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INFLUENCE OF DENSITY OF DISLOCATIONS ON SELECTED

MECHANICAL PROPERTIES OF DIRECTIONALLY CRYSTALLISED Ni3Al BASED ALLOY

VLIV HUSTOTY DISLOKACÍ NA VYBRANÉ MECHANICKÉ

VLASTNOSTI SMĚROVĚ KRYSTALIZOVANÉ SLITINY NA BÁZI Ni3Al

Jitka Malcharczikováa Miroslav Kursab

a Department of Non-Ferrous Metals, Refining and Recycling, FMMI, Technical University of Mining and Metallurgy in Ostrava(VŠB-TUO), 17. listopadu 15/2172, CZ

708 33 Ostrava, Czech Republic; jitka.malcharczikova@vsb.cz b Department of Non-Ferrous Metals, Refining and Recycling, FMMI, Technical

University of Mining and Metallurgy in Ostrava(VŠB-TUO), 17. listopadu 15/2172, CZ 708 33 Ostrava, Czech Republic; miroslav.kursa@vsb.cz

Abstract

The paper is oriented on investigation of influence of deformation behaviour of Ni3Al based alloy in dependence on chemical composition and conditions of preparation with focus on structure. Density of dislocations was determined in deformed and non-deformed condition of Ni3Al based material. The samples containing 22 at.% of aluminium and 25 at.% of aluminium were chosen for analysis. All the samples were used in directionally oriented state. Directional crystallisation was made with use of the Bridgman’s method with vertical arrangement. Castings were crystallised at various speeds ranging from 10 mm/h to 108 mm/h with different temperature gradient. The samples were submitted to tensile tests, metallographic analysis. They were afterwards used for manufacture of thin foils for investigation by method of transmission electron microscopy. The targets were prepared with use of parts of the rods after the tensile test. The head of the tensile rod was always used as a non-deformed part of the sample and the body of the tensile rod at the area of fracture was used as a deformed part of the sample. The targets with thickness of 0.5 mm were cut from the appropriate part of the samples by a diamond disc saw. They were then used for manufacture of targets with diameter of 2.8 mm on a spark cutter. Thickness of these targets was further reduced by grinding to 0.15 mm. The foils for investigation were afterwards made from them electrolytically. A dependence on selected mechanical properties was established from the ascertained facts. Dependence of ductility on the ratio of dislocations density in deformed and non-deformed parts appears to be highly remarkable.

Abstrakt

Příspěvek je zaměřen na studium vlivu deformačního chování slitiny na bázi Ni3Al v závislosti na chemickém složení a podmínkách přípravy se zaměřením na strukturu. Bylo provedeno stanovení hustoty dislokací v deformovaném a nedeformovaném stavu materiálu na bázi Ni3Al. K analýze byly vybrány vzorky

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s chemickým složením 22 at.% hliníku a 25 at.% hliníku. Všechny vzorky byly použity ve stavu po směrová krystalizaci Bridgmanovou metodou s vertikálním uspořádáním. Odlitky byly krystalizovány při různých rychlostech od 10 mm/h až po 108 mm/h s odlišným teplotním gradientem. Vzorky byly podrobeny tahovým zkouškám, metalografické analýze a následně použity pro výrobu tenkých folií ke studiu metodou transmisní elektronové mikroskopie. Pro přípravu terčíků byly použity části tyče po tahové zkoušce. Jako nedeformovaná část vzorku byla vždy použita hlava tahové tyče a jako deformovaná část vzorku bylo použito tělo tahové tyče v oblasti přetržení. Ze vzorků byly na diamantové pile nařezány z příslušné části terčíky tloušťky 0,5 mm. Z nich byly na elektrojiskrové řezačce připraveny terčíky o průměru 2,8 mm a tyto terčíky byly dále ztenčeny broušením na tloušťku 0,15 mm. Z nich pak byly elektrolyticky připravovány folie pro pozorování. Ze zjištěných skutečností byla sestavena závislost na vybraných mechanických vlastnostech. Velmi pozoruhodnou se jeví závislost tažnosti na poměru hustot dislokací v deformovaných a nedeformovaných částech.

INTRODUCTION

Inter-metallic phases can be generally considered as type of material, the properties of which are somewhere between metallic and ceramic materials. High strength of inter-metallic materials is their inherent property. They have in connection to bonding parameters high values of Young’s modulus, low coefficient of auto-diffusion, which means also higher temperature stability of properties of inter-metallic materials. Moreover, it is possible to achieve in inter-metallic materials formed on the basis of “light” elements higher specific strength characteristics [1]. Drawback of inter-metallic materials consists in low level of their plastic properties, namely at low and intermediary temperature. Reasons for this degradation are similar as e.g. in the case of structural ceramics.

The most frequently investigated inter-metallic material is Ni3Al, the melting point of which is reported in literature to be within the interval from 1362 to 1395°C [2, 3]. Its strength changes untypically with temperature, its yield value increases up to the temperature of 700°C, which corresponds approximately to 60% of the value of the fusing point. Another favourable property of the inter-metallic material Ni3Al is its plasticity in mono-crystalline form. It can be deformable also in poly-crystalline form, if stoichiometry and content of admixtures are controlled [4].

1. PREPARATION OF EXPERIMENTAL SAMPLES

The samples that were prepared from foundry alloys and then re-melted by method of directional crystallisation were used for experimental purposes. The foundry alloys were prepared in the vacuum induction furnace LEYBOLD type IS3/1. Prior to melting itself the vacuum treatment was applied twice, namely under the value of 0.04 mbar with use of two-stage rotary and Roots’ pumps. Several basic samples were prepared by casting with various content of aluminium in the form of cylinders with length 95 mm and diameter 10 mm. The melting was performed in a corundum crucible and casting was made into graphite ingot-moulds.

The castings were then directionally crystallised by Bridgman’s method with vertical arrangement. Melting of the sample 1-2 was made in the two-zone superkanthal resistance furnace Classic with automatic feed at the VŠB-TU Ostrava at the temperature of heating of 1435°C and with the temperature gradient of 30°C/cm. The sample 3-5 was prepared in collaboration with the company Magneton in Vladimir (Russia) at the temperature of heating of 1550°C and with the

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temperature gradient of 80-100°C/cm. Inert argon atmosphere of the purity 4N6 was used at melting. The sample was placed into the corundum tube with closed bottom.

1.1 Determination of dislocations density

Density of dislocations was determined in original non-deformed state and then in the state after deformation of material at the working site of the Charles University in Prague (UK Praha), Department of materials physics. Deformation occurred during the tensile testing, when the applied strain rates were within the interval from 1.15 to 1.20·10-4 s -1.

The targets were prepared from the parts of rod after the tensile test. The head of the tensile rod was always used as a non-deformed part of the sample, and the body of the tensile rod at the place of rupture was used as a deformed part of the sample. The targets with thickness of 0.5 mm were cut by a diamond saw from the samples. From them the targets with diameter 2.8 mm were prepared on a spark cutter and thickness of these targets was further reduced by grinding to 0.15 mm. The foils for investigation were then prepared from them electrolytically [5]. The tables 1 and 2 contain the obtained values of dislocations density in non-deformed and deformed state.

Tab. 1 Density of dislocations in non-deformed parts of material

Sample No. 1 2 3 4 5 ρ N[1013 m-2] 0.7 0.6 1.2 0.8 0.3

Tab. 2 Density of dislocations in deformed parts of material

Sample No. 1 2 3 4 5 ρ D[1013 m-2] 2.2 8.7 6.1 13.7 10.8

1.2 Determination of ration of dislocations density in non-deformed and

deformed state The table 3 gives the ration of dislocations density in deformed and non-deformed

parts of the sample, aluminium content, rate of directional solidification (DS) and values of ductility, yield value and strength of each sample. The highest calculated value corresponds to the highest value of determined ductility, i.e. to the sample 5. The values of mechanical properties of the sample 4 were probably influenced by premature fracture, which is a serious problem in this type of material.

Tab. 3 Ratio of dislocations density in deformed and non-deformed parts of the

sample

Sample No. 1 2 3 4 5 ρ D/ ρ N 3.1 14.5 5.1 17.1 36.0

Al content [at.%] 25 25 22 22 22 Rate of DS [mm/h] 100 50 108 60 18

A [%] 3.5 6.6 13.9 20.5 53.4 Rp [MPa] 295 255 267 343 245 Rm [MPa] 318 330 392 707 505

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2. DETERMINATION OF DEPENDENCE OF SELECTED MECHANICAL PROPERTIES ON RATIO OF DISLOCATIONS DENSITY The Fig. 1 shows graphically the dependence of the ratio of dislocations density

on ductility. The dependence was established for the samples 3-5 with two-phase structure, where this dependence shows a remarkable trend. The equation of linear regression shown in the Fig. 1 is suitable for notation of this dependence. The confidence coefficient is 0.94 and the correlation coefficient for this dependence is 0.97.

Fig. 1 Dependence of the ratio of dislocations density in deformed and non-deformed parts of the sample on ductility

Comparison of dislocation structure of the samples of the type 1 and 2 with

structure of the samples of the type 3-5 shows that the structures differ [6]. The sample 1 and 2 do not contain any two-phase region, only γ´ phase is present. The Fig. 2 shows a TEM picture from the non-deformed part of the sample 2, and the Fig. 3 shows the deformed parts of this sample. In the samples 3-5 a two-phase region formed by light areas (γ´) and mesh (γ´+γ) is visible both in the non-deformed parts of the sample 3 (Fig. 4) and in the deformed parts of this sample (Fig. 5).

Fig. 2 TEM picture – non-deformed part of the sample 2

Fig. 3 TEM picture – deformed part of the sample 2

ρD/ρ

N

y = 0,7153x - 1,5356

R2 = 0,9441

0

5

10

15

20

25

30

35

40

10 15 20 25 30 35 40 45 50 55

A [%]

500 n

m

500 n

m

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Fig. 4 TEM picture – non-deformed

part of the sample 3 Fig. 5 TEM picture – deformed part

of the sample 3 3. STRUCTURAL CHARACTERISTICS

The samples were subjected also to metallographic investigation. The Figs. 6-9 show micro-structures of selected samples in longitudinal section. Distinctive differences in structures are evident here as well. The Figs. 6 and 7 show a single-phase structure formed only by the phase Ni3Al. The Figs. 8 and 9 show the samples with two-phase structure formed by an inter-metallic phase Ni3Al and solid solution (Ni). Shape of grains in all the samples was influenced by the rate of directional solidification.

Fig. 6 Longitudinal section of the sample 1, Ni25Al

Fig. 7 Longitudinal section of the sample 2, Ni25Al

Fig. 8 Longitudinal section of the sample 3, Ni22Al

Fig. 9 Longitudinal section of the sample 4, Ni22Al

50

0 n

m

50

0 n

m

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4. CONCLUSION The samples with chemical composition containing 22 at.% and 25 at.% of

aluminium were chosen for an analysis. All the samples were used directionally oriented state. Directional solidification was made by the Bridgman’s method with vertical arrangement. The castings were crystallised at various rates from 10 mm/h to 108 mm/h with different temperature gradient. The samples were subjected to tensile tests, metallographic analysis and they were afterwards used for manufacture of thin foils for investigation by method of transmission electron microscopy. Densities of dislocations were determined in deformed and non-deformed state of Ni3Al based material. The obtained data were used for determination of the dependence on selected mechanical properties. The dependence of ductility on the ration of dislocations density in deformed and non-deformed parts is very remarkable, particularly in the samples with aluminium content of 22 at.%. LITERATURE

[1] JONŠTA, Z., MAZANEC, K. Zvyšování užitných vlastností kovových materiálů. Nové typy technických materiálů [Enhancement of service properties of metallic materials. New types of technical materials]. Textbooks of PGS. Ostrava, VŠB-TU Ostrava, 1999.

[2] MICHNA, Š. et al.v Encyklopedie hliníku [Aluminium encyclopedia]. Adin, Prešov, 2005, pp. 183-184. ISBN 80-89041-88-4.

[3] MASSALSKI, T., B. Binary Alloy Phase Diagrams. Amer. Soc. for Metals, Metals Park, Ohio, 1986, pp.140-143.

[4] MAZANEC, K. Materiálově inženýrské charakteristiky vybraných typů technických materiálů [Material engineering characteristics of selected types of technical materials]. Ostrava, VŠB-TU Ostrava, 1994.

[5] KURSA, M., MALCHARCZIKOVÁ, J, PEŠIČKA, J., VODÁREK, V., HYSPECKÁ, L. Microstructural analysis and mechanical properties of polycrystalline Ni-rich Ni3Al alloy prepared by directional solidification. „Kovové materiály“. 2008, vol. 46, No. 6, pp. 351- 359.

[6] MALCHARCZIKOVÁ, J, PEŠIČKA, J., KURSA, M. Structural and micro-structural analysis of directionally crystallised inter-metallic compound Ni3Al realised by TEM. Acta Metallurgica Slovaca. Metallurgical Faculty of Technical University in Košice, Košice, 2007, vol. 13, No. 4, pp. 503-510.

The presented results were obtained during solution of the research plan

No. MSM6198910013 entitled “Processes of preparation and properties of high-purity and structurally defined special materials”.

The authors of the article express their thanks to doc. J. Pešička from the UK Praha for execution of special analyses and assistance at evaluation of the results.