Papers by Keyword: Embrittlement

Abstract: High temperature Ti-alloys are usually sophisticated and hence expensive. To allow the use of cheaper alloys at elevated temperatures an economic and easy to apply procedure was developed to improve their high temperature capability. The treatment consists of a combination of Al-enrichment in a shallow surface region plus additional fluorination. The Al-enrichment at elevated temperatures leads to the formation of intermetallic TiAl-phases. These phases improve the oxidation resistance of Ti-alloys but not to a sufficient extent. An additional fluorine treatment of the Al-enriched surface leads to the formation of a protective alumina scale due to the fluorine effect. In this paper results from high temperature exposure tests performed on different Ti-alloys without any treatment and with a combination of Al-treatment plus fluorination are presented. The results are discussed in the view of the use of the optimized Ti-components for several high temperature applications.

Abstract: Fe₄₀Al₆₀ (at%) intermetallic alloy composition was obtained by conventional casting methods and subsequently subjected to high-energy mechanical milling under different conditions of humidity. All samples were characterized by X-ray diffraction patterns (XRD), transmission electron microcopy (TEM) and DSC-TGA thermogravimetric experiments. After the milling process, the amount of hydrogen generated was determined using thermogravimetric analysis and chemical reactions (stoichiometry). All techniques confirm the formation of bayerite phase which is attributed to the hydrogen embrittlement reaction between the intermetallic material and water to release hydrogen. It was observed that the hydrogen generation is increased as the ball milling time is increased. The quantity of hydrogen evaluated is similar to that obtained in previous reported experiments with pure aluminum and some of its alloys.

Abstract: Grain boundary embrittlement and de-embrittlement observed in age hardening iron alloys were reviewed. Fe-Mn-Ni and Fe-Ni-Ti alloys show excellent hardening response during aging treatment. However these alloys all suffer grain boundary embrittlemnt and show no tensile ductility even after very short aging treatment. Precipitation of intermetallic phases, θ-MnNi in Fe-Mn-Ni alloys and η-Ni3Ti in Fe-Ni-Ti alloys, at grain or lath boundaries was suggested as the reason for the weakening of grain boundary strength. Grain boundary strength recovered when these precipitates transform to austenite after extended aging. Dislocation glide or dislocation climb did critical role in conversion of these grain boundary precipitates to austenite.

Abstract: Thermal aging embrittlement of type 304L stainless steel weld is investigated on the basis of changes in microstructure, microhardness and electrochemical behavior after aging up to 20,000 h at 335, 365 and 400 °C. Spinodal decomposition and G-phase precipitation in the ferrite was observed after thermal aging. Aging led to increase in the hardness of ferrite phase while there was no change in the hardness of austenite. The changes in electrochemical behavior due to aging were studied using double loop electrochemical potentiokinetic reactivation (DL-EPR) test. Aging led to increase in the DL-EPR value which is attributed to Cr depletion in the ferrite phase.

Abstract: Because of its good corrosion resistance and biocompatibility, superelastic Ni-Ti wire alloys have been successfully used in orthodontic clinics. However, delayed fracture in the oral cavity has been observed. The susceptibility of a Ni–Ti shape memory alloy towards hydrogen embrittlement has been examined with respect to the residual stress after a few numbers of cycles and ageing in air at room temperature. Orthodontic wires have been cathodically charged by hydrogen with a current density of 10 A/m2 from 4h in 0.9% NaCl aqueous solution at room temperature. The critical stress for the martensite transformation under a monotonous tensile test of the as-charged specimen has been 30 MPa higher than that without hydrogen charging. However, after 1 to 50 cycles followed by hydrogen charging, the austenite-martensite plateau is decreased outstandingly compared to the monotonous tensile test of the as-charged by hydrogen specimen. Moreover, compared to the non-cycled and hydrogen charged material, the cyclic deformed and charged by hydrogen specimens present an increase of the initial stress for inducing martensite structure. This increase is about 110 MPa after 50 cycles. In addition, an embrittlement has been detected for the specimen submitted to 50 cycles and hydrogen charging for 4h. This behavior is attributed to the generated dislocations during cyclic deformation which act as trapping sites of hydrogen and a barrier for the austenite-martensite transformation.

Abstract: The liquid bismuth penetration from solid bismuth telluride (Bi2Te3) into grain boundaries of copper and copper-based solid solutions was studied. The experiments were performed at 570 oC in hydrogen atmosphere. The copper specimens were annealed from 10 to 90 minutes. Solid Bi2Te3 didn’t contact with copper specimens during experiments, so copper was covered by the decay products of Bi2Te3 through the gas phase. Liquid Bi penetration leads to grain boundary embrittlement of copper. We assumed that the depth of penetration is equal to the length of cracks which are formed after sample bending. It was shown that there is only bismuth at the grain boundaries while on the surface tellurium also exists. Bismuth concentration in the grain boundaries measured by Auger spectroscopy was about 10-20 at. % and the width of penetration channel was less then 10 nm. Bismuth covered the grain boundaries uniformly. Time dependence of the penetration depth was approximately parabolic. In the present work the effect of impurities (silver, iron) on grain boundary liquid bismuth penetration was studied also. It was shown that silver accelerates the penetration in contrast to iron. The probable reasons were discussed.

Abstract: The tensile ductility of the Ti-23Al-17Nb (at.%) alloy laser welded joints is very low at 650°C, so the embrittlement of the joints at high temperature was investigated. The metastable coarse B2 phase of the as-welded metal transformed into coarse O phase in the weld zone when the joints were heated up to 650°C. This type of phase transformation was based on shear transformation. Coarse orthorhombic O phase has lower ductility compared with body-centered cubic B2 phase. Meanwhile, the shear transformation from B2 to O phase could induce stress-strain concentrations in the grain boundary, which seriously weakened the grain boundary of the columnar grains and led to the brittle fracture characteristics in the weld zone during tensile test at 650°C.

Abstract: The microstructure and property of 15Cr1Mo1V steel welded joints after 70,000h service have been researched by OM, SEM, Vickers hardness test and small punch test (SPT). The microstructure of 15Cr1Mo1V steel welded joints after service became coarse and the grain boundary became vague. More carbide particles were observed in welded joints after service. The microhardness gradient curve of 15Cr1Mo1V steel welded joints before service was higher than that after service. The SPT properties gradient curves, including yield strength gradient curve and tensile strength gradient curve by SPT, had the same shape and trend as the microhardness gradient curve before and after service. The SEM fracture appearance of SPT specimens was the characteristic of ductile fracture, irrespectively of before or after service, though the dimple size was a little bit big and deep before service and some cluster particles were observed after service for the ripening of carbides.

Abstract: Stress corrosion cracking (SCC) is a brittle fracture of a ductile material under severe environment. Due to the complexity of mechano-chemical degradation during SCC formation, the scientific community is still far from the complete understanding of this phenomenon. Moreover, it is commonly misunderstood that polymeric materials is ‘SCC-free’, but it should be noticed that SCC is universal phenomenon for all engineering materials including polymers. In this paper, the similarity and differences of SCC in different materials, such as carbon steels and engineering polymers, are observed and reported. The SCC modeling in carbon steels and engineering polymers is also compared and discussed.

Abstract: Hydrogen embrittlement sensitivity of austenitic stainless steels, SUS316L and SUS310S exposed to high compressed hydrogen gas atmospheres was evaluated by means of a slow strain rate testing (SSRT) in air. Hydrogen evolution behavior during tensile deformation and fracture was also investigated by using a testing machine equipped with a quadrupole mass spectrometer installed in an ultrahigh vacuum chamber. When the SUS 316L specimen with hydrogen gas charging were deformed at a very slow crosshead speed of 1.67 nm/s, local deformation was promoted as compared to the specimen without hydrogen gas charging. On the other hand, no decrease of the ductility was observed in the SUS310S specimen with hydrogen gas charging even in the SSRT. In the hydrogen charged SUS316L specimen, the amount of continuous hydrogen evolution throughout deformation was much higher than that in the specimen without hydrogen gas charging. In addition, sudden hydrogen evolutions were sometimes identified in the SUS316L specimen with hydrogen gas charging during the deformation.