Ni Rich Bulk Metallic Glasses with High Glass Forming Ability and Good Metallic Properties

Ni-Rich Bulk Metallic Glasses with High Glass-Forming Ability

and Good Metallic Properties

Yuqiao Zeng

, Nobuyuki Nishiyama

, Tokujiro Yamamoto

and Akihisa Inoue

1

Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

2_{R&D Institute of Metals and Composites for Future Industries (RIMCOF), Sendai 980-8577, Japan}

Glass-forming ability, thermal behavior and mechanical properties of Ni80xPdxP20 (10at%6x635at%) glassy alloys have been

investigated. The alloys with Pd contents higher than 25 at% exhibit high glass-forming ability, as is evidenced from the formation of a cylindrical Ni50Pd30P20glassy rod with a diameter of 21 mm. The Ni-rich bulk metallic glasses show relatively high strength in compression and

tension tests. More notably, plastic strains exceeding 2% were obtained for all the metallic glasses at room temperature in compression test. [doi:10.2320/matertrans.MRA2008453]

(Received December 8, 2008; Accepted July 10, 2009; Published September 2, 2009)

Keywords: bulk metallic glass, nickel-rich alloy, glass-forming ability, ductility

1. Introduction

During the last decade, a number of alloys with high glass-forming ability (GFA) have been developed, such as Al-,1)

Mg-,2) _Zr-,3,4) _{and Pd-Cu-}5) _{rich alloys, which enables the}

formation of bulk metallic glasses (BMGs) with large size. In the case of Ni-rich alloy, after the first success in producing Ni-rich Ni-Nb-Cr-Mo-P-B BMG6)in 1990, BMGs have been formed in various alloy systems, such as Ni-Nb-Ti-Zr,7) Ni-Ti-Zr-(Si,Sn),8) and Ni-Ta-Sn9) etc. These Ni-rich BMGs exhibit favorable properties for practical application, such as outstanding corrosion resistance and ultrahigh strength. However, two serious weaknesses need to be overcome. One is the limited GFA. The critical diameter for glass formation (Dmax) is below 5 mm for most of these

alloys. The other is the limited ductility. Most of Ni-rich BMGs show a compressive plastic strain smaller than 2% at room temperature. The further enhancement of GFA and ductility is expected to enable a new type of Ni-rich BMGs with higher reliability.

We have reported the formation of a Ni-rich Ni50Pd30P20 cylindrical BMG with a diameter of 7 mm by water quenching (WQ) technique associated with B2O3 flux treat-ment.10) The WQ BMG sample shows relatively high strength of about 1780 MPa and a large plastic strain of about 7% in compression test. This result implies that Ni-rich BMGs with high GFA and good mechanical properties can be developed in Ni-Pd-P system. The aim of this paper is to investigate the GFA, thermal behavior and mechanical properties of the Ni-Pd-P BMGs. The relationship among the compositions, GFA, structure and ductility is discussed.

2. Experimental Procedures

Mother alloys with nominal compositions of Ni80xPdxP20

(10x35) were prepared by melting the mixture of pure Ni and Pd metals with pre-alloyed Ni-P ingots in vacuumed fused silica tubes. The rapidly solidified ribbon samples with a cross section of about1:20:02mm2 were prepared by a single-roller melt-spinning technique. For compression test,

bulk samples with a diameter of 2 were prepared by copper mold casting. For tensile test and GFA evaluation, bulk samples with diameters ranging within 6–21 mm were prepared by WQ associated with a repeated B2O3 flux treatment, which has been proved much effective than a one step flux treatment.10–12) The formation of a single glassy phase was identified by X-ray diffraction (XRD) using CuK

radiation. The further local structure was studied by trans-mission electron microscopy (TEM). To avoid the possibility of introducing artificial structure by ion milling, TEM samples were prepared by electrolytic jet thinning using a solution of one part of HClO4, one part of CH3OH and nine parts of C2H5OH. The thermodynamic parameters, including the onset of glass transition temperature (Tg), crystallization

temperature (Tx), melting temperature (Tm), and liquidus

temperature (Tl), were examined by differential scanning

calorimetry (DSC) at a heating rate of 0.67 Ks1_{under a flow} of pure argon gas. Mechanical properties were examined at room temperature with an Instron testing machine. The Young’s modulus was measured using a strain-gauge meter. The morphologies of the samples subjected to fracture were examined by scanning electron microscopy (SEM).

3. Results and Discussion

The DSC curves of the as-spun Ni80xPdxP20 ribbon samples are shown in Fig. 1. During heating, all the samples exhibit a distinct endothermic peak due to glass transition, followed by a supercooled liquid region, and then a sharp exothermic peak due to crystallization. With further increas-ing temperature, the alloys finally melt after several steps of endothermic events. When xincreases from 10 to 35, Tx

increases from 667 K to 674 K, whileTg,TmandTldecrease

gradually from 600 K, 674 K, 944 K and 1107 K to 587 K, 667 K, 883 K and 1002 K, respectively. The most distinct decrease of Tm andTl occurs asxincreases from 25 to 30,

suggesting that there is a dramatic change in GFA.

The supercooled liquid region (Tx¼TxTg) and the

reduced glass transition temperature (Trg¼Tg=Tl), the

the basic thermal data and plotted as a function ofxin Fig. 2. With increasing x from 10 to 35, Tx and Trg increase

from 65 K to 83 K and 0.54 to 0.59, respectively, indicating the improvement of thermal stability and GFA. When x exceeds 20, the Tx and Trg keep relatively high values,

favorable for achieving high GFA.

To confirm the ultimate GFA of Ni80xPdxP20, bulk

samples were prepared by WQ. Different from the one step flux treatment in Ref. 10), a more careful repeated B2O3flux treatment similar to Ref. 12) was employed along with WQ for all samples to avoid the degradation in GFA caused by heterogeneous nucleation. As summarized in Fig. 3, theDmax

determined by this preparation method is below 6 mm for Ni60Pd20P20, between 8–12 mm for Ni50Pd25P20, and higher than 21 mm for Ni50Pd30P20 and Ni45Pd35P20, respectively. Apparently, the GFA increases with increasing Pd content, in agreement with the prediction caused byTx andTrg. It is

worth noting that the Ni50Pd30P20 alloy is the composition containing relatively low palladium but exhibiting excellent GFA. And the Ni50Pd30P20 BMG with a diameter of 21 mm is the largest Ni-rich BMG reported so far. The inset of Fig. 4(a) presents the outer appearance of the D21 mm Ni50Pd30P20 sample which exhibits a lustrous surface. Figure 4(a) shows the corresponding common XRD spectra taken from the transverse plane of the upper side of the D21 mm rod (as shown in the inset of Fig. 4(a)). There is only

a broad diffraction maximum without any observable crys-talline peaks, demonstrating the formation of a fully x-ray amorphous structure. The DSC curve in Fig. 4(b) displays the characteristic feature of glassy alloys from glass transition to crystallization with increasing temperature, confirming the glassy nature of the D21 mm Ni50Pd30P20 sample. In com-Fig. 1 DSC curves of melt-spun Ni80xPdxP20ribbons.

Fig. 2 TxandTrgas a function of Pd content for Ni80xPdxP20 glassy

alloys.

Fig. 3 Formation of Ni80xPdxP20BMGs by WQ along with a B2O3flux

treatment.

Fig. 4 A Ni50Pd30P20 alloy rod with a diameter of 21 mm prepared by

water quenching along with B2O3flux treatment. (a) XRD pattern, the

parison with the melt-spun Ni50Pd30P20 ribbon (Tg ¼594K,

Tx ¼668K, Tx¼74K), the D21 mm Ni50Pd30P20 bulk

sample has the same Tg higher Tx (682 K) and larger Tx

(83 K), which can be attributed to the impurity scavenging by flux treatment of the WQ bulk samples and the surface effect on the crystal nucleation of the thin ribbons.13–15)

We further examined the mechanical properties of Ni80xPdxP20BMGs. The as-cast Ni80xPdxP20samples with the dimension of 2 mm in diameter and 4 mm in length were used for compression test. The nominal stress-strain curves are shown in Fig. 5, where the slopes are calibrated by using a strain-gauge settlement. All Ni80xPdxP20BMGs exhibit an

elastic deformation followed by distinct plastic flow. The data of Young’s modulus (Ec), maxim compressive strength

(c,max) and plastic strain ("c,p) are summarized in Table 1.

With increasing Pd content from 25 to 35 at%, Edecreases from 117 to 106 GPa, being consistent with the composition dependence ofTg. Considering the previous concept thatE

andTg of the glassy alloys reflect the bonding force among

the constituent elements,16) it can be concluded that the

higherEandTgof the high Ni-rich alloys are originated from

the stronger bonding nature among the constituents. The morphology of the BMGs subjected to the final fracture was examined by SEM. The samples exhibit similar fracture features as shown in the inset of Fig. 5 for the Ni50Pd30P20 BMG. The fracture angle is about 41–42 _{(inset 1), in}

agreement with the previous data of other Ni- and Pd-rich glassy alloys.17,18)_{Instead of a single shear band that leads to}

a catastrophic failure, multiple shear bands distribute on the outer surface of the deformed samples (inset 1). In addition to the primary shear bands which are declined by about 42_to

the loading direction, some secondary shear bands are formed with an angle of 30–42 to the primary ones. The similar phenomenon can also be observed in some other ductile glassy alloys.19) _{On the fracture surface (inset 2),}

well-developed vein pattern can be observed.

Mechanical properties of Ni80xPdxP20 BMGs were also examined under a tensile load. Rectangular samples for tension test were cut from WQ BMGs into a gauge dimension of 1mm2mm7mm. Figure 6 presents the nominal stress-strain curves. The samples exhibit an elastic deformation followed by a catastrophic failure. The Young’s modulus (Et) and maxim tensile strength (t,max) are

100 GPa, and 1370 MPa, respectively, for the Ni50Pd30P20 alloy, 95 GPa and 1400 MPa, respectively, for the Ni45Pd35P20 alloy. The morphology of the Ni50Pd30P20 sample subjected to tensile fracture is shown in the inset of Fig. 6 as a representative. The fracture angle is about 50–52 _{to the stress axis (inset 1). On the outer surface}

Fig. 5 Nominal stress-strain curves of the Ni80xPdxP20 BMGs under a

compressive load. The insets are the SEM images of the Ni50Pd30P20BMG

subjected to a compressive fracture.

Table 1 Mechanical properties of Ni80xPdxP20BMGs under a

compres-sive load.

BMGs Ec, GPa c,max, MPa "c,p, %

Ni55Pd25P20 117 1920 2.2

Ni50Pd30P20 110 1800 5.4

Ni45Pd35P20 108 1740 6.1

Fig. 6 Nominal stress-strain curves of the Ni80xPdxP20 BMGs under a

tensile load. The insets are the SEM images of the Ni50Pd30P20 BMG

(inset 1), multiple shear bands are not observed, indicating that the propagation of a single shear band leads to the final fracture under a tensile load. On the fracture surface, vein pattern and radiate cores can be clearly seen (inset 2). Clearly, the Ni80xPdxP20 BMGs show deformation asym-metry (strength, plasticity and fracture morphology) between compression and tension tests, which may be caused by the different condition of normal stress.20)

4. Discussion

The present study shows that Ni80xPdxP20 alloys exhibit

high GFA and the GFA increases with increasing Pd content through the decrease inTmandTl. For a better understanding

of the relationship between GFA and composition, the melt-spun Ni80xPdxP20 ribbons were annealed at temperatures

under Tm for six hours and the crystalline phases are

compared in Fig. 7. The Ni70Pd10P20 sample shows the simplest precipitation phases mainly composed by Ni3P. The Ni60Pd20P20 and Ni65Pd15P20 samples shows the crystalliza-tion mixture of Ni3P, Ni5P2, Pd3P and (Ni,Pd) phases. And the Ni50Pd30P20 and Ni55Pd35P20 samples show the crystal-lization mixture of Ni5P2, Pd3P and (Ni,Pd) phases. Clearly,

the metastable Ni3P phase is gradually replaced by Ni5P2 with increasing Pd content. The Ni5P2 phase has quite large cell parameters (a¼b¼0:6613nm, c¼1:2311nm) when compared with that of Ni3P (a¼b¼0:8954nm, c¼

0:4386nm). Due to the larger cell parameters, the atomic rearrangement for forming N5P2becomes difficult and makes the precipitation of metastable Ni3P easily retarded even when the cooling rate is relatively low. Therefore the undercooled melts become stabilized and BMGs with large diameters can be easily formed.

Another noteworthy result in this work is the high ductility of Ni80xPdxP20 BMGs. Alloys with Pd contents above

25 at% exhibit remarkable plastic strain of 5–6% under a compressive load. With the aim of clarifying the origin of the high ductility and its composition dependence, the local structure of the as-cast Ni55Pd25P20 and Ni50Pd30P20 BMGs with a diameter of 2 mm was examined by TEM. In Fig. 8(a), the HRTEM image reveals that the Ni50Pd30P20 sample consists of a glassy matrix with homogenously dispersed medium-range ordered regions (MROs). These MROs are thought to contribute to the high ductility. Actually the disperse of secondary phases, such as reinforced-fibers,21)

-particles22) _{and even voids}23) _{in BMG matrix, have been}

found to be effective for the improvement of the ductility for BMGs. These second phases can work as effective stress concentrators.24) Therefore the direction of shear slipping will be forced to change at the intersection site between the shear propagation and the second phase. In fact, Yavariet al.

has reported that the direction of shear propagation takes zigzagged and strayed progress by in-situ TEM observa-tion.25) _{The change of the slip direction will cause the}

formation of multiple shear bands and the suppression of shear band propagation. Thus large plastic deformation can be achieved in Ni50Pd30P20BMG. In the case of Ni55Pd25P20 sample, most of the places show the similar structure as Ni50Pd30P20BMG. However in some limited area (Fig. 8(b)), nanocrystals with a size of about 10 nm heterogeneously precipitate in glassy matrix. The formation of these nano-crystals is due to the lower GFA of Ni55Pd25P20. Nanobeam diffraction pattern reveals the structure of the particles as a Ni5P2 phase, corresponding to a brittle intermetallic com-pound. The heterogeneity and brittleness of the intermetallic compound may cause the relatively lower ductility for the Ni55Pd25P20BMG.

Fig. 7 XRD patterns of melt-spun Ni80xPdxP20 ribbons annealed at

temperatures underTm.

Fig. 8 HRTEM images and nanobeam diffraction patterns of as-cast (a) Ni50Pd30P20and (b) Ni55Pd25P20BMG samples.

5. Conclusions

Glass-forming ability, thermal behavior and mechanical properties of Ni80xPdxP20 (10at%6x635at%) glassy alloys have been investigated. The alloys with Pd contents higher than 25 at% exhibit high glass-forming ability, as is evidenced from the formation of a cylindrical Ni50Pd30P20 glassy rod with a diameter of 21 mm. The Ni-rich BMGs show relatively high strength in compression and tensile tests. More notably, BMGs with Pd content above 20% exhibit high ductility. Large plastic strain of 5–7% can be obtained under a compressive load.

Acknowledgement

This work was supported by Global COE program ‘‘Materials Integration, Tohoku University’’, MEXT, Japan.

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