structural relaxation owing to the ordinary glass transition. 65Zr0.75Hf0.25 alloy shows an appreciable sub-peak phenomenon in the ΔCp,a,max(Ta) and ΔHa(Ta) behavior accompanied with the minimum phenomenon around Ta=573 K, and with further increasing Ta to Tg, their values increase rapidly due to the glass transition relaxation. Fig.7 shows the normalized curves of endothermic peak ΔCp,a and relaxation endothermic enthalpy ΔHa as a function of Ta/Tg. The 70Zr and 55Zr alloys also exhibit a monotonous increase in the values of ΔCp,a,max and ΔHa, but the 65Zr0.75Hf0.25 alloy shows an appreciable II-stage phenomenon. It is thus judged that the annealing-induced enthalpy relaxation occurs through a single stage with a peak at Ta=573 K near Tg for 70Zr and 55Zr alloys, while it occurs through distinguishable two stages for 65Zr0.75Hf0.25 alloy.
Fig.6 Changes in the maximum values of endothermic peak ΔCp,a,max and relaxation endothermic enthalpy ΔHa with annealing temperature (Ta): (a) Zr70Al7.5Ni8Cu14.5, (b) Zr55Al10Ni5Cu30, and (c) (Zr0.75Hf0.25)65Al7.5Ni10Cu17.5 alloys
Fig.7 Normalized curves of endothermic peak ΔCp,a,max and relaxation endothermic enthalpy ΔHa as a function of annealing temperature (Ta)/glass transition temperature (Tg): (a) Zr70Al7.5Ni8Cu14.5, (b) Zr55Al10Ni5Cu30, and (c) (Zr0.75Hf0.25)65Al7.5Ni10Cu17.5 alloys
Such a single-stage enthalpy relaxation behavior is similar to that for other multicomponent bulk glassy alloys with an opti- mum alloy component, i.e., La55Al25Ni20[20], Pd40Cu30Ni10P20 [21], Pd42.5Cu30Ni7.5P20[22], Ni40Pd40P16B4 and Ni60Pd20P16B[15]. This is different from the double-stage enthalpy relaxation behavior with distinctly separated sub-peak and main peak for metal-metal-metalloid type amorphous alloys containing two metallic elements or more such as Fe-Ni-P[23], Fe-Ni-B[23] and Fe-Ni-Co-Si-B[24]. On the contrary, the present single-stage relaxation behavior is similar to that for metal-metalloid type amorphous alloys such as Fe-P, Fe-B and Ni-P systems[25], though there is an essential difference in alloy components between the multicomponent bulk glassy alloys consisting of three kinds of metallic elements or more and the amorphous type binary alloys.
Fig.8 shows the change in Vickers hardness (HV) with Ta for 70Zr, 55Zr, 65Zr0.75Hf0.25 and 65Zr0.5Hf0.5 alloys. The HV increases gradually with increasing Ta and significantly at the Ta near Tg for 70Zr and 55Zr alloys, while the Ta dependence of HV for 65Zr0.75Hf0.25 and 65Zr0.5Hf0.5 alloys is different, i.e., the HV increases with Ta, followed by the maximum at Ta=523 K, a slight decrease at Ta=573 K and then an increase at Ta near Tg. The Ta dependence of HV for the four glassy alloys is quite similar to the enthalpy relaxation peak behavior. The confirmation of the double-stage relaxation behavior through the measurement of HV is believed to be the first evidence, indicating that the enthalpy-relaxation is closely related to other properties.
Fig.8 Change in Vickers hardness (HV) with Ta for glassy alloys
It is known that Zr-Al-Cu ternary alloys can have a typical glassy structure with distinct glass transition and wide supercooled liquid region[2], and the most stable glass with nearly eutectic alloy composition and large supercooled liquid region is obtained for Zr50Al10Cu40[16]. Besides, the high stability of supercooled liquid to crystallization is due to the formation of medium-range ordered atomic configurations consisting of three elements of Zr, Al and Cu which include Al-centered first shell clusters[26]. The highest stability of the composition of Zr50Al10Cu40 implies that this atomic ratio is the most suitable for the construction of the medium-range ordered atomic configurations. Besides, it has been reported that the partial replacement of Cu by about 5wt% Ni causes further increase in ΔTx and glass-forming ability[27]. These previous results also suggest that the Zr-rich composition of Zr70Al7.5Ni8Cu14.5 deviates significantly from the optimum atomic ratio to Zr-rich side. The extra Zr element is presumed to allow the generation of Zr-Zr atomic pair with weaker bonding nature and shorter relaxation time in the base structure of the optimum medium-range ordered atomic configurations with longer relaxation time. The cooperative atomic rearrangement of the medium-range ordered atomic configuration corresponds to the ordinary glass transition, while the easy rearrangement of Zr-Zr atomic pair with much shorter relaxation time is presumed to cause the extra endo-thermic sub-peak reaction at the lower annealing temperature side. However, these presumption and expectation are not consistent with the experimental data where no sub-peak behavior is observed.
Here, it is important to point out that the single-stage enthalpy relaxation behavior for the 70Zr and 55Zr glassy alloys with large diameters of centimeter-class is similar to that for Pd42.5Cu30Ni7.5P20 and Ni60Pd20P17B3 bulk glassy alloys with large maximum diameters of 80[28] and 15 mm[29], respectively, belonging to metal-metalloid type. The disappear-ance of the first-stage sub-peak reaction suggests that the present Zr-based bulk glassy alloys can keep the skeleton structure of medium-range ordered atomic configurations even in the significantly deviated alloy composition of 70Zr. The medium-range ordered atomic configuration is dominated by the coexistence of larger atomic size of Zr, medium atomic size of Al, and smaller atomic size TM (Ni and Cu) elements with large negative heats of mixing among Zr, Al and TM elements, and almost independent of Zr content in the range of 55at%‒70at%. The maintenance of the medium-range or-dered atomic configuration in the wide Zr content range is presumed to promote the high glass-forming ability, leading to the formation of bulk glassy alloys with 10 mm in diameter even for 70Zr alloy, in addition to the appearance of the large supercooled liquid region[16].
In addition, as shown in Fig.5–Fig.8, the two-stage relaxations of enthalpy and hardness (HV) are recognized for 65Zr0.75Hf0.25 and 65Zr0.5Hf0.5 glassy alloys. Considering that the single-stage enthalpy and hardness relaxations are observed for 70Zr and 55Zr alloys, the appearance of the two-stage enthalpy and hardness relaxations for 65Zr0.75Hf0.25 and 65Zr0.5Hf0.5 alloys is concluded to originate from the generation of additional Zr-Hf atomic pair with weaker bonding and shorter relaxation time compared with other atomic pairs of Zr-Al, Zr-Ni, Zr-Cu, Hf-Al, Hf-Ni and Hf-Cu with larger negative heats of mixing[30]. The weaker bonding nature of Zr-Hf pair can be easily estimated from the nearly zero heat of mixing[30]. It is thus said that the significant replacement of major metallic element by other elements with nearly zero heat of mixing makes the structural relation easy in the low temperature region of about Tg‒150 K. This information also proves that the component multiplication to synthesize high entropy glassy alloys by adding the elements with nearly zero heat of mixing leads to the formation of glassy alloys with easier structural relation even at low temperatures in conjunction with the change in fundamental properties.
With the aim of investigating the effect of the Hf replacement on the double-stage relaxation of enthalpy and hardness for Zr65Al7.5Ni10Cu17.5 glassy alloy, we calculated five parameters, i.e., chemical mixing enthalpy (ΔHmix), atomic size difference (δ), entropy of mixing (ΔSmix), valence electron concentration (VEC) and electron negativity difference (Δχ) for the present 70Zr, 55Zr, 65Zr0.75Hf0.25 and 65Zr0.5Hf0.5 glassy alloys. The ΔHmix can be calculated according to the extended regular solution mode shown in Ref.[31–32]. The atomic size, electronegativity and VEC for Zr, Hf, Al, Ni and Cu elements used for the calculation are listed in Table 1. The calculated results of five parameters for 55Zr, 70Zr, 65Zr0.75Hf0.25 and 65Zr0.5Hf0.5 alloys are shown in Table 2. As can be seen in Table 2, no distinct difference is recognized for ΔHmix, δ, VEC and Δχ, while the ΔSmix shows clearly distinguishable values. The ΔSmix values of 65Zr0.75Hf0.25 and 65Zr0.5Hf0.5 alloys are 11.43 and 12.14 J·(mol·K)‒1, respectively, which are consider-ably larger than those (7.699‒8.896 J·(mol·K)‒1) for 70Zr and 55Zr alloys. Based on this data, it is confirmed that the multiplication of glassy alloy by the element with nearly zero heat of mixing to increase the mixing entropy value is effective for the enhancement of the first-stage relaxation tendency monitored by enthalpy or hardness at Ta in the vicinity of Tg‒150 K.
Table 1
Atomic size, electronegativity and VEC for Zr, Al, Ni, Cu and Hf elements[28] | Parameter | Zr | Al | Ni | Cu | Hf |
|---|
|
Atomic size/×10-1 nm |
1.603 |
1.432 |
1.246 |
1.278 |
0.158 |
|
Electronegativity |
1.33 |
1.61 |
1.91 |
1.90 |
1.30 |
|
VEC |
4 |
3 |
10 |
11 |
4 |
Table 2
Calculated parameters of ΔHmix, δ, ΔSmix, VEC and Δχ for Zr70Al7.5Ni8Cu14.5, Zr55Al10Ni5Cu30, (Zr0.75Hf0.25)65Al7.5Ni10Cu17.5, (Zr0.5Hf0.5)65Al7.5Ni10Cu17.5 alloys
| Alloy | ΔHmix/kJ·mol-1 | δ | ΔSmix/J·(mol·K)-1 | VEC | Δχ |
|---|
|
Zr55Al10Ni5Cu30 |
‒30.57 |
10.41 |
8.896 |
6.3 |
0.2649 |
|
Zr70Al8Ni16Cu6 |
‒36.66 |
9.56 |
7.597 |
5.3 |
0.2393 |
|
Zr70Al7.5Ni8Cu14.5 |
‒29.94 |
9.31 |
7.699 |
5.42 |
0.2396 |
|
(Zr0.75Hf0.25)65Al7.5Ni10Cu17.5 |
‒30.84 |
9.83 |
11.43 |
5.75 |
0.2574 |
|
(Zr0.5Hf0.5)65Al7.5Ni10Cu17.5 |
‒29.46 |
9.67 |
12.14 |
5.75 |
0.2610 |
It has previously been reported that the double-stage enthalpy relaxation behavior occurs through the generation of Fe-Ni, Fe-Co and Co-Ni atomic bonds with shorter relaxation time as compared with metal-metalloid atomic pairs for Fe41.5Ni41.5P17[23], Fe41.5Ni41.5B17[23] and (Fe, Co, Ni)75Si10B15[24] amorphous alloys where atomic configurations consisting of short-range ordered trigonal prisms are formed in a skeleton structure [33]. The present recognition of the double-stage enthalpy and hardness relaxations is believed to be the first evidence for metal-metal type multicomponent bulk glass-type alloys.
Fig.9 shows a schematic illustration of the changes in the ΔCp, endo, ΔHendo and HV with Ta for 70Zr, 55Zr, 65Zr0.75Hf0.25 and 65Zr0.5Hf0.5 glassy alloys, together with the previous data of other typical bulk glassy alloys such as Zr-Al-Cu[2], Zr-Al-Ni[2] and Pd-Cu-Ni-P[21] systems. Besides, the data of typical amorphous alloys such as Fe-B[25], Fe-P[25], Ni-P[25], Fe-Ni-P[23], Fe-Ni-B[23] and (Fe, Co, Ni)-Si-B[24] systems are also shown for comparison. The features of the figure can be summarized as follows: (1) clear difference in the annealing-induced enthalpy or hardness relaxation behavior between the quaternary 70Zr and 55Zr glassy alloys and the quaternary 65Zr0.75Hf0.25 and 65Zr0.5Hf0.5 glassy alloys, i.e., the single-stage relaxation for the quaternary alloys and the double-stage relaxation for the quaternary alloys; (2) no difference in the relaxation behavior for the quaternary metal-metal type bulk glassy alloys and the quaternary metal-metalloid type (Pd-Ni-Cu-P) bulk glassy alloys; (3) no difference in the relaxation behavior among the ternary and quaternary glassy alloys; (4) no close relation between the relaxation behavior and glass-forming ability; (5) no appreciable difference in relaxation behavior between enthalpy and hardness. Thus, the single-stage relaxation can be observed for the multicomponent glassy type alloys with the three components rule, and the replacement of the main constituent metal by other elements with nearly zero heat of mixing causes the change to the double-stage relaxation mode.
Fig.9 Change in enthalpy relaxation amounts (ΔCp,a and ΔHa) and HV with Ta for glassy alloys (the previous data of other typical bulk glass-type alloys and amorphous-type alloys are also shown for comparison)
In addition, when the enthalpy relaxation data (Fig.2, Fig.5 and Fig.6) of 70Zr alloy are compared with those (Fig.3, Fig.6 and Fig.8) for 55Zr alloy, one can recognize the following differences: (1) the temperature at which the irreversible exothermic relaxation begins to occur is about 395 K for 70Zr alloy, which is much lower than that (about 425 K) for 55Zr alloy; (2) the annealing-induced endothermic quantity for 70Zr alloy is considerably larger than for 55Zr alloy in spite of the much lower annealing temperature condition; (3) the temperature leading to an internal equilibrium supercooled liquid for 70Zr alloy is considerably lower than for 55Zr alloy. These differences also indicate that 70Zr alloy has a more unstable glassy structure which enables the progress of structural relaxation in the lower annealing temperature side, because of a looser medium-range ordered atomic configu-rations resulting from the deviation from the optimum Al and TM concentrations, though no distinct sub-peak relaxation behavior occurs.
Thus, the deviation of alloy component from the nearly optimum multicomponent alloy composition in the Zr-based bulk glassy alloys does not cause distinguishable double-stage reaction in the annealing-induced enthalpy or hardness structural relaxation behavior, which may be useful to understand the glass-forming ability of Zr-based bulk glassy alloys and to provide a guiding principle to search for an optimum alloy composition of highly ductile bulk glassy alloy. This expectation is supported by the favorable experimental evidence that the 70Zr bulk glassy alloy exhibits very high compressive plastic strain without final fracture[16], high fracture toughness[34] and high resistance to stress corrosion cracking[35].
1) Both Zr70Al7.5Ni8Cu14.5 (70Zr) and Zr55Al10Ni5Cu30 (55Zr) alloys show a single-stage enthalpy or hardness relaxation behavior with the peak at the temperature near Tg. No obviously distinguishable sub-peak relaxation behavior is recognized in the wide low Ta range even for the Zr-rich 70Zr alloy where a large number of Zr-Zr atomic pairs are included.
2) (Zr0.75Hf0.25)65Al7.5Ni10Cu17.5 (65Zr0.75Hf0.25) and (Zr0.5Hf0.5)65-Al7.5Ni10Cu17.5 (65Zr0.5Hf0.5) alloys exhibit distinguishable two-stage enthalpy or hardness relaxation behavior where each peak appears at Ta=Tg‒150 K and Ta≌Tg. The first-stage enthalpy or hardness peak is due to the atomic rearrangement of Zr-Hf atomic pair with shorter relaxation time via weaker bonding nature compared with other atomic pairs with longer relaxation time via stronger bonding nature resulting from large negative heats of mixing. The single-stage enthalpy or hardness relaxation as well as their relaxations at the higher temperature side in the second-stage enthalpy relaxation are due to the ordinary glass transition caused by the long-range cooperative atomic rearrangements of the constituent elements.
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