Void Nucleation and Growth in Dual-Phase Steel 600 during Uniaxial Tensile Testing

Abstract

Commercial dual-phase (DP) steel in sheet form and comprised of ferrite, martensite, and bainite was subjected to uniaxial tension up to fracture. The damage characteristics were studied through extensive quantitative metallography and scanning electron microscope (SEM) observations of polished sections and fracture surfaces of failed specimens. The observed void nucleation mechanisms include nucleation at the martensite/ferrite interface or triple junction (most predominant), nucleation due to the cracking of martensite particles, and nucleation at the inclusions. The void characteristics in terms of area fraction, void density, void size ranges, and void orientations were analyzed as a function of thickness strain from various regions of the different uniaxial tensile test specimens taken to fracture. The damage analysis suggests that the void nucleation occurs during the entire deformation process with an almost constant rate and this rate reduces before fracture. A nucleation strain of 0.15 has been estimated for this material.

Failure mechanisms in DP600 steel: Initiation, evolution and fracture

“…In addition to mechanisms that involve crack initiation and propagation within the martensite islands, failure of the ferrite-martensite (F-M) interface is another damage initiation mechanism that is reported for DP steels [5,7]. Uggowitzer and Stuwe [19] related this type of failure to the separation of inclusions from the martensite islands close to the interface with ferrite.…”

“…It is found that voids are associated with either non-metallic inclusions or martensite islands in DP steels [5]. The major void formation mechanisms reported [5][6][7] for DP steels include: 1) decohesion of the ferritemartensite interface, 2) fracture of martensite islands, and 3) voids generated in the martensite islands due to either the separation of prior austenite grain boundaries between adjacent martensite islands or the localized deformation within the martensite phase. The martensite cracking was found to be more frequent in a coarse martensite microstructure [5,7].…”

“…The major void formation mechanisms reported [5][6][7] for DP steels include: 1) decohesion of the ferritemartensite interface, 2) fracture of martensite islands, and 3) voids generated in the martensite islands due to either the separation of prior austenite grain boundaries between adjacent martensite islands or the localized deformation within the martensite phase. The martensite cracking was found to be more frequent in a coarse martensite microstructure [5,7]. The martensite failure may occur due to : i) decohesion of prior austenite grain boundaries [8,9], ii) fracture of martensite islands due to crack initiation [10] and iii) decohesion at the ferrite-martensite interface [11].…”

“…Avramovic-Cingara et al [7] have measured the through thickness strain along the midwidth plane of DP600 samples subjected to uni-axial tensile deformation. They showed that in the martensite, fracture occurs at small strains as low as 0.13.…”

Optical Microscopy-Based Damage Quantification: an Example of Cryogenic Deformation of a Dual-Phase Steel

“…8,[12][13][14] However, most of these works were carried out under limited experimental conditions such as a quasi-static tensile strain rate at room temperature. For further industrial application of the method, damage quantification must be performed at various conditions such as high deformation speed, cryogenic temperature etc.…”