M. Anglada

The influence of plastic hardening on surface deformation modes around Vickers and spherical indents

Abstract Measurements of piling-up and sinking-in of material around Vickers and spherical indentations are conducted in metals and ceramics. The surface displacement at the contact boundary under applied load and in the unloaded state is correlated with the uniaxial strain hardening exponent, n . It is found that sinking-in predominates in materials where n >0.2. A state of non-uniform deformation is detected around Vickers indents in contrast to the case of spherical indentation, where the deformation state is more uniform as a result of the axisymmetry of the contact conditions. The average (mean) surface deformation state around the contact perimeter in Vickers indents follows a similar correlation with n as that found for spherical indentation. The present study of contact profiles enables an accurate assessment of contact area from measurements of depth of penetration which is central to the analysis of instrumented indentation experiments.

Hot deformation of duplex stainless steels

Abstract Duplex stainless steels (DSSs) have become established materials, successfully employed in many industrial applications. Their combination of mechanical properties and corrosion resistance is particularly appreciated in the petrochemical field. Hot deformation of these two-phase materials is still a critical point because the different mechanical response of austenite and ferrite often leads to the formation of edge cracks. In the present research, two DSSs with different nitrogen contents, i.e. EN 1.4462 and EN 1.4410, have been subjected to uniaxial hot compression tests in a wide range of temperatures and strain rates. The microstructural changes produced as a consequence of the distinct test conditions have been analyzed by means of optical and electron microscopy. The characteristics of high temperature plastic flow of both DSSs are interpreted in terms of the classical hyperbolic sine equation. The results are finally discussed considering the intrinsic two-phase nature of the materials studied.

On the fatigue crack growth behavior of WC–Co cemented carbides: kinetics description, microstructural effects and fatigue sensitivity

Abstract The influence of microstructure and load ratio (R) on the fatigue crack growth (FCG) characteristics of WC–Co cemented carbides are studied. In doing so, five hardmetal grades with different combinations of binder content and carbide grain size are investigated. Attempting to rationalize microstructural effects, key two-phase parameters, i.e. binder thickness and carbide contiguity, are used. On the other hand, the effect of load ratio is evaluated from the FCG behavior measured under R values of 0.1, 0.4 and 0.7. Experimental results indicate that: (1) WC–Co cemented carbides are markedly sensitive to fatigue; and (2) their FCG rates exhibit an extremely large dependence on Kmax. Furthermore, both fatigue sensitivity and relative prevalence of Kmax over ΔK, as the controlling fatigue mechanics parameter, are found to be significantly dependent upon microstructure. As mean binder free path increases, predominance of static over cyclic failure modes diminishes and a transition from a ceramic-like FCG behavior to a metallic-like one occurs (conversely in relation to contiguity). Consequently, the trade-off between fracture toughness and FCG resistance becomes more pronounced with increasing binder content and carbide grain size. The observed behavior is attributed to the effective low ductility of the constrained binder and its compromising role as the toughening and fatigue-susceptible agent in hardmetals, the latter on the basis that cyclic loading degrades or inhibits toughening mechanisms operative under monotonic loading, i.e. crack bridging and constrained plastic stretching.

Characterization of the intermetallic G-phase in an AISI 329 duplex stainless steel

Duplex austenite–ferrite stainless steels are susceptible to a variety of decomposition processes when aged within the intermediate range of temperatures (250–500 °C). One of these phenomena is the precipitation of the intermetallic G-phase. In the present investigation, the crystal structure and the chemical composition of the G-phase, precipitated in the ferritic phase of an AISI329 duplex stainless steel, is studied by electron microdiffraction and energy dispersive X-ray spectroscopy. It is determined that the space group of the G-phase is F 3 with a lattice parameter four times that of the ferritic matrix. The precipitation mechanism of the G-phase showed a synergetic relation with the ferrite decomposition in Cr-rich and Fe-rich domains. Based on the obtained results, the structural proximity of ferrite matrix and G-phase has been studied. Further analysis allows to suggest that the spinodal decomposition leads to an interdomain of a ferritic structure which is thermodynamically unstable and serves as a precursory site to the development of the G-phase by atomic position readjustments inferior to the atomic distances.

On the transformation toughening of Y–ZrO2 ceramics with mixed Y–TZP/PSZ microstructures

Abstract Heat treatment of Y–TZP at high temperatures produces materials with a mixed Y–TZP/PSZ phase assemblage, which exhibit a unique combination of high mechanical strength and fracture toughness, uncommon in zirconia ceramics. The microstructure and crack growth resistance of the Y–TZP/PSZ materials developed by treating at 1650°C in air a fine-grained Y–TZP was studied. XRD as well as Raman spectroscopy results indicate that the obtained microstructure allow the retention of large tetragonal grains (up to ∼4 μm), resulting in both phase transformability enhancement and pronounced R-curve behavior. The large transformation zone, discerned from accurate measurements with Raman microprobe spectroscopy, sustains the above assessment and points out tetragonal to monoclinic phase transformation as the main toughening mechanism in the investigated Y–TZP/PSZ microstructures. This was confirmed by satisfactory agreement between the transformation toughening estimated from numerical analysis and the crack shielding experimentally determined from the R-curve measurements.

Fracture toughness of alumina and ZTA ceramics: microstructural coarsening effects

Abstract The influence of zirconia particles addition and microstructural coarsening, as a result of different heat treatments, on the fracture toughness of a monolithic alumina and a zirconia-toughened-alumina (ZTA) composite is studied. It is observed that addition of zirconia particles results in an enhancement of fracture toughness with respect to that exhibited by the alumina matrix. Similarly, microstructural coarsening within ZTA is found to produce a fracture toughness increment, mainly associated with the effect of zirconia particle size on their phase transformability. The obtained mechanical properties and grain growth kinetics are discussed in terms of microstructural features and operative toughening mechanisms.

Fracture toughness evaluation of hardmetals: influence of testing procedure

Abstract This paper describes an examination of the dependence of fracture toughness measurement on test technique for WC–Co hardmetals. Four different testing procedures have been assessed: the single edge notched beam (SENB) method, the single edge V-notched beam (SEVNB) technique, the surface crack in flexure (SCF) procedure and the conventional indentation microfracture (IM) method. The obtained fracture toughness values are compared and discussed on the basis of the particular theoretical and experimental features underlying each test technique investigated. As a result, the measurements obtained applying the SENB and SCF methods, as related to samples where residual tensile stresses induced during the corresponding precracking procedures were first relieved, are suggested to be the most reliable of all the values determined from the test methods evaluated. This assertion is further sustained through fracture mechanics analysis combining fracture toughness evaluation, flexural strength measurement and fractographic examination.

Phase transformation and subsurface damage in 3Y-TZP after sandblasting

OBJECTIVE The goal of this work is to investigate t-m phase transformation, and subsurface damage in 3Y-TZP after sandblasting. METHODS Commercial grade 3Y-TZP powder was conventionally sintered and fully dense specimens were obtained. Specimens were sandblasted using different particle sizes (110 and 250μm) and pressures (2 and 4bar) for 10s. Phase transformation was measured on the surface and in the cross-section using X-ray diffraction and micro Raman spectroscopy, respectively. Subsurface damage was investigated on cross-sections using SEM and in shallow cross-sections machined by focused ion beam. RESULTS Sandblasting induced monoclinic volume fraction is in the range of 12-15% on the surface. In the cross-section, a non-homogeneous phase transformation gradient is found up to the depth of 12±1μm. The subsurface damage observed was plastic deformation in grains with the presence of martensite plates, and this effect is found to be larger in specimens sandblasted with large particles. SIGNIFICANCE The extent of subsurface tetragonal-monoclinic transformation and damage induced by sandblasting are reported for different sandblasting conditions. This knowledge is critical in order to understand the effect of sandblasting on mechanical properties of zirconia used to fabricate dental crowns and frameworks.

Aging effects on the cyclic deformation mechanisms of a duplex stainless steel

Abstract Aging effects on the cyclic deformation mechanisms of an AISI-329 duplex stainless steel have been studied on the basis of the cyclic hardening-softening response, cyclic stress-strain curve and substructure evolution within the individual phases. The cyclic behavior of an unaged and two aged materials shows, in terms of plastic strain amplitude (e pl ), three well-defined stages. In the first regime, at low e pl , no differences are observed among the response of the three materials as a consequence of the dominance of “austenitic-like” deformation mechanisms for all the materials. In the second regime, at intermediate e pl , the cyclic behavior of unaged material is associated with a mixed “austenitic/ferritic-like” character, mainly due to plastic activity of both phases. On the other hand, the cyclic response of aged material within this intermediate e pl range is rather correlated to “austenitic-like” cyclic deformation mechanisms because of the intrinsic brittleness of the ferritic matrix. A third regime, at relatively large e pl , suggests a synergetic phenomenon of dislocation activity, deformation twinning and demodulation of spinodal microstructure in ferrite that enables this phase to sustain plastic deformation. Thus, in this e pl interval, the observed mechanical and substructural behavior within ferrite may be considered as relatively similar to that observed in unaged material at much lower stress levels; and therefore is amenable to be associated with “ferritic-like” cyclic deformation mechanisms. Finally, based on the results presented, the prevalence of “austenitic-like” or “ferritic-like” cyclic deformation mechanisms, for a given plastic strain range, is discussed in terms of the different role played by the ferritic matrix in each material investigated, depending upon its embrittlement degree.

Fracture toughness of zirconia–alumina composites

Abstract The dependence of fracture toughness on microstructure coarsening, induced by thermal treatment at 1600°C, has been studied in zirconia–alumina composites with compositions 5%, 15% and 30% zirconia by volume. The fracture toughness has been evaluated using different methods based on producing small surface cracks by Vickers indenters, or on producing a large through the thickness crack. It is concluded that fracture toughness increases with the volume fraction of zirconia as well as with zirconia particle coarsening. This is explained by the effect of these microstructural parameters on the main operating toughening mechanism, which is found to be stress induced phase transformation.