Powder metallurgy of high-strength Al90.4Y4.4Ni4.3Co0.9 gas-atomized powder K. G. Prashanth1*, K. B. Surreddi1,†, S. Scudino1, M. Samadi Khoshkhoo1, Z. Wang1,2, D. J. Sordelet3 and J. Eckert1,4 1
IFW Dresden, Institut für Komplexe Materialien, Postfach 270116, D-01171 Dresden, Germany 2 School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510640, China 3 Caterpillar Inc., Advanced Materials Technology Group, Mossville, Illinois, USA 4 TU Dresden, Institut für Werkstoffwissenschaft, D-01062 Dresden, Germany. Keywords: Metallic glasses, Powder metallurgy, Microstructure, Mechanical properties
*
Corresponding author; E-Mail:
[email protected]; Tel.: +49-351-4659-685; Fax: +49-351-4659-452. † Present address: Department of Materials and Manufacturing Technology, Chalmers University of Technology, Göteborg 41296, Sweden. Abstract Al90.4Y4.4Ni4.3Co0.9 gas-atomized powder was hot pressed (HP) to produce highly dense bulk samples through combined devitrification and consolidation. The microstructure of the asatomized powder is a mixture of amorphous phase with nanocrystalline fcc Al, whereas the consolidated samples consists of fcc Al and a series of intermetallic phases with or without residual amorphous phase depending on the hot pressing temperature (673 or 723 K). The HP samples exhibit a remarkable high strength of ~ 925 MPa (HP at 673 K) and ~ 820 MPa (HP at 723 K) combined with a plastic strain ranging between 14 and 30%. The reduction in strength for the sample HP at 723 K is linked to the complete crystallization of the powder with no residual amorphous phase. Introduction Materials with high specific strength has kindled the interest of the scientific society due to the demand for weight saving and emission reduction in the automobile sector [1]. High specific strength combined with good corrosion resistance has made amorphous, partially amorphous and nanocrystalline Al alloys a prominent member in this arena [2-3]. For example, Al-based amorphous alloys, such as Al85Y8Ni5Co2 melt-spun ribbons, show tensile fracture strength of about 1200 MPa [4]. Partially crystallized Al-based amorphous materials with uniform dispersion of fcc Al nanoparticles within the residual amorphous matrix exhibit even higher fracture strengths, hardness and wear resistance compared to fully amorphous alloys of the same composition [5-8]. Powder metallurgy (e.g. gas atomization followed by powder consolidation) is regarded as one of the suitable techniques to fabricate such alloys [9-10]. During powder consolidation, amorphous and partially amorphous alloys undergo devitrification with change in viscosity (depending of the temperature used) [11], along with other mechanisms that takes place during consolidation, like mass diffusion, plastic deformation and elastic recovery [12], making the overall process very complex. Surreddi et al. [13] have shown that the temperature during spark
plasma sintering plays a dictating role in determining the microstructure of the Al87Ni8La5 amorphous powder, leading to variation in strength as well as in plasticity. Along this line, this work focuses on the hot pressing (HP) of Al90.4Y4.4Ni4.3Co0.9 gas-atomized powder at 673 and 723 K. The microstructures as well as the mechanical properties of the HP compacts are studied and an attempt is made to understand the structure-property relation in the Al90.4Y4.4Ni4.3Co0.9 alloy. Experimental Gas atomized partially amorphous powder with a nominal composition of Al90.4Y4.4Ni4.3Co0.9 (at.%) was used as the starting material. The phase formation was studied by X-ray diffraction (XRD) using a Philips PW 1050 diffractometer (Co-Kα radiation). Thermal stability of the asatomized powder was studied by differential scanning calorimetry (DSC) at a heating rate of 40 K/min with a Perkin-Elmer DSC7 under a continuous flow of purified argon. Consolidation of the powder was done under argon atmosphere by uniaxial hot pressing at two different temperatures (673 and 723 K) using a pressure of 600 MPa. The microstructure of the consolidated samples were investigated by scanning electron microscopy (SEM) using a Gemini 1530 microscope and by transmission electron microscopy (TEM) using a FEI Tecnai F30 microscope operated in scanning TEM mode at 300 kV. Cylindrical specimens with a length / diameter ratio of 2 (6 mm length and 3 mm diameter) were prepared from the consolidated samples by wire erosion method. The specimens were tested with an INSTRON 8562 testing facility under quasistatic compressive loading (strain rate of 1x10-4 s-1) at room temperature. The strain during the compression tests was measured directly on the specimen using a Fiedler laserextensometer. Both ends of the specimens were polished to make them parallel to each other prior to the compression test. Results and discussion The microstructure of the as-atomized Al90.4Y4.4Ni4.3Co0.9 powder investigated by XRD is shown in Fig. 1. The pattern exhibits the broad maxima typical for an amorphous phase together with the broad diffraction peaks of fcc Al. This indicates that the as-atomized powder is a mixture of amorphous phase along with nanocrystalline fcc Al, which is similar to the structure observed for the Al87Ni8La5 gas-atomized powder [14]. The DSC curve of as-atomized Al90.4Y4.4Ni4.3Co0.9 powder (Fig. 2) exhibits two exothermic peaks due to the crystallization of the glass. The onset temperatures of first and second exothermic peaks are Tx1 = 600 K and Tx2 = 650 K. The XRD patterns of the as-atomized powder after heating above the crystallization events in Fig. 2 are shown in Fig. 1. The pattern taken after heating the powder to 640 K (above the first crystallization event) shows broad diffraction peaks belonging to Al along with diffraction peaks from the Al3Y and Al4Ni3 intermetallic compounds. In addition, an extremely weak and broad maximum due to the residual amorphous phase can be observed. The pattern of the powder heated to 873 K (far above the second crystallization event) shows the diffraction peaks from Al and Al3Y, Al4Ni3 and Al9Co2 intermetallic phases. No amorphous phase is visible at this stage, indicating that complete crystallization of the glass has occurred. The Al90.4Y4.4Ni4.3Co0.9 powder was consolidated into bulk samples by hot pressing at 673 and 723 K. The XRD pattern of the sample HP at 673 K (Fig. 1) reveals the formation of the phases already observed in the sample heated to 640 K in the DSC, i.e. Al and Al3Y and Al4Ni3 intermetallic compounds, along with a weak broad maximum due to the presence of a residual
amorphous phase. The presence of a residual amorphous phase is due to the HP temperature (673 K), which is below the second crystallization event. The pattern of the sample HP at 723 K (above the second crystallization peak in Fig. 2) reveals the formation of Al, Al4Ni3, Al3Y and Al9Co2 phases without any visible residual amorphous phase (Fig. 1).
Fig 1 XRD patterns (Co-Kα radiation) of the Al90.4Y4.4Ni4.3Co0.9 powder: asatomized, after heating to 640 and 873 K in the DSC and bulk samples consolidated by hot pressing at 673 and 723 K.
Fig. 2 DSC scan (40 K/min) of the as-atomized Al90.4Y4.4Ni4.3Co0.9 powder. Fig. 3(a) and 3(b) show the SEM micrographs of the sample HP at 673 K. The images reveal the formation of a bright interface between the particles along with black areas at the intersection of the particles (i.e. the triple point junctions), similar to those observed for the Al87Ni8La5 samples consolidated by SPS [14]. The particles are not homogeneous and can be considered as consisting of a composite microstructure made of grains of about 1-3 µm size separated by a matrix with rod-like morphology (Fig. 3(b)). EDX analysis indicates that the bright interfaces are
rich in Y, Ni and Co (~ 40±10% richer than the original atomic percent), which may correspond to the intermetallic compounds observed by XRD (Fig. 1). A similar composition has been observed for the rod-like matrix between the grains. On the other hand, the 1-3 µm sized grains in Fig. 3(b) show a higher Al content (about 5% higher than the nominal Al content) that, considering the XRD results in Fig. 1, suggests a mixed microstructure consisting of fcc Al and the residual amorphous phase. Fig. 3(c) and 3(d) show the TEM-bright field micrographs corresponding to the black triple point junctions observed in Fig. 3(a). The figures reveal that these black areas are made of Al grains of about 100 – 300 nm and are surrounded by the rodlike features (the intermetallic compounds) having 40 nm thickness and 300 to 500 nm length.
Fig. 3 (a)-(b) SEM and (c)-(d) bright-field TEM micrographs of the Al90.4Y4.4Ni4.3Co0.9 sample hot-pressed at 673 K. Fig. 4 shows the SEM micrographs of the sample HP at 723 K. Similar to the sample HP at 673 K (Fig. 3), the micrographs show the formation of bright interfaces between the particles, indicative of good particle bonding, along with the presence of the rod-like structures. The amount of rod-like features in the sample hot-pressed at 723 K is higher (~ 30 ± 10 % from the image analysis) as compared to the sample HP at 673 K. This is accompanied by the disappearance of the 1-3 µm sized grains visible in Fig. 3(b) and to the formation of a uniform microstructure throughout the particles. The formation of such a microstructure can be related to the high HP temperature (723 K) which leads to the full crystallization of the material and to the formation of additional fcc Al and intermetallic compounds from the residual amorphous phase (Fig. 1). Fig. 5 shows the room temperature compressive stress-strain curves of the samples consolidated by hot pressing at 673 and 723 K. The sample HP at 673 K exhibits yield and compressive strength of about 880 ± 10 MPa and 925 ± 2 MPa, respectively. With increasing stress, the curve displays a softening behavior up to fracture, which occurs at 850 ± 4 MPa stress and 14 ± 1% strain. This softening-like behavior is similar to that observed for the Al-Ni-La alloy [9]. The
stress-strain curve of the sample HP at 723 K exhibits lower yield and compressive strengths (780 ± 10 MPa and 820 ± 2 MPa) as compared to the sample HP at 673 K. However, the fracture strain is remarkably larger, reaching a value of 30 ± 2% at 690 ± 5 MPa stress.
Fig. 4 SEM micrographs of the Al90.4Y4.4Ni4.3Co0.9 samples hot-pressed at 723 K.
Fig. 5 Compression stress-strain curves of the Al90.4Y4.4Ni4.3Co0.9 samples consolidated by hot pressing at 673 and 723 K. The higher stress level of the sample HP at 673 K with respect to the sample HP at 723 K is most likely due to the presence of the residual amorphous phase, as shown by the corresponding XRD pattern in Fig. 1 and the SEM micrograph in Fig. 3(b). Due to the incomplete crystallization of the amorphous phase, the sample consolidated at 673 K displays a lower amount of crystallized fcc Al. As a result, the fracture strain of the sample HP at 673 K is reduced compared to the sample HP at 723 K that, in contrast, is fully crystallized and is not characterized by the hindrance of the dislocations movement induced by the residual amorphous phase. Conclusions Al90.4Y4.4Ni4.3Co0.9 gas-atomized powder has been consolidated by HP. The bulk samples display remarkable mechanical properties, namely, high compressive strength ranging between 820 and 925 MPa combined with plastic strain in the range of 14 – 30%. Strength and plasticity of the HP samples are dictated by their corresponding microstructures. Higher strength and reduced plasticity are related to the presence of a residual amorphous phase, which may hinder the
dislocations movement within the Al phase. On the other hand, reduced strength but enhanced plastic deformation is a result of the complete crystallization of the glass and of the formation of additional fcc Al from the residual amorphous phase. The combined devitrification and consolidation of glassy precursors has led to the development of high-strength deformable bulk samples. Moreover, this route gives room for tuning the mechanical properties by the proper control of the microstructure at the consolidation stage. Thus, the results presented in this work clearly indicate that powder metallurgy, i.e. powder synthesis and consolidation, is a particularly suitable method for the production of novel Al-based materials characterized by high strength combined with considerable plastic strain. References
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