ISSN 1067�8212, Russian Journal of Non�Ferrous Metals, 2010, Vol. 51, No. 1, pp. 59–61. © Allerton Press, Inc., 2010.

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1 1. INTRODUCTION

Severe plastic deformation (SPD) is a hopeful pro�cess to fabricate bulky materials with desirablemechanical properties. The most famous and success�ful SPD processes are the equal channel angular press�ing (ECAP) [1], high pressure torsion (HPT) [2],accumulative roll�bonding (ARB) [3] and repetitivecorrugation and straightening (RCS) [4–6]. The prin�ciple of SPD includes increasing dislocation densityby heavily deforming materials with uniformity, for�mation of dense dislocation walls and transformingdislocation walls into high angle grain boundaries(HAGBs) [7]. At the low strain rates and strain percycle of process, only the density of dislocationincreases and the size of subgrains decrease. In thisstate, the suitable condition to transform the disloca�tion walls into HAGBs is not provided and as a result,the strength improves in medium values [7, 8]. In theRCS process, a work piece is repetitively bent andstraightened without significantly changing the cross�section geometry which large plastic strains areimparted into the materials [4–6].

Duratherm 600 belongs to the group of age�harden�able CoNiCr alloys whose special features are excel�lent spring properties, high corrosion resistance andthermal load ability. By cold�working of the strip orwire, their spring properties can be even furtherenhanced. The spring elements attain maximummechanical properties through subsequent aging.Duratherm 600 sheet has difficult availability in manycountries. Sometimes these materials are availableonly in annealed condition with special thickness that

1 The article is published in the original.

it is impossible to apply cold rolling process forimprovement of strength. In this condition, it is neces�sary to apply an efficient process to enhance thestrength and hardness in sufficient level that RCS issuitable to reach this goal [9].

In the present study, Duratherm 600 superalloythin sheet in annealed condition has been processedusing RCS as a novel technique to increase strengthwith low change of thickness for application in rotaryengines.

2. RCS SETUP AND EXPERIMENTAL PROCEDURE

A basic RCS cycle consists of two steps: Corruga�tion and straightening. The first step is to corrugate thework piece, which is performed using a recently dis�continuous RCS facility shown in Fig. 1. For thisstudy, Duratherm 600 samples with dimension of 15 ×15 × 0.35 mm3 were prepared in annealed condition.The composition of the employed material is given intable. The RCS experimental were carried out at roomtemperature and were repeated on samples to obtain 5,10, 15, 20 and 25 cycles with 90° rotations betweenconsecutive cycles to reach a homogenous distribution

Work Hardening of Duratherm 600 Cobalt Superalloy Using Repetitive Corrugation and Straightening Process1

H. Sheikha, *, E. Paimozdb, and S. M. Hashemib

aDepartment of Materials Science and Engineering, Sharif University of Technology, Azadi Ave., Tehran, Iran, P. O. Box 11365�9466

bDepartment of Materials Science and Engineering, Malek Ashtar University of Technology, Shahin, Shahr, Iran, P. O. Box 83145/115

*e�mail: sheikh_scientific@yahoo.com

Abstract—In this paper, the effect of repetitive corrugation and straightening (RCS) process on hardness ofDuratherm 600 superalloy has been investigated. To do so, the RCS was carried out until 25 cycles using mul�tiple teeth corrugative setup that the rotation of sample with 90° between cycles was utilized. The results showthat the increasing of cycle number enhances the value of hardness. Also, the microstructures of samples arean evidence of slipbands formation during RCS showing the applied strain on the material. As a result, theincreasing of hardness can be attributed to formation of subgrains, LAGBs and hcp martensitic plates at largestrains.

DOI: 10.3103/S1067821210010116

FOUNDRY

Chemical composition of Duratherm 600 used in this work(wt %)

Element

Co Ni Cr Fe W Mo Ti Al

Base 25.12 11.83 8.78 3.00 4.05 2.51 1

60

RUSSIAN JOURNAL OF NON�FERROUS METALS Vol. 51 No. 1 2010

SHEIKH et al.

of strain. For doing so, the hydraulic press wasemployed. Then, the aging process was utilized at650°C for 2 h. Specimens were prepared and hardnessmeasurements were taken using a Vickers microhard�ness tester. Also, optical microscopy performed toexamine structural micrographs.

3. RESULTS AND DISCUSSION

Figure 2 shows the microstructures of the surface oftwo samples in annealed condition and after 25 cycles.It is noteworthy that the grain size of material has notchanged during RCS process. Figure 2a is a typicaloptical micrograph of an annealed sample and showsthe equiaxed fcc grains, many of which containannealing twins. The significant change in the micro�structure that accompanies the pronounced increasein hardness during RCS of the alloy is the formation ofintersecting sets of fine parallel striations (slipbands)within the fcc grain as shown in Fig. 2b. Slipbandschange directions at own preferred slip planes whichhave different orientation from those of neighboringgrain. Figure 3 shows the microstructures of thicknessof sample after 25 cycles. It has elongated grains withparallel slipbands illustrating applied plastic strainduring RCS.

The micostructural evolution during RCS is as fol�lows: at low strains, grains contain dislocation cellswith increasing RCS strain, the dislocation cellsbecome subgrains when the misorientation acrosstheir boundaries are so large that they develop theirown slip systems. The misorientation across subgrainboundaries increases with further RCS strain andeventually becomes larger enough to transform thesubgrain boundaries into low angle grain boundaries(LAGBs) or HAGBs [6] that their misorientation (θm)are 15–15° and larger than 15°, respectively [8].According to below equation, the strain of 0.22 isapplied per cycle [10]:

(1)ε 4

3����� R t+

R 0.5t+����������������⎝ ⎠⎛ ⎞ln=

Where R and t are the radius of die and the thickness ofsheet, respectively. The present parameters in this study(low strain rate: 0.3–0.51 1/s, room temperature: 25°C,low strain: 0.22) only increase the dislocation densityand create subgrains and LAGBs. It means thatdynamic recovery phenomena occurs and the transfor�mation of the dislocation walls into HAGBs is impossi�ble. In Fig. 4, the values of hardness in different RCScycles and after aging have been plotted. At low appliedstrain, the formation of dislocation cells causes hard�ness enhancement. While in high number cycles, thenumber of subgrains and misorientation between ofthem increase by increasing of the applied strain andconsequently the size of subgrains decreases [8]. Thesephenomena cause the increasing of hardness values.The subgrain boundaries have misorientation less than5° which act as barriers for moving of dislocations andas a result the value of hardness increases [8, 11]. Theimprovement of the strength by decreasing subgrain sizeis related according to Eq. 2 [11]:

(2)σG���D

b��� K=

Press

R1.5 m

m

Fig. 1. Die Set up for discontinuous RCS process.27 µm(а)

(b) 27 µm

Slipbands

Fig. 2. Microstructure of surfaces related to (a) annealedcondition (b) after 25 cycles of RCS process.

RUSSIAN JOURNAL OF NON�FERROUS METALS Vol. 51 No. 1 2010

WORK HARDENING OF DURATHERM 600 COBALT SUPERALLOY 61

Where G, D, b and K are the shear modulus, the sub�grain diameter, the burger’s vector of dislocations and adimensionless constant, respectively. It is noteworthythat this superalloy has an interesting phenomenon dur�ing plastic deformation. The fcc hcp transforma�tion in pure cobalt and in many cobalt superalloys isclassified as a martensitic transformation because it isdiffusionless transformation which does not result in achange in phase composition and forms hcp plates infcc matrix. Therefore, a part of hardness increasing dur�ing RCS process of Duratherm 600 can be attributed tothis martensitic transformation [12, 13]. Also, Fig. 4illustrates that the aging process increases the value ofhardness that is due to precipitatation of phases such asCoxW along the twins, grain boundaries, slipbands andsubgrain boundaries [14].

4. CONCLUSIONS

In this study, the ability of discontinuous RCS as asevere plastic deformation process has been evaluatedfor hardness increasing of Duratherm 600 cobaltsuperalloy. The current design of the RCS did not pro�vide enough plastic strain and strain rate per cycle tobe effective in grain refinement. But, it has enoughefficiency to improve hardness. Therefore, this processhas industrial applications for production of springsand other components with desirable strength andhardness with low change in thickness of raw material.

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Fig. 4. The values of hardness after different cycles of RCS.

27 µm

Fig. 3. Microstructure of thickness after 25 cycles of RCSprocess.