Development of constitutive models and hot-working processing map for Al–12Ce–0.4Sc alloys

Abstract

The current study investigates the hot deformation behavior of Al–12Ce–0.4Sc alloy with an isothermal hot compression test at 300–450°C/0.001–1 s−1. Results show that the flow curves exhibit typical dynamic recovery (DRV) and slight flow-softening behavior. Additionally, the flow curves overlap owing to the dynamic strain aging (DSA) phenomenon at 400–450°C/0.01–0.1 s−1. Two different constitutive models were developed using the experimental data for hot deformation: (i) strain-compensated Arrhenius model (Method I) and (ii) logistic regression model (Method II). The average stress exponent (n) and apparent activation energy (Q) are 14.25 and 209.58 kJ·mol–1, respectively. The hot-working processing map shows that the optimal processing condition is 400°C/1 s−1, and the maximum power dissipation efficiency is 22%. Stable and unstable domains indicated by the processing map were correlated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron backscatter diffraction (EBSD) characterization techniques. The unstable domains are primarily associated with pro-eutectic Al11Ce3 intermetallic fracture and interfacial cracks between α-Al and pro-eutectic Al11Ce3.

1. Introduction

Selecting materials for the automobile and aerospace sectors on the basis of weight reduction is essential to minimizing fuel consumption in fast-growing countries. Accordingly, Al alloys are usually preferred owing to their high specific strength and excellent corrosion resistance [1]. Al–Si, Al–Si–Mg, and Al–Ni alloys are mostly used in various applications. Given the improvements in the overall performance of automobiles over the past few years, materials with resistance to high temperatures of up to 350°C and modest strength and formability are needed [2]. Intermetallic strengthened binary Al–Al11Ce3 and Al–Al3Ni alloys can retain their mechanical strength and thermal stability for maximum service time. However, adding Ni to Al decreases fluidity, increases hot tearing tendency, and results in poor casting during large-scale production [3]. By contrast, the addition of Ce to Al alloys enhances the fluidity and strength of Al–Ce alloy [4]. Al–Ce alloys contain micron-sized eutectic and pro-eutectic α-Al11Ce3 intermetallic phases that enhance the strength of matrices, and the intermetallic α-Al11Ce3 phase is extremely stable because of the slow diffusion coefficient of Ce in Al ( = 5.59 10−18 m2/s). The diffusion rate of Ce is more than one order of magnitude lower than the diffusion rates of Cu, Si, and Sc in Al and four orders of magnitude lower than the diffusion rate of Ni in Al at 500°C [3].

Minor alloying elements, such as Si, Mg, Zr, and Sc, in Al–Ce alloys may further enhance solid solution and precipitation strengthening. Adding Sc (0.38wt%) to Al alloys promotes the formation of stable spherical nanoscale L12-Al3Sc precipitates between matrices and Al11Ce3 intermetallic phases during solidification [3]. In addition, Sc effectively inhibits grain growth and recrystallization in Al–Ce alloys [3]. The nearly zero solid solubility of Ce and Sc in Al alloys results in an extremely stable cast microstructure, rendering secondary heat treatment unnecessary. Combining a large volume fraction of Al11Ce3 and a small volume fraction of nanoscale Al3Sc precipitates enhances the mechanical and service performance of aerospace and automobile components [5].

High-strength Al-alloy components are fabricated through various thermomechanical processing routes, such as rolling, forging, and extrusion [6]. Therefore, flow behavior has been explored using various constitutive models, such as the physics-based, phenomenological-based, and artificial neural network (ANN) models [7–10]. The phenomenological Arrhenius constitutive model is widely used because of its high prediction accuracy and is easy to establish. It not only describes how flow stress depends on deformation parameters but also provides some physical meaning to its constants [11]. Hu et al. [12] found that strain plays an essential role in the activation energy of thermal deformation (Q) and other material constants in Al–Si eutectic alloys. Sun et al. [13] reported that the flow behavior of Al–Mg–Si alloy highly depends on deformation strain rate ( ) and temperature (T). The predicted flow stress ( ) is reliable only in a range of 410–510°C/0.001–0.1 s−1.