Compressive behavior of NiAl/(Cr,Mo)Hf alloy prepared by high-pressure die casting and hot isostatic pressing

DU Xing-hao(杜兴蒿)¹, GUO Jian-ting(郭建亭)² , WU Bao-lin(武保林)¹

1. Department of Materials Engineering, Shenyang Institute of Aeronautical Engineering, shenyang 110034, China;

2. Department of superalloys, Institute of Metal Research, Chinese Academy of Sciences, shenyang 110016, China

Received 28 July 2006; accepted 15 September 2006

Abstract: The NiAl-28Cr-5.85Mo-0.15Hf alloy was prepared by high-pressure die casting (HPDC) and subsequent hot isostatic pressing(HIP), and tested for compressible strength and fracture behavior at 300-1 373 K. The results show that the elevated temperature 0.2% compressible yield strength as well as the room-temperature compressible fracture strain of as-HIP alloy are larger than those of the same alloy prepared by directional solidification (DS). It suggests that the fine structures with a homogeneous distribution of fine Cr（Mo）and Hf-rich phase created by high-pressure die casting lead to these improvements.

Key words: NiAl/Cr(Mo,Hf) alloy; high-pressure die casting; hot isostatic pressing; compressive properties

1 Introduction

NiAl has received more attention as a potential structural material because of many superior physical properties, however, its use is limited by the poor high-temperature strength and much low fracture toughness and ductility at ambient temperature[1-2]. Among the various attempts to overcome these shortcomings, multicomponent system based on NiAl is regarded as a logical choice and directionally solidified (DS) NiAl-28Cr-6Mo eutectic alloy has been determined as a promising candidate due to its relatively high melting point, low density, good thermal conductivity and relatively high creep resistance as well as higher fracture toughness [3-5]. Recently, Hafnium (Hf) was found to be very effective in improving the elevated temperature strength of NiAl-Cr(Mo) eutectic alloy further[6-8]. Unfortunately, as the result the segregation, Hf addition seriously weakens the eutectic cell boundaries and interface between NiAl and Cr(Mo) and thus makes the brittle failure of the alloy again[6-8]. Therefore, for NiAl-Cr(Mo)/Hf lamellar eutectic alloy, a feasible way to improve the ductility and strength further is to reduce the interlamellar spacing and segregation of Hf. Rapid solidification is anticipated to attain fine microstructure and evenly distributed Hf-rich phase precipitation. High-pressure die casting (HPDC), as a popular method to fabricate bulk amorphous materials[9-10], can be used to obtain bulk NiAl alloys at a relatively high cooling rate of about 10²-10³ ks^-1. And the research of NiAl based alloy prepared by rapid solidification can provide a practical way to improve its mechanical properties. Hence, the microstructural evolution and compressive properties of NiAl-28Cr-5.5Mo-0.15Hf alloy prepared by high-pressure die casting (HPDC) and subsequent hot isostatic pressing treatment (HIP) were investigated.

2 Experimental

The NiAl-28Cr-5.85Mo-0.15Hf alloy was prepared by induction melting from pure Ni, Al, Cr, Mo and Hf and each sample was approximately 100 g. When an experiment began, the sample was placed in a quartz tube with 3 mm in inside diameter, 15 mm in outside diameter and 160 mm in length, the glass bottom of which had a small orifice about 0.2 mm in diameter. The quartz tube was then installed on the top of the drop tube. Afterwards the drop tube was evacuated to 5.0×10^-5 MPa before being backfilled with a mixture of He and Ar gas to about 0.1 MPa. The sample was melted by induction heating and further overheated to about 100 K above its liquidus temperature for several minutes. The tempera-

ture was measured by an infrared pyrometer. Being blown by a gas flow of highly purified He or Ar into the quartz tube, the melt was ejected out from the orifice of the quartz tube and dispersed into a copper mould of 1.0 cm in diameter and 10 cm in length cooled by circulated water. The cooling rate by using the described method was estimated to be higher than 10²-10³ ks^-1. The obtained samples were HIP-treated at 1 300 ℃ and  150 MPa for 3 h. Microstructural characterization of all samples was determined by scanning electron microscopy (SEM), electronic probe microanalysis (EPMA) and transmission electron microscopy (TEM). The compressive specimens with the size of d3 mm×6 mm were cut from the as-HIP ingot by electro-discharge machining (EDM) and the compressive test was conducted in air with a Gleeble 1500 test machine at a nominal strain rate from 2.0×10^-3 s^-1 to 2.0×10^-5 s^-1. For comparison, the same tests were also performed on some samples which were cut from DS ingots，which were obtained by the Bridgman method[11]. The autographically recorded load—time curves were converted to true stress—true strain curves via assumption of the constant volume.

3 Results and discussion

3.1 Microstructure

The SEM micrograph of as-grown transverse section microstructure of the DS ingot is shown in Fig. 1(a). The composite is composed of three phases, which are a lamellar Cr(Mo) phase, NiAl matrix and a semicontinuously distributed white phase at the cell boundaries. Energy dispersive X-ray spectroscopy (EDXS) results show that this white phase is the Hf-rich phase. According to the NiAl-Cr(Mo) phase diagram[12] and NiAl-Hf pseudobinary phase diagram[13], the maximum solubility of Hf in NiAl is less than 5% at the eutectic temperature of approximately 1 350 ℃, there- fore, Hf can be fully dissolved in the NiAl matrix at the NiAl-Cr(Mo) eutectic temperature. The solidification process of DS NiAl-28Cr-5.6Mo-0.15Hf alloy could be deduced as follows. The Cr(Mo)-NiAl eutectic reaction occurs at about 1 445 ℃ firstly, then the NiAl phase becomes enriched in Hf. Secondly, the Hf rich phase precipitates with decreasing temperature from the NiAl phase. Since the eutectic cell boundaries of NiAl and Cr(Mo) phase possess higher energy, Hf rich phase will nucleate preferentially at the cell boundaries.

The typical microstructure of as-HPDC ingot is quite different from that of DS ingot, as shown in Fig.1(b). Some primary NiAl dendrites can be observed at the eutectic cell interiors and/or cell boundaries. The average eutectic cell size with about 25 mm is smaller than that for DS ingot(100-150 mm). The interlamellar spacing (λ) of eutectic cell interior of as-HPDC alloy prepared is finer than that of DS ingot. Furthermore, it is specific that the Hf-rich phase has been distinctly refined and evenly distributed. RAJ et al[14] 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 μm. The high cooling rate also results in the fine eutectic cell size in this study. The rapid solidification undercooling decreases the segregation of the Hf element and more Hf elements are dissolved in the NiAl dendrites during crystallization process. Thus, under the rapid solidification condition, the disordered Hf-rich phase nucleates and grows both in the eutectic colonies and at the cell boundaries.

Fig.1 SEM micrographs of transverse section for as-DS alloy (a) and as-HPDC alloy (b)

3.2 Compressive behavior

3.2.1 Compressive behavior at room temperature

Fig.2 shows the true compressive stress—strain curves of the DS alloy and HPDC alloy at room temperature, and a summary of the compressive properties is also listed in Table 1. A remarkable difference between the DS alloy and HPDC alloy in the compressive fracture strain, i.e. compressive strain is obtained. The compressive fracture strain at the ultimate compressive strength (UCS) is consistently about 5% for DS alloy, and 16% for HPDC alloy. The test results are also quite reproducible.

Fig.3 shows typical room temperature compressive fractographies of the tested samples. The fracture surface of the DS alloy presents flat cleavage morphologies (Fig.3(a)), i.e. typical stripping of NiAl/Cr(Mo) from the cell boundaries and cleaving of primary NiAl. However, many dimple-like cavities can be observed on the fracture surface of HPDC alloy (Fig.3(b)). These cavities are determined to be formed by thin Cr(Mo) phase and NiAl phase in eutectic cell pulled out from each other. The further improvement of room temperature compressive fracture strain of HPDC alloy can be attributed to the fine microstructure and evenly distributed Hf-rich phase. For DS NiAl/Cr(Mo) alloy, because of the segregation of Hf-rich phase, the cell boundaries become the most weak position, as discussed by GUO et al[7-8]. As for the HPDC alloy, the decreased size and amount of Hf-rich phase at the cell boundaries make for the enhancement of the strength of cell boundaries. Then some thin Cr(Mo) phase and NiAl phase in eutectic cell can pull out from each other during the compressive process, which is propitious for the improvement of room temperature fracture strain, as observed by CHEN et al[4-5].

Fig.2 True stress—true strain relationships at ambient temper- ature for as-DS and as-HPDC alloys with compression deform- ation rate of 2×10^-4 s^-1

Table 1 Compressive properties of tested alloy at room tem-  perature

Fig.3 Typical room temperature fracture surface morphologies of as-DS alloy(a) and as-HPDC alloy(b)

3.2.2 Compressive behavior at elevated temperatures

Similar to other NiAl alloys, the compressive strain of the HPDC alloy is not a problem at elevated temperatures more than 0.4T_m, where T_m is melting point of the NiAl alloy. Here, we discuss the yield strength of the alloy at elevated temperatures. Fig. 4 shows all yield strength of conventionally casting alloy and as-suction-casting alloy with HIP treatment under various test conditions. At 1 273 K and 1 373 K, the yield strength of HPDC alloy is significantly higher than that of DC alloy at all the strain rates. It is inspiring that the HPDC alloy improves the elevated temperature strength of the tested alloy at elevated temperatures.

Based on Refs.[7-8], for NiAl/Cr(Mo) alloy with low addition of Hf, Cr(Mo) precipitating phase in the NiAl matrix is the main reason for the improvement of strength at elevated temperatures by dispersion strengthening. The element Hf can affect the onset of climbing by forming solute atmospheres around mobile dislocations and effectively pin them, then provides additional strengthen effects. Fig.5 shows TEM images of DS and HPDC alloys. From Fig.5, it can be seen that the HPDC+HIP treatment causes higher density fine dispersion of Cr(Mo) particles evenly distributed in the alloy, which may have a more positive influence on the elevated temperature strength. The high cooling rate of high-pressure die casting process can account for this phenomenon. The undercooling of rapid solidification decreases the segregation of Cr(Mo) element and more Cr(Mo) elements solve in the NiAl dendrites. Without the eutectics growing conditions under the rapid solidification, disordered Cr(Mo) phase nucleates and grows independently in NiAl matrix due to the solute imprisonment. Similar phenomenon has been observed on the rapid solidification of Al-Nb-Ni ternary eutectic alloy[15]. On the other hand, besides the additional strengthening effects from the evenly distributed Hf-rich phase, the decreased segregation of Hf can also lead to the improvement of the cell boundary strength. Accordingly, the elevated temperature yield strength of HPDC alloy should be higher than that of DS alloy.

Fig.4 0.2% offset yield strength as a function of temperature and strain rate(r) for differently processed alloys

Fig.5 Fine precipitates of Cr(Mo) in primary NiAl phase: (a) As-DS alloy; (b) As-HPDC alloy

4 Conclusions

NiAl/(Cr,Mo)Hf alloy by the high pressure die casting and subsequent hot isostatic pressing treatment presents a fine microstructure with more homogeneously distributed Hf-rich phase and more fine Cr(Mo) precipitates. The prospective microstructure is suggested to be responsible for the significant improvement of the room temperature ductility and elevated temperature strength for the alloy tested.

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(Edited by YANG You-ping).

Foundation item: Project (05YB31) supported by the Scientific Research Initial Foundation for Doctor of Shenyang Institute of Aeronautical Engineering, China

Corresponding author: WU Bao-lin; Tel: +86-24-89723976; E-mail: wubaolin@syiae.edu.cn