Study on liquid structure feature of Al100 − xNix alloy with resistivity and rapid solidification method

Journal of Non-Crystalline Solids 411 (2015) 26–34

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Study on liquid structure feature of Al100 − xNix alloy with resistivity and rapid solidification method Mingyang Li a, Songzhao Du a, Yangbo Hou c, Haoran Geng a,b,⁎, Peng Jia a, Degang Zhao a a b c

School of Materials Science and Engineering, University of Jinan, Jinan 250022, China Shandong Provincial Key Laboratory of Preparation and Measurement of Building Materials, University of Jinan, Jinan 250022, China Heze Branch of Shandong Special Equipment Inspection Institute, Heze 274000, China

a r t i c l e

i n f o

Article history: Received 16 August 2014 Received in revised form 23 September 2014 Accepted 25 November 2014 Available online xxxx Keywords: Al–Ni alloys; Microstructure; Resistivity; Solidification

a b s t r a c t In this paper, the liquid structure of Al100 − xNix alloys (x = 0, 2, 2.7, 5) was mainly studied with resistivity and rapid solidification method, the resistivity of liquid alloy increases with the increase of the temperature and Ni content, and shows the linear and nonlinear change successively; the various tests on melt-spun Al–Ni alloy show that the alloys, which contained amorphous structure and treated with higher melt temperature, have a favorable corrosion resistance and hardness, which can reflect and verify the liquid feature from rapidly solidified alloy and amorphous structure. These results indicate that the melt structure becomes homogeneous and disordered with rising temperature, and temperature-induced liquid–liquid structure change occurs at 850–950 °C. Based on these results explored the melt structure from different aspects, we describe this melt structure and discuss shallowly the relationship between the melt and amorphous structure. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Liquid structure has been a fascinating subject in the field of materials science, condensed matter physics and related chemistry field, but it still has many unanswered questions to solve, such as the existence of liquid–liquid structure change and its nature, the mechanism of temperature-induced structure change [1,2]. The resistivity method reflects the melt structure change along with temperature as a physical performance sensitive to the structure, but this method has strict requirements for experimental conditions, which limits its application. In addition, the alloy melt features have a direct and important impact on the structure and properties of the alloy; the melt and amorphous structure both belong to the disordered state and there are some relations between them [3–5], the amorphous alloys would remain more structure features than other solidified alloys by rapidly “freezing” it. As an important high melting point material, the Al–Ni alloy has good performance such as light weight, high strength, and high temperature stability, and is widely used as high temperature material [6], fuel battery [7,8], gasless high-energy density material [9], functional thin film [10,11], phase catalytic material [12] and so on; but Al–Ni alloy with high Al content has a poor ductility at room temperature, weak high temperature strength and creep resistance [13,14], some mechanisms on the liquid structure of Al–Ni alloy are not clear [15,16], there is an urgent need to solve those problems. In addition, there is little ⁎ Corresponding author at: School of Materials Science and Engineering, University of Jinan, Jinan 250022, China. E-mail address: [email protected] (H. Geng).

http://dx.doi.org/10.1016/j.jnoncrysol.2014.11.031 0022-3093/© 2014 Elsevier B.V. All rights reserved.

precise measurement on the resistivity of Al–Ni alloy melt, especially from liquidus temperature (TL) to high temperature above 1000 °C, and the Al–Ni–RE (RE = rare earth metal) system has attracted widely attention due to its wide glass formation range and performance which still remains to be improved [17]. The study on the melt structure of Al–Ni alloy with resistivity and rapidly solidified alloy is novel and significant. In this trial, the Al100 − xNix alloys (x = 0, 2, 2.7, 5) were selected according to their relatively high melting point (Tm, 640–700 °C) and the fact that Al97.3Ni2.7 is eutectic alloy while the others are hypoeutectic and hypereutectic alloy. The melt-spun Al100 − xNix alloys were prepared with different temperatures to reflect and verify the liquid structure change. These experiments provide useful guidance and preliminary attempt to the study of melt structure features and the enhancement of Al–Ni alloy performance. 2. Experimental section 2.1. Resistivity, viscosity and high-temperature XRD experiments of melt The alloy ingots with the nominal composition of Al98Ni2, Al97.3Ni2.7 and Al95Ni5 were prepared by arc-melting pure metal Al (99.99 wt.%), Ni (99.97 wt.%) under an argon atmosphere through using arc melting furnace (model YSU-ZZ). Each alloy ingot re-melted 3–4 times to ensure homogeneity. The electrical resistivity was measured by DC four-probe method to cancel the influence of thermoelectric potential and contact potential. The geometrical parameter of quartz cell was calibrated experimentally using high purity mercury (99.99 wt.%) of known resistivity at

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room temperature. During the measurement, a stable constant direct current was provided by PF66M current source, the potential drop and the temperature were measured continuously by Keithley 2182 nanovolt-meter. All measurements were under the protection of pure argon gas (99.99 wt.%). The sample was heated up to 1100 °C and then held for 10 min, then cooled to room temperature with the heating rate of 4 °C/min. The viscosity of alloy melts was measured by a torsional oscillation viscometer, the alloy ingots were melted in an alumina crucible with a diameter of 30 mm and a height of 60 mm by using a high frequency induction electric furnace and superheated to different temperatures, then they were hold 30 min at the selected temperature to measure the viscosity in the cooling process. The alloy ingots with nominal composition of Al97.3Ni2.7 was prepared by arc-melting pure metal Al, Ni under an argon atmosphere (99.99 wt.%) using arc melting furnace (model YSU-ZZ). The samples were placed in alumina crucibles (25 × 30 × 8 mm3) which can reduce the effects of surface tension and a resistant heater inserted into the high temperature chamber was used for progressive heating up to experimental temperature under the protection of pure helium gas (99.99 wt.%). The high-temperature X-ray diffraction measurements for the liquid samples were followed out by means of a θ–θ diffractometer at temperatures approximately 50 °C above the TL, using a traditional X-ray generator with sealed-off Mo-tube run at 50 kV, 30 mA. Mo-Kα radiation (λ = 0.71069 Å) was reflected from the free surface of the liquid specimen, the X-ray then reached the detector through a graphite monochromator in the diffraction beam. During the measurements, the surface of the specimen was fixed to one horizontal position by means of a laser calibrator. The magnitude of experimental diffraction vector Q was ranged from 5 nm−1–120 nm−1. Three sets of diffraction data were collected for each sample in order to minimize random errors.

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repeatability and the error was limited within 1%. Fig. 1(a) displays the resistivity-temperature curves (ρ–T curves) of Al97.3Ni2.7 alloy melt in the heating process, while Fig. 1(b) shows ρ–T curves of the cooling process. As can be seen, the resistivity versus temperature is basically the same. In order to make these curves more clear, we add 15 μΩ·cm to the resistivity values of each curve successively to keep them separate, the curve marked with “1H” represents the first heating process and that of “1C” represents the first cooling process and so on. As shown in Fig. 1, the resistivity of Al97.3Ni2.7 alloy increases with rising temperature, a sharp peak appears at 640–670 °C during the first heating process, corresponding to the Tm and TL, i.e., the endothermic peak of melting process in the DSC curves (see Fig. 7 and Table 1). The relationship between resistivity and temperature is approximate to linearity during 670–880 °C, then turns into the other linear tendency during 880–1035 °C and turns into another tendency above 1035 °C, indicating that some relatively large changes of structure features occur at about 880 °C and 1035 °C in Al97.3Ni2.7 alloy. As for the cooling process, one tendency between resistivity and temperature turns into the other tendency at about 935 °C and 850 °C. The resistivity of Al97.3Ni2.7 alloy increases with temperature below 640 °C because the vibration of lattice atoms becomes violent, which increases the collision frequency between atoms and electrons and hinders the directional movement of electrons; while the alloy is in the solid–liquid mixing state during 640–670 °C, the randomness and chaos of the atoms are even larger than the liquid state, leading to the greater impediment to electron movement and the sharp rise of resistivity. In addition, some extrinsic structure and heterogeneous

2.2. Experiments of rapidly solidified alloy The alloy ingots were re-melted in a quartz tube by low-frequency induction heating in a single roller melt spinning apparatus and superheated to different temperatures with holding time of 30 s, then the alloy melts were ejected onto a copper roller with a diameter of 0.22 m at rotation speed of 2000 and 2500 revolutions per minute (rpm) respectively to obtain the melt-spun alloy ribbons. The temperature was measured by laser thermodetector (model IR-AL). The structure of Al–Ni melt-spun alloys was identified by X-ray diffraction (XRD, Germany, D8-advance): the patterns were obtained utilizing this diffractometer operated at 40 kV and 40 mA with Cu Kα radiation and a wavelength of 0.15406 nm. The thermodynamic property and crystallization process of the alloy ribbons were investigated by differential scanning calorimetry (DSC, Germany, STA409EP) at a heating rate of 0.33 °C/s under a continuous flow of purified argon. The microstructure of melt-spun alloys was examined by field emission scanning electron microscope (FESEM, America, QUANTA, FEG250) and the composition of the phases was determined by energy dispersive spectrum (EDS, Britain, INCA, X-MAX-50X). The corrosion resistance of alloy ribbon was evaluated through electrochemical potentiodynamic polarization test (EC500) in aqueous chloride solution (3.5%, NaCl) by using Chinese standard [18]. The hardness of the alloy ribbons was measured through HV-10B Vickers hardness tester with a load of 500 g and an upload time of 10 s by following the Chinese standard [19] as much as possible. 3. Results and discussion 3.1. Resistivity of alloy melts versus temperature The same sample was tested in heating and cooling processes for three times to measure the resistivity to obtain ρ–T curves with good

Fig. 1. ρ–T curves of Al97.3Ni2.7 alloy in heating and cooling processes for three times: a, the heating processes; b, the cooling processes.

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Table 1 Thermal values of Al100 − xNix (x = 2, 2.7, 5) alloys prepared with 750 °C in roller rotation speed of 2000 and 2500 rpm. Al–Ni alloy 2500/rpm

2000/rpm

Al95Ni5 Al97.3Ni2.7 Al98Ni2 Al95Ni5 Al97.3Ni2.7 Al98Ni2

Tx/°C (±3)

Tm/°C (±2)

TL/°C (±2)

263.37 253.42 261.99 – – –

699.38 638.94 644.16 697.28 639.23 642.98

739.96 670.34 676.28 728.81 668.87 681.50

changes during the melting process also have an effect on the resistivity, resulting in the peak in the ρ–T curves. The resistivity of alloy melts is mainly the function of structure factor S(q) and component, the theory of Faber–Ziman [20] has received wide acceptance and can illuminate the resistivity of alloy melts quite well, which yields the following expression:
ρ¼

3π he2



1 Vν2F

!Z



 K 3 K 2 jU ðK Þj 4 d 2k F 2k F 0 1

 2 2 2 2  2 jU ðK Þj ¼ cA cB μ A −μ B  þ cA μ A  SAA þ cB μ B  SBB þ 2cA cB μ A μ B SAB : According to this equation, the electrical behavior of alloy melts mainly depends on those electrons which have energy closed to the Fermi surface. With the increase of the temperature, those electrons obtain more energy from the lattice waves, then the more electron transitions above the Fermi surface will make the Fermi level decrease, the vF and kF will become smaller; moreover, the curves of S(q) become wide and short, leading to the change of integral for s(q). These factors make the resistivity of alloy melts increase as the temperatures rising. The atoms in alloy melts obtain more energy, and their movements become more violent, accompanying the weakening of electronic movement and the decline of the conductivity; more atoms in clusters would break away from the clusters and become free atoms, also increasing the number of free atoms and the impediment of conductivity, hence the resistivity of Al97.3Ni2.7 alloy melt increases with temperature [21,22]. The structure factor curve of Al97.3Ni2.7 alloy melt at different temperatures is shown in Fig. 2. From a to f, the left side of the first peak appears pre-peak which is the symbol of MRO [23]. The pre-peak is weaker than before with the increase of temperature. The reduction of pre-peak relative area at 900–950 °C indicates that the disappearance of MRO or dramatic changes of SRO. The pre-peak has some relations with chemical range ordering (CRO), or it comes down to a larger

Fig. 2. Structure factor curve of Al97.3Ni2.7 alloy melts at different temperatures.

atomic clusters which formed by heterogeneous atoms [24,25]. Fig. 2 displays the pre-peak at the position of small Q, the intensity of prepeak is strengthened with decreasing temperature expected for greater number of clusters of MRO. While the position of pre-peak is unchanged on the whole, since the position and heights of the pre-peak reflect the size and the number of the clusters in the liquid. It shows that the size of cluster corresponding to the MRO changes little and only the number is increased. The second peak flattens gradually in shape with rising temperature. As the temperature decreases to 900 °C, the obvious splitting of the second peak of S(Q) gives strong evidence for an amorphous structure. What's more, the second peak of structure factor splits into two peaks below 900 °C, which reflects the existence of icosahedral local order in the system [26]. The network consisting of the icosahedra and defect icosahedra clusters can be formed in the amorphous system [24]. Comparing to the liquid, the atoms in the amorphous state are restricted to move relatively. It should be noted that this difference has an influence on the formation of a characteristic structure for the amorphous state, although the basic arrangement of the atoms in the amorphous state is similar to that in the liquid. In both amorphous and liquid states, the atoms are randomly distributed in a nearly closed-packed structure, and the mean free path is short and comparable to the atomic size. This implies that the positional correlation of atoms is relatively strong within the near-neighbor region. However, the average atomic configuration in the liquid is more homogeneous than that of the amorphous state because the atomic vibration is high. In other words, the atomic configuration in the amorphous state shows a slight inhomogeneity, which gives a deformed pattern in the S(Q) frequently. Fig. 3 shows this clear change of resistivity with temperature in the heating (Fig. 3(a)) and cooling processes (Fig. 3(b)), accompanying with the data of coordination numbers (CN) and viscosity of Al97.3Ni2.7 liquid alloy. The dramatic growth of the CN of Al97.3Ni2.7 alloy melt at 900 °C in Fig. 3(a) and the disappearance of pre-peak at 900–950 °C in Fig. 2 indicate the dramatic changes of SRO or the disappearance of MRO. These results are consistent with the conclusions of above resistivity experiments. Coincided with the resistivity of Al97.3Ni2.7 alloy melt, the viscosity of this alloy melt decreases with rising temperature. As shown in Fig. 3(b), the relation between viscosity and temperature conforms the Arrhenius equation, an abnormal change tendency appears around 920 °C, corresponding to the turning point on the ρ–T curves. The viscosity of liquid metal is a reflection of the atom moving ability and the atom binding force, the reduction of viscosity means that the order degree of atomic distribution declines as well as the CN in liquid alloy. The type of clusters changes and the scale of clusters contracts, more free atoms would be released, and then the alloy melt is easy to flow with this uniform and disordered structure. Accordingly, it can be concluded that dramatic changes of SRO or the appearance of MRO occurs around the temperature of 935 °C in cooling process. Fig. 4 depicts the ρ–T curves of the second and third processes of the Al100 − xNix (x = 0, 2, 2.7, 5) alloy in order to study the influence of component on the resistivity, Fig. 4(a) shows the heating process and Fig. 4(b) shows the cooling process. As can be seen, the relation between resistivity and temperature also turns from linearity into non-linearity at the temperature range of 850–950 °C for the alloys with different components. The heating process and cooling process show the similar change tendency. As shown in Fig. 4, the resistivity of Al–Ni alloy melts increases with the rise of Ni content in varying degrees. The higher Ni content means that the clusters in the alloy melt contain more Ni atoms, then the amount of Al atoms that surrounded the central atoms Ni will reduce comparatively and the CN decline, the melt structure contained coordination polyhedrons with lower CN have higher resistivity value. The work of Brillo [27] shows this change tendency clearly by measuring and calculating the CN of Al97.3Ni2.7 and Al75Ni25 alloy melt, their CN declines with the rise of temperature and Ni content, as shown in Fig. 3(a).

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Fig. 3. ρ–T curves of Al97.3Ni2.7 alloy in the second heating and cooling processes accompanying with other data: a, CN [27] and resistivity in heating process; b, viscosity and resistivity in the cooling process.

The coordination polyhedrons with high CN will transform into that of low CN with rising temperature, since the main coordination polyhedrons in melt structure are tetrahedron and octahedron [28], and these changes of polyhedron types are mainly the transformations of the octahedron into tetrahedron. Some researchers also believe that the tetrahedron structure will become the predominant composition gradually while the octahedron reduces with the increase of temperature, because the existence of the tetrahedron can reduce Gibbs free energy of the system [21,28]. The disappearance of some heterogeneous structure and the transformation from coordination polyhedrons with high CN into low CN play an important role in the process that the melt structure becomes uniform and disordered.

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Fig. 4. ρ–T curves of Al100 − xNix (x = 0, 2, 2.7, 5) alloys at the second and third processes: a, heating processes; b, cooling processes.

750 °C. It can be seen that the main crystal phase is Al phases, there also exists a little of Al1.1Ni0.9 and Al3Ni2 phases whose diffraction peaks overlap with the peak of Al phase. Compared with the alloys prepared by the permanent mold (steel mold), this alloy ribbon prepared by melt-spun only has Al1.1Ni0.9 and Al3Ni2 phases but no Al3Ni phase. It can be inferred that the Al1.1Ni0.9 and Al3Ni2 phase form firstly and then transform into Al3Ni phase during the solidification process,

3.2. Influence of melt structure feature on rapidly solidified alloys In consideration of the discussions above, the Al100 − xNix (x = 2, 2.7, 5) alloy melts were superheated to 750, 850, 950 and 1050 °C with holding time of 30 s and then melt-spun onto single roller with different rotation speeds respectively, afterward, the alloy ribbons with a width of 2 mm and thickness of 20 μm were obtained. In this trial, the objective is to study the melt structure feature from rapidly solidified alloy and amorphous structure. 3.2.1. Structure and organization of rapidly solidified alloys Fig. 5 shows the XRD patterns of Al100 − xNix (x = 2, 2.7, 5) alloy ribbons prepared at roller rotation speed of 2000 and 2500 rpm with

Fig. 5. XRD patterns of Al100 − xNix (x = 2, 2.7, 5) alloys prepared with the roller rotation speed of 2000 rpm and 2500 rpm at 750 °C.

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however, at the high cooling rate the solidification process is so fast that the Al1.1Ni0.9 and Al3Ni2 phases solidify quickly before they transform and vanish, resulting in the existence of these phases. The phase near the TL is mainly Al1.1Ni0.9, Al3Ni2 phase and then Al3Ni phase at relatively low temperature. As shown in Fig. 5(a)–(c), the amorphous characteristics of diffuse scattering peaks appear in its XRD patterns at about 2θ = 15–25° when the roller rotation speed increases to 2500 rpm. The appearance of diffuse scattering peaks is the existence of a small number of amorphous structure even the binary Al–Ni alloy with this composition is hard to prepare amorphous alloy. This result is consistent with the conclusions of high-temperature XRD experiments. However, the amorphous structure can be obtained more easily at higher cooling rate, the Al84Ni10La6 amorphous alloy has been prepared by using the same method and equipment in more relaxed conditions in our previous work [29]. In order to verify this conclusion and explore its microstructure, the experiments of SEM and EDS were undertaken on these Al–Ni alloy ribbons. The backscattered electron images (BEIs) of Al100 − xNix alloy ribbons prepared with 750 °C at the roller rotation speed of 2500 rpm were displayed in Fig. 6. Fig. 6(a) shows the sub-microscopic structure of Al98Ni2 alloy, indicating that the structure of these melt-spun alloys is quite uniform. Fig. 6(b)–(d) shows the microscopic structure of Al98Ni2, Al97.3Ni2.7 and Al95Ni5 melt-spun alloys respectively, the regions of the dark gray background are the matrix while the small regions of gray-white are the crystalline phases which distribute randomly among the matrix. The EDS results are marked in the BEI, as can be seen, the components of the matrix are approximate to their nominal composition. These crystalline phases are mainly Al, Al1.1Ni0.9 and Al3Ni2 phases, leading to a large deviation of the element content of crystalline phases from their nominal composition. Coincided with the deduction of Figs. 2 and 5, this result indicates that the structure of Al–Ni melt-spun alloy prepared with the above conditions can be viewed as the uniform and random distribution of crystalline phases among matrix which contains amorphous structure partly.

Fig. 7. DSC patterns of Al100 − xNix (x = 2, 2.7, 5) alloys prepared with 750 °C in roller rotation speed of 2000 and 2500 rpm.

At the same time, the DSC tests of Al100 − xNix (x = 2, 2.7, 5) alloy ribbons also confirm the conclusion above. As shown in Fig. 7, the DSC curves of samples are corresponding to the XRD patterns in Fig. 5 respectively; the related values are listed in Table 1. The curves form (a) to (c) display the Al–Ni alloys prepared at the roller rotation speed of 2500 rpm, a small exothermic peak appears at about 260 °C. These exothermic peaks appear in three curves and indicate that some structure changes occur indeed, considering that there is almost no other reaction at this temperature, it can be deduced that these exothermic peaks are the crystallization of amorphous structure. The result of DSC shows that the Al–Ni alloys prepared at rotation speed of 2500 rpm contain amorphous structure; taking into account of these results, we reach the conclusion that this Al100 − xNix alloy prepared by melt-spun method with the roller rotation speed of 2500 rpm contains partly amorphous structure. The values of Tm and TL from Fig. 7 and Table 1

Fig. 6. BEIs with EDS results of Al–Ni alloy ribbons prepared in roller rotation speed of 2500 rpm at 750 °C: a, Al98Ni2; b, Al98Ni2; c, Al97.3Ni2.7; d, Al95Ni5.

M. Li et al. / Journal of Non-Crystalline Solids 411 (2015) 26–34

Fig. 8. Potentiodynamic polarization curves of Al97.3Ni2.7 alloy prepared with different melt temperatures in roller rotation speed of 2500 rpm.

are consistent with the experimental data of resistivity as mentioned above, then the temperature ranges of melting peaks are consistent with the peaks appeared in the ρ–T curves (Fig. 1) even they have little deviations. 3.2.2. Corrosion resistance The corrosion resistance of Al100 − xNix (x = 2, 2.7, 5) melt-spun alloys was evaluated through electrochemical potentiodynamic polarization test to reflect its organization and structure. Fig. 8 displays the potentiodynamic polarization curves of Al97.3Ni2.7 alloy ribbons prepared with different melt temperatures at the roller rotation speed of 2500 rpm in the aqueous chloride solution (3.5%, NaCl) with a potential between −1100 and 250 mV (SCE). The curve marked by (a) represents the Al97.3Ni2.7 alloy ribbons prepared with the temperature of TL, the curves from (b) to (e) represent the Al97.3Ni2.7 alloy ribbons prepared with 750, 850, 950 and 1050 °C respectively. Their corresponding values are listed in Table 2. As can be seen, the Al97.3Ni2.7 alloy ribbon prepared with the temperature of TL has higher corrosion potential (Ecorr) and lower corrosion current density (icorr) than the alloy prepared with 750 °C, and the difference of Ecorr and icorr between them is quite small. The change tendencies of various experimental data from (b) to (e) are not consistent: the most important data icorr becomes small gradually while the Ecorr and primary passivation potential (Epp) increase, indicating that their corrosion resistance improves with the rise of prepared temperature; while their primary passive current (ipp) becomes large and the passive region (Etp-p) reduces, which shows the opposite conclusion. The higher Ecorr means that the melt-spun alloy is corroded at higher voltage while the lower icorr means the slower corrosion rate, which indicates their good corrosion resistance as well as compact and uniform structure, the higher Epp means that the passive film is corroded at higher voltage and also indicates the favorable structure. However, this structure

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Fig. 9. Potentiodynamic polarization curves of Al100 − xNix (x = 2, 2.7, 5) alloys prepared with 750 and 850 °C in the roller rotation speed of 2500 rpm.

contained fine grains and the other small organizations would form a passive film with more grain boundary and large specific area, leading to the corrosion at a very fast rate once the passive film begins to break, as a result, the ipp becomes large and Etp-p reduces gradually. Since the icorr and Ecorr are more convincing while the Epp and ipp tend to explore the passive film, it can be concluded that the corrosion resistance improves with the rise of prepared temperature. The potentiodynamic polarization curves of Al97.3Ni2.7 alloy ribbons prepared with 750 °C and 850 °C melt have a clear passivation region, while that of 950 and 1050 °C melt have almost no passivation region. In addition, the icorr of Al97.3Ni2.7 alloy shows a large change which spans one order of magnitude between the alloy ribbons prepared with 750–850 °C melt and 950–1050 °C melt, as shown in Fig. 8 and Table 2. These obvious changes imply that the corrosion process and reaction mechanism of Al97.3Ni2.7 alloy ribbons prepared with 750–850 °C melt and 950–1050 °C melt are quite different, that is, the structure of 750–850 °C melt and 950–1050 °C melt is different. As mentioned above, some rather large changes occur in the Al–Ni alloy melts during 850–950 °C, leading to the transformation of ρ–T curves from linear into non-linear and the difference between the melt-spun Al97.3Ni2.7 alloy ribbons prepared at different melt temperatures, thus they follow different electrochemical reaction mechanisms and then show this obvious change. The corrosion resistance of Al100 − xNix (x = 2, 2.7, 5) alloy ribbons prepared by 750 and 850 °C with the roller rotation speed of 2500 rpm is also tested by the same method to inquiry the influence of component on the structure and performance of Al–Ni alloy; the related results are listed in Fig. 9 and Table 2. As can be seen, the Ecorr of Al100 − xNix alloy ribbons prepared with 750 °C (from (a) to (c)) increases with the rise of Ni content while the icorr value decreases, indicating that their corrosion resistance improves with the addition of Ni gradually. The three components of Al–Ni alloys prepared with 850 °C show the same change

Table 2 Corrosion resistance parameters of Al100 − xNix (x = 2, 2.7, 5) alloys prepared with different temperatures in roller rotation speed of 2000 and 2500 rpm. Alloy

icorr/mA·cm−2

Ecorr/V

Epp/V

ipp/mA

Etp-p/V

Al98Ni2 750 850 Al97.3Ni2.7 TL 750 850 950 1050 Al95Ni5 750 850

0.88 ± 0.03 0.65 ± 0.02 0.68 ± 0.02 0.72 ± 0.02 0.62 ± 0.02 0.05 ± 0.001 0.02 ± 0.001 0.72 ± 0.02 0.61 ± 0.02

−0.59 ± 0.02 −0.54 ± 0.02 −0.52 ± 0.01 −0.55 ± 0.02 −0.49 ± 0.01 −0.33 ± 0.01 −0.31 ± 0.01 −0.55 ± 0.02 −0.48 ± 0.01

−0.59 ± 0.02 −0.53 ± 0.02 – −0.52 ± 0.02 −0.47 ± 0.02 −0.31 ± 0.01 – −0.54 ± 0.02 −0.46 ± 0.02

0.74 ± 0.03 0.74 ± 0.02 – 0.57 ± 0.02 0.59 ± 0.02 0.65 ± 0.01 – 0.72 ± 0.03 0.54 ± 0.02

0.19 ± 0.01 0.17 ± 0.01 – 0.24 ± 0.01 0.22 ± 0.01 0.05 ± 0.002 – 0.18 ± 0.01 0.18 ± 0.006

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tendency, moreover, they have higher Ecorr and lower icorr than that of 750 °C for the same component, indicating their better corrosion resistance. The values of Epp, ipp and Etp-p show the same conclusion according to the discussion above even their change tendencies are not so coincident. This result shows that the corrosion resistance of Al–Ni melt-spun alloy improves with the increase of Ni component, and the higher melt temperature can enhance the corrosion resistance for the three components of Al–Ni alloys. As mentioned above, with the rise of temperature and Ni content, the clusters become small and their distribution becomes more uniform and dispersive, the structure of Al97.3Ni2.7 melt becomes more uniform and disordered, thereby the melt-spun alloys prepared under such a favorable melt condition form a dense and uniform structure and exhibit excellent corrosion resistance. In addition, the more uniform and disordered melt structure is more similar to the amorphous structure than crystal structure, and less nucleating center appears during the rapid solidification process, resulting in the tendency to form amorphous structure, thereby the corrosion resistance improves with the more uniform structure contained amorphous structure.

3.2.3. Hardness The hardness of alloy mainly depends on the atomic binding force, while the atomic binding force is closely linked to the structure and organization. Fig. 10 shows the hardness of the Al100 − xNix (x = 2, 2.7, 5) alloy ribbons prepared with 750, 850, 950 and 1050 °C in the roller rotation speed of 2500 rpm. As can be seen, the hardness of melt-spun Al–Ni alloy increases with the rise of the prepared melt temperature and Ni content. As mentioned above, the alloy melt structure becomes uniform and dense at the higher temperature, the melt-spun alloy that remained part of this melt features forms a more compact and uniform structure with less defects and fine grains, thereby its hardness increases. The Ni content has the same effect on the hardness of Al–Ni alloy. In addition, as a strengthening phase, the amorphous structure also enhances the hardness. In addition, the increments of hardness with the rise of the prepared temperature are different. As shown in Fig. 10, the hardness increment of Al–Ni alloy from prepared temperature 850 to 950 °C is large while that from to 750 to 850 °C and from 950 to 1050 °C are small, indicating that the structure of melt-spun Al–Ni alloy prepared below the 850 °C is quite different with that prepared above 950 °C. In accordance with the resistivity experiment and electrochemical potentiodynamic polarization test, the hardness measure also shows that the melt structure change of Al–Ni alloy with the temperature is non-linear, and the liquid–liquid structure change occurs during 850–950 °C.

Fig. 10. Hardness of Al100 − xNix (x = 2, 2.7, 5) alloys prepared with different melt temperatures in the roller rotation speed of 2500 rpm.

3.3. Details and connections between melt and amorphous structure This Al–Ni alloy melt is consisted of atom clusters and large numbers of free atoms, the central atoms (Ni or Al) and the nearest neighbor atoms (Al) form coordinating polyhedron, then several polyhedrons connect with each other to form clusters. The Al–Ni alloy melts structure stays in a dynamic equilibrium because of the existence of structural fluctuation, composition fluctuation and energy fluctuation, the atoms of clusters can break away from clusters and the free atoms can also be absorbed into the clusters. With the rise of temperature, the atoms have more energy and more drastic movements, leading to the increment of free atoms and volume contraction of clusters, the coordinating polyhedrons with high CN transform into that with low CN, resulting in the transformations of cluster types, increments of cluster numbers and more uniform distribution of clusters. The resistivity experiments show that the temperature dependence of melt structure gradually while certain relatively large changes have taken place during 850–950 °C. The corrosion resistance and hardness of melt-spun Al–Ni alloy prepared with different melt temperatures also have quite large changes from 850 to 950 °C, indicating the difference of melt structure between 850 and 950 °C. The melt structure becomes more uniform, disorder and dense with the rise of temperature, the SRO and MRO change along with the melt structure and their scale contracts gradually, when this decrement reaches to a critical point the MRO even can disappear since the MRO only exists in the relatively low temperature. Accordingly, the relatively large structure change in 880 °C, i.e., the temperature-induced liquid–liquid structure change, is the disappearance of MRO or the large changes of SRO. The features of this favorable melt structure are remained in Al–Ni alloy partly by melt-spun process; thereby the alloys prepared with higher melt temperature exhibit the stronger corrosion resistance and higher hardness. Some studies indicate that the liquid Al alloy still maintains the solid-like clusters that based on the face-centered cubic (fcc) form; then its SRO changes and the fcc structure transforms into the bodycentered cubic (bcc) structure at about 800 °C; moreover, some large changes occur during 770–900 °C such as viscosity and density of liquid Al [30–32]. Those studies coincide with the results in this paper, as clearly depicted in Fig. 11, the change of melt structure with temperature can be described simply as follows: with the rise of temperature, the Al–Ni alloy melt structure undergoes the decomposition and disappearance of solid-like clusters, the formation, increment and transformation of coordinating polyhedron with high CN, the increment and conversion of coordinating polyhedron clusters structure with low CN, the disappearance of certain heterogeneous structures; this process is accompanied by the reduction of SRO, reduction and disappearance of MRO. As a physical quantity sensitive to melt structure, the resistivity depicts this change tendency intuitively by the ρ–T curve as clearly shown in Fig. 11(a)–(b). The different properties of Al97.3Ni2.7 meltspun alloy prepared with different melt temperatures also show the similar change tendency and verify the accuracy of the melt experiment as shown in Fig. 11(c)–(e). Amorphous structure and melt structure both belong to disordered state and their structure are similar, they are both hard to describe because of their disordered atomic arrangement within SRO and MRO. The melt structure and amorphous structure have similarity to a certain degree and two structures also have something in common. Some comprehensive previous works show that the amorphous structure and melt structure are quite similar, and the CN for the amorphous and liquid samples is practically the same as for the crystalline fcc structure [33]. The studies [34,35] show that the amorphous Al–Ni alloys can be prepared by rapid quenching and their structures are similar to those of liquid alloys although amorphous alloys have more ordered structures, and in both liquid and amorphous Al–Ni alloys, the tetrahedral local order is dominant.

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Fig. 11. Change tendency with temperature about different properties of Al97.3Ni2.7 alloy: (a)–(b), CN and resistivity of liquid alloy [27], (c)–(e), the hardness, Ecorr and icorr of melt-spun alloy.

In this paper, we try to study the melt structure and its change with resistivity and rapid solidification method. As clearly shown in Fig. 11, these experimental results of melt-spun alloy verify with the experimental results of the alloy melts, indicating that this method is useful. The mutual authentication of these experiments reflects the Al–Ni alloys melt structure and its changes with the temperature from different angles. However, the method that explored the melt structure from rapidly solidified alloys and amorphous structure is not yet mature, which still needs lots of work to improve and perfect it. 4. Conclusions The resistivity and viscosity of Al100 − xNix alloy melts (x = 0, 2, 2.7, 5) were studied at different temperatures, and the structure and properties of melt-spun Al–Ni alloy contained amorphous structure were also explored to research the melt feature and its influence on the Al–Ni alloy. From these results and discussions, the major conclusions are summarized in the following: (1) The resistivity of Al–Ni melt increases with the rise of the melt temperature and Ni content, and shows a liner change from TL to 850–950 °C and then turns into another tendency. With the rise of the temperature and Ni content, the melt structure becomes homogeneous, dense and disordered. (2) The corrosion resistance and hardness of melt-spun alloy improve with the rise of melt temperature and Ni content. The homogeneous and dense melt structure features are remained partly in this melt-spun alloy and have a beneficial influence on its corrosion resistance and hardness. The various tests on melt-spun alloy verify the results of the resistivity, high-temperature XRD and viscosity. (3) The resistivity, high-temperature XRD and viscosity of the alloy melt and the properties of melt-spun alloy indicate that the melt structure versus the temperature and Ni content is nonlinear, and certain temperature-induced liquid–liquid structure change (the dramatic

changes of SRO or the disappearance of MRO) occurs during 850–950 °C. The study of melt structure from the rapid solidified alloy and amorphous structure is useful even the further work should be done to improve and perfect it. Acknowledgment The authors would like to acknowledge the National Natural Science Foundation of China (51271087, 51471076, 51401085) with this work. References [1] E. Axinte, Metallic glasses from “alchemy” to pure science: present and future of design, processing and applications of glassy metals, Mater. Des. 35 (2012) 518–556. [2] X.F. Li, F.Q. Zu, L.J. Liu, J.G. Li, J. Chen, C.M. Hu, Effect of Sn on reversibility of liquid– liquid transition in Bi–Sb–Sn alloys, J. Alloys Compd. 453 (2008) 508–512. [3] E. Karaköse, M. Keskin, Structural investigations of mechanical properties of Al based rapidly solidified alloys, Mater. Des. 32 (2011) 4970–4979. [4] N.I. Noskova, V.V. Shulika, A.G. Lavrentev, A.P. Potapov, G.S. Korzunin, Structure and Barkhausen effect parameters of amorphous alloys after various heat treatments, Russ. J. Nondestruct. Test. 40 (2004) 620–624. [5] A.I. Zaitsev, N.E. Shelkova, A.D. Litvina, Association in metallic melts and its relation to amorphization: Fe–P alloys, Dokl. Phys. 45 (2000) 359–362. [6] Z.P. Chen, H. Yu, Y. Wu, H. Wang, X.J. Liu, Z.P. Lu, Nano-network mediated high strength and large plasticity in an Al-based alloy, Mater. Lett. 84 (2012) 59–62. [7] H. Devianto, Z.L. Li, S.P. Yoon, J. Han, S.W. Nam, T.H. Limb, et al., The effect of Al addition on the prevention of Ni sintering in bio-ethanol steam reforming for molten carbonate fuel cells, Int. J. Hydrogen Energy 35 (2010) 2591–2596. [8] Y.L. Wang, H.Q. Ji, W. Peng, L. Liu, F. Gao, M.G. Li, Gold nanoparticle-coated Ni/Al layered double hydroxides on glassy carbon electrode for enhanced methanol electrooxidation, Int. J. Hydrogen Energy 37 (2012) 9324–9329. [9] R.V. Reeves, A.S. Mukasyan, S.F. Son, Thermal and impact reaction initiation in Ni/Al heterogeneous reactive systems, J. Phys. Chem. C 114 (2010) 14772–14780. [10] T. Weidlich, A. Krejčová, L. Prokeš, Study of dehalogenation of halogenoanilines using Raney Al–Ni alloy in aqueous medium at room temperature, Monatsh. Chem. 141 (2010) 1015–1020. [11] R. Banerjee, P. Ayyub, G.B. Thompson, R. Chandra, P. Taneja, H.L. Fraser, Microstructure and magnetic, transport, and optical properties of ordered and disordered Ni–25Al alloy thin films, Thin Solid Films 441 (2003) 255–260.

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