H A R D E N I N G A N D S O F T E N I N G OF N I C K E L A L L O Y S

C O N T A I N I N G A L U M I N U M O X I D E

(UDC 669. 24)

V. A. B o r o k , R. D. Z a i t s e v a , G. M. K a r p m a n , a n d M. D. P e r k a s

Central Scientific Research Institute of Ferrous Metallurgy Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 3, pp. 29-32, March, 1966

Interest in strengthening metals with small insoluble particles was aroused when it was found that particles of aluminum oxide dispersed in aluminum render aluminum resistant to high temperatures. During recent years there have been several publications concerning the possibility of using this method to harden metals which have a higher melting point than aluminum [1-4].

We investigated the influence of aluminum oxide on different modifications of the structure and the mechan- ical properties of nickel hardened by different methods and also on its softening during heating.

The initiat material was a powder of nickel carbonyl consisting of particles smaller than 1 micron. The in- soluble phase was c~- or y-aluminum oxide. The powder of the ct-aluminum oxide consists of particles with an average diameter of 0.1 microns; the powder of the y-aluminum oxide consists of particles with an average diameter of 0.014 microns. The alloys* were mixed in ball mills, compressed, and sintered in hydrogen. The sintered sam- pies were extruded at 1050~ We obtained nickel alloys containing 0.5-7% y-A1203 and 3.0% c~-A120 a. The sam- ples containing the a-oxide were sintered at 1300-1400~ to obtain smaller inclusions of c~-AlzO 3. At these t em- peratures the T'A1203 is completely transformed into ct-A1203 and the particles do not increase in size to any signif- icant degree.

The study of the mechanical properties of extruded samples showed that the hardness and the yield strength of the alloy increases with increasing concentrations of y-A120 a. Thus, extruded samples of nickel have a hardness of HRB 45 and a yield strength of 18 k g / m m 2, while the alloys with 3 and 7% y-AlzOahave a hardness of HRB 76 and 87 respectively and yield strengths of 29.4 and 40 k g / m m 2. The increase in hardness is accompanied by a decrease in the ductility of the alloy (Fig. 1). The presence of c~-Al203 has little influence on the strength of the sintered alloy of nickel + ~x-A120 a (alloy c~1) or the alloy containing y-oxide and sintered at high temperature to transform the T-AlzOa into c~-AlzO 3 (alloy cr

Measurements of the width of the x-ray fringes"f for extruded samples of nickel and nickel alloys with alumi- num oxide showed that the widening of the fringes is very slight. The width of the (222) fringe for nickel as well as for its alloys is 14-16"10 -3 tad. This means that if extrusion is responsible for the deformation of the crystal struc- ture then most of the deformations are eliminated either during deformation or during subsequent cooling. Accord- ing to the values of the hardness, the hardening of nickel by extrusion is about the same as that produced by 5%cold deformation (compression).

Fig. 2 shows the dependence of the hardness of extruded samples of alloys with 3~ AIRO a on the annealing t em- perature. For nickel and its alloys with r the hardness decreases after annealing at 400-600~ The sharp decrease of the hardness of the alloy with y -A�89 a occurs after annealing at temperatures above 1100~ Thus, the small particles of y-A1203, unlike those of cz-A12Oa, considerably increase the softening temperature of nickel.

* The term alloy is used arbitrarily to indicate the metal containing small inclusions of oxides. ~- The x-ray photographs were made with the URS-50I apparatus and iron Kc~ radiation. We measured the widths of the (111) and (222) lines.

206

~0 50 I j

f0

1

Fig. 1. Dependence of the hardness,

yield strength, and re la t ive elongat ion after extrusion on the concentrat ion of y-Al~Oa.

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~0 , ' ] . i _ J t

Fig. 2. Dependence of the hardness of extruded samples on the anneal/ng t e m - perature.

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200 ON SN 8OO ~000 ~

Fig. 3. Dependence of the hardness of cold-deformed samples on the annealing temperature . 1) Extruded; 2) extruded with 20% cold deformation; 3) extruded with 40% cold deformation; 4) extruded with 805 cold deformation.

The softening of the n ickel alloys with a luminum oxides during

annealing is due to the change (coarsening) of the substructure of the

matr ix , to the increase in the size of the part icles of the second phase

(if the oxides are not par t iaI ly dissolved during heating), or, in the case

of alloys with y -AI2Q , to the polymorphic transformation y ~ c~-At203, which is accompanied by a sharp decrease in volume.

The influence of the smal l part icles of AI20 a on the coarsening of the substructure of nickel during heat ing can be studied by the change in the width of the x- ray fringes. But the widening of the fringes m e a - su_red with samples d i rec t ly after extrusion is small , and therefore these data do not allow us to draw any conclusions on the influence of a lumi -

num oxides on the substructure of extruded samples during heating.

The de te rmina t ion of the temperature of the beginning of crys ta l - l i za t ion by x - ray analysis showed that the sharp point reflections on the background of continuous x- ray fringes ( indicat ing the beginning of the recrys ta l l iza t ion process) occur in al l alloys after heat ing at t empera -

tures above 400-500~ Thus, unlike s imilar a luminum alloys, where the presence of oxides leads to a sharp increase in the temperature of the beginning of recrysta l l izat ion, in nickel alloys a luminum oxides do not affect the temperature of the beginning of recrystalI izat ion. This result Ieads us to conclude that most of the imperfections in the in t ra- granular structure in nickel and its alloys with oxides are e l imina ted at 400-600~

The presence of aluminum oxides handicaps the increase of the grain size during annealing. There is no marked coagulat ion of a lumi - num oxide part icles up to 1200~

The exper imenta l results indicate that the type of a luminum oxide has a great influence on the softening of alloys after extrusion. The presence of the ),-AI203 modif icat ion, which, l ike nickel, has a face-cen te red cubic la t t ice , considerably retards the softening of n ic- ke l during heat ing. Possibly in this case the bond between the base and the oxide is greater than in alloys with c~-aluminum oxide, and there-

fore the presence of the inclusions of c~-aluminum oxide has no effect on the type of softening of nickel but is related essential ly to the coars- ening of the substructure of the matrix.

The sharp decrease of the hardness of the n ickel al loy with y-

A120 a at temperatures above 1000-1100~ is apparently due main ly not to the coarsening of the structure and not to the coagulat ion of the par- t icles of oxides but to the y ~ o~-AI20 a transformation at these t empera - tures [5]. X-ray analysis of a luminum oxide precipi ta ted e l ec t ro ly t i ca l - ly out of annealed samples of nickel al loy with ),-A1203 showed that only c~-AlzO a is present in the al loy after annealing at 1200~

Since the strengthening of alloys after extrusion is small , we made experiments to de termine the strengthening of nickel alloys with y- a luminum oxide after cold-plast ic deformat ion and the loss of strength during heating of cold-worked samples.

After 80% cold-p las t ic deformation the hardness of the alloys is much higher than after extrusion and is p rac t ica l ly independent of the concentrat ion of y-AI20 a. The hardness of pure n ickel and of the alloy with 5% A�89 a is the same (about HRB 98). The widths of their x- ray fringes are also s imilar (28-30.10 -3 tad), which is apparently an ind ica- t ion of a s imi lar substructure.

207

"N j r-~tz 0~

100 L I // - &

I

Fig. 4. Dependence of the time before rupture of alloys at 800~ under astress of 3 k g / m m 2 on the concentration of aluminum oxide.

The presence of insoluble particles of y-a luminum oxide has, in this case, no influence on the resistance to plastic deformation. We may assume that as the result of high degrees of plastic deformation the bond between the base and the oxide particles is disrupted and, consequently, these particles have no effect on the strength of the alloy.

The type of softening of the cold -deformed samples of the nickel alloy with y-Al20 a and of extruded samples of the same alloy is quite different. In cold-deformed samples the hardness decreases sharply after annealing at 400-450~ while in extruded samples it decreases at t em- peratures above 1000~ Also, after annealing at temperatures above 450 ~ C the hardness of cold-deformed samples becomes lower than the hard- ness of extruded samples annealed at the same temperature (Fig. 3). The decrease in the width of x-ray fringes and the beginning of recrystalliza- tion occurs at the same temperatures (400-450~ for cold-deformed and for extruded samples).

This type of loss of strength of cold-deformed samples apparently confirms the assumption that high degrees of deformation disrupt the bond between the matrix of the alloy and the y-A1203 in the same way as in nickel alloys with c~-A120 a.

If as the result of high degrees of deformation (80%) the bond between the matrix and the particles of y-A1203 is disrupted we may expect that at lower degrees of deformation the bond may not be disrupted.

In fact, the samples subjected to 22% cold deformation begin to lose their strength at the same temperature as the extruded samples. Samples subjected to 40% deformation lose their strength at lower annealing temperatures than those which induce loss of strength in samples deformed 80% (Fig. 3).

These experimental results are in agreement with the assmnption that cold deformation (more than 20-30%) may lead to the disruption of the bond between the matrix and the particles of y-AleO3, and therefore the loss of strength of the alloy begins at lower temperatures.

The study of the mechanical properties at room temperature and at high temperature showed that the alloys containing particles of y-A120 a have considerable advantages which are manifest during prolonged tests at high tem- peratures.

Fig. 4 shows the dependence of the time preceding rupture on the concentration of aluminum oxide in nickel at 800~ under a stress of 3 k g / m m 2. The addition of 1% y-A1203 results in a much stronger alloy than nickel. The effect of c~-AlzO 3 is much weaker: the alloys with 5% y-A1203 have the best properties. The t ime before rupture of this ahoy is 625 h, i.e., almost 70 times longer than for pure nickel.

C O N C L U S I O N S

1. The type of aluminum oxide has a great effect on the loss of strength of nickel alloys with aluminum oxide. The y-A1203 considerably increases the strength of the alloy. Possibly in this case the bond between the matrix and the oxide particles is stronger than the bond between nickel and c~-A120 a. Further experiments are needed to deter- mine the type of bond between nickel and aluminum oxide.

2. It is assumed that a high degree of cold-plastic deformation leads to the loss of strength of nickel + y-A1203 at the same temperature as the toss of strength of nickel + ~-A1203 because of the very different deformation capac- ities of nickel and the y-A1203.

1.

2. 3, 4. 5.

L I T E R A T U R E CITED

K. Zwilsky and N. Grant, Iournal of Metals, 9, No. 10 (1957). A. Gatti, TMSAIME, 215, No. 5 (1959). K. Cremens and N. Grant, Proeedings ASTM, 58 (1958-1959). L. Bonis and N. Grant, TMSAIME, 224, No. 2 (1962). S. G. Tresvyatskii and A. M. C~aerepanov, High-Strength Materials and Articles Made of Oxide [in Russian], Metallurgizdat, Moscow (1957).

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