Structural superplasticity at higher strain rates of hypereutectoid Fe-5.5Al-1Sn-1Cr-1.3C steel
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TRODUCTION
SUPERPLASTICITY is the ability of crystalline solids to achieve extremely large uniform elongations in tensile samples strained in uniaxial tension tests and comprises some characteristic phenomena.[1,2,3] The most typical forms of superplasticity are structural superplasticity, which is observed in microcrystalline, two-, or multiphase materials, and internalstress superplasticity, which occurs during phase transformations or thermal cycling under an imposed stress state or even under neutron radiation. Structural superplasticity at high strain rates is associated with very fine-grained microstructures and higher deformation temperatures Tdef 0.85 Tt related to a respective phase transformation. In this case, Tt represents the quasi-eutectoid reaction temperature where -ferrite plus cementite (Fe3C) transforms to -austenite or more generally is the incipient melting of low melting point alloys. One remarkable example is hypereutectoid plain carbon steels that exhibit superplasticity at 675 °C slightly below the eutectoid transformation temperature of 723 °C, where -ferrite and cementite are the coexisting phases. The second phase—Fe3C— stabilizes the microstructure and prevents grain growth.[1] Superplastic flow can be described by the Dorn equation:[3] AD0G |b| p s0 n Qc # a b a b exp a b kT d E RT
[1]
where D0 is the diffusion coefficient, E is the elastic modulus, G is the shear modulus, k is the Boltzmann’s constant, and R is the universal gas constant; A is a microstructurerelated coefficient. The influence of the grain size is considered |b| P by the expression a b , where d is the average grain size, d G. FROMMEYER, Department Head, is with the Max-Planck Institute for Iron and Steel Research, Dusseldorf, Germany. Contact e-mail: frommeyer@ mpie J.A. JIMÉNEZ, Group Leader, is with the Centro National de Investigaciones Metalurgicas CENIM, 28040 Madrid, Spain. Manuscript submitted June 4, 2003. METALLURGICAL AND MATERIALS TRANSACTIONS A
b is the Burgers vector, and p is the grain size exponent. The ratio, flow stress 0 over Young’s modulus ( 0/E), is temperature compensated; n is the stress exponent and the reciprocal value is the strain-rate-sensitivity exponent m 1/n. From the exponential term exp (Qc /RT), the thermal activation energy of superplastic flow can be determined using the relationship # #
ln / 0 [2] Q R a b
1/T (s/E,d ) # where is the actual strain rate at given absolute test temperature T (K) and constant Young’s modulus-compensated flow stress ( /E). The structure or average grain size d is also considered to be constant. The normalized strain rate # 0 1 s1 is defined for mathematical reasons to make the # # argument of the natural logarithm ln (/0) “dimensionless.” Superplastic UHC steels and some modified UHC steels with additions of aluminum or silicon, and also kappa carbidebased material (Fe3AlCx), have been developed and extensively studied by Sherby and co-workers in the past.[4–7] By adding the ferrite formers Si and Al in medium concentratio
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