Effect of the Vanadium Addition on the Grain Size and Mechanical Properties of the Copper-Aluminium-Zinc Shape Memory Alloys K

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EFFECT OF THE VANADIUM ADDITION ON THE GRAIN SIZE AND MECHANICAL PROPERTIES OF THE COPPER-ALUMINIUM-ZINC SHAPE MEMORY ALLOYS K. Enami, N. Takimoto, S. Nenno

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K. Enami, N. Takimoto, S. Nenno. EFFECT OF THE VANADIUM ADDITION ON THE GRAIN SIZE AND MECHANICAL PROPERTIES OF THE COPPER-ALUMINIUM-ZINC SHAPE MEMORY ALLOYS. Journal de Physique Colloques, 1982, 43 (C4), pp.C4-773-C4-778. 10.1051/jphyscol:19824126. jpa-00222109

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EFFECT OF THE VANADIUM ADDITION ON THE GRAIN SIZE AND MECHANICAL PROPERTIES OF THE COPPER-ALUMINIUM-ZINC SHAPE MEMORY ALLOYS

K. Enami, N. Takimoto and S. Nenno

Department of Materials Science and Engineering, Osaka University, Suita, Osaka, Japan

(Revised text accepted 27 September 1982)

Abstract. - The effects of vanadium addition on the 0-grain size, pseudo- elastic and shape memory behaviour and fracture mode of the copper-aluminium- zinc shape memory alloys with different vanadium contents were investigated. It was found that the vanadium addition is very effective in reducing 0-grain size. In the specimen hot-rolled and recrystallized for several ten minutes at 1073K, the maximum mean grain size was only 300 pm. When the annealing time is shorter, i.e. a few minutes, it reaches 200 pm. The minimum 6-grain size 100 to 150 of the specimen in the recrystallized state was obtained by combination of vanadium addition and cold-rolling. The recoverable pseud* elastic strain of the specimen having four-five grains along its thickness was found to be up to 5%, and recoverable shape memory strain up to 5.5%, both are near the mean theoretical values of polycrystalline materials. It was found that grain boundary cracking, which is usually observed in copper- base 0-brass alloys and is considered to be the most undesirable failure of these alloys, was remarkably suppressed by vanadium addition.

Introduction.- Although the copper-aluminium-based shape memory alloys have technical and economical advantages over other shape memory alloys, there are still some important problems which must be solved in order to develop more stable andreliable shape memory aluminium-bronze. Among them,the large B-grain size (typically its diameter is 0.5 to 2 mm) and tendency of grain boundary cracking, both typically observed in copper-base B-brass type alloys, have undesirable effects on mechanical properties and shape memory behaviour of these alloys. The difficulty in reducing 0-grain size limits the technically applicable range of those shape memory alloys. Since Khan and Delaey (1) first suggested that less-soluble alloying elements in copper-base 0-brass alloys, such as iron, would be effective inhibitors of 0-grain growth, several works have been reported on the effects of the 3rd or 4th alloying elements on 0-grain size and mechanical properties (2,3). According to the copper- aluminium-vanadium phase diagram (4,5), it is expected that vanadium is also one of those elements which are less-soluble in copper-aluminium 0-phase. In the present work the effects of vanadium addition on the 0-grain size, pseudo-elastic and shape memory behaviour, the tensile fracture mode in the copper-aluminium-zinc B-brass alloys were investigated, and it was found that the vanadium addition is very effective in reducing the 0-grain size and also the tendency of intergranular cracking, without lowering shape memory properties.

Experimental Procedures.- Ingots withapproximate size 18mm x 150mm of seven Cu-Al-Zn alloys with different vanadium contents (from about 0.2 to 2.0 mass%) were melted in vacuum-sealed quartz capsules. The chemical compositions and transforma- tion temperatures found by electrical resistivity measurement are shown in Tables 1 and 2, respectively. Ingots of these alloys were hot-forged slightly and hot-rolled at about 900K to 1.5 to 2mm thick. Strips for the tensile testing were cut out from the hot-rolled sheets, mechanically polished and finished by electropolishing to lmm thick x 3mm wide x 50mm long to remove any surface failure. All specimens were then annealed in quartz capsules under argon atmosphere at 973K or 1073K from a few tens to a few thousands seconds followed by water-quenching. Optical microscope

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19824126 C4-774 JOURSAL Dic PHYSIQUE

Table 1. Chemical Composition of Alloys, Tab lc 2. 'l'ransfor~nation'femperatures (K) . (mass%). Al* Zn 1' * Cu Ms As Af alloy': A- 1 8.5 2.4 0.42 bal. above 373 U- 1 7.2 17.8 0.37 bal. 213 208 223 B- 2 7.0 17.9 1.83 bal. 262 253 271 C- 1 6.1 21.3 0.29 bal. 231 220 242 C-2 6.0 21.7 0.55 bal. 215 206 224 D- I 2.6 31.2 0.15 bal. 221 210 229 D- 2 2.6 34.9 1.75 bal. below 77 - " by ICI' method. observation on the whole surface of each tensile specimen was carried out to mcasure the 6-grain size. The 6-grain size was measured by the usual linear interccpt ncthod. Tensile test was carried out in each spccimcn between liquid nitrogen temperature and room tempcraturc. Fractographic study was carried out to obscrvc the tensile fracture surface by scanning clcctron microscopy.

-Results and Discussion. 1. 0-grain size Fig.1 shows the typical fine-grained polycrystalline specimens of alloys B-1 and C-2, both were hot-rolled at 900K and recrystallized at 1073K for 1.8ks, followed by water-quenching. l'hc avcrage grain size of B-1 was found to be 260 Um. Tt was observed that across the thickness of the tensilc specimen 111m thick there are four-five grains, and the a-grains tiavc alniost cqui-axed shapc. It seems that 13-grain size scarecely varies with thc concentration of vanariur~in the alloys used in the prcsent experiment.

IFig.1 Optical micrographs of rccrystall izcd speci~r~cnsoC vanadium-addcd Cu-Al-Zn f3-brass type alloys. (a) Alloy B-1, hot-rollcd at 900K and recrystallized at 1073K for 1.8 ks, followed by water-quenching. (b) Alloy C-2, as above.

Fig.2 shows the feature of 6-grain growth of alloy 13-1 (luring rccrystalli- zation at 1073K. It is striking that the 6-grain gowth is effectively retarted hy v;inadium addition and the size of 6-grains never grows over 300 um even after recrystallizing for 1.92 ks, Fig.2(d).

Itlien thc specin~ensizc is within 21nm thick x lOmm wide, it was found that the present alloys can bc cold-rolled at room tempcr;~turcup to 10% in the 6-phase state and to 20% in the martensitic state without failure. Thus, it is possible to obtain even finer 6-grains. Fig.3 shows the effect of cold work on the 6-grain sizc rcducing in thc rccrystallized statc. Fig.3(a) is the original as-recrystallized state and shows relatively large grain sizc with tli:imeter 200 to 250 urn of alloy D-2. Fig.3(1?) shows the spccirllen of the same tilloy after being cold-rolled at r.t. and recryst;rllizcd at 973K for 900 s. Thc 6-grain sizc in this case is reduced to 100 Urn. In (c), the case of alloy A-1, cold-rolled in the martensitic statc, is shown. In this case also, the 6-grain size is reduced to near 100 Urn. b. . c., i '* - :v ...... <. ' -

1:ig.L Optical micrographs showing thc effect of annealing time on the 3-grain size in alloy B-1. (a) Annealed :it 1073K for 60s in ;I salt bath, followed by watcr- quenching. (h) for 120s. (c) for 900s. (d) for 1.92ks.

Fjg.3 0ptic:ll micrographs shoving the effect of cold-rolling on the @-grain size. (:I! Ilot-rolled and recrystallized at 973K for 900s, Alloy D-2. (b) as above + cold-rolling at r.t. and recrystallizcd at 973K for 900s. (c) Alloy A-1, as above (cold-rolled in the martensitic state).

Usually the @-grain size in copper-hase B-brass type alloy is 0.5 to a few mm ;~ndto obtainc the grain size lcss than 500 Dm, annealing time should he limited to a few tens of seconds :it the recryst;illizntion temperature, while in the present vanadi~~m-addedalloys the grain size 200 to 300 um can be obtained by ~lsualanncalinz treatment for a few ks, without any special heat- or ther~nonlcchanical treatment. 'The above ohserv:ltion suggests that the vanadium addition reduces effectively the 6-grain size or copper-aluminium-b:tsed B-hrass alloys.

Kilmei et :11. (3) showed that vanadiun~ has a retardation effect against grain grokth of Cu-Al-hi high damping 6-phase cast alloys, and sr~ggestcdthat the effect isdue to suppress i on of grain 1,oundat-y migrat i on by reducing the diffusion coefficient by dissolving of vsnadi trn~ into B-phase. Ilowever, it is clearly scen in the present experiment that vanadium is rattler lcss- soluble in the @-phase and makes fine precipitates in the B-grain, 1:igs.l to 3.

-- In :iddition to those optical micro- scopic scale l~recipitatcs,it was found hy Fig. 4 TEM j mage of vanadium-added transmission clectron microscopy that nruch Cu-!\I-7n alloy, alloy 11-1, showing finer precipitates, iqitt~size about 50 nnr, fine vanadium precipi tates in exist within the %-grains. I-lowcvcr, B-phasc. JOURNAL DE PHYSIQUE

no clear evidence of the precipitation at grain boundaries was obtained by TEM. XMA experiment revealed that the precipitates found in the optical micrographs are vanadium-rich aV-Alphase. Therefore, from all the above observations, it is natural to consider that the 6-grain size reduction is due to a dispersion effect of the precipitates found in the OM or TEM, at least in the alloys used here.

2. Pseudo-elasticity and SME.

Fig.5 Stress-strain curves for alloy B-2, deformed by tension at 29SK (above Af) .

Fig.6 Stress-strain curves for alloy D-1, deformed by tension at 233K (above Af), (a) and (b) , at 183K (below Mf) , (c) .

Figs.5 and 6 show the typical tensilc bchaviour of the present vanadium-added Cu-Al-Zn '6-brass alloys. Fig.5 shows the stress-strain curves of alloy B-2 tested at 295K by tension (e in all cases was about 0.017/min). In this alloy small amount of plastic strain (about 0.17%) remains after unloading the tensile stress, (a). The residual strain increases with increasing applied strain, (b) and reaches about 0.7% when the applied strain is over 6%, (c). It was usually observed that the residual strain reaches zero when the applied strain or stress in the succesive deformation cycle is lower than that of the immediately previous one. The maximum recoverable pseudo-elastic strain was found to be about 5%. Figs.6 (a) and (b) are the stress-strain curves of alloy D-1. Apparently the alloy is mechanically softer than the above alloy B-2. The mean difference between these results is the differ ence between Ttest. and Af, i.e. A(Ttest. - Af), in both alloys. Within 3% applicd strain pseudo-elastic strain recovers almost completely when unloading the tensile stress, (a) and about 0.4% strain remains after being clongated over 4%, (b). h'hen the specimen was deformed below Mf, it showcd usual shape memory effect, (c). It was found that the maximum shape memory strain reached 5.5%. The pseudo-elastic and shape memory behaviour shown above was commonly observed in the other alloys used in the present experimcnt and it was found that the vanadium addtion does not cause any negative effect on shape memory properties. Both pseudo-elastic and shape memory strains obtained in the prcscnt investigation are near the average theoretical values of polycrystalline materials (6).

3. Fractography. Fig.7 shows the scanning electron micrographs of the tensile fracture surface of alloys B-1 and D-1. The remarkable effect of vanadium addition on the Cu-Al-Zn 6-phase alloys appeared in the tensile fracture mode of these alloys. In the present experiment, all alloys showed almost transgranular type fracture when deformed either in the 6-phase state or in thc martensitic statc. Figs. 7(a) and (b) are the SE micrographs of the tensile fracture surface of alloy 8-1, deformed at 235K (6-phase). It can bc clearly seen that the fracture mode is entirely transgranular and there are some areas showing dimple patterns in the enlarged photograph (b). When the specimen R-1 was fractured at liquid nitrogen temperature (martensitic state), therc are a few intergranular fracture areas, while most of the fracture is still transgranular, (c) and (d). In alloy D-1, cven when the specimen was deformed Fig.7 SEM images of the tensile fracture surface of the vanadium-added Cu-Al-Zn alloys. (a) and (b), alloy B-1, fractured at 235K (above Af) , fracture strain 7.8%. (c) and (d), alloy B-1, fractured at 77K (below Mf), fracture strain 6.6%. (e) and (f), alloy D-1, fractured at 77K (below Mf), fracture strain 10.1%. and fractured at liquid nitrogen temperature (martensitic state), there is no intergranular fracture, (e) and typical dimple patterns are seen in some areas, in the enlarged photograph (f) .

In the polycrystalline specimen of the copper-base B-brass type shape memory alloys, the fracture mode is usually intergranular and this leads to the low mechanical stability, low fatigue life and is the main shortcoming of this type alloy for the practical technical use. However, in the present vanadium-added Cu-A1 Zn shape memory alloys, most of the fracture mode is transgranular and sometimes dimple patterns are observed. Therefore, it is expected that the vanadium addition is one of the effective and useful method to develop good shape memory materials based on copper-aluminium bronze. JOURNAL DE PHYSIQUE

References 1. A.Q.Khan and L.Delaey: Z.Metallkde., g(1969), 949. 2. H.Warlimont and L.Delaey: Progress in Materials Science, vo1.18(1974), Pergamon Press, Oxford. 3. K.Kamei, K.Sugimoto, H.Matsumoto, T.Sugimoto and T.Irisawa: Shindo Gijutsu Kenkyukaishi, g(-1981), 260, (in Japanese). 4. R.Aravamudham and S.Kokrae1: Z.Metallkde., E(1965), 99. 5. "Binary and Multicomponent Systems based on Copper", M.E.Dritz ed., Nauka Pub., Moscow, 1979, (in Russian) . 6. T.Saburi and S.Nenno: Proc. International Conf. on "Solid-Solid Phase Trans- formations", Pittsburgh, 1981, in press.