CC BY
24
I.V. Belyaev, 2007
УДК 621.793.3/.4
M. Kursa, J. Malcharczikova, I. V. Belyaev INFLUENCE OF CONDITIONS OF DIRECTIONAL CRYSTALLISATION BY BRIDGMANS METHOD ON THE STRUCTURE AND MECHANICAL PROPERTIES OF Ni3Al
1. Introduction
71 7*-Al-based intermetallic compounds are of great interest from physical and metallurgical
1 y # viewpoints. The possibility of their application in demanding environment, namely at increased and high temperatures under influence of an oxidation atmosphere, is of high importance. The directional crystallisation (DC) offers the possibility to affect the structure and properties of the final product. This process can be used to control the formation of shrinkage cavities, for influencing grain size and manner of their growth [1]. The directional crystallisation by Bridgman’s method is one of the simplest and most frequently used methods. A tube containing the charge in its bottom part is placed into the temperature field of the furnace. During slow passing of the tube with a melted metal through a steep temperature gradient, the progressive directional crystallisation of the melt and, thus, the growth of a crystal occur [2].
2. Preparation of samples
For experimental purposes, castings and samples subjected to directional crystallisation were used. The castings were prepared in a vacuum induction furnace LEYBOLD of the type IS3/1. Before the melting, the two-fold vacuum treatment at a pressure of less than 0.04 mbar with the use of two-stage rotary and Roots’ pumps was performed. Several basic samples containing 25, 24.5, and 24 at. % Al were prepared by casting into cylinders 95 mm length and 10 mm in diameter. Melting was realised in a corundum crucible; graphite moulds were used for casting.
3. Directional crystallisation by Bridgman’s method realised in a two-zone crystallisation furnace “Classic”
These castings were subjected to directional crystallisation (DC) by Bridgman’s method with the vertical arrangement. The process was realised in two-zone super-kanthal resistance furnace Classic with the automatic feed for crystallisation rates 10, 50 and 100 mm/hour at the Technical University VSB-TU Ostrava, Czech Republic. The melting was made under an inert argon atmosphere of purity 6N. The sample was placed into a corundum tube with the closed bottom and, after melting, it was moved down at a required speed. Table 1 gives the composition of samples, used rates of directional crystallisation; final results of the measured values of porosity and micro-hardness are also given.
3.1. Tensile tests with measurement of acoustic emissions
For the purposes of evaluation of the influence of various parameters of directional crystallisation, such as chemical composition and rate of crystallisation, tensile tests were performed. For these tests, short tensile bars of circular section 55 mm length and 5 mm in diameter of the central part of the bar were used. Table 1 summarises the measured values for 6 samples of three different chemical compositions crystallized at two different rates. The acoustic emission was measured on a commercial apparatus DAKEL XEDO made by DAKEL-ZD Rpety, Czech Republic. The two-threshold detection according to the ASTM standard was used, as it enables a simple amplitude discrimination of signals.
During all measurements, the intensive acoustic emission with characteristic time dependence and correlation with individual stages of deformation was observed. In all measurements, it is possible to discern several areas in the tensile diagram and acoustic emission characteristics corresponding to them. First of all, an area of low loads with a low level of acoustic emission appears. With growing load, a characteristic increase in the number of emission overshoots appear, which can be related to the activation of Table 1
Composition of samples, rates used speed of directional crystallisation and resulting values of tensile test
SAMPLE No. Al contents rate of DC Rm RP 0.2 Description of fractured part
[at. %] [mm/hour] [MPa] [MPa]
128.1 25 100 318 295 transverse fracture, short needles
130.1 25 10 284 251 longitudinal long fracture, needles
132.1 24.5 100 374 278 transverse fracture, short needles
134.1 24.5 10 415 286 longitudinal long fracture, needles
138.1 24 100 308 238 longitudinal fracture, short needles
136.2 24 10 255 250 transverse fracture, short needles
Fig. 2 Load diagram, measurement AE, rod after tensile test and detail of fracture surface of the sample No. 132.1 -Ni24.5Al, 100 mm/h
macro-plastic deformation. It is followed by the area of degradation with a progressive decrease of tension, which is characterised by
intensive acoustic emission. Particularly strong signals appear at the moments of step declines of tension.
Figures 1 and 2 show load diagrams recordings for the measurement of acoustic emissions and details of fracture surfaces of samples Nos. 130.1 and 132.1. The photos show characteristic fibrous fracture.
3.2. Metallographic evaluation of structures
Table 2
Composition of samples and rate used speed of directional crystallisation
Sample No. rate of DC [mm/hour] Ni contents [at.%] Al contents [at.%]
139.1 o 00 75 25
139.2 60 75 25
139.3 18 75 25
141.1 0 00 76.5 24.5
141.2 60 76.5 24.5
141.3 18 76.5 24.5
143.1 105 76 24
143.2 18 76 24
143.3 60 76 24
Figures 3-6 show micrographs of the structure of the cross section of samples Nos. 130.1 and 132.1. The influence of the directional crystallisation is apparent. Figures 3 and 5 show the as-cast state. Figures 4 and 6 show the state with the directed structure: the grain growth occurs; the number of grains decreases and their structure becomes more homogenous.
Fig. 3. As cast state, sample No. 130.1, Fig. 4. Directed state, sample No. 130.1, cross section, Z=50x cross section, Z=100x
-V ’* ■ Ck \ t
7\ > ,'pTt *
/i ir f)
\\ v7
\ ;■ - vO/N r • 1
A- j r ^8-
Fig. 5. As cast state, sample No. 132.1, Fig. 6 Directed state, sample No. 132.1, cross section, Z=50x cross section, Z=100x
4. Directional crystallisation by Bridgman's method
Next series of samples was used for the directional crystallisation by Bridgman's method with the use of an apparatus in Vladimir, Russia. The crystallisation process took place in corundum tubes with a vertex angle of 60-70°.
Figure 7 shows the samples marked as 139 after directional crystallisation. The castings were placed into electro-corundum tubes, the tip of which had the angle 60-70°.
Fig. 7. Samples 139.1,139.2 and 139.3 after directional crystallisation 4.1. Metallographic evaluation of the structures
Figures 8, 10 and 12 show the microstructure of cross-sections corresponding to the central part of samples Nos. 139.1, 139.2, and 139.3. This structure is very homogenous; this was confirmed also by X-ray analysis. Figures 9, 11 and 13 show the macrostructure of these samples.
Fig. 8. Sample No. 139.1, cross Fig. 9. Sample No. 139.1
section, Z=50x
Fig. 10. Sample No. 139.2, cross Fig. 11. Sample No. 139.2
section, Z=50x
Fig. 12. Sample No. 139.3, cross Fig. 13. Sample No. 139.3
section, Z=50x
4.2. X-RAY evaluation of structures
Cross sections of central parts of samples marked as 139 and 143 were used for the determination of grain orientation. X-ray diffraction by method of back reflection was applied using Fe-radiation without filter; for homogenous samples 139, the measurements were performed at 3 different spots of cross section and for polycrystalline samples 143 - at various grains. The obtained Laue patterns were evaluated by determination of angles between pairs of zones, to which the traces are lying on intersecting hyperbolas [3]. Table 3 summarises the results obtained for samples 139.1, 139.2 and 139.3.
Table 3
Determination of crystals orientation
Sample No. Orientation Note
139.1 a <221 > deviation < 5°
139.1 b <221 > deviation < 5°
139.1 c <221 > deviation < 5°
139.2 a <311> deviation 8°, general orientation
139.2 b <311> deviation 8°, general orientation
139.2 c <311> deviation 8°, general orientation
139.3 a <111> deviation 4°
139.3 b <111> deviation 4°
139.3 c <111> deviation 4°
The orientation in samples 139 is always identical at all three spots that were measured. The structure of these samples approaches the structure of preferentially oriented polycrystals. In samples 143, some grains at random were selected. In sample No. 143.2, the orientation of the grain axis <511> was determined without any deviation and with the deviation less than 2°. In sample No. 143.3, the grain orientation <221 > was determined with the deviation less than 5°.
4.3. Tensile tests with measurement of acoustic emissions
Samples 139 and 143 were used for the determination of mechanical properties. Table 4 summarises the measured values of the yield strength, strength, and ductility till yield strength for the given samples. Table 4
Final values obtained at tensile tests
Sample No. Al [at.%] rate of DC [mm/hour] Rp0.2 [MPa] Rm [MPa] ductility till Rm [%]
139.1 25 108 267 392 13.9
139.2 25 60 343 707 20.5
139.3 25 18 245 505 53.4
143.1 24 105 222 501 13.1
143.3 24 60 208 430 17.3
, the conditions of directional crystal isation have manifested themselve
In sample 139.1, no significant changes in the values already measured for the samples of the same type occur. On the other hand, in samples 139.2 and 139.3, a substantial increase in the strength and a primarily unexpectedly high increase in the ductility, even up to 53 % in sample 139.3 subjected to directional crystallisation at a rate of 18 mm/hour took place.
Figures 14-17 show load diagrams for samples 139.1, 139.2, 139.3, and 143.3 with recordings of acoustic emissions.
Time [s] Time [s]
Fig. 14 Load diagram for the Sample Fig. 15 Load diagram for the Sample
Time [s] Time [s]
Fig. 16 Load diagram for the Sample Fig. 17 Load diagram for the Sample No. 139.3 No. 143.3
5. Conclusion
This work compares 2 sets of the samples prepared by Bridgman’s method with different apparatuses. The samples prepared in the laboratories in Vladimir show the highly homogenous structure approaching the targeted required structure. Very interesting results were obtained from tensile tests with measurement of acoustic emissions, when a fibrous failure of material occurs. In samples 139.2 and
139.3, a surprising increase in the ductility even up to 20 and 53 % occurs, respectively.
The further investigation will be focused on the study of the fracture surfaces and densities of dislocations in material before and after mechanical load.
The presented results were obtained upon solution of the research plan No. MSM6198910013 entitled “Processes of preparation and properties of high-purity and special structurally defined materials”.
------------------------------------------------ СПИСОК ЛИТЕРАТУРЫ
1. Kursa M. Technologicke a fyzikalne metalurgicke charakteristiky intermetalicke slouceniny Ni3Al [Technological and physical-metallurgical characteristics of intermetallic alloy Ni3Al]. Sbornik vёdeckych praci Vysoke skoly banske-Technicke univerzity Ostrava [Proceedings of scientific works of the Technical University of Mining and Metallurgy in Ostrava], No. 1, vol. 46, Ostrava 2000.
2. Kuchar L., Drapala J. Metalurgie cistych kovii. Metody rafinace cistych latek [Metallurgy of pure metals. Methods of refining pure substances]. NADACIA R. KAMMELA, Kosice, 2000, pp. 63. ISBN 80-7099-471-1.
3. Cizek L. Praktikum ze zkouseni kovi II. Specialni zkusebni metody [Practical textbook on testing of metals II. Special testing methods]. Ostrava 1992, pp. 18-19.
— Коротко об авторах ----------------------------------------------------
Miroslav Kursa, Jitka Malcharczikova - Technical University of Mining and Metallurgy in Ostrava (VSB-TU Ostrava), 17.listopadu 15, 708 33 Ostrava -Poruba, Czech Republic
BelyaevI.V. - OAO NPO “Magneton”, Kuibysheva 26, Vladimir, Russia.
