Zinc-iron phases formed on galvannealed steel

Mössbauer spectroscopy has been used to identify the zinc-iron intermetallic phases present in the coating of three galvannealed steels, two of which were produced on commercial galvanizing lines and one in the laboratory. Both CEMS and XMS have been used in order to determine the depth dependence of each phase. Three main zinc-iron phases have been identified with the proportion of each dependent on the preparation conditions. In the commercially produced coatings, CEMS, probing near the top of the coating, indicates that the zinc rich χ-FeZn₁₃ and δ-FeZn₁₀ alloys are present. XMS indicates the presence of some Π-Fe₃Zn₁₀ closer to the steel. In contrast, the laboratory produced sample contains nearly pure χ-FeZn₁₃ through the entire coating thickness. Subphases of the delta and gamma alloys were also identified. These subphases appear to be mixed with a preference for the higher zinc subphase of each to form closer to the surface of the coating.     

Structure of intermetallic phases in Al-free galvannealed zinc coatings

Galvannealed coatings of thickness of (20–40) μm were prepared on the ultra low carbon (ULC) steel substrate. Metalography analysis was carried out to obtain the phase composition of coatings. Coatings were then transfered onto a polyacrylate foil. Transmission spectra yielding all the positions of iron within the coating thickness were measured. The doublet of zeta phase occured only after short annealing times, lower annealing temperatures and longer dipping times. Parameters of three sites of delta phase were observed to approach equilibrium values at higher annealing temperatures and longer annealing times. These changes are ascribed to diffusion transformations in the coatings during annelaing. Presented at International Colloquium “M?ssbauer Spectroscopy in Materials Science”, Všemina, Czech Republic, June 1–4, 2004.     

Thermal stress-induced fracture of coating layer of galvannealed steel

The thermal stress-induced multiple fracture of the Fe-Zn intermetallic compound coating layer of galvannealed steel was investigated for samples with IF (interstitial free steel) and SPCC (steel plate cold commercial, Japanese Industrial Standard) as substrates. The analysis of the measured length of the multiply fractured coating layer showed that the fracture strength of the coating layer was 260–280 MPa and the residual stress around 720–740 MPa of the coating layer was reduced to around 130–140 MPa through the multiple fracture. The influences of the coating layer thickness and the stress–strain curve of the substrate on the multiple fracture behavior of the coating layer were also clarified.     

In-situ observations and acoustic measurements upon fragmentation of free-floating intermetallics under ultrasonic cavitation in water

Grain refinement in alloys is a well-known effect of ultrasonic melt processing. Fragmentation of primary crystals by cavitation-induced action in liquid metals is considered as one of the main driving mechanisms for producing finer and equiaxed grain structures. However, in-situ observations of the fragmentation process are generally complex and difficult to follow in opaque liquid metals, especially for the free-floating crystals. In the present study, we develop a transparent test rig to observe in real time the fragmentation potential of free-floating primary Al₃Zr particles under ultrasonic excitation in water (an established analogue medium to liquid aluminium for cavitation studies). An effective treatment domain was identified and fragmentation time determined using acoustic pressure field mapping. For the first time, real-time high-speed imaging captured the dynamic interaction of shock waves from the collapsing bubbles with floating intermetallic particles that led to their fragmentation. The breakage sequence as well as the cavitation erosion pattern were studied by means of post-treatment microscopic characterisation of the fragments. Fragment size distribution and crack patterns on the fractured surface were then analysed and quantified. Application of ultrasound is shown to rapidly (<10 s) reduce intermetallic size (from 5 mm down to 10 μm), thereby increasing the number of potential nucleation sites for the grain refinement of aluminium alloys during melt treatment.     

Hydrogen absorption and its effect on the magnetic properties of rare-earth iron intermetallics

X-ray diffraction studies of the hydrogen absorption in several YFe and CeFe intermetallic compounds showed that no structural changes occur upon hydrogen absorption in Y₆Fe₂₃, YFe₃, YFe₂. The lattice constants of the hydrides were found to be appreciably larger than those of the pure intermetallic compounds. The magnetic properties of the hydrides were determined and compared with the original compounds. In all cases the magnetic moment per Fe atom proved to be much larger in the hydride phases. Hydrogen absorption can lead to a decrease as well as to an increase of the magnetic ordering temperature (T_c). These changes in T_c could adequately be explained in terms of the observed increases in lattice constant and the data available for the pressure derivative of T_c of these compounds.     

Magnetic properties and spin reorientation of iron rare earth based intermetallics

The Curie temperature and saturation magnetization M_s(μ_B/f.u.) of R₂Fe₁₄B have been discussed. The spin reorientations of Nd₂Fe₁₄B compounds have been studied by many authors with various methods, but have not been checked with the neutron diffraction method. We investigated the spin reorientation of Nd₁₅Fe₇₈B₇ by neutron diffraction and obtained θ = 26°34' at 77 K which is in good agreement with other authors' results. The small amount substitution of Si for Fe in Nd₂(Fe_1−xSi_x)₁₄B increases T_c and _cH_c of the compound. These will be able to make an advantage for Nd-Fe -B magnets.     

The effects of iron oxides on proton relaxivity

The magnetic properties and relaxivities of superparamagnetic, ferromagnetic and paramagnetic iron oxides are presented and compared. The iron in colloids of ferromagnetic iron oxide has a large spin-spin relaxivity and a small spin-lattice relaxivity. The iron in colloids of paramagnetic iron oxide has a low spin-spin and spin-lattice relaxivity. The iron in colloids of highly dispersed superparamagnetic iron oxides has a large spin-spin relaxivity and a large spin-lattice relaxivity. Superparamagnetic colloids with various particle sizes in solution have been made by varying the number of superparamagnetic iron oxide crystals per particles in solution. Superparamagnetic colloids of larger solution particle size have a lower spin-lattice relaxivity than colloids comprised of smaller solution particle sizes.     

Hard carbon coatings deposited on steel

We report results of steel substrates coated with hard amorphous carbon and with diamond films. In order to enhance the adherence to the substrate, steel substrates were pretreated by means of a silicon ion beam. Raman spectroscopy was used to analyze the structure of silicon interface while the elastic recoil detection analysis method was applied to determine their composition and thickness. The a-C adherence to the substrate and hardness were also tested. The diamond films were observed by SEM.     

Studies on some alkali-phosphate glasses containing antimony and iron oxides

The effect of heat treatment on the Fe²⁺/Fe³⁺ ratio in some alkali phosphate glasses containing antimony and iron oxides has been investigated. Differential thermal analysis, Mossbauer spectroscopy, magnetic susceptibility and density measurements as well as the molar volume calculations were used in this study. The obtained data indicate that all glasses show approximately unchangeable stability. It is found, from the area under the Mossbauer absorption spectra that ferrous transformed gradually to ferric ions and the transformation rate constant was found to be 48 min⁻¹. The elapsed time was found to be 160 min. This transformation represents a first-order solid state reaction. The molar magnetic susceptibility measurements confirm the above results.     

Nanowires of iron oxides embedded in SiO2 templates

To attain the complete filling of the channels of MCM-41 with magnetite and maghemite, we have tried out an alternative method to the incipient wetness impregnation. The mesoporous material was instilled with a Fe-carrying organic salt after subjecting the matrix to a silylation treatment. Thus, a solid of 7.7 wt.% iron-loaded MCM-41 was obtained. Different subsequent thermal treatments were used to produce γ-Fe₂O₃ or Fe₃O₄. The Mössbauer and magnetic results show that after this method, the as-prepared composite displays a size-distribution of magnetic particles. It is mainly made up of fine particles that display a superparamagnetic relaxation at room temperature and get blocked at ≈42 K for the AC susceptibility time-scale measurements both for γ-Fe₂O₃ and Fe₃O₄ particles. For both samples, about 24% of larger iron-containing phases are magnetically blocked at room temperature. For the Fe₃O₄ particles, this fraction undergoes the Verwey transition at about 110 K; in addition, there is a minor Fe (III) fraction that remains paramagnetic down to 4.2 K.     

Surface oxidation phenomena of boride coatings grown on iron

Single phase and polyphase boride coatings were grown on iron using B₄C-base powder mixures, oxidized for 1 h in gascous oxygen at temperatures up to 850°C and then studied by means of surface Mössbauer measurements, X-ray diffraction analyses and metallographic observations.     

Study of coatings based on titanium oxides and oxynitrides using a set of methods

A set of methods has been applied to study the properties of titanium oxide and oxynitride coatings on steel. It is established that the coatings consist of two phases with a nanocrystalline and an amorphous structure with a high proportion (～50 %) of grain boundaries; anatase is the dominant crystalline phase. The obtained results can be applied in the development of coatings for stents as well as for manufacturing hydrogen accumulators.     

光学薄膜激光损伤的光热偏转法实时研究

本文首次把调制与脉冲光热偏转技术同时置于激光损伤测试装置之中,用以进行光学薄膜激光损伤的实时研究.初步实验结果表明,该方法不仅有助于确定损伤阀值,也有助于分析损伤形貌和损伤过程,包括样品损伤前的行为及样品在重复频率激光作用下损伤的累积效应.     

In-situ SEM observations of ultrasonic cavitation erosion behavior of HVOF-sprayed coatings

Several typical high-velocity oxy-fuel (HVOF)-sprayed coatings, including WC-10Co4Cr coatings, Co-based coatings, WC-10Co4Cr/Co-based composite coatings, and Fe-based amorphous/nanocrystalline coatings were fabricated, and their cavitation behavior was evaluated in deionized water. Further, in-situ SEM surface observations were used to understand the microstructure of tested coatings. The results show that cavitation erosion initially occurred at pre-existing defects in the coatings. Meanwhile, it was found that cavitation erosion damage of the WC-10Co4Cr/Co-based composite coating, which contained a hard reinforcing phase (WC-10Co4Cr phase) and a soft matrix phase (Co-based phase), preferentially occurred at or around pores and microcracks in the reinforcement, rather than in the defect free matrix. This suggested that defects were a critical contributing factor to cavitation damage of the composite coatings. Furthermore, a mechanism was suggested to explicate the cavitation behavior of composite coatings. The approach of using in-situ SEM surface observations proved to be useful for the analysis of the cavitation mechanism of engineering materials and protective coatings.     

Cavitation erosion mechanisms in Co-based coatings exposed to seawater

The cavitation erosion (CE) of most materials in seawater is more serious than in fresh water due to the onset of corrosion; however, in a previous study we reported results that contradict this widely accepted trend. In this research our objective is to provide fundamental insight into the mechanisms that may be responsible for these earlier results. To accomplish this objective, two types of Co-based coatings, prepared by high velocity oxygen fuel (HVOF) spraying system, were used to further investigate the underlying corrosion-mitigating CE mechanism in seawater. Accordingly, the influence of spraying parameters on microstructure, composition and mechanical properties of the coatings was analyzed on the basis of SEM, XRD, Raman spectroscopy, Vicker’s hardness and nano-indentation results. Electrochemical corrosion tests were used to evaluate the corrosion behavior of the Co-based coatings. Their CE performances in seawater and deionized water were comparatively studied by a vibratory apparatus. Results demonstrated that a higher flame temperature facilitated the oxides formation with associated improvements in compactness, hardness and toughness of the coatings. The presence of alumina in combination with the oxides formed in-situ facilitated the formation of an oxidation film on surfaces, and effectively enhanced the charge transfer resistance of the coating, thereby significantly improving the corrosion resistance in seawater. Metallic Co was not only more easily oxidized but also more readily corroded than the alloyed Co. Compactness was identified as an important factor affecting CE resistance of coatings in deionized water, because defects facilitate the nucleation and eventual collapse of bubbles. Moreover, bubble collapse produced a transient high temperature spike in excess of 600 °C that also caused Co and Cr elements to oxidize. Because the CE tests were carried out in seawater, additional Co₃O₄ and Cr₂O₃ were generated owing to corrosion that more effectively increased the surface compactness and mechanical properties of the coatings. This behavior was particular notable for coatings with metallic Co and Cr, which should be why seawater corrosion could weaken the CE of Co-based coatings.     

Magnetism in iron implanted oxides: a status report

Emission Mössbauer spectroscopy on ⁵⁷Fe fed by ⁵⁷Mn ions implanted in the metal oxides ZnO, MgO and Al₂O₃ has been performed. The implanted ions occupy different lattice sites and charge states. A magnetic part of the spectra in each oxide can be assigned to Fe^3?+? ions in a paramagnetic state with unusually long relaxation time observable to temperatures up to several hundreds Kelvin. Earlier expectations that the magnetic spectra could correspond to an ordered magnetic state could not be confirmed. A clear decision for paramagnetism and against an ordered magnetic state was achieved by applying a strong magnetic field of 0.6 Tesla. The relaxation times deduced were compared to spin–lattice relaxation times from electron paramagnetic resonance (EPR).     

Characterization of iron oxides and hydroxides in the sand of Flaming Mountain

A sand sample collected at the foot of Flaming Mountain was studied further. On the basis of the change in the relative intensity after heat treatment, one of the sextets observed in its Mössbauer spectrum at liquid nitrogen temperature was ascribed to goethite. This result provides a useful information on the geological history of the region where the mountain is located.