Analysis of mechanical properties and its associated fracture surfaces in dual‐phase austempered ductile iron

Abstract

This work aims at evaluating the fracture surfaces of tensile samples taken from a new kind of ductile iron referred to as ‘dual‐phase Austempered Ductile Iron (ADI)’, a material composed of ausferrite (regular ADI microstructure) and free (or allotriomorphic) ferrite. The tensile fracture surface characteristics and tensile properties of eight dual‐phase ADI microstructures, containing different relative quantities of ferrite and ausferrite, were studied in an alloyed ductile cast iron. Additionally, samples with fully ferritic and fully ausferritic (ADI) matrices were produced to be used as reference. Ferritic–pearlitic ductile irons (DI) were evaluated as well. For dual‐phase ADI microstructures, when the amount of ausferrite increases, tensile strength, yield stress and hardness do so too. Interesting combinations of strength and elongation until failure were found. The mechanisms of fracture that characterise DI under static uniaxial loading at room temperature are nucleation, growth and coalescence of microvoids. The fracture surface of fully ferritic DI exhibited an irregular topography with dimples and large deformation of the nodular cavities, characteristic of ductile fracture. Microstructures with small percentages of ausferrite (less than 20%) yielded better mechanical properties in relation to fully ferritic matrices. These microstructures presented regions of quasi‐cleavage fracture around last‐to‐freeze zones, related to the presence of ausferrite in those areas. As the amount of ausferrite increased, a decrease in nodular cavities deformation and a flatter fracture surface topography were noticed, which were ascribed to a higher amount of quasi‐cleavage zones. By means of a special thermal cycle, microstructures with pearlitic matrices containing a continuous and well‐defined net of allotriomorphic ferrite, located at the grain boundaries of recrystallised austenite, were obtained. The results of the mechanical tests leading to these microstructures revealed a significant enhancement of mechanical properties with respect to completely pearlitic matrices. The topographies of the fracture surfaces revealed a flat aspect and slightly or undeformed nodular cavities, as a result of high amount of pearlite. Still isolated dimple patterns associated to ferritic regions were observed.

Microstructural damage evaluation of ferritic-ausferritic spheroidal graphite cast iron

“…Under this experimental procedure, the relative amounts and morphology of ferrite and ausferrite can be controlled by using different austenitizing temperatures and thermal cycles. The literature reports several experimental procedures and considerations to obtain IADI [2][3][4][5] and different mechanical properties based on the amount of ferrite and ausferrite [6][7][8]. In general, tensile strength, yield stress, and fracture toughness increase when the amount of ausferrite increases, while elongation diminishes.…”

“…Nevertheless, such a decrease is very small and the IADI keep having the minimum value of elongation required by ASTM A 536 standard. The fracture surfaces of IADI for different amounts of ferrite-ausferrite were also evaluated by several authors [8][9], nevertheless, there is not a complete understanding about the sequence and occurrence of damage mechanisms, nor about the influence of the ausferrite (in terms of morphology and volume fraction) on crack propagation and damage evolution. Recent studies carried out by the authors improved the understanding of the role of different microconstituents during fatigue and tensile testing by means of in-situ analysis [10][11][12][13].…”

Crosshead Speed Effects on Ferro Casting Ductile (Fcd) Tensile Strength and Fault Morphology

“…Cleavage can be found inside the particles of crystallography area. Cleavage fracture changes its direction when crossing the intergranular fracture and graphite nodules, similar to a river pattern [28]. Cleavage fracture generally occurred in high microstructure strength (with low plasticity) and tended to take place in the inclusion around the eutectic cell boundary rather than in the distance between nodular interfaces [27].…”

Influence of casting heterogeneities on microstructure and mechanical properties of austempered ductile iron (ADI)

“…There was no significant increase in ductility with increasing volume fractions of retained austenite when compared to samples austempered at 300°C. This result can be attributed to the effect of as-cast heterogeneities in the microstructure of ADI such as eutectic carbides and martensite formation during cooling after austempering [19,20,21]. The fracture surfaces of tensile test specimens were analyzed by SEM.…”