Effect of electric current pulses on the microstructure and niobium carbide precipitates in a ferritic-pearlitic steel a
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R.S. Qin Department of Engineering and Innovation, The Open University, Walton Hall, Milton Keynes MK7 6AA, Buckinghamshire, United Kingdom (Received 6 May 2015; accepted 14 August 2015)
Niobium is an important alloying element in steels. In the present work an effort has been made to investigate the effect of electropulsing on the niobium carbide (NbC) at an elevated temperature (800 °C). The results show that the electropulsing treatment can generate an evenly distributed NbC by decreasing the kinetics barriers for precipitation. It has been also found that a semitransformed pearlite structure forms in such a way that the grains are oriented toward a direction parallel to that of the electric current flow. Furthermore, the electropulsed sample benefits from refined grain size. This is thought to be due to the electropulse-enhanced nucleation rate. Tensile testing has been carried out to compare the properties of electropulsed sample with that of without electropulsing. The results show that the sample with treatment has greater yield strength and ultimate tensile stress while its elongation is only 1% less that of the unelectropulsed samples. The improved mechanical properties of the sample with pulsing are attributed to its finer grain sizes as well as the elimination of precipitation free zones caused by the electropulsing treatment.
I. INTRODUCTION
The application of electropulsing to metallic materials and its effects on the microstructure and mechanical properties have been studied extensively in past decade. It has been reported that electropulsing can improve the corrosion behavior1 and remove detrimental inclusion such as MnS and Al2O3 from metal matrix.2 The treatment has also been applied to various aspects of materials processing, e.g., to generate plasticity,3 to refine microstructure,4 and to expedite structural relaxation.5,6 Studies have shown that the effect of electropulsing on both substitutional and interstitial precipitations is complex and depends on various factors such as the alloy system, the solutionizing time and temperature, the aging time and temperature, the magnitude of the current density and its frequency.5 Since precipitation is a diffusion-controlled phase transformation, it is expected that electromigration may play a significant role when considering the effect of electric current pulses. The atomic drift flux due to the application of electropulsing to a metal can be expressed by the modified Nernst–Einstein equation as5:
Contributing Editor: Yang-T. Cheng a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.268 J. Mater. Res., Vol. 30, No. 20, Oct 28, 2015
fi ¼
ni Di @lnNi @r þ Z eqj kT X ; @x kT 2x
ð1Þ
where N is the atomic density, D the pertinent diffusion coefficient, Z∗ an effective valance, e the charge on an electron, q the resistivity, and j the current density. The second term in the right hand side of the equality is related to the back force due to an opposing chemical potential gradient generated by the applicat
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