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Transient creep behaviour of Ni3Al polycrystals Tomas Kruml, Birgit Lo Piccolo and Jean-Luc Martin Département de Physique, Ecole Polytechnique Fédérale de Lausanne (EPFL) CH 1015 Lausanne, SWITZERLAND ABSTRACT Repeated creep tests were used for measuring various constant strain-rate deformation parameters. The results are consistent with those of repeated stress relaxations, although the precision is lower for creep in the present case. The small yield point observed in reloading after the transient is directly related to the amount of exhausted mobile dislocations, i.e. it originates from multiplication processes. During the transient test (180s total), the total exhaustion rate of mobile dislocations can be as high as 99%. It exhibits a maximum at the same T (about 500 K) as the work hardening. This supports the validity of a model which considers the work-hardening peak temperature to correspond to the stress under which incomplete Kear-Wilsdorf locks yield.

INTRODUCTION The technique of repeated stress relaxations is well established and has been used frequently since the sixties for the determination of some material parameters, above all the activation volume of the microscopic deformation mechanisms [1-4]. The repeated creep technique was proposed recently as an alternative [5-7] and its abilities are shown below.

EXPERIMENTAL DETAILS Polycrystalline binary Ni3Al rods with a nominal composition of 24 at.% aluminium and 76 % nickel were kindly provided by Dr. T. Khan (Onera, Paris). The material was homogenised at 1583 K for 48 hours. The metallurgical inspection revealed a single-phase material with large equiaxed grains of mean size about 800 micrometers. Parallelpipedic compression specimens with a length of 7 mm and a gauge section of 3.5 x 3.5 mm2 were cut by a diamond saw. The Schenck RMC 100 machine was used for the compression tests. These were performed between room temperature and 800K under an inert helium atmosphere. A nominal strain rate of 5x10-5 s-1 was kept constant.

THEORY OF REPEATED CREEP TESTS When the machine is switched from the constant strain rate mode to the constant force mode, a logarithmic dependence of plastic strain on time is experimentally observed : ∆ γ p = (kT / MVc )ln(1+ t / Cc )

N5.13.1

(1)

T is the absolute testing temperature, M is the elastic modulus of the machine-specimen assembly, Vc has the dimension of a volume, t is the time, Cc is a time constant. The Orowan equation is: dγ p / dt = αρ m bv (2) ρm being the mobile dislocations density, v their average velocity, b the Burgers vector and α a geometrical parameter. The applied stress τ is decomposed according to: τ = τ * + τi (3) * the effective stress τ depending only on temperature and strain-rate, τi being the internal stress. The velocity of mobile dislocations is a function of the effective stress: * v = νd exp − ∆G(τ ) / kT (4)

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∆G is the activation free enthalpy and ν and d have the respective dimensions of a frequency and a distance. The microscopic activation volume V is: V = −∂∆G / ∂τ * (5) Co