Precipitation behavior of a CuAlNi shape memory alloy at elevated temperatures

Precipitation behavior of a CuAlNi shape memory alloy at elevated temperatures

Scripta METALLURGICA Vol. 19, pp. 231-234, 1985 Printed in the U.S.A. Pergamon Press Ltd. All rights reserved PRECIPITATION BEHAVIOR OF A CU-AL-NI ...

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Scripta METALLURGICA

Vol. 19, pp. 231-234, 1985 Printed in the U.S.A.

Pergamon Press Ltd. All rights reserved

PRECIPITATION BEHAVIOR OF A CU-AL-NI SHAPEMEMORYALLOY AT ELEVATED TEMPERATURES Jogender Singh, Haydn Chen and C. M. Wayman Department of Metallurgy and Mining Engineering and Materials Research Laboratory University of I l l i n o i s at Urbana-Champaign Urbana, I l l i n o i s 61801

CReceived December 3, 1984)

Introduction There have been several investigations of thermal aging effects in copper base shape memory alloys. (1-5) Most e a r l i e r investigations in the Cu-AI-Ni alloy system were carried out below 400°C and were concerned with the precipitation ofyj-phase in the matrix. (1-3) In a recent study Singh et a l . (6) observed the sequence of transformations in the as-quenched Cu-14%AI-4%Ni (wt.%) alloy to be 2H ÷ DO3 + yp during aging up to 500°C. In the present report we w i l l concentrate on the growth behavior of the a-phase (F.C.C.) during prolonged annealing at or above 500°C. Experimental Procedures A single crystal of composition Cu-14Al-4Ni (wt.%) alloy was prepared using the Bridgeman technique. Slices were cut from the crystal with [001] surface orientation. After mechanical and chemical polishing to a thickness of O.3mm, 3mm discs were punched out. The samples were then homogenized at 850°C for 48 hours in evacuated quartz capsules and rapidly quenched into iced water by breaking the capsule after contact with water in order to avoid precipitation of high temperature phases. The as-quenched discs were electropolished at room temperature using a conventional j e t polishing technique with an electrolyte containing phosphoric acid and water. Heat treatments and in situ observations were carried out for the as-quenched specimens using the hot stage of a Hitachi H500 and Philips 420 (attached with EDAX) microscopes operated at 125 and 120 KV respectively. Specimens were aged up to 600°C. The heating stage could be t i l t e d to ±60 degrees about a single axis. Someof the samples were examined in the microscope at room temperature using a ±60 degree t i l t , 360° rotation gonimeter. Results and Discussion The as-quenched alloy shows APB's in the matrix (Fig. I ) . (The early stage transformation in this ternary alloy w i l l be published elsewhere (6).) The as-quenched specimen was i n i t i a l l y heated to 500% and kept there for 2-3 hours. The continuous growth of theY2-phase was observed. I t was found by STEM x-ray microanalysis that they,-phase is rich in Al solute content as compared with the remaining matrix (6). On further agihg between 500550°C for short times (< 30 min), development of strain contrast was seen in the matrix along with the growth of y~-phase, (Fig. 2). Aging for a longer period (about I hour) at the same temperatures reveal e~ spherical shaped precipitates (Fig. 3). These precipitates have been identified as :-phase having a F.C.C. structure; the matrix is the B-phase having a B.C.C. structure. Preferred growth of thea-phase precipitates along the <022> direction of the matrix suggests that certain crystallographic orientation relationship exists between the two phases. Upon aging for more than 1 hour at the same temperature, the a-phase coarsens in the form of interconnected rods (dark elongated particles in Fig. 4). In addition, the coarsening and alignment of the~-phase appeared to take place about 0.2 um away from theyg-phase interface (Fig. 5). On coupling Fig. 1-4, it is suggested that during the growth ofy~-phase, partitioning of Cu and Ni atoms takes place from theY2-phase towards the remaTning matrix. When the matrix reached supersaturated conditions, the a-phase would be

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precipitated out. In addition, these micrographs also suggest the long range redistribution of Cu and Ni atoms in the matrix, away From the interface of theyp-phase. I t was found by STEM x-ray microanalysis that the~-phase is rich in Ni and Al ato~ms and that the depleted matrix contains mainly Cu atoms with s m a l ] amounts of Ni and Al. Once the~-precipitates reached equilibrium on aging at 500-550°C for a longer period (> 2h), these precipitates coarsen further by coalescing into long rods as shown in Fig. 5 and 6. The dark field image (Fig. 6) also shows that the a-phase has some crystallographic relationship with theyp-phase. From the diffraction pattern analysis, the orientation relationships of they~ ands-phases to theB-depleted matrix are obtained as follows: (llO)y2 II ( 0 2 0 ) I I [I12]y2 If [ l O l l

(Oil) B

II [ I I [ ] B

During the course of this investigation, i t was found that the~-precipitates redissolved in the matrix above 600°C whereas they,-phase was s t i l l present. Therefore,this temperature may be considered as a solvus boundary ~for ~-precipitation in this ternary alloy. Consequently, the above investigations concerning the growth behavior of rhea precipitates were a l l carried out below 550°C. On cooling the alloy from 550°C to room temperature, small DO3 domains were observed only in the depleted matrix as shown in Fig. 7. The size of the DO3 domains is less than 2mn. The precipitation behavior of the Cu-AI-Ni alloy above 500°C may be explained with the aid of Fig. 8 and a pseudo binary Cu-Al phase diagram such as the one constructed by Dvorack et a l . (7). The precipitation and growth of the yp-phase, which was rich in Al solute, upon heating to 500°C would cause the composition of thee remaining matrix to become deficient in Al, i . e . , shifting from point A to point B in Fig. 8. Once the remaining matrix becomes supersaturated with excess Cu and Ni atoms,the ~-phase begins to be precipitated out which, in turn, leaves the matrix high in A] content. The matrix concentration w i l l then move from point B back to C Fig. 8. The composition of the remaining matrix might now l i e in the B-phase region. The B-phase with the B.C.C. structure would then transform to DO3 phase upon cooling from 550°C to room temperature. Conclusion On aging the ternary Cu-14%Al-4%Ni alloy between 500-550°C, precipitation of the~-phase was observed. The growth and alignment of ~-phase took place away from the interface of they,-phase suggesting the long range redistribution of Cu and Al atoms in the matrix. The~-pha~-e is rich in Ni and Cu atoms compared with the depleted matrix. The solvus line for the~-phase is about 600°C. DO3 domains appear in the depleted matrix during cooling from 550°C to room temperature. P

AcknowledQements This work was p a r t i a l l y supported by the National Science Foundation through the Materials Research Laboratory at the University of I l l i n o i s , Grant NSF/DMR 83-16981. A portion of the TEM experiments was conducted at the University of I l l i n o i s Materials Research Laboratory's Center for Microanalysis of Materials which is supported by the Materials Research Division of the Department of Energy. 1. 2. 3. 4. 5. 6. 7.

Referenc~ N. Kumano and C. M. Wayman, Met. Trans., 15A (1984) 621. M. A. Dvorack and H. Chen, Scripta Met., 1--7--(1983) 131. N. F. Kennon, D. P. Dunne and L. Middleto-~, Met. Trans., 13A (1982) 551. Miroshi Kuboo, A. Miyake and K. Shimizu, Trans. J.I.M., 24---(-a) (1983) 603. N. Kuwano and C. M. Wayman, Trans. J.I.M., 2_4_4(8) (1983)~61. Jogender Singh, H. Chen and C. M. Wayman, Acta Met. (1984) submitted. M. A. Dvorack, N. Kuwano, S. Polar, H. Chen and C. M. Wayman, Scripta Met., I_.7.7(1983) 1333.

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Fig. 1. Electron micrograph of as-quenched sample showing the anti phase boundaries in the matrix. Fig. 2. Electron micrograph taken at 525°C for about 30 min showing the development of the strain in the matrix along with the growth of Y2phase. Fig. 3. DF taken at 550°C for specimen aged at 550°C for I hr showing the precipitation of a-phase. Fig. 4. BF taken at 550°C for specimen aged at 550°C (~ 2 hours) showing the coarsening and alignment of the ~-phase. Fig. 5. DF taken at 550°C for specimen aged at 550°C (~ 3 hours) showing further coarsening and coalescing of the e-Phase.

Fig. 6.

Electron micrograph taken at room temperature f o r specimen aged at ~ 550°C f o r 3 hours showing precipitation of T?, and a-phase together (a) BF (b) DF and (c) corresponding d i f f r a c t i o n pattern.

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Fig. 7. Electron micrographstakenat roomtemperaturefor specimenaged at 550°C (~ 3 hours) (a) showingthe precipitation of s-phase. (b) showingthe precipitation of DORphase. (c) correspondingdiffraction pattern.

Ni

Precipitation of / /

dePleted matrix I I ' ~

/ Cu

/

~ ~%

,i~

%~ m a i n i n g M a t r i x

I

"\

C'| ~

"~ A / ~

~ 's r'ch `n A1 atOms Al

!

formation of DO3 phase or domains Fig. 8. Schematicrepresentation of hypothetical ternary phase diagram. For moredetails, see the text.