Peculiar mechanical properties of aluminium-magnesium, aluminium-copper and aluminium-zinc alloys at low temperatures

The paper reports on work hardening o f solid solutions under tension at a constant rate of 7 x 10-4 s-1, at temperatures o f 1.6 to 300 K. The materials studied were single crystals o f AI (0.62-9.0 at % Mg) and polycrystals of AI (0.62-10.4 at % Mg), AI (0.42 at % Zn) and AI (0.62 at % Zn). A t very low temperatures, in addition to the softening due to maximum magnesium doping of aluminium, the authors found a softening associated with, first, an anomaly o f the temperature dependence on the flow stress, second, an anomaly o f the temperature dependence on the relative lengthening, and third, discontinuous flow.

Peculiar mechanical properties of aluminiummagnesium, aluminium-copper and aluminium-zinc alloys at low temperatures V.P. Podkuyko, and V.V. Pustovalov Low temperature plasticity and strength of various materials have been studied, ahiminium alloys being among the most common. The reason is their high plasticity, specific strength, good weldability and high strength of welded joints. However, efficient application of such promising structural materials as aluminium alloys of magnesium, copper and zink, requires a thorough investigation into low temperature work hardening, l' 2

Material and Procedure The investigation used single crystals of AI (0.50, 1.40, 1.78, 3.41,5.00 and 8.20 % Mg), a and polycrystals of AI, 1.40 % Mg, A1, 1.00 % Cu, and AI, 1.52 % Zn, smelted from 99.99% pure aluminium, 99.8% magnesium, 99.8% copper and 99.8% zinc. The alloys contained impurities: silicon (0.1%) and iron (0.009%). The average polycrystal grain size was 0.03-0.035 ram. The test samples were shaped like two-bladed fiat trowels, the test part being 25 mm long and 2 x 5 or 2 x 8 mm in cross section (polycrystals). The experiments enabled us to plot tension curves in load-time coordinates, which were eventually recalculated to stressstrain curves. Deformation at a constant rate of 7 x 10 -4 s -I was performed by means of a low temperature breaking machine, designed at the Physico-Technical Institute of Low Temperatures in Kharkov. 4 Low temperatures in the range 1.6-300 K were produced using liquid nitrogen and helium by the common method, s

Temperature dependences o f characteristic shear stresses~ Fig. 2 presents critical shear stress ro and other characteristic shear stresses: rn (for two sections of stage II - rn~ and rn_ ), Tm and rmax, as a function of temperature. Curves z - T f0*r AI - (0.62 - 9.0) at. % Mg are similar, and, for pure aluminium, 7 include three sections. The temperature-work hardening relation being different in each section: from 300 to 150 K, r is fairly constant; from 150 to 10-25 K, r increases linearly with falling temperature; below 10-25 K, r decreases appreciably which is a significant feature of plastic deformation at low temperatures. Effect o f temperature on the characteristic shear stress vs concentration. Fig. 3 illustrates concentration dependences of the characteristic stresses in the temperature range where T does not depend on temperature (athermic range) and where it does (normally observable thermally activated and anomalous ranges). The critical shear stress, z0, (Fig. 3a) is seen to decrease linearly with Mg concentration for all the temperatures used. Stress Tu (Fig. 3b) also displays a linear rise with concentration, the work hardening coefficient, dZli/dc, however, is different in the single and two-phase

I,!,l

3

- . -

;,<

Experimental results Stress-strain curve. Fig. 1 shows stress-strain curves for AI-Mg crystals containing 0.62, 5.52 and 9 at % Mg for different temperatures. For very low temperatures, firstly, the 4.2 K curve lies completely outside the curve for 20 K, secondly, the general deformation before failure at 4.2 and 1.6 K is lower than at 20 K, and, thirdly, below a certain temperature, deformation is discontinuous: the discontinuous nature was observed not only at stage III (as in AI),6 and it began much closer to the yield stress. The said features were observed on all the AI-Mg single crystals and AI-Mg, AI-Cu and AI-Zn polycrystals we tested. The authors are at Physico-Technical Institute of Low Temperatures of the UkrSSR Academy of Sciences, Kharkov, USSR. Received 5July 1978.

CRYOGENICS. NOVEMBER 1 9 7 8

293K

Fig. 1 Work hardening curves for single crystals of AI with 0.62 (1), 5.52 (2) and 9 at. % Mg (3) at different temperatures

0011-2275/78/110589-07 $02.00 © 1978 IPC Business Press

589

I E E a¢

:

I a

~

0

50

25

I00

150 20O T, K

250

bl. 0

I

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C o

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T,K

12 ~,

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Fig. 2 Temperature dependences of characteristic shear stresses for single crystals of AI alloys with 0.62 (©), 1.55 (A), 3.78 (A), 5.52 (El), 9.0 at. % Mg (•); and concentration dependences of anomalies AT = r ( m a x ) -- r 1,6:a -- ~ r 0 and A'/'II ; b -- AT"II 1 and AZ 1t2; C -- ATma x

regions. The reduction of work hardening in the transition to the two-phase region must be due to the softening action of/3-phase particles (berthollide AI3 Mg2). Dependences rli! (c) and rmax(C) are fairly well described over a wide range of concentrations b~ a different relation, namely a parabolic function r ~ c '/~ (Figs 3c and 3d). Results for the alloy containing 9 at. % Mg are outstanding, the additional softening depends on enlargement of the/3-phase particles (to about 1/am)./3-phase particle enlargement (to 3 - 4/am), which reduces strength at low temperatures was observed earlier on polycrystals with 8.32 and 9.55 wt. % Mg.SThe concentration dependences of rlti and rmax, as well as those of to, are insensitive to the second phase presence in the alloy; though, for different stresses, the nature of the

+4

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The linearity of the function r(c) is due to the Mott-Nabarro work hardening mechanism, 9 based on dissimilar sizes of the atoms of the solvent and of the dissolved material, the latter being distributed at random: 8o = 1/a da/dc (a is the lattice constant). The relation between concentration and higher characteristic stresses can be described by the relation r ~ c v2, implying mechanisms allowing for the local variation of the shear modulus 7? = 1/G dG/dc 1 o are a result of alloying in addition to size dissimilarity 8o, assuming that elongated and compressed regions exist in the lattice, u Weakening in the work hardening dr/dc with increasing r may be due to a change in the deformation

/

I

I

~4

c - rrelationship can vary, owing to participation of different mechanisms.

3Corn+ 9 I

I

o

I

I

v-c

2

i

// /

l

4 1

d

12

.-~

/

2

a o

3

6 Mg,am %

co

t

v-c

~

3

e

o

I

v-c

~

3

Fig. 3 Concentration dependences of characteristic stresses for AI--Mg single crystals at various temperatures: • 300; u 77; o 4.2; o 1.6 K; • temperature T ~(max)The values for 99.99~ aluminium are borrowed from Didenko 7

590

CRYOGENICS.

NOVEMBER

1978

mechanism to a deformation which can enhance the effects of local variation of shear moduli and presence of eleongated and compressed regions in lattice. It should be pointed out, that though temperature decreases cause dr/de to rise, the concentration dependences are, only slightly dependent on temperature. Thus, the regularities of hardening in case of Mg alloying of AI vary with increasing stress, however, for each particular value of r, the concentration dependence is the same for all temperatures.

Anomaly in o(T). At temperatures below 10 - 25 K, we revealed an anomalous decrease of the yield stress and ultimate strength that was found to be associated with a variety of factors. The anomaly in o(T) was evidenced by its parameters, ie (Fig. 4) temperature To(max) at which reversal of the sign do/dT occurred and the amount of reduction in o below To(max) (absolute Ao = Crmax - - 0"1, 6 and relative A0"/Omax). The effect o f magnesium concentration in annealed single crystals. The value of Tr(max) decreases as Mg concentration rises. Tr(max) (see Table 1) is independent of the stress. In contrast to Tr0nax), Ar is sensitive not only to the Mg concentration, but also to the stress. For stresses re and TII, AT ~ C ~/2 (Fig. 2a), while ro ~ c and TII ~. e. For high stresses ('rll I in Fig. 2c and rmax in Fig. 2d), Az(c) is a nonmonotonic function with a maximum around 2 kg mm -2 , occurring at a limited Mg content in the solid solution (ca. 2 at %). The relative value of the anomalous softening displays a linear decrease with growing magnesium concentration (Fig. 5). The maximum value of AT/T(max) is observed at the yield stress, and in A1-1.55 at. % Mg it is as high as ~20%.

The importance of grain boundaries. Experiments which could evaluate the role of grain boundaries in the anomalous softening have not been run. However, comparative analysis of z(T) of single crystals and o(T) of polycrystals of alloy A1 1.55 at. % Mg shows, that in polycrystals the anomaly is less conspicuous (Fig. 6, Table 2). Temperature To(max) of the anomaly onset 'tends to be lower in polycrystals, than in single crystals. The results obtained from AI-Mg agree well with the earlier observations ~z on lead, aluminium, copper and silver. The influence o f the alloying element. Fig. 7 shows the relation between temperature, yield stress and ultimate strength for single-phase aluminium solid solutions and 99.97 % AI. The variation of the yield stress and ultimate strength in the temperature range dealt with (1.6 - 77.3 K) are qualitatively similar and display peculiar regularities of the o(T) anomaly, depending on the alloying element type. The value of To(max) seems to be slightly dependent on the alloying element and deformation stress, and is about 1 0 - 1 5 K. In contrast, for To(ma~) the value Ao is sensitive to the alloying element type and the deforming stress. If the solid solution consists of Table 1. Mg

Values of Tr(max)for several concentrations of

Mg, at,

%

0.62 1.55 1.97 3.78 5.52 9.0

Tr(max)

K

25

22

CRYOGENICS. NOVEMBER 1978

22

18

15

10

8mox

K ~rmi" b ~min

I P

I I Tc,mox

T~,

To T

Fig. 4 Schematic representation of anomalies of 6 ( T ) a n d 6 ( T ) aluminium alloys

5O

O

I0

0

~_ _

i

_ _

4

i

6

i

Mg, om %

Mg,am% Fig. 5 Concentration dependences of the relative values characterizing the anomaly of T(T): o ~'0; z~ r l I1 ; O r l l 2; A rl II; • T(max )

alloying element atoms the sizes of which are close to those of the solvent atoms (diameter DAlOf AI atoms is 2.86, Zn atoms Dzn = 2.74 A), a(T) continuously rises with dropping temperature, though there is some reduction of hardening, do/dT, below ~ 15 K, as against higher temperatures. If aluminium is alloyed with Mg or Cu atoms, which differ in size from the solvent atoms (Dig = 3.20, Dcu = 2.54 A), then the deforming stress decreases at T < 15 K. In this case, the Ao value depends only on the absolute value of the size dissimilarity, 60, irrespective of whether alloying causes the AI lattice to contract (Cu) or expand (Mg) (Fig. 8). eg, A6o,2for the melt of AI-Mg is about 0.5 kg mm -2 , For AI Cu it is ~ 0.6 and Aoo,2/Oo,z(max) are 8 and 13%, respectively. With the deforming stress increasing, softening Ao/a(max ) decreases and is ~ 4 - 5% for aB. Neither a nor Aa have been found to be affected by the environment (to test this, AI-Mg polycrystals were deformed at 20 K under liquid hydrogen and in helium vapour). It is to be emphasized, that Fig. 8 is based on the assumption that Ao ~ e '/2 (because Ar ~ e v2 for AI-Mg single crystals). Thus, the alloying element type appreciably affects the anomalous softening magnitude.

Anomaly in o(T). Fig. 9 shows the temperature dependences of lengthening (6) for AI-Mg, AI-Cu and A I - Z n polycrystals and for aluminium. Table 3 lists the values characterizing the decrease of 6. As shown, 6 increases with

591

Table 2 Parameters of the anomaly of o(T) in single and polycrystals of A1-1.55 at % M9 alloy* T6(max), Aao, AOb. Ado/ AOb[ K kgmm -= kgmm -= ao(max), Ob(max ) % %

Material

3C-

~ ~ a

Single crystals 22

0.74

3.34

--~ 21

15

Polycrystals

0.5

1.9

8

5

10-15

* -- For single crystals, cr = r/k, where k ~ 0.5 is the orientation factor

Table 3

ro

i

0

2.0

i

L

40

60

r,K Fig. 6 Effect of crystallinity on the anomalous softening in alloy A I - 1 . 5 5 at. % Mg; z~single crystal; o polycrystal

40

35

Alloy material

T6 (max) ' 6(max ) K %

61,6, %

A6, %

6300 %

A~/ 8 (max)

AI-99.97 % polycrystal

40-45

68

50

18

53

-

AI-1.52 % Zn polycrystal 20-25

64

35

29

45

0.45

AI-I.0 % Cu polycrystal 25-30

53

25

28

28

0.53

A I - 1.4 % Mg polycrystal 30-35

57

29

28

30

0.5

A I - 1.4 % Mg single crystal

80-95

55-70

25

40-50

0.31 -0.26

25-30

t -- Aluminium was tested to 4.2 K t t -- For single crystals, 6 = e.k,'where e is the shear deformation, k ~ 0.5

30

10 K. Thus, at low temperatures aluminium alloys can display decreasing relative lengthening in addition to decreasing deforming stress.

25 E E

Discontinuous deformation. At temperatures below ~ 12 K,

.at

b" 20

0

40 60 T,K Fig. 7 Temperature dependences ot the y~eld stress, 60,2, and ultimate strength, a b, in comparison to those for 99.97 % AI. shows data for tests under liquid hydrogen (20.4 K)

20

decreasing temperature only to a certain T6(max). The absolute magnitude of the ~ decrease, A6 = 6(max) 1,6, was observed to be as high as ~ 30%. The temperature T~(max), below which there is an anomalous decrease of 6, being 25 - 30 K. These values are slightly sensitive to the alloying element type (at least, for the concentrations used). It will be noted, that Ta(max) is lower than T~ (max) by about

592

Decreaseof S values at low temperaturesf t t

plastic deformation of mono- and polycrystals is discontin. uous (Fig. 1), proceeding in asymmetrical jumps of the stress in work hardening curves. Low temperature plastic deformation was analyzed for AI-Mg single crystals and AI-Mg and A I - Z n polycrystals, and also for pure aluminium in ~a , the parameters being the strain magnitude, ecK, the discontinuous run start stress, rCK, the minimum stress, A~'min, the maximum stress Al"max o f the registered amplitude, and the jump number, N. We shall now consider the effects of various factors on the characteristics chosen.

Effect of temperature. In the temperature range where deformation is discontinuous, temperature decrease leads to lower ec~and rCK. For instance, for AI alloy containing 0.62 at. % Mg, a temperature drop from 4.2 to 1.6 K causes ecK to decrease from 75 - 80% to 65 - 70%, r c K from 7.5 7.8 to 6.4 - 6.8 kg mm -2 , the jump number rises from 45 - 50 to 180 - 200, and the jump amplitudes becoming smaller. For other concentrations, the general regularities of dicontinuity parameter behaviour at falling temperature are qualitatively similar.

CRYOGENICS. NOVEMBER 1978

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2O 0

v Ol04

Ol 0 8

Oil2

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Fig. 10 C o n c e n t r a t i o n dependences o f d e f o r m a t i o n , a -- eCK and stress b - TCK at the beginning o f d i s c o n t i n u o u s d e f o r m a t i o n in annealed A I - M g crystals at 1 -- 4.2 and 2 -- 1.6 K: - - - - - shows the y i e l d stress, ~'0, at 1 -- 4.2 and 2 - 1.6 K; o quenching; • ageing. T h e values f o r a l u m i n i u m were b o r r o w e d f r o m D i d e n k o = 3

Fig. 8 A n o m a l o u s s o f t e n i n g at the y i e l d stress in b i n a r y a l u m i n i u m alloys as a f u n c t i o n o f the a t o m size d i s s i m i l a r i t y , 50

70

6(3

Role of the alloying element concentration in an annealed crystal. In Fig. I0, the Mg concentration dependences of eCK (Fig. lOa) and rCK (Fig. 10b) are given. As the Mg concentration increases, the discontinuous deformation start stress becomes gradually lower so that from certain T's and c's it coincides with the yield stress, At 1.6 K, it does so for c = 5.52%, at 4.2 K for 9% Mg. The concentration range studied included both the single-phase region (crystals with 0.62, 1.55, and 1.97 at. %Mg), and twophase region (crystals with 3.78.5.52, and 9 at. % Mg). Transition from the single- to two-phase region (particle size in the/3-phase in alloys with 3.78 and 5.52 % Mg was 1 /am and in alloys with 9 at. % Mg it was 3 to 4/am) caused no qualitative changes in the concentration dependences of ec~ and TCK. The phase ratio does not affect (within the data point scatter) the general trend of ATtain and Armax. The jump amplitude does not decrease as a result of Mg concentration increase either. Thus, transition to the twophase region does not give rise to conditions which would prevent discontinuous deformation in an annealed crystal.

.o

50

r *~ 4 0

c , - A1(99,97%) • - AI - 1.52°/*Z.n 0 - AI - 1"4%Mg

Effect of quenching. The investigation of discontinuous deformation included experiments on alloys quenched from the annealling temperature 723 K in water. For quenching, a single-phase alloy crystal (0.62 at. % Mg) and a two-phase crystal (3.78 at. % Mg) were used. Deformation was performed at 4.2 K. Quenching of a single-phase crystal not only entails some quantitative changes in the work hardening curve (viz., increase of the yield stress and duration of the stage I), but also smoothes out the discontinuity (Fig. 11, Table 4), except for a few major jumps (AT -~ 4 kg mm -2)

• - AI - 1.0%Cu

2

0 0

~ 20

40

60

T,K Fig. 9 T e m p e r a t u r e dependences o f the relative lengthening 5 f o r alloys A I - - 1 . 4 % Mg, A I - - 1 . 0 % Cu and A I - - 1 . 5 2 % Zn c o m p a r e d to those f o r 99.97 % a l u m i n i u m @ tests in l i q u i d H

Table 4 Effect of heat treatment on parameters of low temperature discontinuous deformation in A I - 0 . 6 2 Mg and A I - 3 . 7 8 Mg crystals Jump amplitude* Number Mg at. %

Heat treatment

r o, I'CK , cCK , kg m m -2 kg m m -2 %

rmax, eTmax , kg m m -2 %

A1"1 ~'CK

ATrr~ Trnax

0.62

annealing

1.2

9.3

80

10.0

170

0.02

0.087

0.62

quenching

1.4

-

-

8.8

192

-

-

-

2

3.78

annealing

2.8

7.15

48

14.3

168

0.016

0.056

255

3

3.78

quenching

3.2

7.81

60

12.8

160

0.033

0.078

200

2

3.78

ageing

3.5

8.15

38

15.6

125

0.002

0.068

328

3

of

N

tests

48

3

* -- A~-~ is the a m p l i t u d e o f the f i r s t registered j u m p

C R Y O G E N I C S . N O V E M B E R 1978

593

O0 0000

._=801

~ O~ "-70~ T=4"2K

6 5~Al(e~

o!

0

,

i

t

, iO

i

IOO

Fig. 12 Crystal hardness against time of ageing at 453 K for A I - 3 . 7 8 Mg alloy, z~ is the annealed crystal hardness

_~kF mm - 2

Fig. 11

Effect o f thermal treatment on low temperature of aluminium single crystals with 0.62 at. % Mg (curves 1, 2) and with 3.78 at. % Mg: 1,3 - annealing, 2, 4 - quenching, 5 - - ageing. T = 4.2 K.

,%

deformation

at the failure moment. These continued to occur representing sample cross-section diminution as a result of developing constriction in some places, which gave rise to shear steps visible by an unaided eye. Two-phase crystal quenching also results in rising yield stress, a prolonged easy glide stage, shortened linear hardening stage and somewhat decreased discontinuity of the deformation (see Table 4, Fig. 11). Since after quenching from 723 K, the solid solution (oversaturated) is fixed in the two-phase crystal, the changes mentioned must be due to the same reasons as in case of single phase crystals.

Effect of ageing. This trial was applied to crystals with 3.78 at. % Mg ageing lasted for 20 hours at temperature of 452 K. It was controlled, as usual, by measuring hardness. The measurements suggest (Fig. 12), that the maximum effect of ageing was observed after a 20 hour exposure to a temperature of 453 K. Discontinuous deformation was studied on samples with the maximum ageing effect for this technique. Fig. 11 presents work hardening curve (5) for an aged crystal. Table 5 shows average values of some parameters. The ageing is seen to have caused an appreciable increase in yield stress, ultimate strength and decrease in the maximum deformation prior to failure; the jump number as well as discontinuity start stress have become higher, while the deformation corresponding to the first jump has reduced. Thus, quenching leads to smaller discontinuity at higher deformations, whereas ageing causes greater discontinuity under stresses closer to the yield stress. Effect of grain boundaries. A comparison of results (see Table 5) for single and polycrystals of AI-1.4 % Mg shows, that discontinuity is less developed in polycrystals. The smaller number of jumps and amplitudes (AOmin/OCK and AOmax/OB in polycrystais indicate that conditions unfavourable to discontinuous deformation, have developed, in agreement with detailed investigations of A1 single and polycrystals, 13 whose grain size ranged from 0.05 to 5 ram.

594

Fig. 13 Effect of Zn and Mg alloying on discontinuous deformation of aluminium

Thus, grain boundaries decrease low temperature deformation discontinuity.

Effect of size dissimilarity. Fig. 13 shows work hardening curves for A I - Z n and AI-Mg polycrystals in comparison to those for pure aluminium. At 4.2 K, there is no discontinuity in aluminium yet. Alloying of AI with zinc or magnesium results in onset of discontinuous deformation at higher temperatures, at about 12 to 15 K for the concentrations studied. In the discontinuous deformation temperature range, decreasing temperature leads, like in single crystals, to lower 6CK and oCK and more numerous jumps (see Table 5). However, the effects of alloying are not only quantitative changes in aluminium discontinuous deformation. There may be changes in the character of discontinuity, which depends on the alloying element type and size dissimilarity. For instance, if AI is doped with Zn, whose atoms have a very small size effect (60 = - 0.02), only one type of jumps is realized, the large one (Ao = 2.65 kg mm-~). Each jump in the o - 6 curve corresponds roughly to a single necking in the sample. In the case of alloying with Mg, the atoms of which give rise to a significant size effect (6 = 0.1), the o - 6 curve displays two jump types, one of them being large as those in A1-Zn (Ao = 2.32 kg ram-2), the other much smaller (Ao ~ 0.4 kg ram-2). Both in AI-Zn and A1-Mg alloys the number of large jumps roughly coincides with the number of neckings. Singlephase polycrystals generally have much fewer neckings than two-phase ones? In the meantime, single crystals of A1-(0.62 - 9.0) at. % Mg display neither more than one necking shape, nor the two types of jumps, though the amplitudes of the minimum and maximum jumps are very close to those in polycrystals. Therefore, minor stress jumps may be interpreted as being due to discontinuous slip inside a grain, while the large ones to the Luders deformation, for which grain boundaries are no bar. Thus, the atom size dissimilarity, 60, seems to be of great importance for the conditions of

CRYOGENICS. NOVEMBER 1978

Table 5

Effect of alloying on discontinuous deformation parameters in aluminium f it

Test

temperMaterial

ature

AI, 99.97 polycrystal

AO'min

00,2,

~b,

aCK, 8CK, kg mm -2 kg mm -2 kg rnm -2 %

4.2

3.41

30.5

AI-1.52 % Zn potycrystal 12 4.2

3.8 3.87

30.2 31.4

A1-1.4

6.86

38.8

6.61 3.56

12

% Mg polycrystal 4.2 Ai-1.4 % Mg

4.2

Aamin* A°max, oCK kg mm -= kg mm -2 %

8,%

1 8o -~da dc

0

54.1

0

2

58.3 50.2

-0.02 -0.02

3 3

AGmax o"b %

N

25.1 29.6

34.2 37.5

~0 2.65

0.29 2.65

~0 8.96

0.96 8.45

16 24

29.1 31.6 23.6 27.8

~0 1.83 0.37 2.32

0.27 3.04 0.88 3.95

0 5.2 1.2 10.1

0.7 7.9 2.31 10.5

6 3 13 4

44.2

0.1

38.0

32.3 35.3 30.9 32.9

40.8

0.1

23.6

16.6

30

0.4

0.92

2.4

4.0

81

~80

0.1

Number of tests

t -- F o r single crystals, o = r K, a = e K tt

-- F o r A I - - M g p o l y c r y s t a l s , parameters o f the m i n o r and large j u m p s are listed

avalanche-type dislocation movement in a solid solution in case of catastrophical disruption of barriers by dislocation clusters.

Metallography. Metallographic observations on surfaces of annealed, quenched and aged mono- and polycrystals, deformed at 4.2 K and then warmed to room temperature, showed that no preferential ship band localization takes place. Conclusion

A systematic investigation of single and polycrystals of A I - M g , A I - C u and A I - Z n alloys in 1.6 to 77.3 K temperature range revealed a decrease of the deforming stress o and relative lengthening 6 below a certain temperature T6(max) and established the relation of the said anomalies to the type and concentration of the alloying element and the role o f grain boundaries. Values of Ta(max) vary from 10 to 25 K for different alloys, Ts(max) ranging from 20 to 40 K. Softening, Ao/o(max ), at the yield stress may be as high as ~ 20% in single crystals and ~ 13% in polycrystals, but decreases with growing stress and alloying element concentration. The fall o f f below Ts(max) in polycrystals can be by about 30%. Values ofAo/o(max ) and A6/f(max ) depend also on the atom size dissimilarity, 60 = 1/a da/dc, decreasing as 6 0 increases. Significant features of low temperature plastic deformation are anomalies of o(T) and 6(T). We found that it was possible to make more efficient use of such promising cryogenic structural materials, as aluminium alloys, specifically magnalium, duralumin and more complicated alloys. A third peculiarity of low temperature plastic deformation is the discontinuous character of work hardening which, at a certain temperature and concentration of alloying element can start above the yield stress parameters of this effect (stress decrease in a j u m p of Ao, deformation 6o~ and discontinuous deformation start stress aCK) depend on the type, state and concentration of the alloying element. By varying thermal treatment parameters, one

CRYOGENICS. NOVEMBER 1978

can increase or decrease the discontinuous nature o f the deformation, shift it to higher deformations or suppress it. The dependence of the described peculiarities on the strength and concentration of obstacles for moving dislocations seems to be indicative of a common nature of the effects observed.

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