PROPERTIES OF NANOCRYSTALLINE Al-Cr-Fe-Ti ALLOYS PREPARED BY POWDER METALLURGY. Karel DÁM, Dalibor VOJTĚCH, Filip PRŮŠA

Transcription

1 PROPERTIES OF NANOCRYSTALLINE Al-Cr-Fe-Ti ALLOYS PREPARED BY POWDER METALLURGY Karel DÁM, Dalibor VOJTĚCH, Filip PRŮŠA Department of Metals and Corrosion Engineering, Institute of Chemical Thechnology, Prague, Technická 5, Prague 6, Czech Republic, Abstract Al-Cr alloys have been extensively studied because of their superb mechanical properties at elevated temperatures that can compete with steels or titanium alloys. An Al-6Cr-2Fe-1Ti (in wt.%) was prepared by powder metallurgy. Rapidly solidified powder was made by melt atomisation and compacted using hot ultrahigh pressure (6 GPa). Samples were uni-axially pressed 400 C for 1 h using hydraulic press. The purpose was to examine its structures, room and elevated temperature mechanical properties and thermal stability during static annealing and creep tests. The increased pressing temperature could have significant impact on the final structure, since it can reduce the porosity and the quality of powder particles bonding. The examined Al-6Cr-2Fe-1Ti alloy showed excellent thermal stability its mechanical properties did not change significantly even after 100 h of annealing at 400 C. In addition, the Al 6Cr 2Fe 1Ti alloy exhibited very good creep resistance. Compared to the commercial Al-12Si-1Cu-1Mg-1Ni alloy the mechanical properties as well as the thermal stability were considerably better. The mechanical properties were discussed in relation to their structures and diffusivities of the alloying elements. Key words: Rapid solidification, Powder metallurgy, Aluminium, Thermal stability, Mechanical properties 1. INTRODUCTION Aluminium alloys for elevated temperature applications have become a subject of considerable interest especially in automotive and aerospace industry due to their good strength to weight ratio. For this purpose Al-Cr alloys have been extensively studied. Very promising materials on this base are Al Cr Fe Tibased alloys, which have shown excellent mechanical properties at elevated temperatures, and can thus compete with steels or titanium alloys. These alloys are characterised by a very low solubility and diffusivity of the alloying elements in solid aluminium [1]. Since concentrations of these elements should be relatively high, it is almost impossible to prepare Al Cr Fe Ti alloys by conventional casting metallurgy. Therefore, powder metallurgy (PM) is widely used. For this purpose, both rapid solidification and mechanical alloying are used to prepare metal powders. The structure of powders contains sub-micrometer grains of the fcc Alphase and also nanoscaled crystalline or quasi-crystalline (Q) intermetallic phases [2-7]. Q phases are known to decompose at temperatures exceeding 500 C [8] and are essential for the thermal stability of Al Cr Fe Ti alloys. Due to a similar crystallographic orientation between Al and Q-phases, they positively affect the strength [7]. Several techniques have been used for compacting of metal powders, such as hot extrusion, hot isostatic pressing (HIP) or uni-axial pressing. The hot extrusion has been most commonly used but high temperatures negatively affect final mechanical properties because of the decomposition of supersaturated solid solutions and growth of Al grains and intermetallic phases. The HIP process is generally performed at relatively low pressures (in order of 100 MPa), which is not very suitable for aluminium alloys due to insufficient bonding of oxidized aluminium powder particles. However, recently reported experiments have shown, that using pressures exceeding 1 GPa can produce compact bulk materials almost porosity-free [9]. Previous experiments investigated a similar Al Cr Fe Ti alloy prepared by cold pressure compaction of a

2 rapidly solidified powder, at an ultra-high pressure of 6 GPa [10]. Material obtained was compact, pore-free and exhibited good mechanical properties even at higher temperatures. However, it appeared as mechanically unstable due to internal stresses induced by cold pressing at 6 GPa. The aim of this study is to provide information about an Al 6Cr 2Fe 1Ti alloy prepared by hot ultrahigh pressure compaction of an atomized powder. The purpose was to examine its structure, room and elevated temperature mechanical properties and thermal stability during static annealing and creep tests. The increased pressing temperature could have significant impact on the final structure, since it can reduce the porosity and the quality of powder particles bonding. Simultaneously, it can support relaxation of internal stresses induced by pressing and increase the mechanical stability of the material. The chemical composition of the alloy used in this experiment differs from the commonly investigated materials based on Al-Cr-Fe-Ti system [1-8, 10-14]. The content of high-cost titanium was lowered to approximately 1 wt.% to reduce the cost of the alloy and, at the same time, to retain the stabilizing effect of Ti on quasi-crystalline phases in the structure. Ti is known to stabilize Q-phases by occupying preferential positions in their structural units [12]. This is the reason why titanium is widely used in thermally stable Albased alloys. 2. EXPERIMENTAL The nominal composition of the investigated alloy was Al-6Cr-2Fe 1Ti (in wt.%). The ingot was prepared by melting of pure Al, Al-30Fe, Al-10Cr and Al-10Ti (all in wt.%) master-alloys and casting of the melt into a metal mould. Afterwards, the ingot was remelted in an induction furnace and atomized by pressured nitrogen (500 kpa) into a RS powder, with particle size ranging from 1 to 500 µm. Powder fraction of less than 100 µm was used for further processing. The powder was compacted using ultra-high pressure of 6 GPa in a Norton hydraulic press. Samples were uni-axially pressed at 400 C for 1 h in a tungsten carbide (WC) compression mould to prepare cylindrical samples of 20mm in diameter and 10mm in height. Mechanical properties were examined using uni-axial compression tests at room temperature and 250 C at a deformation rate of 1 mm/min. Thermal stability was also tested by Vickers microhardness measurements (HV 5) during static annealing of samples at 400 C for up to 100 h. Compressive creep behaviour of the tested materials at 250 C was also measured using three different compressive stress levels: 200, 250 and 300 MPa. All compressive measurements were conducted using LabTest SP1 VM testing machine. Light microscopy (LM) and scanning electron microscopy (SEM Tescan Vega 3 with EDS detector) were used for structural examinations of as prepared and annealed samples. All the tests mentioned above were also performed on an alloy with a chemical composition of Al-12Si- 1Cu-1Mg-1Ni (in wt.%) in order to compare mechanical properties and thermal stability of the investigated PM material with a commercial alloy. The Al-Si based alloy is considered to be thermally stable and is commonly used for pistons for combustion engines. This alloy was provided by an industrial supplier. The T6 heat treatment included solution treatment at 510 C/5 h, followed by water quenching and aging at 230 C/6 h [15]. 3. RESULTS AND DISCUSSION 3.1. Structures Fig. 1 shows the backscattered electron (BSE) SEM images of the structure of the Al 6Cr 2Fe 1Ti alloy taken in a plane parallel to the pressing direction. Fig. 1a corresponds to the as-pressed sample which was compacted at 400 C for 1 hour. The almost porosity-free structure contained fcc-al matrix (dark) and intermetallic phases of various sizes and shapes (light). Regions with different structure correspond to the

3 initial powder particles, forming heterogeneous grain structure. The high pressure affects the shape of the grains which became elongated in one direction. Smaller particles are characterized by very refined structure, consisting of nanoscaled phases. This was caused by higher cooling rate and a deeper undercooling of such particles. The powder particles were deformed; however, the obtained images indicate that the applied deformation led to a good contact between the original particles. During the pressing, friction between the particles breaks the surface oxide layer, which is necessary for a good diffusion bonding. Ultra high pressure may also cause a vacancy gradient in the fcc-al lattice, which is a driving force for diffusion. Similar results were reported in [9]. It can be seen that the pressing temperature did not have a significant influence on the grain size or size and shape of the intermetallic phases which denotes the excellent stability of the intermetallic phases at high temperature. Fig. 1b shows the structures of the same sample as Fig. 1a but after subsequent static annealing (400 C/100h) which was performed in order to test the thermal stability. These results will be discussed later in the Chapter 3.3. Fig. 1 Structure (SEM) of the Al 6Cr 2Fe 1Ti alloy (a) in the as-pressed condition 1 hour at 400 C; (b) after subsequent static annealing for 100 hours at 400 C Mechanical properties The compressive stress-strain diagram of the Al 6Cr 2Fe 1Ti and Al-12Si-1Cu-1Mg-1Ni alloys at room temperature and at 250 C is shown in Fig. 2. At room temperature the compressive yield strength of the Al 6Cr 2Fe 1Ti alloy (490 MPa) was significantly higher than that of the casting Al-12Si- 1Cu-1Mg-1Ni (350 MPa) (see Fig. 2). This can be explained by different strengthening mechanisms of these types of alloys. In the Al 6Cr 2Fe 1Ti alloy, strengthening is primarily caused by the Hall-Petch mechanism. A part of the strengthening effect can be associated with the presence of sub-micrometer sized intermetallic phases (see Fig. 1) and lattice defects caused by incomplete recovery after work hardening. In contrast, the casting Al-12Si-1Cu-1Mg-1Ni alloy contains large grains and eutectic silicon particles [16]. Previous experiments [10] have shown the Fig. 2 Compressive stress-strain diagrams of the Al 6Cr 2Fe 1Ti and Al-12Si-1Cu-1Mg-1Ni alloys measured at room temperature (RT) and at 250 C.

4 mechanical properties of a ultra-high pressure compressed alloy with similar composition. The yield strength of the cold compressed material was 547 MPa which is slightly above the values measured for the hot compressed materials investigated in this study. The reason is that the cold deformation of the powder particles increases the concentration of lattice defects and introduces a large internal stress in the fcc- Al lattice. The hot compressed alloy showed improved ductility (Fig. 2). This is attributed to recovery processes which occurred during the compaction at increased temperatures. The compressive stress-strain curves, measured at room temperature, also suggest that the casting Al-12Si-1Cu-1Mg-1Ni alloy exhibited the most pronounced work hardening and plasticity. This is attributed to the fact that the strengthening effect of the intermetallic particles in this alloy is weaker compared to the Al 6Cr 2Fe 1Ti alloy. The Al-Cr based alloy shows a good plasticity and some work hardening during the compression test. This behaviour suggests that some of defects induced by the ultra-high pressure annihilate during the compaction at such a high temperature and part of the stress is relieved. As a result, defects, formed during the loading and the plastic deformation, have more space to migrate within the structure. It means there is a lower probability that moving dislocations will accumulate at barriers and introduce local stresses resulting in the formation of macroscopic fractures Thermal stability The compressive testing at 250 C reveals that the elevated temperature yield strength of the Al 6Cr 2Fe 1Ti alloy is higher than that of the casting alloy (see Fig. 2). The Al 6Cr 2Fe 1Ti alloy compacted at all temperatures exhibits better results compared to the casting Al-12Si-1Cu-1Mg-1Ni alloy. Therefore, this material can be considered as an alternative to the commercially used Al-Si based alloy, since it retains its excellent mechanical properties even at a relatively high temperature, due to the presence of thermally stable intermetallic phases. The achieved values of compressive yield strength are similar with reported values for the Al 93 Ti 2 Fe 3 Cr 2 (at.%) alloy prepared by mechanical alloying and hot extrusion [5]. However, our samples contain a lower total concentration of alloying elements. Apparently, using an ultra-high pressure for powder compaction results in a material with sufficient density where positive structural features of the rapid solidification are retained. The thermal stability of the investigated materials was also examined by measuring hardness changes during the long time (100 h) static annealing at 400 C (Fig. 3). It should be noted that the used temperature considerably exceeds the maximum operating temperatures of most aluminium components. From Fig. 3, it can be seen that the initial hardness of the Al 6Cr 2Fe 1Ti alloy is around 150 HV. Within the first few hours the values drops to approximately 115 HV and remains almost constant during the whole annealing period. In contrast, the initial hardness of the Al-12Si-1Cu-1Mg-1Ni alloy is 121 HV and during the annealing it is reduced to a value of 63, which makes a difference of more than 50 Vickers units compared to the Al 6Cr 2Fe 1Ti. Although the Al-Si based alloy is generally accepted as thermally stable and suitable for applications as pistons in combustion engines, its stability at these conditions appears to be poor. In Fig. 3, it can be seen that the hardness of the Al 6Cr 2Fe 1Ti alloy changes significantly only at the beginning of the annealing period and then it remains essentially constant. From the images of the structures which were taken after annealing (Fig. 1b) it is apparent that the

5 Fig. 3 Room temperature Vickers hardness of the Al 6Cr 2Fe 1Ti and Al-12Si-1Cu-1Mg-1Ni alloys as a function of time at 400 C. Fig. 4 The creep behaviour of the Al 6Cr 2Fe 1Ti and Al-12Si-1Cu-1Mg-1Ni alloys measured at 250 C and three levels of compressive stress 200, 250 and 250MPa. structure does not change significantly in terms of size and shape of intermetallic phases. This means that the initial drop in hardness is not caused by structural changes. It can be assumed that an ultrahigh pressure compaction causes deformation of the originally spherical particles and induces formation of internal stress in the material. This is manifested as an elastic strain of the fcc-al lattice. Shaw et al. [13] showed that relaxation of internal stresses in a mechanically alloyed Al-Cr-Fe-Ti material begins at 150 C after 1 hour and it is accompanied by a reduction in hardness. Thus, it indicates that the observed decrease in hardness of the Al 6Cr 2Fe 1Ti alloy compacted at a pressure of 6 GPa is caused by relaxation of the elastic strain in the fcc-al lattice. After the relaxation process mechanical properties become stable (Fig. 3). Another part of thermal stability testing was compressive creep tests. The creep behaviour provides additional information about the performance of a material at high temperatures and its usability for applications, in which it is subjected to such long term conditions. Results of creep tests are shown in Fig. 4. In this figure the Al 6Cr 2Fe 1Ti alloy compacted at 400 C is compared with the casting Al-12Si-1Cu-1Mg-1Ni alloy. The difference in the creep behaviour of the two alloys is evident. The Al 6Cr 2Fe 1Ti alloy shows a compressive strain of 50% after 65 hours at 300 MPa. In contrast, for the Al-12Si-1Cu-1Mg-1Ni alloy this value is exceeded at the lowest stress level used (200MPa) already after 30 hours. At 300 MPa the level of deformation reached is ~70 %. It can be also seen in Fig. 4 that the linear creep flow is achieved within a few tens of hours of the experiment. The steady state creep rate is relatively low and similar for all the experiments. It indicates a relative mechanical stability of both materials after certain initial periods of thermal and mechanical exposition. The observed excellent creep resistance of the Al 6Cr 2Fe 1Ti alloy is attributed to the presence of very fine, thermally stable intermetallic phases, which stabilize its microstructure and assure that the materials retains its mechanical properties. In addition, these phases effectively hinder dislocation slip and thermally activated processes like grain boundary sliding and dislocation climbing. All these processes have been shown to control the creep of Al alloys [17].

6 4. CONCLUSIONS In this study the ultra-high uni-axial pressure and high temperature of 400 C were used for compaction of the rapidly solidified Al 6Cr 2Fe 1Ti powder alloy. It was shown that this process results in compact and porosity-free materials with excellent mechanical properties. The hardness and compressive yield strength of the investigated alloy is comparable with alloys prepared by most commonly used hot extrusion method. Using elevated temperatures for the compaction appeared to be efficient for increasing the mechanical stability after applying the ultra-high pressure. It was shown that the compaction temperature of 400 C resulted in a material with a good mechanical stability, sufficient mechanical properties and excellent thermal stability. The powder metallurgy Al-Cr-Fe-Ti alloy was compared to the Al-12Si-1Cu-1Mg- 1Ni alloy, which is currently used for pistons in combustion engines. The mechanical properties as well as the thermal stability of the Al-Cr based material were considerably better; therefore, this material can be considered as an alternative to the commercially used Al-Si based alloy. ACKNOWLEDGEMENTS The authors wish to thank the Czech Science Foundation (project no. P108/12/G043) and the Specific University Research (MSMT No 21/2012) for the financial support for this research. REFERENCES [1] NAGAISHI, Y., YAMASAKI, M., et al. Effect of process atmosphere on the mechanical properties of rapidly solidified powder metallurgy Al Ti Fe Cr alloys. Materials Science and Engineering: A, 2007,vol , pp [2] GALANO, M., AUDEBERT, F., et al. Nanoquasicrystalline Al Fe Cr-based alloys with high strength at elevated temperature. Journal of Alloys and Compounds, 2010,vol. 495, nr. 2, pp [3] INOUE, A., KIMURA, H. High-strength aluminum alloys containing nanoquasicrystalline particles. Materials Science and Engineering: A, 2000,vol. 286, nr. 1, pp [4] INOUE, A., KIMURA, H.M., et al. High-strength aluminum- and zirconium-based alloys containing nanoquasicrystalline particles. Materials Science and Engineering: A, 2000,vol , pp [5] LUO, H., SHAW, L., et al. On tension/compression asymmetry of an extruded nanocrystalline Al Fe Cr Ti alloy. Materials Science and Engineering: A, 2005,vol. 409, nr. 1 2, pp [6] SHAW, L., LUO, H., et al. Compressive behavior of an extruded nanocrystalline Al Fe Cr Ti alloy. Scripta Materialia, 2004,vol. 50, nr. 7, pp [7] YAMASAKI, M., NAGAISHI, Y., et al. Inhibition of Al grain coarsening by quasicrystalline icosahedral phase in the rapidly solidified powder metallurgy Al Fe Ti Cr alloy. Scripta Materialia, 2007,vol. 56, nr. 9, pp [8] GARGARELLA, P., ALMEIDA, A., et al. Microstructural characterization of a laser remelted coating of Al91Fe4Cr3Ti2 quasicrystalline alloy. Scripta Materialia, 2009,vol. 61, nr. 7, pp [9] CIEŚLAK, G., LATUCH, J., et al. Quality of compaction of bulk nanocrystalline Al73Si19 Ni7mischmetal1 alloy produced by different pressure at high temperature. Reviews on Advanced Materials Science, 2008,vol. 18, nr. 4, pp [10] VOJTĚCH, D., MICHALCOVÁ, A., et al. Properties of the thermally stable Al95Cr3.1Fe1.1Ti0.8 alloy prepared by cold-compression at ultra-high pressure and by hot-extrusion. Materials Characterization, 2012,vol. 66, pp [11] GALANO, M., AUDEBERT, F., et al. Nanoquasicrystalline Al Fe Cr-based alloys. Part II. Mechanical properties. Acta Materialia, 2009,vol. 57, nr. 17, pp [12] GALANO, M., AUDEBERT, F., et al. Nanoquasicrystalline Al Fe Cr-based alloys. Part I: Phase transformations. Acta Materialia, 2009,vol. 57, nr. 17, pp [13] SHAW, L., LUO, H., et al. Effects of internal strains on hardness of nanocrystalline Al Fe Cr Ti alloys. Scripta Materialia, 2004,vol. 51, nr. 5, pp [14] ZAWRAH, M., SHAW, L. Microstructure and hardness of nanostructured Al Fe Cr Ti alloys through mechanical alloying. Materials Science and Engineering: A, 2003,vol. 355, nr. 1 2, pp

7 [15] VOJTĚCH, D., MICHALCOVÁ, A., et al. Structural characteristics and thermal stability of Al 5.7Cr 2.5Fe 1.3Ti alloy produced by powder metallurgy. Journal of Alloys and Compounds, 2009,vol. 475, nr. 1 2, pp [16] VOJTĚCH, D., VERNER, J., et al. Properties of thermally stable PM Al Cr based alloy. Materials Science and Engineering: A, 2007,vol. 458, nr. 1 2, pp [17] CAVOJSKY, M., BALOG, M., et al. Microstructure and properties of extruded rapidly solidified AlCr4.7Fe1.1Si0.3 (at.%) alloys. Materials Science and Engineering: A, 2012,vol. 549, pp