L. Llanes

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.

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.

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.

Cyclic stress-strain response and dislocation substructure evolution of a ferrite-austenite stainless steel

The hardening-softening response, the cyclic stress-strain behavior and the evolution of dislocation structures of an AISI-329 ferrite-austenite stainless steel have been studied. Fatigue testing has been conducted under fully reversed total strain control and constant total strain rate. Detailed transmission electron microscopy studies have been carried out in order to determine the individual substructure evolution, as a function of increasing imposed strain amplitude, in each constitutive phase. In general, the cyclic response of the studied material may be described in terms of three different regimes within the plastic strain amplitude (ϵpl) range investigated, i.e. from 2 × 10−5 to 6 × 10−3:at ϵpl below 10−4 the dominant cyclic deformation mechanisms are those correlated to planar glide of dislocations within the austenite which is the phase which carries a large part of the macroscopic strain in this first regime. On the other hand, at ϵpl higher than 6 × 10−4 the dominant substructure evolution is observed inside the ferritic matrix. In this case, strain localization is enhanced, within the ferritic grains, through the development of veins into the wall structure. Such evolution induces a pronounced decrease of the cyclic strain hardening rate in the cyclic stress-strain curve. At ϵpl in-between these values, the cyclic behavior is characterized by a relatively high strain hardening rate and may be classified as a mixed “ferritic/austenitic-like” behavior. In this intermediate regime substructural changes are observed in both phases and the dislocation activity in each of them seems to be strongly influenced by their particular cyclic strain hardening behaviors. Finally, the results are analyzed and compared with data from the literature in terms of volume fraction and chemical composition of the constitutive phases.

Influence of electrical discharge machining on the sliding contact response of cemented carbides

Abstract The correlation between electrical-discharge machining (EDM) and surface integrity and its influence on the sliding contact response of a fine-grained WC-10% wt Co hardmetal have been studied. Different surface finish conditions were evaluated corresponding to sequential EDM as well as grinding and polishing employing diamond as abrasive. Surface integrity was characterized in detail for all conditions through scanning electron microscopy (SEM). Sliding contact response was determined through microscratch measurements using a nanoindentation system. Contrary to the results obtained from conventional hardness evaluation, the above testing technique allows discrimination of EDM effects on the contact behavior of the studied cemented carbide. The experimental results, given in terms of scratch penetration-load curves and friction coefficient, indicate that microscratch resistance increases with finer-executed EDM, up to values similar to those exhibited by the reference ground and polished condition. The findings are discussed in terms of contact damage features as related to surface integrity and intrinsic microstructural features.

Oxidation-induced strength degradation of WC-Co hardmetals

Abstract The oxidation behavior at 700°C and its influence on room-temperature mechanical strength of three grades of WC–Co cemented carbides containing different binder content were studied. Oxidation kinetics, determined by thermogravimetric measurement, were found to follow a linear time dependence of weight gain per unit area in all the cases. It was also observed to rise with an increase in the oxygen concentration of the atmosphere and a reduction of the cobalt content. Three oxidation times of 10 min, 1 and 6 h were chosen to evaluate changes in the mechanical strength. Fracture resistance of oxidized samples were measured at room temperature under four-point bending and compared to that of non-oxidized specimens. In all cases, a pronounced degradation in strength was observed after oxidation. The mechanical characterization studies were complemented by detailed fractographic examination and the results were analyzed by linear-elastic fracture mechanics (LEFM). It is conjectured that strength reduction results from a combination of load-bearing section reduction (extrinsic effect) and a concomitant interplay of several factors including increasing size and severity of critical flaws, wedge-effects in the case of heterogeneous oxide penetration and residual tensile stresses associated with volume and thermal differences between the substrate and the oxide scale (intrinsic effects).