Investigations and quality improvement of structural steels with high resistance to impact loading, used for automobile parts, are critically important because of minimizing passenger's injuries during a crash. Extensive microstructural investigations revealed that high-strain-rate deformation under impact loading leads to the formation of hard and narrow adiabatic shear band in a soft matrix 1. The formation of such inhomogeneous microstructure results from excessive thermal softening and thermo-viscous instability, leading to strain localization. Despite the recent growing understanding of the formation mechanism, effects on physical properties were not understood because of few reports available. Here, we report results of measurements of magnetic first-order-reversal curves (FORCs) for high-strain rate deformed steels for the possible application to non-destructive evaluation (NDE) of inhomogeneous microstructural changes. FORCs are one kind of asymmetrical minor loops and provide useful information on interaction field (Hu) and coercivity (Hc) distribution, which can not be obtained from conventional major hysteresis loop.
Cylindrical samples of high-tensile AISI4340 steels were subjected to high-strain-rate deformation using a split Hopkinson pressure bar. We prepared samples with different strain rates up to 4500 s-1. FORCs were measured at room temperature using a SQUID magnetometer. Figure 1(a) and 1(b) show FORC diagrams after high-strain rate deformation with a strain rate of 1800 and 4500 s-1, respectively. In addition to a large FORC distribution peak which extends along Hu direction (peak 1), we found that a small additional peak appears at Hc~400 Oe (peak 2 in Fig.1), whose position gradually shifts toward a higher Hc with increasing strain rate. This indicates that a small fraction of magnetically hard regions (i.e. adiabatic shear bands) are formed in a soft matrix and the regions become magnetically harder at higher strain rate. These observations clearly demonstrate that FORCs can be a useful technique of investigating inhomogeneous microstructures under impact loading.
References:
1 A. G. Odeshi, M. N. Basim, et al., J. Mater. Proc. Tech. vol.169 (2005) 150.
2 B.C. Dodrill, J. Lindemuth, C. Radu, H. Reichard, MRS Bulletin vol.40 (2015) 903.

Fig. 1 FORC diagram after high-strain rate deformation with a strain rate (SR) of (a) 1800 and (b) 4500 s-1.