L O W - T E M P E R A T U R E M E C H A N I C O T H E R M A L

T R E A T M E N T OF A 1 - - M g - - S i A L L O Y S

M. M. S h t e i n b e r g , M. A. S m i r n o v , N. T . K a r e v a , S. N. A n a n ' i n , G. E . G o l ' d b u k h t , a n d A. M. T o l s t o y

UDC 669.71'721'782:621.789

Weakly precipi ta t ion-hardening alloys of the A1-Mg-Si sys tem, used for manufacturing conductors, have reIat ively low strength. To increase the s trength of the alloy with 0.50% Mg, 0.014% B, 0.44% Fe, 0.50% St, 0.014% Mn, < 0.01% Cu, and t races of Ti, Cr , and Zn we used low-temperature mechanieother- mal t reatment (LTMTT), which consis ted of quenching + cold working + aging. Extruded rod 10 mm in diameter was used to prepare wire with different d iameters in order to obtain wire with the same diameter (2.12 mm) but different degrees of deformation. The samples of the alloy were heated at 525~ for 30 min and quenched in water . The deformation in LTMTT consisted of drawing at a speed of 0.5 m/min with r e - ductions of 8-95%. The drawn samples were aged at 140-180~ for 0.5-48 h. The time between quenching and the beginning of deformation and the time between the end of deformation and the beginning of aging did not exceed 10 min.

After LTMTT the physical and mechanical proper t ies were determined. Reerysta l l izat ion and the texture were determined f rom x - r a y pat terns made in axial cameras with KaFe radiation.

The variat ion of the mechanical proper t ies and the res is t ivi ty with the degree of deformation is shown in Fig. 1, where it can be seen that the ultimate s trength increases great ly at large deformations. Plast ic deformation leads to an increase of resis t ivi ty, evidently due to the increasing density of defects in the c rys ta l lattice and the development of the zone stage of aging [1].

After drawing with around 60% reduction the x - r a y photographs show texture peaks, which become more distinct with increasing deformation. This leads to the conclusion that the anomalous increase of the ultimate s trength at large deformations is due to < 111 > texture. It may be affected by decomposition of the solid solution, the rate of which should increase with the degree of deformation. This is confirmed by

P, a ~-mm2/m

o la 2a se *a 5a 50 70 8~ 9o % D e g r e e o f d e f o r m a t i o n

Fig, 1. Mechanical and phys- ical proper t ies in relation to reduction in drawing.

TABLE 1

8

'~t Aging conditions

f i r s t s e c o n d ) $

?

0 C~

c ~

95 140~ ?h -- 28,8 4,5 0,0334 5,1 �9 10 - 3

140~ 2 h 35 5 5,6 0,0328 5,9.10 - 3 40 140~ 5h 160eC 2 h 36:0 5,0 0,0325 - -

180~ 2 h 28,6 4,2 0,0303 - -

140~ ~h 33 8 4,5 0,0312 40 I60~ 5h 160~ 2 h 32:5 5,1 0 0312

180~ 2 h 29,5 4,5 0,0301

72 140~ 5h 140~ 2h 30,0 2,0 i 0,0337 -- L

I I

I |

Chelyabinsk Polytechnical Institute. Trans la ted f rom Metallovedenie i Termicheskaya Obrabotka Metallov, No. 8, pp. 26-29, August, 1973.

�9 1974 Consultants Bureau, a division o f Plenum Publishing Corporation, 227 g'est 17th Street, \'ew York, V. Y. lO01l. ,'Vo part o f this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission o f the publis]zer. [ copy o f this article is available from the publisher for $]J.00.

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P, l~ -mmZ/m

%, kg ~ram ~ Y '~ ~ ~ ~"~"~--~------~-------:--~--"-~-~-~' dO

~ ~ , j , . 7 , . - - - - l . . . . . . . . . i

25

20 +"~ r '~ "

}}

5

II lO 12 14 16

Aging time

~ ~ ~ I

2 4 6 8 18 20 22 2~ 48 h

p,f~-mm2/m

0.O330 ,. ,

0,o32o '~;

%, kg/mm;

r

I I I I I t I I PQ

- , ~ ' T - T T ~ 1" T-I" F ] T i l l " 0 2 ~ b 8 #0 12 I~ 16 20 2# ~Sh

Aging time b

Fig, 2. Change in the mechanical p rope r t i e s and r e s i s t iv i ty with aging at 140~ (a) and 180~ (b). O) Quenching; t ) 8% deformation; V) 22% deformation; i ) 60% deformation; I ) 95% deformat ion.

the fact that with over 60% reduction the ultimate

strength and mierohardness increase, which should

not be affected by the texture.

The variation of the resistivity and mechanical properties with aging time at 140~ is shown in Fig.

2a~ Aging for 30 min at this temperature leads to an

increase in the resistivity of the quenched samples.

With Longer aging this property gradually decreases.

although it is still higher after aging for 24 h than after quenching. This leads to the assumption that decomposition of the solid solution is limited essen-

tially to the zone stage [2]. A similar change in the

resistivity with aging time at 140 ~ occurs in samples

subjected to 8% deformation. For samples with large

degrees of deformation the original value of p is con-

siderably higher and remains unchanged with increas-

ing degrees of deformation. The resistivity decreases

smoothly with increasing aging time, and with aging

for more than 4 h it is lower than for the undeformed

samples. The high values of the resistivity in the

original conditions and its rapid decrease in the first

minutes of aging indicate that with large degrees of

deformation the zone stage of decomposition of the

solid solution develops greatly in the process of plas-

tic deformation itself. Further aging leads primarily

to an increase of the previously developed Guinier- Preston zones, and possibly the development of the

later stages of decomposition.

The intensification of precipitation processes and the retention of preliminary strain hardening leads to

the fact that the deformed samples have a higher ul-

timate strength than undeformed samples with aging

at 140 ~ the effect increasing with the degree of de-

formation. The favorable effect of the intensification of precipitation processes on the strengthening of the alloy

is particularly evident at small deformations. For ex- ample, in the unaged condition the diffe rence in the ulti-

mate strengths of undeformed samples and samples de- formed 22~ is no more than I. 5 kg/mm 2, while after aging

for 8 hthe difference is 4 kg/mm 2. The elongation of the

deformed samples increase somewhat during aging, but iris lower than that of undeformed samples.

With an increase of the aging temperature to 160 and 180 ~ the zone stage is shortened in undeformed samples and samples deformed 8% (Fig. 2b) - a relative increase is observed with aging for no more than

2 and 1 h respectively. Judging from the rate at which the resistivity decreases, the decomposition of the

solid solution is accelerated with increasing deformation. In this case an increase of aging temperature from 140 to 160-180 ~ eliminates the "anomalous" behavior of the alloy deformed 95%, since it is probable that finely dispersed GP zones grow at a fairly high rate at these temperatures.

A difference in the ultimate strength of the deformed and undeformed samples at 160 ~ is observed only in aging for no more than 6 h. At longer aging times the strength increases very slightly as compared

with the undeformed condition only for 95% deformation. The data obtained lead to the assumption that

with aging at 160 ~ the accelerated decomposition of the deformed solid solution leads to an undesirable effect - the formation of particles with a low degree of dispersity. This, in turn, must weaken the matrix.

At 180 ~ the rate of weakening increases for deformed samples, and with aging for 6 h none of the drawing conditions ensures an increase of the ultimate strength as compared with the undeformed condition~

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The x-ray analysis showed that recrystallization of the deformed alloy occurs only with heating to 240 ~ . It follows that weakening of the deformed matrix during aging at 160 and 180 ~ is limited by the stage of recovery. It is possible that this is one of the reasons that samples deformed 95% usually retain a higher

ultimate strength at these aging temperatures than samples with smaller degrees of deformation. This should also be facilitated by texture retained during aging.

To increase the strength of the alloy, we tested LTMTT in which deformation was alternated with aging in two stages. The degree of deformation and aging temperature were varied, while the total defor- mation (95%) and the total aging time (7 h) were constant. The data given in Table 1 indicate that the use of this treatment produces a higher ultimate strength than deformation in one stage, and the ductility of the alloy is retained.

The increase in strength can be explained by the fact that finely dispersed precipitates formed during the first aging treatment and the pinning of previously existing dislocations produce more refining of the structure during subsequent deformation. This assumption is confirmed by the results of x-ray analysis - line broadening - (202)c~ - is larger after this treatment than after deformation in one stage.

The highest ultimate strength is attained by the treatment with 55% deformation in the second stage. With 23% or 80% deformation in the second stage the effect is smaller. The maximum effect is attained by aging at 140 ~ . Higher temperatures in the first as well as the second stage of aging lead to lower ultimate strengths.

C ONC L U S I ONS

1. Considerable strengthening of weakly precipi tat ion-hardening alloys of the A1-Mg-Si sys t emcan be obtained by LTMTT (e = 95%), leading to the development of texture and intensive zone decomposition of the solid solution during subsequent aging at 140 ~ .

2. The increase in s trength due to LTMTT can be increased by deformation in two stages with ap- proximately equal deformation in each stage and totalling 95%, alternating with aging at 140 ~

IB

2.

LITERATURE CITED

A~ Kelly and R. Nicholson, Precipitation Hardening [Russian translation], Metallurgiya, Moscow (1965), p. 23. I. N. Fridlyander, Tekhnologiya Legkikh Splavov, 5, 78-82 (1967).

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