E L S E V I E R Materials Science and Engineering A214 (1996) 177-180

MATERIALS SCIENCE &

ENGINEERING

A

Letter

Threshold stresses in high temperature deformation of dispersion strengthened aluminum alloys

Fernando Carrefio 1, Oscar A. Ruano* Departmem of Physical Metallurgy, CENIM, C.S.I.C., Av. de Gregorio del Amo 8, 28040 Madrid, Spa#z

Received 19 March 1996; revised 30 April 1996

Abstract

Creep data of five dispersion-strengthened aluminum materials were analyzed by means of well known diffusion controlled slip creep mechanisms incorporating a threshold stress. It is shown that the threshold stress that has to be introduced into such mechanisms to fit the creep data is strongly temperature dependent. Such temperature dependence cannot be explained by the various models used in the literature.

Keywords: Dispersion strengthened materials: Aluminum alloys; Threshold stress; Creep behavior

1. Introduction

The high temperature deformation behavior of dis- persion strengthened materials is characterized by a high stress exponent n and a high activation energy for creep, (2. Experimentally, these two creep parame- ters are usually determined from the relation [1,2]

~= A e x p ( - Q ~((r ~" -~--~,) \~] (1)

where D is the steady state strain rate, A is a material constant, R is the gas constant, T is the absolute temperature, ~ is the creep stress, and E is Young's modulus. The steady creep rate for a large number of polycrystalline pure metals, including aluminium, and also for many other alloy systems tested at high tem- perature have been observed to be associated with n = 5 and 0 = QL, where OL is the activation energy for lattice self-diffusion [1-4]. If the structure remains constant during the creep test, an equation similar to Eq. (1) with a substructure term and n = 8 has been commonly used to describe the creep behavior of many dispersion strengthened materials [5,6].

:¢ cr Correspondin~ author. 1 Present address: Department of IVlaterials Engineering, University

of Wales, Swansea, Singleton Park, Swansea SA2 8PP UK.

The creep data of dispersion strengthened materials are often interpreted on the basis of the contribution of a threshold stress which must be overcome in order for dislocations to pass over the obstacles represented by the dispersoids. The introduction of this threshold stress in a creep equation, such as Eq. (1), in the stress term as [(~-~o/E)]", causes a decrease in the values of n and Q allowing the use of established creep relations to model the creep behavior of these materials.

The aim of this study is to analyse the creep data of five dispersion-strengthened aluminum materials in light of diffusion controlled slip creep mechanisms in- corporating a threshold stress, under constant and non-constant structure conditions, to gain some in- sight on the temperature dependence of the threshold stress.

Table 1 Abbreviated denomination of the materials

Composition (mass %) Abbreviated denomination

AI-7.6Fe- 1.2Cr- 1.5Si AI- 7.6Fe- i .2Mn- I. 5Si A1- 7.6Fe- 1.2Mo- 1.5Si AI-8.5Fe- 1.3V- 1.7Si

AFCr-24% AFMn-24% AFMo-24% AFV-27%

178 F. Carrefio, O.A. Ruano / Materials Science and Engineering A214 (1996) 177-I80

Table 2 Experimental tensile tests data together with values of nap, Qap and %, for n = 5 and n = 8, at various temperatures for the five materials investigated

T cy (10-2 s -1) O" (10-3 S - I ) O" (I0-4 S - I ) CY (10-5 S - t ) O" (2,5X10-6 S - I ) /lap Q~p Cro (n=5) % (n=8) (K) (MPa) (MPa) (MPa) (MPa) (MPa) (kJ tool- ~) (MPa) (MPa)

Extruded AFV-27% 481 319 306 289 271 262 42 64 256 235 523 311 290 273 254 244 34 171 235 211 576 262 241 222 201 190 26 228 180 155 623 223 202 184 164 153 22 251 144 I19 674 I72 I54 141 I3I i24 26 365 116 99 725 135 122 112 I01 97 25 451 90 77 774 101 92 83 75 72 24 447 67 56 827 82 73 67 61 56 23 352 53 44

Extruded AFCr-24% 576 294 257 229 199 180 17 292 165 126 630 210 183 162 136 123 16 277 111 81 674 170 148 126 102 87 12 301 78 48 725 111 94 78 65 59 i3 371 50 31 775 82 69 57 45 39 I1 452 33 17 824 43 36 31 26 22 13 747 20 12

Extruded AFMn-24% 576 267 231 201 167 15 204 143 101 624 207 179 154 129 115 14 258 102 69 676 155 130 107 83 70 11 257 59 29 724 105 88 72 59 52 12 339 43 24 775 68 56 47 39 13 491 31 19 822 40 34 29 25 22 14 675 20 14

Extruded AFMo-24% 575 291 246 214 187 16 212 157 113 620 220 188 171 150 18 284 131 101 672 170 144 124 15 306 95 65 728 115 104 90 73 63 13 376 58 39 773 85 74 60 46 11 472 38 21 823 48 4i 32 26 23 11 598 19 10

RolIed AFV-27% (10 -6 s -~) 523 302 273 255 225 24 139 208 176 573 253 228 206 185 167 23 238 160 132 623 199 177 158 142 127 21 301 120 97 673 148 133 121 110 96 22 419 93 77 723 I09 99 88 76 64 17 491 63 48 773 76 67 57 47 40 14 650 37 25 823 43 36 29 25 21 13 773 18 I1

o f the al loys are given in Table 1. Al loy denomina t ions are given by the init ials o f the elements present in the

mater ia ls , assuming that all of them conta in silicon.

The percentage fol lowing those initials cor responds to

the volume f rac t ion o f d ispersoids in the mater ia l . The

mater ia ls , ob ta ined at Al l ied-Signal l abora to r ies by

rap id solidification, powder meta l lurgy and s t anda rd

process ing routes, have been descr ibed elsewhere [7].

Al l mater ia l s were ex t ruded to ob ta in bars o f 89 m m in

diameter . One of the mater ia ls , denomina t ed " 'rolled

A F V - 2 7 % " was, in addi t ion , hot rol led perpendicu la r ly to the ext rus ion direct ion.

Tensile tests were carr ied out on flat or cyl indrical

dogbone tensile samples mechanized para l le l to the

rol l ing or ext rus ion direct ion in o rde r to s tudy the creep

behav ior of the mater ials .

Specimens were tested at bo th cons tan t s train rate

and with strain rate changes dur ing de format ion . The

t empera tu res ranged f rom 481 to 823 K, and the s train rates ranged f rom 10 . 6 to 10 . 2 s -1.

F. Carrego, O.A. Ruano /Materials Science and Engineering A214 (1996) 177-180 I79 /

3 . R e s u l t s a n d d i s c u s s i o n

The values of ~ and a at the various testing tempera- tures as well as the values of the apparent stress expo- nent, n~p, and the apparent activation energy for creep, O~p, obtained from creep data are given in Table 2. The table also contains values of the threshold stress, a0, calculated by plotting the creep data as ~1/5 and ~1/8 versus o- on linear scales at the various temperatures as is often done in the literature [8-16]. The exponents 1/5 and 1/8 correspond to n values of 5 and 8 respectively. These stress exponents correspond to well known slip creep relations described in the introduction.

Fig. l(a) and (b) show a representation of the threshold stress values given in Table 2, compensated by the Young's modulus, as a function of the tempera- ture assuming a stress exponent of 5 and 8, respectively. It is shown that the threshold stress, for both stress exponents, is strongly temperature dependent and con- tinuously decreases with increasing temperature.

In order to predict the high temperature behavior of these materials, it is important to know the dependence of cr 0 with T. Mohamed et al. [10,13-16] analyzed the data of various reinforced and superplastic alloys by

/ ' " ~ ' " extruded AFV-27% 4 10 .3 1:]. ~ ~ exlnMed AFCr-24%

• '~ l - - o - extruded AFMn-24% 4 , / ~ extruded AFMo-24%

3 10 "3 ' ~ , , , for n=5

tz '~ ¢~ ~ z ,

2 10 .3 % "~,

% ~ - . . 1 10"3 a)

0 t l l r l t l l l r r l l l l r , I . . . . I . . . . t , , ,111~ I 450 500 550 600 650 700 750 800 850

T , K

510"3 " ' 1 . . . . ~ . . . . i . . . . I . . . . i . . . . I . . . . I ' "

O rolled AFV-27% - - ~ - - extruded AFV-27%

4 10 .3 ~ extruded AFCr-24% / - < ' - extruded AFMn-24%

rn•. ~ , [ --.N-.-- extruded AFMo-24%

r~ 3 10 -3 ~ for n=8 - % e° 2 10 .3 % r - L

1 10 -3 ~"'C3

b) ¢ ' - ~ 0 , , i . . . . F . . . . I l l t t l , , , , I , , , , I . . . . I , , , 450 500 550 600 650 700 750 800 850

T , K

Fig. I. ModuIus-compensated threshold stress as a function of tem- perature for the various dispersion strengthened aluminum alioys investigated. For determination of the threshold stress it was as- sumed, (a) n = 5 and (b) n = 8.

510 .3 ~ , l , , , ~ 4 , , , , i , , , , i , , , , a , , , , i , , , , i , , ,

C roiled AFV-27% . --,C]-- extruded AFV-27%

4 10 .3 ~ exmJded AFCr-24% - - ~ - extruded AFMn-24%

r n . . + extruded AFMo-24%

'%t 3 10 .3 for n=8 - % e° 2 10 .3 ""i2%

I 10 .3

0 ,b,)r ~,

450 500 550 600 650 700 750 800 850

T, K

Fig. 2. Activation energy for the threshold stress as a function of temperature.

means of a threshold stress with a temperature depen- dence given by

ex [Q~o~ cr°= B0 p | - = = / (2) E \ /< l /

where B0 is a constant and Q~o has the meaning of an activation energy for the threshold stress. This activa- tion energy for our materials was determined from Table 2 using the relation

O~o = R (3)

Fig. 2 is a representation of Q~o as a function of temperature for a stress exponent of n = 5 for our five dispersion strengthened materials. A similar curve is obtained for n--8. The values of Q~o strongly vary from approximately 3 kJ mol-1 at 481 K to approxi- mately 68 kJ mol- i at 823 K. Therefore, a single value of Qo0, as required by the concept of Mohamed et al., does not describe the creep behavior of these materials.

Another approach to understanding the high temper- ature creep behavior of dispersion strengthened materi- als is that of Mishra and Pandey [8]. These authors use a dependence of the threshold stress with the tempera- ture given by

C' ~0 = - C + - - (4 )

T

where C and C' are constants. This dependence of ~r 0 with temperature is much too weak to describe the creep behavior of our materials. Furthermore, these authors attributed such temperature dependence to a change in Young's modulus with temperature which is far from true in our materials as shown in Fig. l(a) or (b).

Another approach is that given by Gonzfilez-Doncel and Sherby [11]. These authors use a constant structure

180 F. Carrei~o. O.A. Ruano /Materials Scie~zee and Enghwerhzg A214 (1996) 177-180

equation with ~z = 8 and a threshold stress for analyses of the creep data of AI-SiC composites. This is an interesting approach since it permits the prediction of the creep behavior by considering two additive contri- butions to the flow stress, a thermally activated term and an athermal term. They obtain a linear dependence of ~0 versus T up to a critical temperature above which no threshold stress is present. Tl'~is temperature was approximately 743 K for their materials. However, in relation to our materials, as shown in Fig. l(b), the va!ues of ~0 continuously decrease with temperature, even at high temperatures, and a temperature beyond which ~0 vanishes is never reached.

In conclusion, the models more often used in the literature to explain the creep behavior of dispersion strengthened materials that include a threshold stress are unable to describe the creep behavior of our disper- sion strengthened A1-Fe-X-Si materials.

The creep results of the five dispersion strengthened materials were also compared with predictions from the detachment model [17]. This model is inappropriate to predict these results [7].

A model considering the dislocation-precipitate inter- action has been recently developed [7]. This model is being currently evaluated using dispersion strengthened materials to determine its predictive capability.

Acknowledgements

The authors gratefully acknowledge the support of

the CICYT (Grant MAT89/0554). In addition, a British-Spanish Joint Research Programme and a Hu- man Capital and Mobility Fellowship is sincerely ap- preciated.

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