Jesús Chao

Microstructure and fracture properties of an ultrahigh carbon steel–mild steel laminated composite

Abstract A seven layer steel based (mild steel and ultrahigh carbon steel, UHCS) laminated composite was processed by roll bonding. Impact properties were improved in comparison with the UHCS. Delamination plays an important role by deflecting cracks, absorbing energy and imposing the nucleation of new cracks in the next material layer.

The high temperature oxidation behaviour of an ODS FeAl alloy

The oxidation behaviour of an oxide dispersion-strengthened (ODS) FeAl intermetallic, microalloyed with Zr and B and strengthened by a fine dispersion of Y2O3, is investigated at 1100°C for exposures of up to 200 h. The results show that a pure alumina scale is formed irrespective of the exposure time. The oxidation rate is far inferior to that found on PM 2000, a commercial alumina forming ODS ferritic superalloy. Limited scale spallation is observed in the intermetallic alloy from the early stages of oxidation. Scale failure, which is shown to occur during the cooling stage after oxidation and not at the high temperature of oxidation itself, results from the high compressive residual stresses in the scale induced by the misfit in the thermal expansion coefficients of the scale and the substrate. Failure of the scale may be supressed by using a very low cooling rate after oxidation.

The influence of some microstructural and test parameters on the tensile behaviour and the ductility of a mechanically-alloyed Fe–40Al alloy

Abstract The present study examines the influence of a wide range of microstructural parameters and tensile test conditions on the tensile behaviour of a mechanically-alloyed, fine grained Fe–40Al intermetallic. Major changes of tensile strength and ductility are obtained by reducing the grain size (with the ductility increasing from 1 to 10% for grain sizes of 100 and 1 μm), by avoiding environmental attack during the test, and by avoiding premature stress/strain concentrators (with the ductility increasing from 5 to 10% as imperfectly machined samples have their sample surfaces polished). Ductility variations are interpreted using a model based on the slow propagation of an initial crack which eventually reaches a condition of instability, and where the respective roles of environment, plastic deformation processes, and fracture mechanisms can be distinguished. The tensile ductility is highly sensitive to the surface state, meaning the degree of exposure to the environment, the extent of geometrical stress raisers, and the microstructure made up of the grain and particle size and distribution.

Influence of bainite morphology on impact toughness of continuously cooled cementite free bainitic steels

Abstract The influence of bainite morphology on the impact toughness behaviour of continuously cooled cementite free low carbon bainitic steels has been examined. In these steels, bainitic microstructures formed mainly by lath-like upper bainite, consisting of thin and long parallel ferrite laths, were shown to exhibit higher impact toughness values than those with a granular bainite, consisting of equiaxed ferrite structure and discrete island of martensite/austenite constituent. Results suggest that the mechanism of brittle fracture of cementite free bainitic steels involves the nucleation of microcracks in martensite/austenite islands but is controlled by the bainite packet size.

The role of the surface roughness on the integrity of thermally generated oxide scales. Application to the Al2O3/MA956 system

This work deals with the effect of the metal roughness on the integrity of thermally generated oxide scales. For illustrative purposes, experimental evidence is shown for the alumina forming MA 956 alloy. The experimental results reveal that scale spallation occurs more readily in scales with rough surfaces than in scales with smooth surfaces, preferentially at the crests of the scale profile. In order to explain this feature, the effect of the roughness of both the gas/scale and scale/metal interfaces on the thermal stress distribution was analyzed by the finite element method. This analysis shows that with increasing roughness a gradient of compression stresses develops in the scale, being the maximum value of stresses located near the gas/scale interface. In general, the higher the roughness the higher the difference between the maximum and minimum values of the stresses. However, the average value of the stress distribution through the scale thickness decreases with increasing surface roughness. The effect of a planar gas/scale and a rough scale/metal interfaces was also modelled. In this case, the stress gradient in the scale was found to monotonically increase with increasing roughness although in a lower extension than when a rough gas/scale interface was also considered. On the basis of the experimental results and the stress distribution analyses a sequence of the scale failure during the cooling stage are proposed for both cases. It is concluded that the stress component that is normal to the interface and the shear stress play a key role on the scale integrity.

Obtaining good ductility in an FeAl intermetallic

Abstract Deformation and fracture mechanisms leading to tensile failure have been examined in a mechanically-alloyed and a recrystallized Fe–40Al intermetallic. It is shown that tensile failure begins by the initiation of a small crack near the sample surface at a given level of strain, and crack propagation occurs rather quickly thereafter such that the total strain at failure is only slightly larger than the crack initiation strain. Crack propagation is initially slow, and affected by the test environment, and later it is faster and unaffected by test environment. Deformation localizes into slip bands and crack nucleation takes place easily, at a small strain, in the recrystallized material of large grain size. In the as-extruded material, deformation remains homogeneous under the influence of the small distributed particles, and crack initiation over a scale of many grain dimensions is slow. Possibilities for further ductility improvements by microstructure control are discussed.

Effects of the alumina scale on the room-temperature tensile behavior of preoxidized MA 956

This article deals with the effects of theα-Al2O3 scale (∼5µm) developed during preoxidation (1100 °C/100 hours) of MA 956 on its room-temperature tensile behavior. The tensile tests were made in the strain-rate range of 10−5 to 10−1 s−1. It is shown that the scale, fine and tightly adherent to the substrate, affects the tensile behavior in two relevant ways. First, the yield strength and the tensile strength are lowered with respect to those of the scale-free material. This is explained in terms of the residual stresses generated in the scale during preoxidation. From the analysis of the differences in the yield strength of preoxidized MA 956 with respect to the scale-free material, residual compression stresses in the scale of about 5500 MPa were obtained. These high stresses account for the surprisingly high tensile strain achieved (1.4 pct) before scale spallation occurs. Second, a ductile to brittle transition (DBT), which is not observed in the scale-free samples, occurs at intermediate strain rates (10−3 s−1). The brittle fracture is related to the increase of the triaxiality state in the substrate near the scale/metal interface.

Fatigue behavior of Ti6Al4V and 316 LVM blasted with ceramic particles of interest for medical devices

Grit blasting is used as a cost-effective method to increase the surface roughness of metallic biomaterials, as Ti6Al4V and 316 LVM, to enhance the osteointegration, fixation and stability of implants. Samples of these two alloys were blasted by using alumina and zirconia particles, yielding rough (up to Ra~8μm) and nearly smooth (up to Ra~1μm) surfaces, respectively. In this work, we investigate the sub-surface induced microstructural effects and its correlation with the mechanical properties, with special emphasis in the fatigue behavior. Blasting with zirconia particles increases the fatigue resistance whereas the opposite effect is observed using alumina ones. As in a conventional shot penning process, the use of rounded zirconia particles for blasting led to the development of residual compressive stresses at the surface layer, without zones of stress concentrators. Alumina particles are harder and have an angular shape, which confers a higher capability to abrade the surface, but also a high rate of breaking down on impact. The higher roughness and the presence of a high amount of embedded alumina particles make the blasted alloy prone to crack nucleation. Interestingly, the beneficial or detrimental role of blasting is more intense for the Ti6Al4V alloy than for the 316 steel. It is proposed that this behavior is related to their different strain hardening exponents and the higher mass fraction of particles contaminating the surface. The low value of this exponent for the Ti6Al4V alloy justifies the expected low sub-surface hardening during the severe plastic deformation, enhancing its capability to soft during cyclic loading.

Comparative study of the alumina-scale integrity on MA 956 and PM 2000 alloys

The present work analyzes the oxidation kinetics of MA 956 and PM 2000 alloys at 900 and 1100°C for exposure times up to 1000 hr. Special emphasis was placed on a comparison of the alumina-scale integrity formed at 1100°C by means of electrochemical tests at room temperature, which have been shown to be very reliable methods to detect the presence of microdefects within oxide scales. To check whether a preoxidation treatment makes these materials corrosion resistant against aggressive fluids, an electrolyte containing chloride ions was chosen. The mass gain of MA 956 was found to be slightly lower than that of PM 2000 up to 200 hr exposure at 1100°C and for the whole exposure range at 900°C. A subparabolic time dependence (n=0.3) of the oxide growth rate was determined for both alloys at both temperatures. On the other hand, the electrochemical-impedance spectroscopy (EIS) and anodic-polarization tests performed on preoxidized alloys (1100°C/100 hr) revealed good room-temperature corrosion behavior for both alloys, the corrosion resistance and polarization values being somewhat higher for preoxidized PM 2000. Consideration of these results and those of both surface and cross-section examinations of the scale, the better room-temperature corrosion behavior of preoxidized PM 2000 denotes the formation of a denser and mechanically more stable alumina scale containing a lower number of microdefects. This could result from the higher aluminum content of this alloy and the lower density of chemical heterogeneities within the scale. The higher mass gain of PM 2000 could be related to the higher concentration of oxide nodules on top of the alumina scale, as deduced from SEM examination.

Effect of substrate roughness on the corrosion behaviour of the Al2O3/MA 956 system.

This paper presents the influence of substrate roughness on the corrosion behaviour of the Al2O3/MA 956 system. An alumina layer of thickness 1-5 microm was generated of the MA956 alloy by thermal oxidation at 1100 degrees C using different exposure times. This Al2O3/MA 956 system with a polished substrate has shown excellent corrosion behaviour in a physiological fluid, due to the fact that the alpha-Al2O3 layer formed is dense, continuous and firmly adhered to the substrate, irrespective of the scale thickness. This good adherence allows it to withstand potentials above 1.7 V. Specimens with rough finish substrate and treatment times above 10 h present spallation of the alumina layer at the crests of the roughness profile. In this case a mixed corrosion behaviour between an alumina coated material and one with a passive layer is observed. In both types of specimens, rough and smooth, once the passivation layer is broken the repassivation capacity of the substrate is ensured due to the high chromium content of the alloy, under oxygenation conditions.