International Journal on Mechanical Engineering and Robotics (IJMER)
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Microstructure Characteristics and Mechanical Properties of the NiAl/Cr (Hf, Ho) in-situ Composite
1K.Vinaya, 2Ratnakar Pandu, 3Vijay John, 4Kranthi Kumar Guduru
1Ace Engineering College, Hyderabad.
2,3MLR Institute of Technology and Management, Hyderabad.
4Christu Jyothi Institute of Technology and Sciences, Jangon.
Abstract : The microstructure and mechanical properties of NiAl/Cr in-situ composite with minor hafnium and holmium were investigated by using of SEM, TEM and compression tests. The results revealed that trace Ho addition could optimize the microstructure, by refining the lamella inside of eutectic cell and controlling the coarsening of intercellular region. However the Ho addition results in the Ni2Al3Ho phase, which has hexagonal crystal structure and an orientation relationship with NiAl phase. Moreover the Ni17Ho2 phase is found in the Ni2Al3Ho phase, which has twin crystal inside and an orientation relationship with Ni2Al3Ho phase. In addition, the Ho addition promotes the precipitation of Ni2AlHf Heusler phase. More Ho addition coarsened the α-Cr phases along the intercellular and resulted in more strengthening precipitates inside eutectic cell. The mechanical properties exhibits that minor addition of Hf and Ho could increase the strength and hardness of the NiAl/Cr composite.
Keywords: NiAl/Cr in-situ composite; Microstructure;
Transmission electron microscopy; Mechanical properties
I. INTRODUCTION
The B2 structured intermetallic compound NiAl has excellent properties, such as high melting point, low density, good thermal conductivity and excellent oxidation resistance, which make it as a potential high temperature structure material [1-3]. However the room temperature (RT) brittleness and inadequate elevated
temperature strength handicap it's application in industry.
Recent studies on the NiAl find that its mechanical properties can get well improvement by the introduction of toughness phases [4,5]. Among these NiAl based alloys and composites, the in-situ composite of NiAl/Cr with lamellar eutectic structure is much attractive, due to its well combination of RT and elevated temperature properties [6, 7]. Though the NiAl/Cr in-situ eutectic composite exhibits relative good room temperature fracture toughness and elevated temperature strength [8, 9], its elevated high temperature creep property still can not reach the Ni-based superalloy [10]. The recent studies [11, 12] reveal that addition of Hf could improve its high temperature mechanical property, but the segregation of Hf based phase along eutectic cell boundary lead to the decreas of its ductility greatly.
Based on the previous researches [13,14], the minor addition of rare earth elements (REEs) could improve the size and distribution of hard phase along boundary and is beneficial to the room temperature mechanical properties. In addition, the REEs prefer to arrest the S, P elements and purify the boundary, which could enhance the coalescence of the grain or phase. Therefore in the present paper minor Hf and trace Ho element was added to the Ni-33Al-33Cr eutectic alloy. The microstructure, precipitates and mechanical properties of NiAl/Cr in-situ composite with and without Hf, Ho addition are investigated as well.
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II. EXPERIMENTAL
The materials used for this investigation were Ni-33Al-33Cr0.2Hf-0.1Ho (at.%) nominal composition.
The alloy were arc-melted under argon atmosphere with starting material of 99.9% Ni, 99.9% Al, 99.8% Cr, 99.6%
Hf and 99.7% Ho, using a non-consumable tungsten electrode. The alloy was turned over and remelted more than three times to get a homogeneous specimen. Since the weight losses were generally less than 0.5%, the alloy composition was considered to be equal to their nominal composition. The alloy buttons were homogenized at 1523 K for 24 hours in evacuated silica capsules followed by furnace cooling to RT.
The samples for microstructure observation and mechanical properties test were cut from similar parts of the composite sample. Microstructure observations were carried out by a S-3400 Scanning Electro Microscopy (SEM) with energy-dispersive spectrometer (EDS) analyzer and the compositions of constitute phases were detected by EPMA-1610 electronic probe microanalysis (EPMA). The samples for transmission electron microscope (TEM) observation were cut from different parts by electro-discharge machining (EDM).
Transmission Electro Microscopy (TEM) were employed to certify the new phases formed in the alloy.
The TEM slices of thickness about 0.5mm were cut by EDM, and then polished to 50μm. The polished slices were shaped into 3 mm in diameter followed by ion milling to perforation. The TEM observation was performed by a JEM-2010 high-resolution transmission electron microscope operated at 200 kV. The
compression specimens with the size of 4×4×6 mm were cut from the composite sample by EDM and all major surface were mechanical ground with 600-grit SiC abrasive prior to compression test. The compression test was conducted in air with a Gleeble 1500 test machine at RT and 1273 K at a initial strain rate of 1×10-3/s. The autographically recorded load-time charts were converted to true stress-true strain curves assuming a conservation of volume. Microhardness measurement was carried out on a Vickers microhardness tester (AMH43) using a load of 25g and dwell time of 15 s.
Seven measurements were performed to obtain an average value.
III. RESULTS AND DISCUSSION
3.1 Microstructure characteristics
The typical microstructure of the NiAl/Cr in-situ composite without Hf and Ho addition is shown in Fig.1 (a) and (b). It can be seen that the composite is composed of eutectic cell and coarse NiAl dendrites along cell boundary. Within eutectic cells, NiAl and α-Cr phases form the lamellar structure radiating from the cell interior to the boundaries. The interlamellar spacing of NiAl and α-Cr near the cell center is finer than that near the cell boundary. Further observations on the NiAl and α-Cr phases find that fine arrays of coherent NiAl and α-Cr particles precipitate in α-Cr and NiAl phase respectively, as shown in Fig. 1(c) and (d).
In addition, the interface dislocations are also observed along the NiAl/Cr(Mo) phase boundary.
Fig.1. (a) SEM micrograph of NiAl/Cr in-situ compoiste, (b) TEM image of the lamellar structure and eutectic cell, (c) Precipitates of Cr(Mo) in NiAl phase and the morphology of interface dislocation, (d) Morphology of NiA precipitates
in Cr(Mo) phase The typical microstructure of the NiAl/Cr(Hf,Ho) in-situ
composite is shown in Fig.2 (a). It can be seen that the Hf and Ho addition changes the microstructure of the NiAl/Cr composite obviously. Firstly, the NiAl/Cr lamella structure in the eutectic cell is refined greatly.
The interlamellar spacing in the eutectic cell is much smaller than that of the composite without Ho addition, but more coarser NiAl and Cr(Mo) phases form in the intercellular region. Secondly, many white phases are precipitated along the eutectic cell boundary. The EDS tests on the white phases show that they are Hf-rich phase and Ho-rich phase. TEM observations on the composite exhibit that they are Ni2AlHf phase and Ni2Al3Ho phase, respectively. Fig.3 (c) shows the selected area electron diffraction (SAED) pattern of the
Ho-rich phase along [111] zone axis. It certify that the Ho-rich phases is Ni2Al3Ho, which has a hexagonal crystal structure with a=0.902 nm, c=0.4052 nm and the space group of P62/mmm. Fig.3 (d) shows the SAED pattern of the Hf-rich phase along [110] zone axis. It certify that the Hf-rich phase is Ni2AlHf, which has a cubic crystal structure with a=b=c=0.6081 nm and the space group of Fm3m. What is interesting is that the Ni2AlHf is semi-coherent with the NiAl phase but incoherent with Ni2Al3Ho phase, as shown in Fig.2 (b).
The high resolution TEM (HRTEM) observation shows the NiAl/Ni2AlHf interface is straight and clear. No nano particles and amorphous layer are found along the interface.
Fig.2. (a) SEM micrograph of NiAl/Cr(Hf,Ho) in-situ composite, (b) Bright-field TEM micrograph of Ni2AlHf and
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Ni2Al3Ho precipitates along NiAl phase boundary, (c) SAED pattern of Ni2Al3Ho phase along [111], (d) SAED pattern of Ni2AlHf along [110], (e) HRTEM image showing the NiAl and Ni2AlHf phases interface Further observations on the Ni2Al3Ho phase reveal that
it has an orientation relationship with the NiAl phase of
NiAl Ho
3 Al 2
Ni //[111] ]
120
[ and (002)Ni2Al3Ho//(110)NiAl , as shown in Fig.3 (a). The HRTEM image shows the Ni2Al3Ho phase and its interface with NiAl, as shown in Fig.3 (b). It is clear that the interface of NiAl and Ni2Al3Ho phases is straight and without small precipitates or amorphous layer, but there is a transition region about several nanometers in size along the interface. The atoms in this region are some disorder, which may be resulted by the great difference in crystal parameters. Additionally, the SAED pattern in Fig.3 (a) also shows some dim dula-spots arranging along the main diffraction spots, which is the characteristic of the twin crystal. When the incident beam parallels to the axis [124], the SAED pattern shows that there is another phase inside the Ni2Al3Ho phase, as shown in Fig.3 (c).
The TEM image also shows that some phases with strip shape precipitate in the Ni2Al3Ho phase, as shown in Fig.3 (d). The analysis on the SAED pattern reveals that the phase is Al17Ho2 phase which has a hexagonal crystal structure with a=0.9807 nm, c=0.8757 nm and
the space group of P63/mmc. The SAED pattern also shows that the Al17Ho2 phase has an orientation relationship with the Ni2Al3Ho phase of
Ho 3 Al 2 Ni 2
Ho 17
Al //[124] ]
123
[ and (111)Al17Ho2//(210)Ni2Al3Ho. In addition, the SAED pattern suggests that there is twin crystal in the Al17Ho2 phase. The HRTEM observation confirms the result, as shown in Fig.3 (e). The presence of Al17Ho2 phase may be ascribed to the Ho addition.
Based on the observation above, most Ho addition would be rejected to the intercellular region that solidifies at the end of the whole alloy. The segregation of Ho elements would change the composition of this region and then results in the formation of Al17Ho2 phase. The formation of microtwins in the Al17Ho2 phase may be attributed to the internal stress. Since the phase forms at the end of solidification, the lack of liquid metal and crystal shrinking will result in great stress. And the previous research [15] has reported that the stress can induce the generation of twin crystal. In addition, in eutectic cell the Ho2O3 oxides were observed along NiAl and Cr(Mo) grain boundary, as shown in Fig.3 (f).
Fig.3. (a) SAED pattern of Ni2Al3Ho phase along [120], (b) HRTEM image of NiAl and Ni2Al3Ho phases, (c) SAED pattern of Al17Ho2 phase along [123], (d) Bright-field TEM micrograph of Al17Ho2 phase precipitated in Ni2Al3Ho
phases, (e) HRTEM image of microtwins in the Al17Ho2 phase, (f) Bright-field TEM micrograph of Ho2O3 particles (Inset picture showing it SAED pattern along [211])
The observation on the NiAl/Cr(Hf,Ho) in-situ composite reveal that the minor Hf and Ho addition also exert great influence on the precipitates in the NiAl and α-Cr phase, as shown in Fig.4. Firstly, coarse and big α-Cr precipitates appear in the NiAl phase, which are full of interface dislocation inside, as shown in Fig.4 (a).
The change of NiAl precipitate in the α-Cr phase is much greater, as shown in Fig.4 (b). The size of big NiAl particle is about five hundreds nanometers and the small one is about one hundred nanometers. Moreover they are all full of interface dislocations inside. These changes should be attributed to the Hf and Ho addition.
As shown in Table 1, the compositions of NiAl and α-Cr phases in the NiAl/Cr and NiAl/Cr(Hf,Ho) in-situ composites are displayed. Obviously, the α-Cr content of NiAl phase in the NiAl/Cr(Hf,Ho) in-situ composites is more than that in the NiAl/Cr composite. Moreover, there are some Hf and Ho solid soluted in the NiAl phase. In addition, the Hf and Ho addition also results in the increase of Ni and Al in the α-Cr phase. According to the recent research [16,17], the solid solution of big atom in the NiAl phase can cause great stress inside, which would generate more interface dislocation along precipitates and matrix phase.
Fig.5. (a) Bright-field TEM micrograph of Cr precipitates in NiAl phase in the NiAl/Cr(Hf,Ho) in-situ composite, (b) Bright-field TEM micrograph of NiAl precipitates in Cr phase in the NiAl/Cr(Hf,Ho) in-situ composite Table 1 Composition of constituent phases in the
NiAl/Cr and NiAl/Cr(Hf,Ho) in-situ composites (at.%)
Phase Ni Al Cr Hf Ho
NiAl/ Cr in-situ composite
NiAl 48.62 47.77 3.61 — — α-Cr 6.96 5.51 87.53 — —
NiAl/Cr (Hf,Ho) in-situ composite
NiAl 49.13 46.21 4.3 0.23 0.13 α-Cr 7.28 7.92 84.8 — —
According to the former researches [18,19], the growth rate exerts main influence on the microstructure in the solidification of eutectic alloy. With the increase of growth rate, the size of eutectic cell, the interlamellar spacing and the intercellular region decrease. In the present study, the alloy with minor Ho addition has
refined microstructure, which may mainly contribute to the effect of Ho on growth rate. During the solidification, the Hf and Ho were rejected in front of the L/S interface.
Due to the tip radius, the concentration of Ho and Ho at the interface of NiAl/Cr is higher, which decrease the equilibrium crystallization temperature and undercooling, and then handicap the phase growth perpendicular to the lamella. So the NiAl and α-Cr phases grow faster along the lamella direction, which suppress the adjacent α-Cr phases to amalgamate and contribute to the lamella refinement. In addition, the rare element Ho is a kind of surface-active element, which can help to reduce the critical radius of crystal nucleus and the critical nucleation power. Therefore it can refine the eutectic cell by increasing crystallization nuclei.
However, it can be seen that the microstructure become coarse greatly, especially the intercellular region. That
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can be explained by the constitutional supercooling caused by excess Ho. At the end of the solidification, so many Ho and Hf elements were rejected in the residual liquid metal. The high concentration of Hf and Ho leads to high constitutional supercooling, which make the nuclei generated in the liquid metal and grow up rapidly.
Hence the α-Cr and NiAl phases in the intercellular region grow large.
3.2 Mechanical properties
The mechanical properties of the NiAl/Cr and NiAl/Cr(Hf,Ho) in-situ composites at room temperature are summarized in Table 2. Clearly, with the addition of Hf and Ho, the yield strength, compressive strength and compressive strain all increases obviously. Compared with the increase of strength, the improvement of the compressive ductility is much greater, which increases more than one time. The microhardness tests on the NiAl/Cr eutectic show the Hf and Ho addition increase the hardness of the eutectic structure. Such a change may be attributed to the increase of the interface dislocation, which impedes the movement of the mobile dislocations.
Table 2 Mechanical properties of the NiAl/Cr and NiAl/Cr(Hf,Ho) in-situ composites
Yield strengt h (MPa)
Compres sive strain (%)
Compres sive strength (MPa)
Microhardnes s of NiAl/Cr eutectic (HV)
NiAl/Cr in-situ composite
890 11 1210 490
NiAl/Cr (Hf,Ho) in-situ composite
1070 23 1430 610
Based on the former researches [20-22], the optimizing microstructure is very important to the mechanical properties of the NiAl based eutectic composite. The fine microstructure is beneficial to the RT strength of alloys but detrimental to high temperature strength.
However, if lamella is so coarse, the RT mechanical properties will decrease, especially the ductility. Hence it is easy to understood why the addition of Hf and Ho addition improves the NiAl/Cr composite obviously,
because the NiAl/Cr(Hf,Ho) composite owns finest NiAl/Cr lamella inside of eutectic cell and the appropriate coarse phases in intercellular region.
Moreover, the uniformly distributed Ni2AlHf and Ni2Al3Ho phases also contribute to improve the compressive properties, which could impede the movement of the dislocations. However, it also can find that the 0.1% Ho addition already results in many Ho based phases and promote the precipitation of Hf based phases along eutectic cell boundary. If more Hf and Ho are added, it will be detrimental to the compressive properties, due to their segregation along eutectic cell boundary. According to the recent research [23], the compressive ductility of eutectic alloy is sensitive to intercellular region. Because this is the last solidified region, the impurities and strengthen particles prefer to gather here, which increase its cracking sensitivity.
Appropriate existence of Ho element can react with the impurities, such as S, O, etc, which decreases the crack sources and contributes to the RT mechanical properties.
Furthermore, the previous investigations [13] found that the rare earth elements tended to segregate to the grain boundary and phase boundary, reducing the harmful effect of impurities on the cohesive strength, by purifying the grain boundary. All these factors contribute to the improvement of compressive properties.
V. CONCLUSIONS
(1) The trace Ho addition in the NiAl/Cr composite leads to the formation of Ni2Al3Ho phase, which has hexagonal crystal structure and has an orientation relationship with NiAl phase of [120]Ni2Al3Ho//[111]NiAl and (002)Ni2Al3Ho//(110)NiAl
. Additionally, the Ni17Ho2 phase is found in the Ni2Al3Ho phase, which has twin crystal inside and an orientation relationship with Ni2Al3Ho phase of [123]Al17Ho2//[124]Ni2Al3Ho and (111)Al17Ho2//(210)Ni2Al3Ho
. The Ho addition promotes the precipitation of Ni2AlHf Heusler phase, and most Ni2Al3Ho phases coexist with the Ni2AlHf Heusler phase along NiAl/Cr phase interface.
(2) Minor Ho and Hf addition optimizes the microstructure of the NiAl/Cr composite obviously, by refining the lamella inside of eutectic cell and controlling the coarsening of intercellular region.
However, the Ho and Hf promote the coarseness of NiAl and α-Cr precipitates in the composite and increase the interface disloactions.
(3) The Ho and Hf addition improve the compressive properties and microhardness of the NiAl/Cr composite obviously, which may be attributed to the refined NiAl/Cr eutectic lamella and increased strengthening phases.
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