V. Di Cocco

Pearlitic ductile cast iron: damaging micromechanisms at crack tip

Ductile cast irons (DCIs) are characterized by a wide range of mechanical properties, mainly depending on microstructural factors, as matrix microstructure (characterized by phases volume fraction, grains size and grain distribution), graphite nodules (characterized by size, shape, density and distribution) and defects presence (e.g., porosity, inclusions, etc.). Versatility and higher performances at lower cost if compared to steels with analogous performances are the main DCIs advantages. In the last years, the role played by graphite nodules was deeply investigated by means of tensile and fatigue tests, performing scanning electron microscope (SEM) observations of specimens lateral surfaces during the tests (“in situ” tests) and identifying different damaging micromechanisms. In this work, a pearlitic DCIs fatigue resistance is investigated considering both fatigue crack propagation (by means of Compact Type specimens and according to ASTM E399 standard) and overload effects, focusing the interaction between the crack and the investigated DCI microstructure (pearlitic matrix and graphite nodules). On the basis of experimental results, and considering loading conditions and damaging micromechanisms, the applicability of ASTM E399 standard on the characterization of fatigue crack propagation resistance in ferritic DCIs is critically analyzed, mainly focusing the stress intensity factor amplitude role.

Influence of dipping time on cracking during bending of hot dip galvanized coatings with Sn and Ti contents

In the last years, the attention to environmental topics led a new approach solution in classical protection techniques, introducing innovative way oriented to optimize different coating properties. Hot-dip galvanizing is a classical process aimed to generate coatings on iron-based surfaces, used unchanged since 200 years: some chemical elements are added in the bath with different aims (e.g., Pb is really important for its fluidizing properties, sometimes replaced by Sn) but sometimes these elements are dangerous for human health (e.g. … Pb!). In this work, the influence of dipping time and coatings chemical compositions on damaging micromechanisms was investigated considering different Sn and Ti contents. Main damaging micromechanisms in hot dip zinc coated ipersandelin steel specimens were investigated by means of bending tests. Longitudinal sections of bended specimens were observed by means of a LOM (Light Optical Microscope): main damage micromechanisms were identified as longitudinal and radial cracks.

Fatigue crack behavior on a Cu-Zn-Al SMA

In recent years, mechanical property of many SMA has improved in order to introduce these alloysin specific field of industry. Main examples of these alloys are the NiTi, Cu-Zn-Al and Cu-Al-Ni which are usedin many fields of engineering such as aerospace or mechanical systems. Cu-Zn-Al alloys are characterized bygood shape memory properties due to a bcc disordered structure stable at high temperature called ?-phase,which is able to change by means of a reversible transition to a B2 structure after appropriate cooling, andreversible transition from B2 secondary to DO3 order, under other types of cooling. In ?-Cu-Zn-Al shapememory alloys, the martensitic transformation is not in equilibrium at room temperature. It is therefore oftennecessary to obtain the martensitic structure, using a thermal treatment at high temperature followed byquenching. The martensitic phases can be either thermally-induced spontaneous transformation, or stressinduced,or cooling, or stressing the ?- phase. Direct quenching from high temperatures to the martensite phaseis the most effective because of the non-diffusive character of the transformation. The martensite inherits theatomic order from the ?-phase. Precipitation of many kinds of intermetallic phases is the main problem oftreatment on cu-based shape memory alloy. For instance, a precipitation of ?-phase occurs in many lowaluminum copper based SMA alloy and presence of ?-phase implies a strong degradation of shape recovery.However, Cu-Zn-Al SMA alloys characterized by aluminum contents less than 5% cover a good cold machiningand cost is lower than traditional NiTi SMA alloys. In order to improve the SMA performance, it is alwaysnecessary to identify the microstructural changing in mechanical and thermal conditions, using X-Ray analyses.In this work a Cu-Zn-Al SMA alloy obtained in laboratory has been microstructurally and metallographicallycharacterized by means of X-Ray diffraction and Light Optical Microscope (LOM) observations. Furthermorea fatigue crack propagation and fracture surface scanning electron microscope (SEM) observations have beenperformed in order to evaluate the crack path and the main crack micromechanisms.

Structural transitions in a NiTi alloy: a multistage loading-unload cycle

NiTi shape memory alloys (SMAs) are increasingly used in many engineering and medical applications, because they combine special functional properties, such as shape memory effect and pseudoelasticity, with good mechanical strength and biocompatibility. However, the microstructural changes associated with these functional properties are not yet completely known. In this work a NiTi pseudo-elastic alloy was investigated by means of X-ray diffraction in order to assess micro-structural transformations under mechanical uniaxial deformation. The structure after complete shape recovery have been compared with initial state.