A Study of the Deformation and Fracture of a Dual-Phase Steel
- PDF / 1,839,398 Bytes
- 6 Pages / 594 x 774 pts Page_size
- 12 Downloads / 155 Views
I.
INTRODUCTION
THE results of a microstructural investigation of the tensile deformation and fracture of a commercial dualphase steel are presented here with particular attention to the partition of stress and strain between the ferrite and martensite constituents and the mode of fracture initiation in the microstructure. Considering that dual-phase steels contain 15 to 20 vol pct of martensite, it is likely that the stress carried by the martensite could significantly affect the strength of the alloy. However, the respective values of stress and strain in each of the two constituents are not well known, and the factors determining their relative magnitudes are not well understood. For instance, it has been reported by Davies ~'2 that a continuum theory3'4 based on isostrain behavior of the constituents adequately describes the true stress-true strain relation of dual-phase steels. A consequence of the theory, namely that the ultimate tensile strength is to a first approximation proportional to the volume fraction of the harder phase, was experimentally verified by several investigators. 1,2.5,6However, the validity of the main assumption that the ferrite and martensite deform by equal strains, has been questioned. 7'8 Speich and Miller 9 showed that good agreement results between the experimental and theoretical stress-strain curves for a continuum mechanical model 1~ which assumes that the composite tensile behavior lies between the states of isostrain and isostress, with very appreciable strain difference between the two phases. Also, Karlsson and Sundstrom 11 applied finite element analysis to a two-dimensional model of the ferrite-martensite microstructure and concluded that large differences in strain exist within each phase, as well as between the phases. Micromechanistic theories developed for dual-phase steels5'12'13 are based on dispersion hardening models, such as those of Ashby 14and Brown and Stobbs. 15In general, the use of these models also results in good agreement between the predicted stress-strain curves and experimental data, although they deal mainly with the work-hardening rate of the soft matrix, without explicitly considering the load carrying contribution of the harder martensite constituent. However, the models show that a hard, nondeforming inclusion or particle is subject to incompatibility stresses which eventually lead to local yield or fracture. A. E SZEWCZYK, Engineer, is with IBM East Fishkill Facility, Hopewell Junction, NY 12533. J. GURLAND, Professor, is with the Division of Engineering, Brown University, Providence, RI 02912. Manuscript submitted January 12, 1982. METALLURGICAL TRANSACTIONS A
9
Consideration of the fracture behavior has been mainly descriptive. Investigators have observed that void formation arises from martensite-ferrite interface decohesion 7'~3 or by both martensite particle fracture and interface decohesion9'16 at high levels of strain, with void growth and coalescence leading to subsequent failure. Stevenson t7 demonstrated that the mechanisms of crack
Data Loading...
