Materials & Design Materials and Design 28 (2007) 1940–1944 www.elsevier.com/locate/matdes
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Microstructure and mechanical properties of NiAl–Cr(Mo)/Hf alloy prepared by injection casting K.W. Huai, J.T. Guo *, X.H. Du, R. Yang Institute of Metals Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, PR China Received 11 August 2005; accepted 28 April 2006 Available online 21 June 2006
Abstract In the present paper, the microstructure, compressive properties at different temperatures and hardness of NiAl–Cr(Mo)/Hf 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 Heusler phase Ni2AlHf 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 ductility and yield strength at room temperature increased by about 100% and 25% 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. 2006 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]. Hafnium (Hf) was found to be very effective in improving the elevated temperature strength of NiAl–Cr(Mo) eutectic alloy. However, the addition of Hf weakened the fracture toughness and compressive ductility at room temperature (RT) severely [4,5]. Hence, the RT ductility and toughness of Hf-doped NiAl–Cr(Mo) eutectic alloy need to be improved when it is 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 *
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0261-3069/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2006.04.024
ductility and strength of materials. For NiAl–Cr(Mo)/Hf alloy, the feasible way to improve the toughness and strength further is to reduce lamellar spacing and microsegregation of solute element Hf that will fine the Heusler 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 [6,7], can also be adopted to prepare bulk NiAl alloys at a relatively high cooling rate of about 102 K/s that is higher than the conventional cast. Almost no work has been done about the effect of injection casting on NiAl–Cr(Mo) eutectic alloys so far. Hence, the microstructural evolution and mechanical properties of NiAl–28Cr–5.5Mo–0.5Hf alloy (at.%) prepared by injection casting were investigated. 2. Experimental procedure Alloy ingots with a composition of NiAl–28Cr–5.5Mo–0.5Hf were prepared in a vacuum induction furnace with starting materials of 99.9% Ni, 99.9% Al, 99.8% Hf, 99.5% Cr and 99.9% Mo and cast into rods with 50 mm in diameter. The slices were cut from the conventionally-cast ingot as prealloyed ingot. The injection-cast specimen was prepared by remelting the prealloyed ingot in quartz tube, and injected through a nozzle with a diameter of 0.5 mm into Cu mold with an inner cavity with 10 mm in
K.W. Huai et al. / Materials and Design 28 (2007) 1940–1944 diameter. Structural characterizations of all alloys were determined by optical microscopy (OM), scanning electron microscopy (SEM) and electronic probe microanalysis (EPMA). The interlamellar spacing (k) of the alloys was measured using the line-intercept method at the eutectic cell interior. The region with a fine 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 compressive specimens with the size of 4 · 4 · 6 mm3 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 compressive test. The compressive test was conducted in air with a gleeble 1500 test machine at a nominal strain rate of 2.0 · 10 3 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 micro-hardness tester under 50 g mass hold for 30 s. The hardness values were the average of at least 5 indentations for each load.
3. Results and discussion 3.1. Microstructure The SEM micrographs of conventionally-cast NiAl– Cr(Mo)/Hf alloy are presented in Fig. 1. The alloy consists of gray eutectic cell with an average size of 100–150 lm, intercellular zone with the width of 10–20 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 phase and white Heusler phase 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 typical microstructure of injection-cast NiAl– Cr(Mo)/Hf alloy is shown in Fig. 2. Apparently, the micro-
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structure of injection-cast alloy is quite different from that of conventionally-cast alloy as a result of rapid cooling rate during injection casting. The average eutectic cell size in injection-cast alloy, about 20–25 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 to 0.39 lm. The width of intercellular region with 1–2 lm for injection-cast alloy is far more narrow than 10–20 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 13% 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 Heusler phase has been distinctly fined and evenly distributes at cell boundaries. Raj SV etc. 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 to 508 mm/h and the average width of the intercellular region was essentially independent of growth rate and varied between 20 and 25 lm [8]. However, it is surprised to find that the rapid cooling rate distinctly decreased the width of intercellular region. The possible reason can be attributed to the large amount of primary NiAl dendrite formed during injection casting process. On the one hand, all the composition of injection-cast alloy lies in the eutectic point after the formation of primary
Fig. 1. (a, b) SEM micrographs of conventionally-cast alloy.
Fig. 2. (a, b) SEM micrographs of the injection-cast alloy.
K.W. Huai et al. / Materials and Design 28 (2007) 1940–1944
NiAl phase. On the other hand, these fine NiAl particles can act as nucleating center for cell eutectic, which will lead to the rapid formation of NiAl–Cr(Mo) eutectic cell. For injection-cast alloy, no obviously precipitations were observed in NiAl phase and Cr(Mo) phase, even for coarser primary NiAl phase by SEM. Reviews [9,4] show that even by melt-spinning or rapidly cooling rate by water cooling during heat treatment, fine a-Cr phase can separate out from the NiAl phase in NiAl–Cr eutectic alloy. Hence, no Cr(Mo) particle in primary NiAl phase maybe a gloss due to the lack of great resolution for SEM and further investigation is needed by TEM. The EDS results of different phase in conventionallycast and injection-cast alloys are enumerated in Table 1. For injection-cast alloy, the solubility of Cr, Mo and Hf in primary NiAl phase, Ni, Al in Cr(Mo) phase and Cr in Heusler phase are higher than that of conventionallycast alloy. Although 7.47% Cr was detected in primary NiAl phase of injection-cast alloy by EPMA, the solid solution of Cr in NiAl phase should be less than the measured result [9]. Considering the above analysis that many fine Cr(Mo) particle is likely to exist in the NiAl phase, the Cr content measured by EPMA maybe be affected by the Cr(Mo) particle. 3.2. Compressive properties The true stress–strain curves and mechanical test data at RT of conventionally-cast and injection-cast alloys are shown in Fig. 3 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 1445 MPa and compressive strength of 2031 MPa, both of which are about 25% higher than those of conventionallycast 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 14% than about 7% for conventionally-cast alloy. It means that the RT ductility and strength of injection-cast alloy have been improved at the same time. Fig. 4 shows the typical RT compressive fractographies of conventionally-cast alloy and injection-cast alloy, both of which exhibit similar fracture morphologies (Fig. 4a and b), i.e. typical stripping of NiAl–Cr(Mo) interface and cleavTable 1 Composition of different phases in conventionally-cast and injection-cast alloys (in at.%) Alloys
Phase
Ni
Al
Cr
Mo
Hf
Conventional cast
Primary NiAl Cr(Mo) Heusler
47.22 3.10 50.48
50.18 4.50 23.27
2.60 80.78 3.40
0.10 11.62 –
0.20 – 22.85
Injection casting
Primary NiAl Cr(Mo) Heusler
44.76 9.00 47.17
45.92 10.08 26.58
7.47 70.38 6.17
0.85 10.54 –
0.36 – 20.08
2000
Injection-cast alloy
True stress (MPa)
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1500
Conventionally-cast alloy
1000
500
0 0.00
0.05
0.10
0.15
0.20
0.25
True strain
Fig. 3. True stress–strain curves compressed at RT of conventionally-cast and injection-cast alloy.
ing of primary NiAl. However, many dimple-like cavities can be observed on the fracture surface of SC 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 Heusler 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 [1,3] demonstrated that the dislocation network between NiAl and Cr(Mo) phase interface plays an important role on the strength of NiAlbased 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 Heusler phase per unit area of cell boundaries that is beneficial to the strength improvement by 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 compressive test at 1273 and 1373 K of conventionally-cast and injection-cast alloys (Table 2) show that both alloys have the similar high temperature strength. For injection-cast NiAl–Cr(Mo)/Hf 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 propitious to the improvement of the strength at 1273 and 1373 K. However, it exhibits a similar high temperature strength to the conventionally-cast alloy. The causation can be ascribed to the followed factors, i.e. the weak intercellular zone and
K.W. Huai et al. / Materials and Design 28 (2007) 1940–1944 Table 2 Results of compressive tests under the nominal strain rate of 2 · 10
3
s
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1
Alloy
Test temperature (K)
Yield strength (MPa)
Compressive strength (MPa)
Compressive strain (%)
Conventionally cast
RT 1273 K 1373 K
1168 384 229
1577 434 254
7 >30 >30
Injection casting
RT 1273 K 1373 K
1445 390 227
2031 450 268
14 >30 >30
Fig. 5. Crack propagation path of injection-cast alloy compressed at 1273 K.
Table 3 Room temperature Vickers hardness of NiAl–Cr(Mo)/Hf alloys
Fig. 4. Typical RT fracture surface of: (a) conventionally-cast alloy; (b) injection-cast NiAl–Cr(Mo)/Hf alloy.
large amount of primary NiAl phase. For NiAl–Cr(Mo)/ Hf 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. 5. 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 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 was counteracted by the weaken effect from the weak cell boundaries and large amount of primary NiAl phases. 3.3. Vickers hardness Due to the fine microstructure of injection-cast alloy, only the Vickers hardness of primary NiAl phase and
Materials
Phase
Vickers hardness (kg/mm2)
Conventional cast
Primary NiAl Eutectic cell
433 454
Injection casting
Primary NiAl Eutectic cell
531 596
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)/Hf 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 [4], 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 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 Heusler phase Ni2AlHf. In addition, the extension of solid solubility occurred in the alloy.
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2. The injection-cast alloy attains room temperature yield strength of 1445 MPa and compressive ductility of about 14%, which are higher than 1168 MPa and 7% for conventionally-cast alloy respectively. While both alloys possess the similar high temperature strength 3. The extended solid solution of alloying element and fine microstructure markedly increased the Vickers hardness of injection-cast alloy.
[2]
[3]
[4] [5]
Acknowledgements [6]
The authors acknowledge the Natural Science Foundation of China (contract No. 59895152) and the National High Technology Committee of China (contract No. 863715-005-0030) for financial supports.
[7] [8]
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