Morphology and lattice distortions of nitrided iron and iron-chromium alloys and steels

Specimens prepared from iron, iron-chromium alloys (0 to 4 wt% Cr) and commercial steels (C45, 41Cr4, 15CrNi6 and 24CrMo13) were powder nitrided at 818 K for 0.25 to 32 h. After cooling to room temperature the resulting morphology, lattice distortions and compositional variations were determined by X-ray diffraction analysis, metallographic methods and electron microprobe analysis. In the diffusion zone of iron and iron-chromium (0.54 wt% Cr), -(Fe₁₆N₂)- and -(Fe₄N)-nitrides were observed, whereas specimens of the iron-chromium alloys with a higher chromium content showed a finely dispersed submicroscopical precipitation of CrN in the matrix and precipitates of Cr₂N at the grain boundaries. With increasing nitriding times for the iron-chromium alloy with the highest chromium content (3.82 wt% Cr) a discontinuous precipitation starting from the grain boundaries occurred. For the first time recrystallization phenomena in the diffusion zone were observed, indicating that the inward diffusion of nitrogen introduces large lattice distortions. Large distortions were determined from X-ray diffraction line shift and line broadening respectively. The behaviour of macro- and micro-strain as a function of nitriding time was interpreted in terms of the volume changes caused by nitriding and subsequent precipitation. The residual surface stresses were calculated from the macrostrains applying the Voigt-Reuss-mean model. The experimentally determined ratio of the macrostrains in the (100) and the (110) directions was in good agreement with the value predicted from the model. The corresponding ratio of the microstrains was significantly larger than this theoretical prediction, which can be attributed to precipitates growing along (100) planes in the matrix (such as -nitride and CrN). In contrast with the macrostrain, a strong relation was found between the microstrain and the chromium content of the specimen.     

Phases in as-spun and aged binary aluminium-lithium ribbons

As-spun aluminium-lithium ribbons, containing up to 10wt% Li, and an Al-1.79 wt % Li ribbon aged at different temperatures (room temperature, 66, 175 and 200 °C) were investigated by transmission electron microscopy (TEM) and microhardness testing. It was found that the metastable phase Al₃Li () did not form either in the as-cast ribbons containing less than 1.79 wt% Li or in the featureless zone of Li-rich ribbons (Li content >3 wt%) but it did form, as the stable phase AlLi (), in the segregated zone of the later ribbons. The study of the early stage of precipitation showed that the mechanism of formation of depended on the ageing temperature. At a low temperature (66°C), the phase occurred by homogeneous spinodal decomposition. Whereas, it formed by heterogeneous nucleation at higher temperatures (175 and 200°C). These results are consistent with the presence of a spinodal curve within the miscibility gap ( + ). All investigations performed both on the as-cast and aged ribbons did not reveal any evidence for the formation of G.P. zones. It was also observed in this study that contamination of aluminium-lithium ribbons by impurities such as O₂ and CO₂ could alter the precipitation process and lead to the formation of lithium carbonate.     

Effects of lithium addition and wheel velocity on the microstructure of aluminium-lithium alloy ribbons

The effects of Li addition and wheel velocity on the microstructure of as-cast AI-Li ribbons were studied by optical metallography, scanning and transmission electron microscopy and by X-ray diffraction. Li addition has a marked effect on the ribbon solidification mechanism. Ribbons spun at 12.5 ms^–1 and which contained up to 1.79 wt% Li experienced a cooling rate of about 10⁴ Ks^–1 and solidified under the melt jet with a growth rate greater than 0.3 m s^–1. However, when Li content exceeded about 3 wt%, the ribbons were undercooled (cooling rate 10⁶ Ks^–1) and formed far from the melt puddle effect. Under these conditions, unusual secondary featureless zones occurred through the ribbon thickness and the detected phases corresponded to those expected to be formed under equilibrium conditions. Nevertheless, (AI₃Li) phase did not occur in ribbons containing up to 1.79 wt % Li or in the featureless zones of the most Li-rich ribbons (Li >about 3 wt%). Changing of the wheel velocity from 12.5 to 22.2 or 35.7 ms^–1 did not affect the ribbon formation mechanism, but it favoured the precipitation and decreased slightly the grain size.     

Structure,properties and response to heat treatment of melt-spun Al-Y and Al-La alloys

Al-Y and Al-La binary alloys containing 0.7–18 wt% (0.2–6.3 at%) Y and 0.9–18 wt% (0.2–4.2 at%) La, were rapidly solidified by chill-block melt-spinning to produce ribbons between 35 and 70 m thick. Microstructures were of the classical zone A/zone B type with a notable increase in Al lattice parameter for the Al-6.3 at% Y composition, which exhibited a Knoop hardness of 430±30 kg mm^–2 as-spun. Isochronal ageing for 2 h at 200–500 °C gave significant hardening at 200 and/or 300 °C for all of the more concentrated alloys, the largest responses being produced by Al-6.3 at % Y and Al-4.2 at % La at 200 °C. X-ray diffraction asspun indicated the presence of only Al and equilibrium Al₁₁La₃ in the Al-La alloy ribbons and Al and a non-equilibrium Al₄Y/Al₁₁Y₃ in the Al-Y ribbons. This non-equilibrium Al-Y phase was identified by X-ray diffraction as isomorphous with orthorhombic or tetragonal Al₁₁ La₃ with lattice parameters determined as a _o = 0.42 ± 0.02 nm (b _o = 1.26 ± 0.06 nm) and c _o = 0.97 ± 0.05 nm. TEM showed that it was present as an intercellular network with Energy dispersive spectroscopic analysis indicating an average composition Al-46 wt% Y consistent with the Al₄Y/Al₁₁Y₃ stoichiometry and diffraction patterns consistent with an orthorhombic or tetragonal cell with these lattice parameters. While no significant change in phase constitution of the Al-La ribbons was detected by X-ray diffraction as a result of heat treatment, the Al₁₁Y₃ in Al-Y ribbons was seen to be replaced by AI₃Y on heat treatment at 400 and 500 °C.     

Oxidation and corrosion behaviour of mild steel laser alloyed with nickel and chromium

Surface alloys were made on mild steel, coated with nickel and chromium using laser surface alloying. Mild steel was coated with a composite layer of nickel and chromium using the plasma technique. This was followed by laser irradiation using a continuous carbon dioxide laser. Oxidation and corrosion behaviour of these alloys was then determined by carrying out oxidation in air at 800 °C and corrosion tests at room temperatures in 1 n H₂SO₄. With a 75 m layer of nickel and chromium each, it was possible to make surface alloys on mild steel, which had a chromium concentration of 6–7 wt%, but the nickel concentration varied from 10–20 wt%. Oxidation behaviour improved significantly over the as-coated specimen and aqueous corrosion improved considerably.     

Role of aluminium addition on structure of Fe substituted Fe73·5 − x Si13·5B9Nb3Cu1Al x alloy ribbons

The investigation has dealt with the structure and magnetic properties of rapidly solidified and annealed Fe_73·5???x Si_13·5B₉Nb₃Cu₁Al _x (x?=?0, 2, 4, 6 at%) ribbons prepared by melt spinning. Complete amorphous structure was obtained in as-spun ribbons of x?=?0 and 2 at% compositions, whereas structure of ribbons containing higher Al was found to be partially crystalline. Detailed thermal analyses of the alloys and the melt spun ribbons revealed that the glass forming ability in the form of ${{ \textit{T}}}_{\mathbf{x}}{/}{{ \textit{T}}}_{\mathbf{l}}$ (ratio between crystallization and liquidus temperature) is the highest for 2 at% Al alloys and decreases with further addition of Al. Annealing of all as spun ribbons resulted in the precipitation of nanocrystalline phase embedded in amorphous matrix in the form of either ${ \textit{DO}}_{{ 3}}$ phase or bcc ${\upalpha}$ -Fe(Si/Al) solid solution depending on the initial composition of the alloy. Only bcc ${\upalpha}$ -Fe(Si/Al) solid solution was formed in 2 at% Al ribbons whereas ordered DO₃ structure was found to be stabilized in other ribbons including 0 at% Al. A detailed study on determination of precision lattice parameter of nanocrystalline phase revealed that the lattice parameter increases with the addition of Al indicating the partitioning behaviour of Al in nanocrystalline phase.     

FeCrNiAl alloy intended for using as blade material fabricated by rapid solidification method

We explored fabrication of ribbons of the FeCrNiAl alloy by rapid solidification method and succeeded in producing ribbons of 0.6 mm width and 30 m thickness. The obtained ribbon-shape product is prospective for usage as blade material. The as-solidified product, however, is too hard and brittle for the succeeding punching operation to be applied for a final-product shape. This was found to be due to existence of extremely fine particles of NiAl based B2 phase uniformly dispersed in the matrix. We, therefore, searched for a heat-treatment for improving ductility and found an appropriate one. By this heat-treatment ductility was improved to such an extent that the punching operation was applicable. We then searched for a heat-treatment for producing Al₂O₃ on the surface which facilitates a hard and sharp blade-edge, and found an appropriate one. This heat-treatment simultaneously hardens the matrix once softened to an extent sufficient for supporting the Al₂O₃ layers. The final product is suitable for applications as blades, such as razors.     

Preparation and properties of some alumina-chrome refractories

Preparative procedures, grain growth, densification and change in corundum lattice dimensions in sintered compacts made from chromium (III) oxide/aluminium (III) oxide mixtures were evaluated in a study of some alumina-chrome refractories. High-energy milling using A17 reactive alumina gave densification at low temperature and rapid grain growth, also a low variability in modulus of rupture. X-ray diffraction studies confirmed solid solution formation, the corundum lattice dimensions decreasing with 7 wt% chromium (III) oxide, other compositions giving lattice expansion. The modulus of rupture at 1150 °C for a series of ethyl silicate-bonded alumina-chrome refractories containing 5 to 12 wt% chromium (III) oxide was also minimal at 7 wt% chromium (III) oxide. The composition of this series is typical of ethyl silicate-bonded alumina-chrome refractories used in the steel industry.     

The effects of magnesium additions on the structure and properties of Al-7 Si-10 SiCp composites

The additions of magnesium to an aluminium alloy matrix, which contains insufficient magnesium, was found to be essential during the synthesis of composites by the stir-casting technique. Magnesium promotes interfacial wetting between the dispersoid surface and the matrix. Dispersion of SiC_p in Al-7 Si-0.3 Mg (356) alloy matrix without agglomeration and rejection was not possible. Hence, the addition of up to 3 wt% Mg was made to the alloy matrix during the dispersion of 10 wt% SiC_p (34 m), and the microstructure and mechanical properties of the composites were investigated with a view to optimize the magnesium content. With a magnesium content less than 1 wt% in the matrix, the SiC_p particles were essentially in agglomerated form. The highest UTS of 280–300 MPa was obtained with 1 wt% Mg content and SiC_p was uniformly distributed in the matrix. A higher magnesium content (>1.0 wt%) did not further improve the uniformity in the dispersion of SiC_p but the ultimate tensile strength properties deteriorated. This decrease in strength was attributed to the observed coarseness of the Mg₂Si phase, the precipitation of Mg₅Al₈ phase and the presence of a higher amount of porosity in the composites in the heat-treated condition. The aspect ratio (length/width) of precipitates changed from 1–3 for 1% Mg to 3–9 for 3.2% Mg in the matrix. Corresponding values for per cent porosity were 2% and 6%, respectively.     

Mössbauer studies of a-Fe74Co10−xCrxB16 alloys

Fe⁵⁷ Mössbauer spectroscopy has been employed to study hyperfine interactions in melt-spun a-Fe₇₄Co_10–x Cr _x B₁₆ (x = 0, 2.5, 5, 7.5 and 10) ribbons. The Curie temperature and the effective hyperfine fields observed for the spectra at 80 K show a systematic fall with increasing chromium content. Increasing chromium content also brings about a change in the orientation of the magnetization axis in that it comes closer to the direction of -ray propagation. The temperature dependence of hyperfine fields in a-Fe₇₄Co₅Cr₅B₁₆ alloy is accounted for using the existing theoretical models. The field distribution curves show a temperature dependence, while the e.f.g. distribution evaluated from the paramagnetic doublet spectra aboveT _c are temperature independent. The spin-wave coefficients B (B_3/2) and C (C_5/2) and the critical exponent, , were determined and their significances explained.     

The structure of a cast dental Ni-Cr-Be alloy

The structure of a typical dental Ni-Cr-Be alloy with 1.8 wt% Be has been investigated by SEM and TEM as well as by quantitative X-ray microanalyses in both instruments. Due to its low atomic weight the atomic fraction of Be is as high as 0.10. During solidification beryllium segregates substantially, and a large volume fraction of the casting is made up of a eutectic with coarse ( 1 µm diameter) alternating rods of fcc Ni-Cr and NiBe with a CsCl-type structure (ordered bcc). Smaller ( 0.1 µm diameter) rods of NiBe are precipitated in matrix in the solid state. Microanalyses of the NiBe rods show that they have a low chromium content ( 1.5 wt%). The cube boundary planes of the ordered b cc and fcc structures have a slight difference in orientation of about 7° which is most probably due to a small coherency misfit of the two types of lattices. The 100 directions in cube boundary plane of the fcc structure are nearly parallel to the 110 directions of the ordered bcc cube boundary plane. Sometimes another and more complex relationship between the two lattices occurs. The alloy contains 3.9 wt% Al which gives rise to numerous small ( 10 nm), spherical, ordered particles of Ni₃Al both in matrix as well as in the fcc eutectic rods.     

Cyclic oxidation of Haynes 230 alloy

The cyclic oxidation of Haynes 230 alloy (Ni-Cr-W-Mo alloy) was investigated in air at three different temperatures, 871, 982 and 1093 °C. Studies indicated that during cyclic oxidation, protective scales formed which were predominantly Cr₂O₃, with Kirkendall voids formed both at the scale/alloy interface and grain boundaries. Intergranular oxides were observed at temperatures above 982 °C while internal oxide particles were found above 1093 °C. Both intergranular and internal oxides were identified as aluminium oxide. A 50 m chromium-depleted zone developed after 70 h exposure at 1093 °C and was accompanied by disastrous scale spalling. The lowest chromium concentration within the depleted zone was 14 wt% which still provided a sufficient supply of chromium for development of a continuous Cr₂O₃ rich scale. Decarburization was observed at the higher temperature of 1093 °C, and a carbide-free zone developed. Also, it was found that Haynes 230 is subject to a sensitization process. At the lower exposure temperature of 871 °C, large amounts of chromium carbide formed preferentially at the grain boundaries. While at the surface region chromium carbide precipitation occurred at the twin boundaries.     

Synthesis of boron-dispersed carbon by pressure pyrolysis of organoborane copolymer

Boron-dispersed carbon was synthesized by pressure pyrolysis of divinylbenzene-tris(allyl)-borane and styrene-tris(allyl)borane at 125 MPa below 650° C. Amorphous boron dispersed in a carbon matrix was oxidized easily to yield boric acid by heat treatment under air at 300° C. The BK image of the product showed that boron was dispersed uniformly in a carbon matrix. Boron-dispersed carbon had the morphology of coalescing spherulite and polyhedra depending upon the concentration of boron in the parent copolymer. The grain size of carbon polyhedra decreased from 2.0m to 0.2m with an increase in the boron concentration from 1.3 to 5.7 wt%. The presence of 0.5 wt% boron in a carbon matrix enhanced the graphitization at 4.0 GPa and 1200° C, decreasing the lattice spacing with an increase in the crystallite size. The crystallite sizes were comparable to each other after heat treatment at 1100° C and 4.0 GPa when the specimen contained boron from 0.5 to 2.5 wt%. The lattice constant (c ₀) and crystallite size (L _c) of boron-dispersed carbon containing 2.5 wt% boron were 677.0 pm and 30 nm, respectively, after heat treatment at 1200° C and 4.0 GPa.     

Fe-Cr-C hardfacing alloys for high-temperature applications

The microstructure and phase chemistry of a Fe-34Cr-4.5C wt% hardfacing alloy has been investigated using transmission electron microscopy and microanalytical techniques. The microstructure is found to consist of large primary M₇C₃. carbides in a eutectic mixture of austenite and more M₇C₃. The results indicate that the microstructure of the undiluted alloy becomes configurationally frozen at a temperature of about 1150° C during deposition by the manual metal arc welding technique. This allows the metastable austenite phase to contain a large chromium concentration ( 16 to 17 wt %), thus imparting good corrosion and oxidation resistance. Experimental data on the partitioning of chromium, manganese and silicon between the carbide phases are discussed in the context of the high-temperature stability of the alloy.     

Banding structures in induced morphology crystal aggregates of CaCO3

Average radial growth rates in induced morphology crystal aggregates (IMCA) of CaCO₃ with sheaf-of-wheat morphologies range between 33.4 and 111 m/day. The growth process has not been found to be diffusion-controlled. Banding structures appearing in this kind of IMCA are explained just like rhythmic structures similar in origin to Liesegang rings, but the banding law in this case being modified by the matrix effect. Genetical models explaining the formation of ribbons with different particle density are also proposed.     

Microwave sintering of titanium diboride

Using a 2.45 GHz, 6 kW microwave furnace adapted for inert gas sintering, titanium diboride (TiB₂) can be rapidly microwave-sintered to >90% of theoretical density with sintering temperatures of 1900 to 2100 °C and soak times of 30 min or less. Densification behaviour with low-level additives was evaluated; 3 wt% chromium diboride (CrB₂) was an excellent sintering aid-grain growth inhibitor. A special covering system was required to produce oxidefree TiB₂. Specimen surface and interior temperatures were determined with a hole experiment. Comparison with conventional sintering indicates that microwave sintering of TiB₂-3 wt% CrB₂ occurs at lower temperatures (i.e., 200 °C lower) and can yield material with improved hardness, grain size, and fracture toughness.A portion of the material in this article was presented at a Symposium on Microwave Processing of Ceramics, 91st Annual Meeting of the American Ceramic Society, Indianapolis, Indiana, April 1989.Operated for the U.S. Department of Energy by Martin Marietta Energy Systems, Inc., under contract DE-AC05-84OR21400.     

Duplex spinel-ZrO2 ceramics

Duplex spinel-ZrO₂ ceramic composites were produced by an emulsion-hot kerosene drying technique. The sintered duplex spinel-ZrO₂ ceramics which had the composition of 55 wt% Al₂O₃-20 wt% ZrO₂-25 wt% MgO, consisted of a spinel matrix, whose grain size was in the range of 1.5 to 2.0 m, and uniformly dispersed zirconia agglomerates having grain sizes ranging from 1.0 to 2.0 m. Zirconia agglomerates began to appear at a temperature of 1500 °C and the duplex spinel-ZrO₂ structure was formed with the weight ratio of Al₂O₃/MgO being within 1.67 to 2.20 and the amount of ZrO₂ addition being within 5 to 25 wt %. The relative density, fracture toughness, flexural strength, and critical temperature difference of the spinel-ZrO₂ composite were 97.8%, 1.98 MPam^0.5, 390 MPa, and 275 °C, respectively.     

Preparation of amorphous composite materials

Composite meterialoconuioting of an amorphous Ni₇₅B₁₇Si₈ matrix and containing one or two reinforcing tungsten wire(s) were prepared by a modified melt-spinning technique, With this method it was possible to incorporate a single 10, 26 and 60m wire in the entire length of an amorphous ribbon, as well as two 26m wires. The process is continuous and allows the praparation of arbitrarily long composite ribbons.     

The influence of quenched-in crystals on the thermal stability of partially amorphous Zr76Ni24 alloy

Fully glassy and part-crystalline ribbons were prepared by melt spinning the Zr₇₆Ni₂₄ melt at different rates. X-ray and STEM studies confirmed that the quenched-in crystals are -zirconium. It is shown that these pre-existing crystals do not grow during annealing treatment nor do they act as heterogeneous nucleation sites, and remain inertly during the crystallization of the surrounding amorphous matrix. Crystallization studies showed that the existence of a fraction of crystallinity within the glassy matrix increases the stability of the matrix against further crystallization. Possible explanations for this behaviour are presented.Work carried out while the authors were in the University of Sussex, School of Engineering and Applied Sciences, Brighton, BN1 9QT, UK.     

Manufacturing and friction welding properties of particulate reinforced 7005 Al

This study has successfully incorporated Al₂O₃, SiC particulates into the 7005Al alloy matrix by using a drag-push method. The reinforced particulates are uniformly distributed in the matrix. This study also discusses the influence of aging treatment on the friction welding properties of 7005Al/10 wt%, 15 m and 6 m SiC_(p) composites and 7005Al/10 wt%, 15 m Al₂O_3(p) composites joint system. Experimental results show that after peak aging treatment was performed, if SiC particulates were used in the strengthening phase, the heat-affected zone (HAZ) had higher density of strengthening particulate, this resulted an increase in the hardness and stress concentration at the fully plasticized zone (Zpl) of the HAZ region, but a decrease in the width of the Zpl zone and the welding strength. And the welded fracture surface morphology had a low-ductile fracture.