Materials and Design

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1 Materials and Design 30 (2009) Contents lists available at ScienceDirect Materials and Design journal homepage: Microstructure and mechanical properties of NiAl Cr(Mo)/Nb eutectic alloy prepared by injection-casting L.Y. Sheng a,b, J.T. Guo a, *, H.Q. Ye b a Superalloys Division, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Liaoning, Shenyang , China b Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang , China article info abstract Article history: Received 16 May 2008 Accepted 30 June 2008 Available online 5 July 2008 Keywords: Injection-casting NiAl Cr(Mo)/Nb eutectic alloy Microstructure Mechanical properties In the present paper, the microstructure, compressive properties at different temperatures and hardness of NiAl Cr(Mo)/Nb alloy prepared by injection-casting were investigated. Compared with the conventionally cast alloy, the injection-cast alloy exhibited a fine microstructure, i.e. the fine eutectic cell and interlamellar spacing as well as fine primary NiAl phase and Laves phase Cr 2 Nb due to the high cooling rate. In addition, the area fraction of primary NiAl phase at the cell interior or cell boundaries and eutectic cell increased. The compressive ductility and yield strength at room temperature increased by about 100% and 50% over those of conventionally cast alloy, respectively. However, both alloys possessed the similar high temperature strength. The Vickers hardness of injection-cast alloy also increased markedly. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Compared with many NiAl-based alloys, NiAl 28Cr 6Mo (NiAl Cr(Mo) for short) eutectic alloys are regarded as the most logical choice of the multielement system examined to date because of their relatively high melting point, good thermal conductivity and high elevated temperature creep resistance as well as higher fracture toughness [1 3]. Niobium (Nb) was found to be very effective in improving the elevated temperature strength of NiAl alloy [4,5]. However, the addition of Nb in NiAl Cr(Mo) eutectic alloy decreased the fracture toughness and compressive ductility at room temperature (RT) severely, due to the Laves phase wakened the eutectic cell boundary [6]. Hence, the RT ductility and toughness of Nb doped NiAl Cr(Mo) eutectic alloy need to be improved when it can be applied as high-temperature structure material. Microstructural control is well known to be an excellent way to improve the ductility and toughness without deteriorating the strength of material, among which, grain refinement plays an important role on the improvement of ductility and strength of materials. For NiAl Cr(Mo)/Nb alloy, the feasible way to improve its toughness and strength further is to reduce lamellar spacing and microsegregation of solute element Nb that will fine the Laves phase at the cell boundaries. Solidification at high cooling rate is a feasible way to attain the aim. Injection-casting, as a popular method to fabricate bulk amorphous [7,8], can also be adopted to prepare bulk NiAl alloys at a relatively high cooling rate of about 10 2 K/s that is higher than the conventional cast. * Corresponding author. Tel.: ; fax: address: jtguo@imr.ac.cn (J.T. Guo). Almost no work has been done about the effect of injectioncasting on NiAl Cr(Mo) eutectic alloys so far. Hence, the microstructural evolution and mechanical properties of Ni 32.5Al 28Cr 6Mo 1Nb alloy (at.%) prepared by injection-casting were investigated. 2. Experimental procedure The master alloy with a composition of Ni 32.5Al 28Cr 6Mo 1Nb (at.%) (NiAl Cr(Mo)/Nb for short) were prepared by induction melting with starting materials of 99.99% Ni, 99.9% Al, 99.9% Cr, 99.9% Mo and 99.9% Nb, respectively. The melted alloy was casted into rods with 50 mm in diameter. These rods fabricated by the conventionally casting technique were cut into slices. Some of them were investigated at as-cast state, and the remaining ones were crushed for injection-casting. The injection-casting was conducted with water-cooled copper mold method, which was usually utilized to prepare bulk metallic glasses, having significant undercooling capacity. Microstructural characterization and fracture surface investigation of alloys fabricated by conventionally casting and injection casting techniques were carried out by S-3400 scanning electron microscope (SEM) with energy dispersive spectrometer (EDS) and the compositions of constitute phases were detected by EPMA electronic probe microanalysis (EPMA). The resultant phases in the different alloys were characterized by X-ray diffraction (XRD) with a Cu radiation at 40 kv and 40 ma. The interlamellar spacing (k) of the alloys was measured using the line-intercept method at the eutectic cell interior. The region with a fine /$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi: /j.matdes

2 L.Y. Sheng et al. / Materials and Design 30 (2009) interlamellar spacing is defined as the cell size, while the region between neighboring cells with a coarser interlamellar spacing is defined as the width of intercellular zone. The foils for transmission electron microscope (TEM) observation were prepared by the conventional twin jet polishing technique using an electrolyte of 10% perchloric acid in methnol at 20 C after mechanical polishing to 50 lm and cutting into disc with a diameter of 3.0 mm. The TEM observation was performed by a JEM-2010 transmission electron microscope operated at 200 kv. The compression specimens with the size of 4 4 6mm 3 were cut from conventionally cast alloy and injection-cast alloy by electro-discharge machining (EDM) and all surfaces were mechanically ground with 600-grit SiC abrasive prior to compression test. The compression tests were conducted in air with a Fig. 1. SEM micrographs of the conventionally cast alloy: (a) low magnification; and (b) high magnification. Fig. 2. (a) XRD of the NiAl Cr(Mo)/Hf alloy; (b) TEM micrograph of Cr 2 Nb phase; (c) SAED pattern of Cr 2 Nb phase with beam direction B = [011] and (d) SAED pattern of Cr 2 Nb phase with beam direction B = ½ 111Š.

3 966 L.Y. Sheng et al. / Materials and Design 30 (2009) gleeble 1500 test machine at a nominal strain rate of s 1. The autographically recorded load time curves were converted to true stress strain curves via the assumption of constant volume. Vickers hardness tests for different phases in alloys were carries out by MICROMET microhardness tester under 100 g mass hold for 30 s. Seven measurements were performed to evaluate an average value. 3. Results and discussion 3.1. Microstructure The SEM micrographs of conventionally cast NiAl Cr(Mo)/Nb alloy are presented in Fig. 1. The alloy consists of gray eutectic cell with an average size of lm, intercellular zone with the width of lm. In the eutectic cell, black NiAl and gray Cr(Mo) plates exhibited a radially emanating pattern from the cell interior to its boundaries. Coarser Cr(Mo) rods, primary NiAl phases and white phases mainly distribute at the intercellular zone. By close observation (Fig. 1b), there are many Cr(Mo) precipitations in the NiAl phase and NiAl precipitations in the Cr(Mo) phase in the conventionally cast alloy. The white phases are identified by a combination of XRD and TEM. The EDS test reveals that the white phase is rich of Nb and Cr. The XRD patter shows that the peak of Laves phase appeares in the NiAl Cr(Mo)/Nb eutectic alloys, which indicate the Nb addition results in the formation of Cr 2 Nb Laves phase. Also, this has been further confirmed by TEM. Fig. 2b shows the bright-field TEM image of Laves phase. These phases mainly distributed at the tip of Cr(Mo) or between Cr(Mo) phase. Figs. 2c and d show the selected area electron diffraction (SAED) patterns of [011] and ½ 111Š zone axis of the Laves phase, respectively. From the results, the observed white phase can be determined as Cr 2 Nb the Laves phase with C14 structure. The typical microstructure of injection-cast NiAl Cr(Mo)/Nb alloy is shown in Fig. 3. Apparently, the microstructure of injectioncast alloy is quite different from that of conventionally cast alloy as a result of high cooling rate during injection-casting. The average eutectic cell size in injection-cast alloy, about lm, is smaller than that of conventionally cast alloy. The interlamellar spacing (k) in eutectic cell interior of injection-cast alloy is finer than that of conventionally cast alloy, ranged from 5.62 lm to 0.39 lm. The width of intercellular region with 1 2 lm for injection-cast alloy is far narrower than lm for conventionally cast alloy. The primary NiAl dendrites in injection-cast alloy with an average size of 10 lm mainly distribute at the eutectic cell interior or cell boundary. Although the area fraction of primary NiAl phase in injection-cast alloy with about 19% is more than 9% for conventionally cast alloy, the area fraction of eutectic cell still attains increase from 60% for conventionally cast alloy to 79% for injection-cast alloy as a result of the decreased width of interlamellar zone. By comparing Fig. 1a with Fig. 2a, it is specific that the Laves phase has been distinctly fined and evenly distributes at eutectic cell boundaries. Raj et al. investigated the directionally solidified NiAl Cr(Mo) eutectic alloy and found that the eutectic cell size and lamellar spacing decreased with increasing growth rate from 12.7 mm/h to 508 mm/h and the average width of the intercellular region was essentially independent of growth rate and varied between 20 lm and 25 lm [9]. However, it is surprised to find that the high cooling rate distinctly decreased the width of intercellular region in the present investigation. The possible reason can be attributed to the large amount of primary NiAl dendrite formed during injection-casting process. On the one hand, the composition of injection-cast alloy lies in the eutectic point after the formation of Fig. 3. SEM micrographs of the injection-cast alloy: (a) low magnification; and (b) high magnification. primary NiAl phase. On the other hand, these fine NiAl particle can act as nucleating center for cell eutectic, which will lead to the rapid formation of NiAl Cr(Mo) eutectic cell. TEM observation on the injecting cast alloy shows that there are abundant interface dislocation networks along the interfaces of NiAl and Cr(Mo) phases, as shown in Fig. 4a. Such high-density interface dislocation networks well demonstrate the extension of solid solubility. As shown in Table 1, in the injecting cast alloy the amount of Ni and Al solid soluted in Cr(Mo) phase is higher than that of Cr and Mo in NiAl phase. It is no doubt that this will increase the difference of crystal lattice parameters between the NiAl and Cr(Mo) phases, which can result in more interface dislocations formation along NiAl and Cr(Mo) phases boundaries. According to the report of Hein et al. [10], such high-density interface dislocations is beneficial to improve the strength of the injecting cast alloy. In addition, a great amount of fine NiAl particles precipitate in the Cr(Mo) phase with an average size of 20 nm, which also demonstrates the high cooling rate inhibits elements diffusions. However in the primary NiAl phase, there are just some fine a-cr particles precipitating with several decade nanometers, as shown in Fig. 4b. This exhibits the extension of solid solubility resulted by the injection-casting. The EPMA test results of constituent phases in the conventionally cast and injection-cast alloys were enumerated in Table 1. For injection-cast alloy, the solubility of Cr, Mo in primary NiAl phase, Ni, Al in Cr(Mo) phase and Ni in Laves phase are higher than that of conventionally cast alloy. As shown in Table 1, the 7.47% Cr was detected in primary NiAl phase of injection-cast alloy by EPMA,

4 L.Y. Sheng et al. / Materials and Design 30 (2009) Fig. 5. True stress strain curves compressed at RT of the conventionally cast and injection-cast alloy. Table 2 Results of compression tests of the conventionally cast and injection-cast alloys under the nominal strain rate of s 1 Alloys Conventionally cast alloy Injection-cast alloy Test temperature (K) Yield strength (MPa) Compressive strength (MPa) RT K > K >25 RT K > K >30 Compressive strain (%) Fig. 4. (a) TEM micrograph of Interface dislocation networks along the NiAl and Cr(Mo) phases boundaries in the injection-cast alloy; and (b) fine Cr particles precipitating inside primary NiAl. Table 1 Composition of constituent phases in conventionally cast and injection-cast alloys (at.%) Alloys Phase Ni Al Cr Mo Nb Conventionally cast alloy Primary NiAl Cr(Mo) Laves Injection-cast alloy Primary NiAl Cr(Mo) Laves which is more than its solid solubility in equilibrium solidification state [11] Compressive properties The true stress strain curves and mechanical test data at RT of conventionally cast and injection-cast alloys were shown in Fig. 5 and Table 2, respectively. It can be seen that the alloys fabricated by different technologies have the similar stress strain curves, which exhibit continuous work hardening. The injection-cast alloy attains yield strength of 1525 MPa and compressive strength of 2137 MPa, which are about 50% and 40% higher than those of the conventionally cast alloy. Compressive strain values at RT are calculated based on practical plastic deformation in order to eliminate the contribution from the compliance of the testing system. The injection-cast alloy has a better RT compressive ductility with about 37% than about 18% for conventionally cast alloy. It means that the RT compressive ductility and strength of injection-cast alloy have been improved at the same time. The typical RT compressive fractographies of conventionally cast alloy and injection-cast alloy are shown in Fig. 6a and b. Both of the fractographies exhibit similar fracture morphologies, i.e. typical stripping of NiAl Cr(Mo) interface and cleaving of primary NiAl. However, many dimple-like cavities can be observed on the fracture surface of the injection-cast alloy. These cavities were formed by thin Cr(Mo) phase and NiAl phase in eutectic cell pulling out from each other, which are propitious to attain good compressive ductility and strength. The refinement of eutectic cells, the decrease of lamellar spacing, the extended solubility and the increased area fraction of cell eutectic as well as the refinement of Laves phase particles may be the main factors that are relevant to the improved strength and ductility of injection-cast alloy at RT. The decrease of lamellar spacing and increase of cell eutectic zone produce more interfaces between NiAl and Cr(Mo) phases. Investigations [1,10] demonstrated that the dislocation network between NiAl and Cr(Mo) phase interface plays an important role on the strength of NiAl based alloys. The more NiAl Cr(Mo) interfaces result in more dislocation network at the interface, so the strength of alloys can be enhanced accordingly. The increscent solubility of alloying elements in NiAl and Cr(Mo) is beneficial to the room temperature strength by solution strengthening. Besides, the remarkable increase in the total area of cell boundaries or phase boundaries by injection-casting also induces a significant decrease in the segregated concentration of Laves phase per unit area of cell boundaries that is beneficial to the strength improvement by

5 968 L.Y. Sheng et al. / Materials and Design 30 (2009) Fig. 7. Crack propagation path of conventionally cast alloy compressed at 1273 K. Table 3 Microhardness of NiAl-Cr(Mo)/Nb alloys prepared by conventionally-casting injecting-casting Alloys Phase Vickers hardness (HV) Conventionally cast alloy Primary NiAl 463 Eutectic cell 534 Injection-cast alloy Primary NiAl 581 Eutectic cell 656 was counteracted by the weaken effect from the weak cell boundaries and large amount of primary NiAl phases Microhardness Fig. 6. Typical RT fracture surface of (a) the conventionally-cast alloy; and (b) the injection-cast alloy. particle strengthening. The improvement of ductility can be attributed to the increased area fraction of eutectic cell as well as the fine NiAl and Cr(Mo) plates. The mechanical test data by compression test at 1273 K and 1373 K of conventionally cast and injection-cast alloys (Table 2) shows that both alloys almost have the similar elevated temperature strength. For injection-cast NiAl Cr(Mo)/Nb alloy, the high solid solution of alloying element in NiAl and Cr(Mo) phase, the great area fraction of eutectic cell and fine lamellar spacing should be beneficial to the improvement of the strength at 1273 K and 1373 K. However, it exhibits just a little higher elevated temperature strength compared with the conventionally cast alloy. The causation can be ascribed to the followed factors, i.e. the weak intercellular zone and large amount of primary NiAl phase. For NiAl Cr(Mo)/Nb lamellar eutectic alloy, crack is easy to pass along the cell boundary when stress is applied to the eutectic alloy due to the coarser NiAl Cr(Mo) plate and primary NiAl at intercellular zone, as shown in Fig. 7. During injection-casting process, the eutectic cell fined due to high cooling rate and the total area of cell boundary increases distinctly. The failure of the boundaries may become the dominant factor in determining the high temperature strength. At 1273 K and 1373 K, the strength of NiAl is weaker than the cell eutectic and Cr(Mo) phase. For injection-cast alloy during high temperature deformation, the large amount of primary NiAl phase will yield first, which will accordingly cause the weak high temperature strength. It can be concluded that the strengthening effect from more dislocation networks and large solid solution Due to the fine microstructure of injection-cast alloy, only the microhardness of primary NiAl phase and eutectic cell in injection-cast and conventionally-cast alloys are measured (enumerated in Table 3). It is specific that the hardness of primary NiAl phase and eutectic cell in NiAl Cr(Mo)/Nb alloy prepared by injection-casting increase significantly. Generally, the hardness of eutectic cell is higher than the NiAl phase in NiAl Cr(Mo) eutectic alloys [6], however, the hardness of primary NiAl phase in injection-cast alloy even exceeds the hardness of the eutectic cell in conventionally cast alloy. The increased hardness for the injection-cast alloy can be attributed to the extended solid solution of alloying element in NiAl and Cr(Mo) phase and fine microstructure induced by high cooling rate. 4. Conclusions 1. The microstructure of the injection-cast alloy presents a fine microstructure including the refinement of eutectic cell size, interlamellar spacing and intercellular zone as well as primary NiAl phase and Cr 2 Nb Laves phase. In addition, the extension of solid solubility occurred in the alloy. 2. The injection-cast alloy attains room temperature yield strength of 1525 MPa and compressive ductility of about 37%, which are higher than 995 MPa and 18% for conventionally cast alloy, respectively. Moreover the injection-cast alloy possesses a little higher elevated temperature strength than the conventionally cast alloy. 3. The extended solid solution of alloying element and fine microstructure markedly increased the microhardness of the injection-cast alloy.

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