STRUCTURE AND MECHANICAL PROPERTIES OF NICKEL ALLOYS

Transcription

1 STRUCTURE AND MECHANICAL PROPERTIES OF NICKEL ALLOYS Martin POHLUDKA a, Jitka MALCHARCZIKOVÁ a, Vít MICHENKA b, Miroslav KURSA a, Tomáš ČEGAN a, Ivo SZURMAN a a VŠB Technical University of Ostrava, 17. listopadu 15/2172, Ostrava Poruba, Czech Republic b VÚHŽ a.s., Dobrá 240, Czech Republic martin.pohludka@vsb.cz, jitka.malcharczikova@vsb.cz, michenka@vuhz.cz, miroslav.kursa@vsb.cz, tomas.cegan@vsb.cz, ivo.szurman@vsb.cz Abstract Three different nickel alloys IC221M, IC396 and IC438 were prepared by induction melting followed by centrifugal casting. Metallographic samples were made of bar castings. The samples were used for microstructure documentation, for porosity evaluation and for chemical composition verifying by scanning electron microscope. The bars of nickel alloys were machined to tensile specimens which were strained at standard conditions. Dendritic structure of cast nickel alloys proved that it was unsuitable because it contained large shrinks which caused premature fracture of tensile specimens. This result was opposed to tensile tests of directionally solidified samples because directional solidification orientates a structure and also reduces the presence of shrinks. Therefore, it is important to continue in searching of the processes which reduce a dendritic structure of nickel alloy castings. Keywords: nickel alloys, centrifugal casting, porosity, tensile test 1. INTRODUCTION Materials based on nickel aluminides are used in high-temperature applications [1]. Ni 3 Al intermetallic compound is a base of nickel alloys and it exhibits a positive dependence of deformation stress on temperature [2]. Thereby, strength of nickel alloys increases together with temperature up to 800 C. Unfortunately, polycrystalline Ni 3 Al is brittle at room temperature. Brittleness of Ni 3 Al can be successfully reduced by boron alloying in small quantities [3]. Chrome inhibits a corrosion cracking at high temperatures. Addition of molybdenum and zirconium provides ductility. To carry out tensile tests of commercial nickel alloys with modified composition and to examine effect of chemical composition on mechanical properties being related to these types of test, these were the primary work aims. But they failed, therefore author decided to examine a cause of failure and to propose a treatment which will inhibit the failure in future. 2. EXPERIMENT IC-396, IC-221M and IC-438 nickel alloys were prepared by induction melting in vacuum after which molten alloys were centrifugally cast. All was made in casting apparatus Supercast Titan which is the property of Regional materials science and technology centre in VŠB Technical University of Ostrava. Melting conditions are in Tab. 1.

2 Tab. 1 Preparation conditions of nickel alloys Alloy Casting No. Melting Casting Mould IC-396 N01 vacuum argon graphite IC-221M N02 vacuum argon graphite IC-438 N03 vacuum argon graphite Castings were four oval bars connected by riser. Diameter of the bar was 18 mm and length was 160 mm. After separating of the riser, the bars were not heat-treated. One bar of each nickel alloy was used for cutting of transversal section and for machining of two tensile specimens. Metallographic sample for documentation of microstructure was made from the transversal section. Chemical composition of the IC-396, IC-221M and IC-438 nickel alloys used in this article is modified and it differs from the composition of commercially applied nickel alloys [1]. Tab. 2 gives the modified composition. Tab. 2 Nominal composition of nickel alloys Alloy Ni Al Cr Mo Zr B IC IC-221M IC RESULTS 3.1 Structure and chemical composition All three nickel alloys had a typical cast microstructure. They consisted of long grains oriented in a direction from mould wall to casting centre. The middle of castings was full of pores. Etching of metallographic samples by Marble s etchant revealed that individual coarse grains contained many narrow dendrites with phases in interdendritic space (Figs. 1 till 3). Documentation of microstructure was taken place by inverse metallographic microscope OLYMPUS GX51 equipped with digital camera OLYMPUS DP12. Fig. 1 Microstructure of the IC- 396 alloy Fig. 2 Microstructure of the IC- 221M alloy Fig. 3 Microstructure of the IC- 438 alloy Chemical composition of nickel alloys was at first confirmed by optical emission spectrometer SPECTROMAXx from pure and ground surfaces of the castings. Analysed area had dimension of 12 mm 2 with minimal depth after sparking ( 100 μm). Measurement was carried out several times on different accurately defined places of the sample. Final average values of OES analysis are written in Tab. 3.

3 Tab. 3 Results of OES and EDS analyses Alloy Analysis method Ni Al Cr Mo Zr B IC-396 OES IC-396 EDS IC-221M OES IC-221M EDS IC-438 OES IC-438 EDS Independently of OES analysis, chemical composition of nickel alloys was examined with help of scanning electron microscope QUANTA FEG 450 with a probe EDAX APOLLO X. The electron microscope has one disadvantage the analysis of elements lighter than carbon is impossible. Final average values of EDS analysis of nickel alloy chemical composition are in Tab. 3. The results of both chemical composition analyses are not too different from nominal values from Tab. 2. Chemical analysis of phases present in nickel alloy castings revealed that all nickel alloys contained small phases whose diameter was units of micrometer. The phases were mainly situated in interdendritic space and were formed from the Ni-Zr and the Zr-Mo elements, chrome stayed dissolved in alloy matrix. Sulphur was also identified. It combined with the Zr-Mo phase. Sulphur had an origin in Ni 3 S 2 which contaminated charge of nickel [4]. There were also small carbides in the nickel alloys. 3.2 Tensile tests Two tensile specimens were machined from each nickel alloy. The specimens were strained at room temperature and standard conditions. Tensile specimens had circular cross-section with non-threaded grip sections. Diameter of gauge section was 5 mm and its length was 25 mm. The tests were carried out consistent with the ČSN EN ISO norm in VÚHŽ a.s. on tensile apparatus Tira Test The results of tensile tests are concluded in Tab. 4. Tab. 4 Tensile test results of nickel alloy Alloy Tensile specimen No. YS (MPa) UTS (MPa) E (%) AR (%) IC-396 N01.A IC-396 N01.B IC-221M N02.A IC-221M N02.B IC-438 N03.A IC-438 N03.B non-carried out non-carried out non-carried out non-carried out Unfortunately, all tensile specimens exhibited so low ductility that it was not possible to reach deformation of 0.2 % which is important for determining of nominal yield strength, YS. Therefore, values of elongation, E,

4 and area reduction, AR, have only an informative character. In the case of sample No. N03.B, it fractured during marking of the gauge section. Observation of tensile specimen fracture surfaces on scanning electron microscope (Figs. 4 till 6) revealed that all tensile samples were premature fractured in a place with enhanced concentration of shrinks. This conclusion was confirmed by following defectoscopy analysis. Fig. 4 Fracture surface of the IC- 396 alloy Fig. 5 Fracture surface of the IC- 221M alloy Fig. 6 Fracture surface of the IC- 438 alloy Cast IC-396, IC-221M and IC-438 alloys prepared by above mentioned approach have insufficient mechanical properties for using in commercial practice and they need additional treatment. But even though the alloys were annealed at the conditions of 1100 C/1.5 h/cooling in air, dendritic structure of the alloys with shrinks was not reduced. That was a process of directional solidification which positively affected alloy structure with shrinks [5]. Another solution of casting treatment can be represented by HIP method. 3.3 Porosity By reason of unsuccessful tensile tests, attention was paid to statistical and morphological description of pores and shrinks. Cast structures used in this work were compared with the ones directionally solidified in [5]. Procedure is explained in [4]. Tab. 5 Porosity of nickel alloys after casting and directional solidification Alloy Preparation P (%) n (-) d ( m) IC-396 casting IC-396 directional solidification IC-221M casting IC-221M directional solidification IC-438 casting IC-438 directional solidification Porosity of nickel alloys was quantified with help of the same microscope as their microstructure. Ten photographic images of different places of non-etched sample surface were documented at two hundredfold magnification. These images were evaluated by analysis auto, the computer program for image analysis. Measured parameters are given in [4]. There are the results of porosity, P, together with amount of identified

5 pores, n, and average pore diameter, d, in Tab. 5. Porosity and average pore diameter of directionally solidified nickel alloys are smaller although the alloys can contain more pores than cast alloys (e.g. IC-396). That was identified from 461 to 960 pores in prepared samples of nickel alloys. Diameter of these pores was in range from 1.5 to 21.0 m. Majority of pores was situated in interdendritic space. Predominant character of pore diameter distribution is log-normal. That holds for castings and directionally solidified nickel alloys (Figs. 7 and 8) but the directionally solidified ones contain less pores greater than 10 m. Fig. 7 Pore distribution in nickel alloys after casting Fig. 8 Pore distribution in nickel alloys after directional solidification Results of pore morphology comparison in cast, C, and directionally solidified, DS, nickel alloys are contradictory (Fig. 9). Fully positive effect of directional solidification can be seen in the case of IC-438 alloy whose directionally solidified alloy contains more circular pores with smoother surface than cast alloy. In the case of IC-396 alloy, directional solidification affected positively only pore circularity. Pores in IC-221M alloy after directional solidification had worse morphology than the ones in its casting. Fig. 9 Pore morphology in nickel alloys after casting and directional solidification expressed by medians

6 4. CONCLUSION Cast samples of the IC-396, IC-221M and IC438 nickel alloys were strained by tension at standard conditions. Their fracture happened before reaching of 0.2 % deformation. The cause of premature facture was coarse-grained dendritic structure containing many shrinks. Following heat treatment did not lead to structure refinement and to shrink reduction. This problem was successfully resolved by directional solidification which had an effect on amount and morphology of pores. In future, HIP method can be promised in the challenge of shrink reduction. ACKNOWLEDGEMENT The presented results were obtained within the frame of solution of the research project TA Research and development of centrifugal casting technology of the Ni-based intermetallic compounds and the project CZ.1.05/2.1.00/ Regional materials science and technology centre. LITERATURE [1] DEVI, S. C., SIKKA, V. K. Nickel and iron aluminides: an overview on properties, processing and applications. Intermetallics, 1995, Volume 4, Issue 5, Pages [2] MASAHASHI, N. Physical and mechanical properties in Ni 3 Al with and without boron. Materials Science and Engineering: A, 1997, Volume 223, Issues 1-2, Pages [3] CHAKI, T. K. Boron in polycrystalline Ni 3 Al mechanism of enhancement of ductility and reduction of environmental embrittlement. Materials Science and Engineering: A, 1995, Volume 190, Issues 1-2, Pages [4] POHLUDKA, M. et al. Porosity of Ni 3 Al-based alloys prepared by gravity and centrifugal casting. In Metal 2012: 21 st International Conference on Metallurgy and Materials: May 23 rd 25 th Brno, Hotel Voroněž I, Czech Republic [CD-ROM]. Ostrava: Tanger, May, 2012, pp ISBN [5] MALCHARCZIKOVÁ, J. et al. Structural and fracture characteristics of nickel-based alloys. In Metal 2013: 22 nd International Conference on Metallurgy and Materials: May 15 th - 17 th Brno, Hotel Voroněž I, Czech Republic [CD-ROM]. In press.