Serrated flow in cast ZE43 alloy

Abstract

Plastic deformation of solid solutions is occasionally accompanied by plastic instabilities, i.e. serrated flow or the Partevin-Le Chatelier (PLC) effect. The PLC effect has been observed and investigated in various kinds of aluminum alloys [1–5]. For most aluminum alloys, this effect is observed at ambient temperature. The dynamic interaction between mobile dislocations and diffusing solute atoms, known as dynamic strain aging (DSA), is commonly accepted to account for the observed phenomena [6–11]. Compared to the extensive investigations on serrated flow of aluminum alloys, only a few papers have been concerned with the serrated flow in magnesium alloys. Couling [12] briefly reported the anomalous yielding effect in a Mg–0.5%Th alloy at testing temperatures from 373 K to 663 K. Chaturvedi et al. [13, 14] reported serrated flow in an Mg–10 wt.%Ag solid solution at temperatures between 326 K and 397 K. Zhu et al. [15] recently observed serrated flow in a 16-h-aged Mg–5Y–4Nd alloy and Corby et al. [16–19] found serrations in a AZ91 alloy when tested at room temperature. There seems no other reported work on serrated flow in magnesium alloys. The present paper reports our observations on serrated flow in a cast ZE43 alloy (Mg–4 wt.%Zn– 3 wt.%RE, here RE refers to misch metal). Specimens prepared from the cast ZE43 alloy were annealed at 798 K for 10 h, and quenched from 798 k into hot water (353 K), then taken out to room temperature (T4 treatment). In addition, Half of the hot-water quenched specimens were aged at 523 K for18 h (T6 treatment). Tensile tests were conducted on a screwdriven Instron testing machine at various initial strain rates at temperatures ranging from room temperature to 573 K. Serrations on the stress–strain curves of the specimens in both T4 and T6 conditions were observed in a temperature range from 393 K to 513 K. Outside this temperature range, smooth flow curves were obtained. Figure 1 Shows segments of the flow curves of the specimens in both T4 and T6 conditions at a strain rate of 2.4 · 10s and a temperature of 473 K, where the serrations were most pronounced. It can be seen that periodical serrations develop after about a 1% critical strain, ec. The serrations start to appear as a sequence of small yield points, and with increasing strain, the magnitude (stress drop) of the serrations becomes larger. The higher flow stress for the specimens in T6 condition indicates a precipitation strengthening effect. The power law model r 1⁄4 ke was used to fit the stress–strain curves, giving work hardening exponent n for the two heat treatment conditions a value of 0.31. The specimens in T6 condition develop serrations later than that in T4 condition, but the stress drops seem larger. The possible reason for the higher stress drops of the specimens in T6 condition is due to the effect of C. Y. Wang (&) Department of Materials Engineering, Bohai Shipbuilding Vocational College, Huludao 125000, P.R. China e-mail: cywang1969@yahoo.com.cn