MECHANICAL BEHAVIOUR OF AMORPHOUS Mg-23.5Ni RIBBONS

Transcription

1 VIII Congreso Nacional de Propiedades Mecánicas en Sólidos, Gandía MECHANICAL BEHAVIOUR OF AMORPHOUS Mg-23.5Ni RIBBONS P. Pérez a, G.Garcés a, P. Adeva a and F. Sommer b a Centro Nacional de Investigaciones Metalúrgicas. CSIC. Avda. Gregorio del Amo, Madrid, Spain b Max Planck Institute for Metals Research, Heisenbergstrasse 3, D Sttutgart, Germany. ABSTRACT The tensile properties of amorphous Mg-23Ni (wt. %) ribbons in the ºC temperature range were studied. Three different behaviours depending on the test temperature were found. These regimes can be associated with the microstructural changes occurring from the amorphous to crystalline state. In the 25-70ºC interval, the ribbons exhibited a brittle behaviour. Above 70ºC, the material showed a slight plasticity, about 3 %, decreasing the yield stress up to 125ºC, which can be related to the onset of the crystallization process. At 138 and 150ºC, a substantial increase in the yield stress takes place due to the crystallization of a metastable phase, which is clearly manifested by the appearance of two elastic slopes in the stress-strain curve at 138ºC. The behaviour above 150ºC is characterized by the appearance of a steady state, reaching a maximum elongation of 35% at 200ºC. In this temperature range, the yield stress decreases with increasing the test temperature. Mechanical behaviour in this interval can be explained by the formation of a microstructure consisting of the equilibrium phases, Mg and Mg 2 Ni. Keywords Magnesium alloys, amorphous materials, crystallization, tensile properties 1. INTRODUCTION Magnesium alloys are the lightest metallic structural material and therefore very interesting for using in those applications where the weight saving is a requirement in the design conditions. Studies on some amorphous and partially crystalline magnesium alloys have shown that properties such as tensile strength, toughness, hardness and corrosion resistance are better than for the respective crystalline alloys [1,2]. Higher values of tensile strength with significant plastic deformation have been reported for a Mg-Zn-Ce alloy with a mixed amorphous/crystalline microstructure [3]. Much research has been addressed to systems exhibiting a wide supercooled region with good glass-forming ability in a large compositional range as for Mg-M-Ln alloys, where M is nickel or copper and Ln a lanthanide element. Glass transition and crystallization kinetics of these alloys have been extensively studied by different researchers. It has been observed that these amorphous magnesium alloys display a certain thermal stability which allows to synthesize them as bulk material by different techniques such as metallic mould casting or squeeze casting [4,5]. The mechanical properties of these bulk Mg alloys are similar to those processed by the melt spinning technique. These alloys are interesting because of their high tensile strengths, three times higher than those for the strongest conventional magnesium alloys. This substantial increase in the yield strength turns the material extremely brittle. However, the deformation mechanisms that take place during the mechanical test are not well understood yet. Moreover, the microstructural evolution from the amorphous to the stable crystalline state when these alloys are 61

2 Pérez, Garcés, Adeva and Sommer tested at different temperatures makes it difficult to study the mechanisms controlling the deformation. Different possible transformations steps of a metallic glass like an amorphous+nanocrystalline phase, a nanocrystalline phase or the crystalline phase result in different deformation modes. The aim of this work is to study the tensile behaviour of amorphous Mg-23,5Ni (wt. %) ribbons in the temperature range of 25 to 350ºC, correlating the mechanical properties with the microstructural changes occurring during deformation at these temperatures. The crystallization processes during isochronal and isothermal annealing of this alloy in the investigated temperature range have been extensively established by Kempen et al [6]. 2. EXPERIMENTAL PROCEDURE Amorphous Mg-23,5Ni (wt. %) ribbons with thickness of 39 µm and width of 0.7 mm were produced using the melt-spinning technique. The ribbon exhibits a very smooth surface and show a good strength to be bended. A detailed description of the production of the alloy and the meltspinning procedure is given in reference 6. The ribbons were stored below room temperature to avoid a partial crystallization. Mechanical behaviour was studied by in-situ tensile tests using a microtensile machine coupled to the scanning electron microscope (SEM). This machine has a load cell of 200 N, a sensitivity of 0.1 N, and flat grips. A heating system attached to both grips allows testing up to 500ºC. Length of the samples was 10 mm. Test were conducted in vacuum, between room temperature and 350ºC at an initial strain rate of 10-4 s -1. Since several microstructural changes occur during deformation of the ribbons depending on the temperature and/or deformation, a procedure, in which the same period of heating (6-7 min) was used to reach the temperature prior to run tensile test. This procedure, at least, allows a comparison of the samples tested at the same temperature. Otherwise, crystallization processes could differ from one sample to other. In-situ tensile test allows the observation of the deformation process on the surface of the ribbons and the evolution of the fracture stages. The microstructural changes in the alloy from the amorphous to the crystalline state were observed by SEM and transmission electron microscopy (TEM). TEM samples were prepared by directly argon ion milling of the sample at liquid nitrogen temperature. Fracture surfaces were analyzed by SEM. 3. RESULTS 3.1 Crystallization behaviour of amorphous ribbons Isochronal DSC scan, pertaining to the crystallization of amorphous Mg-23,5Ni (Fig. 1) shows that the crystallization took place in two stages. The first one, which appears at 164 C at a heating rate of 20ºC/min, corresponds to simultaneous crystallization of magnesium and a metastable phase, identified as Mg 5.5 Ni by XRD measurements (Fig.2). The second stage is associated with the transition from the metastable Mg 5.5 Ni phase to the thermodynamically stable phase Mg 2 Ni. This transformation occurs at around 190ºC (see Fig. 1). 3.2 Mechanical properties The tensile behaviour of amorphous ribbons tested from room temperatures up to 350ºC can be divided into three intervals, see Figs 3 and 4, which can be correlated with the microstructural changes occurring in the amorphous ribbon during heating. 62

3 VIII Congreso Nacional de Propiedades Mecánicas en Sólidos Figure 1. Isochronal DSC recorded with a heating rate of 20 K/min Figure 2. X-ray diffractogram of the Mg- 23Ni alloy after completed isothermal crystallization at 150 C. 1. At temperatures below 138ºC the material exhibits high strength. At room temperature, amorphous ribbons are completely brittle, although failure takes place in the elastic regime at stresses of about 400 MPa. At temperatures above 50ºC, the amorphous alloy shows certain plasticity, with a maximum of 3 % at 100ºC. As expected for a magnesium alloy, the yield stress decreased with increasing the temperature from about 440 MPa at 50ºC to 160 MPa at 125ºC. 2. At 138 and 150ºC the material experiences a significant increase in the yield strength. It is worth noting that the σ-ε curve at 138ºC displays two regimes. In the first regime the material deforms plastically at a low stress, with a behaviour similar to that observed in the low temperature interval. The material exhibits a steady state, yielding at about 100 MPa. The amorphous alloy deforms plastically for about 2%, followed by a sudden increase of the stress. It is obvious that under these conditions the material deforms elastically. Further recovery of a plastic behaviour was not observed, and fracture took place at a high stress σ (MPa) C C C 88 C 138 C C 113 C C ε Figure 3. Tensile σ-ε curves of the Mg-23Ni alloy between room temperature and 150 C. 63

4 Pérez, Garcés, Adeva and Sommer (270 MPa). At 150ºC the material is brittle, only 0.25 % of elongation, and its yield stress was 215 MPa. 3. At temperatures above 150ºC, the considerable decrease in the yield stress as the temperature increases is accompanied by a significant increase of the elongation to failure. Maximum values of elongation were attained at 175 and 200ºC, 28 and 35 %, respectively. σ (MPa) C 175 C 200 C C 250 C 350 C Figure 4. Tensile σ-ε curves of the Mg-23Ni alloy between 175 and 350 C. ε 3.3 In situ-sem observation of tensile tests In-situ SEM observation of the ribbon during testing at temperatures below 150ºC did not shown any microstructural changes on the surface of the ribbon during deformation. This might result from the combination of the low temperature, at which no crystallization processes take place, and small elongation. However, at higher temperatures significant variations could be observed throughout the test (Fig. 5). The initial amorphous state underwent different morphological changes during the heating up to the test temperature as well as during deformation (see Fig. 3). It can be checked that at 250ºC crystallization of equilibrium phases took place, as noticed by distinct aspect of the surface after 6 min of preheating up to 250ºC. An equiaxial-like grain structure with an average size of about 10 µm developed with progressing the deformation. The relief between the different grains increases through the deformation process, although their size practically did not change. A close examination of the surface evidences the shifting of the polish scratches between neighbour grains. This suggests, that to some extent grain boundary sliding took place. 3.4 Microstructural characterization of deformed ribbons Fracture surface of the ribbons tested in the first temperature regime, i.e. below 125ºC, shows the typical vein pattern fracture of amorphous materials (see Fig.6). Above this temperature, the crystallization of the ribbon induces a different fracture mode. The roughness observed in some places of the fracture surface tested at 150 C, marked by an arrow in Fig. 7, seems to indicate that crystallization has taken place. Above 150 ºC the ribbon exhibits a high elongation which is manifested by a strong necking, as shown in Fig. 8. The fracture surface 64

5 VIII Congreso Nacional de Propiedades Mecánicas en Sólidos appears with a strong relief (Fig. 8). It is very interesting to note the high deformation capacity of the alloy in this state. The surface of the gauge length after testing is shown in Fig. 9, where a two-phase microstructure and a high plastic deformation manifested by slip bands are visible. a b 0 % 6 % c d 9 % 20 % Figure 5. SEM micrograph sequence of the surface of a ribbon in-situ tensile test at 250 C. a) Non-deformed. b) After 6 % of deformation. c) After 9 % of deformation. d) After 20 % of deformation Figure 6. Fracture surface of the sample tested at 110 C. Figure 7. Fracture surface of the sample tested at 150 C. 65

6 Pérez, Garcés, Adeva and Sommer a b Figure 8. a) SEM micrograph showing necking in the fracture region of the sample deformed at 200 C. b) Fracture surface of the sample tested at 200 C. Figure 9. SEM micrograph showing the surface of the sample deformed at 200 C 4. DISCUSSION The above results show that the mechanical properties of the initially amorphous Mg-23Ni alloy are strongly affected by the different crystallization processes from 20ºC to 350ºC. In order to correlate the mechanical behaviour with the microstructure, TEM observations of the samples before and after tensile tests were performed. In the temperature range below the first crystallization peak (i.e. below 125 C, see Fig. 1), the ribbon deforms in the amorphous state. The observed vein fracture in this temperature range is typical for the amorphous state. The 66

7 VIII Congreso Nacional de Propiedades Mecánicas en Sólidos small plasticity exhibited by the alloy at temperatures above 50ºC, around 2 %, could be explained by structural relaxation of the amorphous phase with increasing temperature. Figure 10. TEM micrograph showing the microstructure of the sample after 7 min preheating to 150 C. Figure 11. TEM micrograph showing the microstructure of the sample deformed 0.2% at 150 C. SADP indicates the nanocrystalline character of the phases The change of the behaviour observed at 138ºC is characterized by a significant strengthening of the alloy. The ribbon, however, becomes extremely brittle. This behaviour has to be associated with the crystallization process. Figs. 10 and 11 show the microstructure of the preheated ribbon at 150ºC and tensile tested at 150 C, respectively. From comparison between both results it can be concluded that ribbon crystallises during preheating. Rings in the Selected Area Diffraction Pattern (SADP) indicate the nanocrystalline character of the phases. From the results shown in Fig. 1 and Fig. 2 it follows that these phases correspond to Mg and metastable Mg 5.5 Ni. The slight coarsening of these phases during the tensile test results from diffusive processes occurring during the deformation. No signs of dislocation activity could be observed. The low ductility of the alloy at these temperatures could be explained by the presence of a high volume fraction of the metastable intermetallic. A mole fraction of 70 % of the Mg 5.5 Ni phase has been reported after isothermal annealing at 127 C for about 4 h [6]. Consequently, any defect in the ribbon such as pores, flaws, etc.. will act as an strong stress concentrator at which any nucleated crack can propagate catastrophically. The stability of this phase is restricted to a narrow range of about 25ºC and, therefore, the metastable phase decomposes in Mg and Mg 2 Ni phases above 150ºC. The decomposition process occurs very slowly at temperatures below 200 C [6], and results in an increase in ductility of the alloy at higher temperatures. The mechanical behaviour of the ribbons deformed above 150 ºC is characterized by a stationary period. At these temperatures an eutectic microstructure develops. The morphology of Mg and Mg 2 Ni phases evolves from small particles at 175 ºC to a lamellar morphology above 200ºC. Fig. 12 shows the microstructure of the ribbon preheated up to 300ºC before testing. It is constituted by magnesium (dark phase) and Mg+Mg 2 Ni lamellar eutectic regions. A high density of stacking faults was observed in the Mg 2 Ni phase. The study of the ribbon deformed at 300 C showed a refinement of the grain size, especially in the regions where the fracture took place. At 67

8 Pérez, Garcés, Adeva and Sommer this temperature, the lamellar eutectic microstructure tends to be broken and be replaced by a more equiaxial one, with small particles of Mg 2 Ni at grain boundaries of magnesium as shown in Fig. 13. The higher ductility and yield strength exhibited by the ribbons deformed at 175 and 200 ºC with respect of those tested above 200ºC, could be attributed to changes in the volume fraction of the different phases, i.e. Mg, Mg 2 Ni and Mg 5.5 Ni, since a long time is needed to attain the equilibrium state. Thus, variations in the volume fraction of these phases, due to changes in temperature and deformation, can be expected during tensile tests. The different microstructures, as the temperature increases, could lead to distinct mechanisms controlling the deformation process. Thus, grain boundary sliding would be a possible deformation mechanism at higher temperatures, as noticed by shifting of polish scratches. This deformation mechanism was observed in nanostructured gold thin films at low homologous temperature [7]. Figure 12. SEM and TEM micrographs showing the microstructure of the sample preheated to 300 C 68 Figure 13. TEM micrograph showing the microstructure of the sample after 20% deformation at 300 C.

9 VIII Congreso Nacional de Propiedades Mecánicas en Sólidos Although dislocations activity could not be observed by TEM, movement of dislocations might also contribute to deformation, as deduced from slip bands observed in Fig. 14. Moreover, it is well known that simultaneous slip in both basal and pyramidal planes operates in magnesium with increasing the temperature. More work is in progress to elucidate the different mechanisms controlling the deformation of this alloy in the temperature range from 175 to 350 C. Figure 14. TEM micrograph showing the microstructure of the sample after 20% deformation at 300 C. 5. CONCLUSIONS From the present study the following conclusions can be drawn: The tensile behaviour of the amorphous ribbon tested from room temperatures up to 350ºC shows three different regimes, which have been associated with the microstructural changes occurring by effect of temperature and deformation from the amorphous to the crystalline state. The tensile behaviour of Mg-23.5Ni in the ºC range is characterized by high strength levels and low ductility. The fracture is the typical vein pattern of amorphous materials. At 138 and 150ºC, the yield stress increases substantially due to the crystallization of Mg and metastable Mg 5.5 Ni. The presence of a high volume fraction of the metastable intermetallic phase causes the embrittlement of the alloy. Since the metastable phase tends to decompose at temperatures above 150 C in Mg and Mg 2 Ni equilibrium phases, the anomalous mechanical behaviour is restricted to a narrow range of about 25ºC. The tensile properties of Mg-23.5Ni above 150ºC are characterized by the appearance of a steady state, reaching a maximum elongation of 35%at 200ºC. In this temperature range, the yield stress decreases with increasing the test temperature. Differences in the mechanical properties at the different temperatures are related to changes in the morphology and in the volume fraction of Mg, Mg 2 Ni and Mg 5.5 Ni. According to SEM observations two possible mechanisms can act during deformation at temperatures above 150 C; grain boundary sliding and traditional slip by dislocation movement. 69

10 Pérez, Garcés, Adeva and Sommer 6. REFERENCES 1. T. Spassov, V. Köster. Microstructure, microhardness and corrosion behaviour of rapidly solified magnesium based Mg-Ni-(Y, NM) Alloys, Z. Metallkd. 91, , M. Avedesian, H. Baker. Magnesium and Magnesium alloys, ASM Specialty Handbook, G. Kim, A. Inoue, T. Masumoto. Increase of mechanical strength of Mg 85 Zn 12 Ce 3 amorphous alloy by dispersion of ultrafine hcp Mg particles, Mater. Trans JIM, 32, , A. Inoue, A. Kato, T. Zhang, S.G. Kim, T. Masumoto. Mg-Cu-Y amorphous alloys with high mechanical strengths produced by a metallic mold casting method, Mater. Trans. JIM, 32, , G. Kim, A. Inoue,T. Masumoto. High mechanical strengths of Mg-Ni-Y and Mg-Cu-Y amorphous alloys with significant supercooled liquid region, Mater. Trans. JIM., 31, , T. W. Kempen, H. Nitsche, F. Sommer, E. J. Mittemeijer. Crystallization kinetics of amorphous magnesium-rich magnesium-copper and magnesium-nickel alloys. Met. Trans A, 33 A, , M. Ke, S.A.Hackney. W. Milligan, E. Aifantis. Observation and measurement of grain rotation and plastic strain in nanostructured metal thin films, Nanostructured Materials, 5, ,