Production of Nd-Fe-B alloy powders using high-pressure gas atomization and their hard magnetic properties

Fine spherical Nd−Fe−B powders with a tetragonal Nd₂Fe₁₄B phase have been produced by high pressure argon or helium atomization. The average size defined by 50 pct cumulative weight fraction is as small as about 25 μm. The Curie temperature of the powders is about 580 K and the intrinsic coercivity (iH _c ) of the bonded products made from the powders increases with decreasing particle size and reaches about 0.581 MA/m for the powder below 25 μm diameter. TheiH _c value increases with an increase in the cooling rate by helium atomization as well as with an increase in neodymium and boron content to 18 at. pct Nd and 12 at. pct B with the highestiH _c value reaching 0.716 MA/m. Annealing in the range of 773 to 1073 K gives rise to a further increase ofiH _c to 1.035 MA/m. The highiH _c value is promising for practical use as isotropic bonded powder magnets. The increase of magnetization for the bonded powder magnets takes place rapidly and the behavior is similar to that for sintered Nd−Fe−B magnets, in goods contrast to a sluggish increase of magnetization for the bonded Nd−Fe−B powder magnets made from the comminuted powders of melt-spun ribbon. From the magnetization behavior, it was presumed that the generation of the large intrinsic coercivity is due to the difficulty of the nucleation of reverse domain rather than the pinning of domain walls.     

Standard enthalpies of formation of neodymium alloys,Nd + Me (Me ≡ Ni,Ru, Rh,Pd, Ir,Pt), by high-temperature direct synthesis calorimetry

The Standard enthalpies of formation of 14 neodymium alloys have been determined by direct synthesis calorimetry at 1477 ± 2 K. The following values of ΔH _f ^o (kJ/g atom) are reported: NdNi₅, −(26.2 ± 1.1); Nd₅Ru₂, −(17.2 ± 1.9); NdRu₂, −(18.8 ± 1.2); Nd₅Rh₄, −(59.9 ± 2.5); NdRh, −(64.2 ± 2.0); NdRh₂, −(59.9 ± 1.1); NdRh₃, −(44.4 ± 1.6); NdPd, −(77.2 ± 2.7); NdPd₃, −(73.3 ± 2.3); Nd₅Ir₃, −(59.7 ± 2.7); NdIr₂, −(67.6 ± 1.5); NdPt, −(104.4 ± 2.6); NdPt₂, −(97.9 ± 2.4); and NdP₅, −(55.0 ± 3.1). The results are compared with available literature data for some of the neodymium alloys and with predicted values from the Miedema model.     

A metallographic study of porosity and fracture behavior in relation to the tensile properties in 319.2 end chill castings

A metallographic study of the porosity and fracture behavior in unidirectionally solidified end chill castings of 319.2 aluminum alloy (Al-6.2 pct Si-3.8 pct Cu-0.5 pct Fe-0.14 pct Mn-0.06 pct Mg-0.073 pct Ti) was carried out using optical microscopy and scanning electron microscopy (SEM) to determine their relationship with the tensile properties. The parameters varied in the production of these castings were the hydrogen (∼0.1 and ∼0.37 mL/100 g Al), modifier (0 and 300 ppm Sr), and grain refiner (0 and 0.02 wt pct Ti) concentrations, as well as the solidification time, which increased with increasing distance from the end chill bottom of the casting, giving dendrite arm spacings (DASs) ranging from ∼15 to ∼95 /im. Image analysis and energy dispersive X-ray (EDX) analysis were employed for quantification of porosity/microstructural constituents and fracture surface analysis (phase identification), respectively. The results showed that the local solidification time(viz. DAS) significantly influences the ductility at low hydrogen levels; at higher levels, however, hydro-gen has a more pronounced effect (porosity related) on the drop in ductility. Porosity is mainly observed in the form of elongated pores along the grain boundaries, with Sr increasing the porosity volume percent and grain refining increasing the probability for pore branching. The beneficial effect of Sr modification, however, improves the alloy ductility. Fracture of the Si, β-Al₅FeSi, α- Al₁₅(Fe,Mn)₃Si₂, and Al₂Cu phases takes place within the phase particles rather than at the particle/Al matrix interface. Sensitivity of tensile properties to DAS allows for the use of the latter as an indicator of the expected properties of the alloy.     

Influence of pressing speed on microstructural development in equal-channel angular pressing

The influence of pressing speed in equal-channel angular (ECA) pressing was investigated using samples of pure Al and an Al-1 pct Mg alloy and a range of pressing speeds from ∼10⁻² to ∼10 mm s⁻¹. The results show that the speed of pressing has no significant influence on the equilibrium grain size, at least over the range used in these experiments. Thus, the equilibrium grain sizes were ∼1.2 μm for pure Al and ∼0.5 μm for the Al-1 pct Mg alloy for all pressing conditions. However, it is shown that the nature of the microstructure is dependent on the pressing speed, because recovery occurs more easily at the slower speeds, so that the microstructure is then more equilibrated. There is also indirect evidence for the advent of frictional effects when the cross-sectional dimensions of the samples are at or below ∼5 mm.     

Short-range order effects on the tensile behavior of a nickel-base alloy

A nickel base weld filler metal alloy with nominal composition of 67 pct Ni, 20 pct Cr, 3 pct Mn, 3 pct Fe, and 2.5 pct Nb (Cb) is used to make austenitic-ferritic dissimilar metal joints. Tensile properties were determined for this alloy over the range 25 to 732°C at strain-rates of 3×10⁻⁶ and 3×10⁻⁴/s. Above about 450°C, both the yield strength and the ultimate tensile strength in the low strain-rate tests showed significant increases over the strengths at the higher strain-rate. The enhanced values for the yield strength persisted to the highest test temperature (732°C), whereas the ultimate tensile strength for the low strain-rate fell below the curve for the higher strain-rate at about 600°C. Above 600°C, the ultimate tensile strength dropped off rapidly and at 677°C approached the yield strength (i.e., the uniform elongation dropped to less than 1 pct). The strain-rate effects have been attributed to “K-state” formation, an effect that investigators have attributed to short range order in other Ni−Cr base alloys.     

Fabrication of bulk ultrafine-grained materials through intense plastic straining

Ultrafine grain sizes were introduced into samples of an Al-3 pct Mg solid solution alloy and a cast Al-Mg-Li-Zr alloy using the process of equal-channel angular (ECA) pressing. The Al-3 pct Mg alloy exhibited a grain size of ∼0.23 μm after pressing at room temperature to a strain of ∼4, but there was significant grain growth when the pressed material was heated to temperatures above ∼450 K. The Al-Mg-Li-Zr alloy exhibited a grain size of ∼1.2 μm, and the microstructure was heterogeneous after pressing to a strain of ∼4 at 673 K and homogeneous after pressing to a strain of ∼8 at 673 K with an additional strain of ∼4 at 473 K. The heterogeneous material exhibited superplastic-like flow, but the homogeneous material exhibited high-strain-rate superplasticity with an elongation of >1000 pct at 623 K at a strain rate of 10⁻² s⁻¹. It is concluded that a homogeneous microstructure is required, and therefore a high pressing strain, in order to attain high-strain-rate superplasticity (HSR SP) in ultrafine-grained materials. This article is based on a presentation made in the symposium “Mechanical Behavior of Bulk Nanocrystalline Solids,” presented at the 1997 Fall TMS Meeting and Materials Week, September 14–18, 1997, in Indianapolis, Indiana, under the auspices of the Mechanical Metallurgy (SMD), Powder Materials (MDMD), and Chemistry and Physics of Materials (EMPMD/SMD) Committees.     

Effect of B on the microstructure and mechanical properties of mechanically milled TiAl alloys

The present study is concerned with γ-(Ti₅₂Al₄₈)_100−x B _x (x=0, 0.5, 2, 5) alloys produced by mechanical milling/vacuum hot pressing (VHPing) using melt-extracted powders. Microstructure of the as-vacuum hot pressed (VHPed) alloys exhibits a duplex equiaxed microstructure of α₂ and γ with a mean grain size of 200 nm. Besides α₂ and γ phases, binary and 0.5 pct B alloys contain Ti₂AlN and Al₂O₃ phases located along the grain boundaries and show appreciable coarsening in grain and dispersoid sizes during annealing treatment at 1300 °C for 5 hours. On the other hand, 2 pct B and 5 pct B alloys contain fine boride particles within the γ grains and show minimal coarsening during annealing. Room-temperature compressing tests of the as-VHPed alloys show low ductility, but very high yield strength >2100 MPa. After annealing treatment, mechanically milled alloys show much higher yield strength than conventional powder metallurgy and ingot metallurgy processed alloys, with equivalent ductility to ingot metallurgy processed alloys. The 5 pct B alloy with the smallest grain size shows higher yield strength than binary alloy up to the test temperature of 700 °C. At 850 °C, 5 pct B alloy shows much lower strength than the binary alloy, indicating that the deformation of fine 5 pct B alloy is dominated by the grain boundary sliding mechanism. This article is based on a presentation made in the symposium “Mechanical Behavior of Bulk Nanocrystalline Solids,” presented at the 1997 Fall TMS Meeting and Materials Week, September 14–18, 1997, in Indianapolis, Indiana, under the auspices of the Mechanical Metallurgy (SMD), Powder Materials (MDMD), and Chemistry and Physics of Materials (EMPMD/SMD) Committees.     

An evaluation of the flow behavior during high strain rate superplasticity in an Al-Mg-Sc alloy

An Al-3 pct Mg-0.2 pct Sc alloy was fabricated by casting and subjected to equal-channel angular pressing to reduce the grain size to ∼0.2 μm. Very high tensile elongations were achieved in this alloy at temperatures over the range from 573 to 723 K, with elongations up to >2000 pct at temperatures of 673 and 723 K and strain rates at and above 10⁻² s⁻¹. By contrast, samples of the same alloy subjected to cold rolling (CR) yielded elongations to failure of <400 pct at 673 K. An analysis of the experimental data for the equal-channel angular (ECA)-pressed samples shows consistency with conventional superplasticity including an activation energy for superplastic flow which is within the range anticipated for grain boundary diffusion in pure Al and interdiffusion in Al-Mg solid solution alloys.     

On the nature of the Fe-bearing particles influencing hard anodizing behavior of AA 7075 extrusion products

The deleterious effects of Fe-bearing constituent particles on the fracture toughness of wrought A1 alloys have been known. Recent studies have shown that the presence of Fe-bearing, constituent particles is also determental to the nature and growth of the hard anodic oxide coating formed on such materials. The present study, using a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron probe microanalysis (EPMA), was made to examine the influence of the nature of the Fe-bearing particles on the hard anodizing behavior of AA 7075 extrusion products containing varying amounts of Si, Mn, and Fe impurities. It was found that, in the alloy containing 0.25 wt pct Si, 0.27 wt pct Mn, and 0.25 wt pct Fe, the Fe-bearing constituent particles are based on the Al₁₂(FeMn)₃Si phase (bcc with α=1.260 nm). These particles survive the hard anodizing treatment, add resistance to the electrical path, causing a rapid rise in the bath voltage with time, and cause a nonuniform growth of the anodic oxide film. In the materials containing 0.05 wt pct Si, 0.04 wt pct Mn, and 0.18 wt pct Fe, on the other hand, the formation of the Al₁₂(FeMn)₃Si-based phase is suppressed, and two different Fe-bearing phases, based on Al−Fe−Cu−Mn-based (simple cubic with a=1.265 nm) and Al₇Cu₂Fe, respectively form. Neither the Al−Fe−Cu−Mn-based phase nor the Al₇Cu₂Fe-based phase survive the hard anodizing treatment, and this results in a steady rise in the bath voltage with time and a relatively uniform growth of the anodic oxide film. Consideration of the size of the Fe-bearing, particles reveals that the smaller the particle, the more uniform the growth of the anodic oxide film.     

An evaluation of the flow behavior during high strain rate superplasticity in an Al−Mg−Sc alloy

An Al-3 pct Mg-0.2 pct Sc alloy was fabricated by casting and subjected to equal-channel angular pressing to reduce the grain size to ∼0.2 μm. Very high tensile elongations were achieved in this alloy at temperatures over the range from 573 to 723 K, with elongations up to >2000 pct at temperatures of 673 and 723 K and strain rates at and above 10⁻² s⁻¹. By contrast, samples of the same alloy subjected to cold rolling (CR) yielded elongations to failure of <400 pct at 673 K. An analysis of the experimental data for the equal-channel angular (ECA)—pressed samples shows consistency with conventional superplasticity including an activation energy for superplastic flow which is within the range anticipated for grain boundary diffusion in pure Al and interdiffusion in Al−Mg solid solution alloys. MINORU NEMOTO, formerly Professor, Department of Materials Science and Engineering, Faculty of Engineering, Kyushu University.     

The role of thermal stabilization in martensite transformations

The variation of the kinetics of the martensite transformation with carbon content and martensite habit plane has been investigated in several Fe−Ni based alloys. Transformation in an Fe-25 wt pct Ni-0.02 wt pct C alloy exhibits predominantly athermal features, but some apparently isothermal transformation also occurs. In a decarburized alloy, on the other hand, the observed kinetic features, such as the dependence ofM _s on cooling rate, were characteristic of an isothermal transformation. In contrast, Fe-29.6 wt pct Ni-10.7 wt pct Co alloys with carbon contents of 0.009 wt pct C and 0.003 wt pct C transform by burst kinetics to {259}_γ plate. At both these carbon levels, theM _b temperatures of the Fe−Ni−Co alloys are independent of cooling rate. It is proposed that the change in kinetic behavior of the Fe-25 pct Ni alloy with the different carbon contents is due to the occurrence of dynamic thermal stabilization in the higher carbon alloy. Dynamic thermal stabilization is relatively unimportant in the Fe−Ni−Co alloys which transform by burst kinetics to {259}_γ plate martensite. P. J. FISHER, formerly with the University of New South Wales D. J. H. CORDEROY, formerly with the University of New South Wales     

The Fe-rich corner of the metastable C-Cr-Fe liquidus surface

The liquidus surface of the C-Cr-Fe system has been experimentally determined in the Fe-rich region —C ≤6 wt pct, Cr ≤40 wt pct —using a sensitive differential thermal analysis technique, along with optical and scanning electron microscopy and X-ray diffraction. Previous liquidus surfaces for this system have differed on the extent of the (Cr,Fe)₂₃C₆ liquidus field, with one version reporting its existence at ∼20 wt pet Cr, and others finding that it did not occur at Cr levels of less than ∼60 wt pct. The present investigation provides evidence in favor of the second contention, with the (Cr,Fe)₂₃C₆ field not being detected at Cr ≤40 wt pct. Changes are proposed to the accepted liquidus surface in respect of the compositions of the invariant reactions—L + αδFe ⇌γFe + (Cr,Fe)₇C₃ andL + (Cr,Fe)₇C₃ ⇌γFe + (Fe,Cr)₃C —and of the monovariant eutectic valley—L⇌ γFe + (Cr,Fe)₇C₃.     

Aluminum deoxidation equilibrium in liquid Fe-16 pct Cr alloy

For thermodynamic prediction, the deoxidation equilibrium of aluminum in liquid Fe-16 pct Cr alloy was studied by employing the electromagnetic levitation technique with a cold crucible in an Ar gas atmosphere at 1923 K. The interaction parameters were determined to be e _Al(Fe) ^Cr =0.0001 (0.19/T, 1823 K≤T<1923 K) and r _Al(Fe) ^O,Cr =−0.001. The calculated relationship between aluminum and oxygen contents in Fe-16 pct Cr alloy by thermodynamic data obtained in this study is in good accordance with the experimental results of the present study and other research.     

The tempering of FeNiN martensite

The tempering behavior of ternary iron-nickel-nitrogen martensitic specimens (∼13.5 at. pct Ni, ∼4.7 at. pct N; ∼4.9 N atoms/100 metal atoms) in the temperature range of 270 to 670 K was investigated by analysis of the corresponding changes in the crystalline structure (X-ray diffraction), volume (dilatometry), and enthalpy (calorimetry). At least three stages of structural change can be distinguished: (1) redistribution of nitrogen atoms, involving segregation and formation of nitrogen enrichments, occurs at temperatures up to 370 K, (2) precipitation of γ′-(Fe, Ni)₄N_1-x nitride takes place in the temperature range of 370 to 430 K, and (3) coarsening of the γ′ precipitates and decomposition of a part of the retained austenite occur at temperatures above 430 K. From a comparison with the tempering behavior of a binary iron-nitrogen martensitic alloy containing a similar amount of interstitials, it was concluded that the presence of nickel suppresses the development of the intermediate α″-(Fe,Ni)₁₆N₂ nitride and advances the precipitation of γ′ nitride.     

Effect of overaging on mechanical properties and stress corrosion cracking of HY-180M steel

The tensile properties, fracture toughness and stress corrosion cracking (SCC) behavior of HY-180 M steel at 22 °C were studied after final 5 h overaging treatments >510 ≤650 °C. SCC tests were conducted for 1000 h with compact tension specimens in aqueous 3.5 pct NaCl solutions at a noble (anodic) potential of −0.28 V_SHE ( −0.48 V_Ag/AgC1) and a cathodic protection potential of −0.80 V_SHE (−1.0 V_Ag/AgC1). The SCC resistance improved at aging temperatures >565 °C, the most significant improvement being at −0.80 V_She, especially after 650 ° aging whereK _ISCC was raised to at least 110 MPa · m^1/2. However, this was at the expense of mechanical properties. Provided low crack propagation rates of ∼3 X 10⁻¹¹ m/s at −0.80V _SHEmay be tolerated, the best compromise between strength, toughness, and SCC resistance was obtained after 594 °C aging. Under these conditions, stress intensities as high as ∼ 110 MPa · m^1/2 can be used, with a yield strength of ∼ 1150 MPa and fracture toughness of ∼ 170 MPa · m^1/2. The retained austenite content after aging increased with aging temperature up to 25 pct by vol at 650 °C. It appeared to correlate with improved SCC resistance, but other microstructural effects associated with aging may be involved. Formerly Research Associate with theDepartment of Metallurgical Engineering , University of BritishColumbia     

Intermetallic phases formed during DC-casting of an Al−0.25 Wt Pct Fe−0.13 Wt Pct Si alloy

The Al−Fe and Al−Fe−Si particles formed during DC-casting of an Al-0.25 wt pct Fe-0.13 wt pct Si alloy have been examined. The particles were analyzed by transmission electron microscopy (TEM) and energy dispersive spectroscopy of X-rays (EDS). Crystal faults were studied by high resolution electron microscopy (HREM). Samples for electron microscopy were taken at various positions in the ingot,i.e., with different local cooling rates during solidification. At a cooling rate of 6 to 8 K/s the dominating phases were bcc α-AlFeSi and bct Al _m Fe. The space group of bcc α-AlFeSi was verified to be Im3. Superstructure reflections from Al _m Fe were caused by faults on {110}-planes. At a cooling rate of 1 K/s the dominating phases were monoclinic Al₃Fe and the incommensurate structure Al _x Fe. In Al₃Fe, stacking faults on {001} were frequently observed. The structure of Al _x Fe is probably related to Al₆Fe. Some amounts of other phases were detected. For EDS-analysis, extracted particles mounted on holey carbon films were examined. Extracted particles were obtained by dissolving aluminum samples in butanol. Accurate compositions of various Al−Fe−Si phases were determined by EDS-analysis of extracted crystals.     

Interdiffusion in Co-Mn alloys

Interdiffusion coefficient in cobalt-manganese alloys has been determined by Matano's method in the temperature range between 1133 and 1423 K on (pure Co)-(Co-30.28 at. pct Mn alloy) and (pure Co)-(Co-51.76 at. pct Mn alloy) couples. This, ∼D, has been found to increase with the increase of manganese content. However, the activation energy (∼Q) and frequency factor ( ₀) show a maximum at about 10 at. pct Mn. The concentration dependence of and has been discussed taking into account the thermodynamic properties of the alloy. The difference in between the ferro- and paramagnetic phases in Co-5 at. pct Mn alloy has been found to be 24 kJ/mol, which is larger, than that for the diffusion of Mn⁵⁴ in this alloy. Further it has been found that the Kirkendall marker moves toward manganese-rich side, showing that manganese atoms diffuse faster than cobalt atoms. From the marker shift, the intrinsic diffusion coefficients,D _Co andD _Mn, at 33 at. pct Mn have been determined as follows:D _Co=0.22×10⁻⁴ exp(−263 kJ mol⁻¹/RT) m²/s, andD _Mn=0.98×10⁻⁴ exp(−229 kJ mol⁻¹/RT) m²/s.     

Aluminum matrix composites: Fabrication and properties

Aluminum alloy matrix composites containing 1 to 30 wt pct of fibrous and particulate nonmetals varying in size from 0.06 μm to 840 μm were fabricated. The composites were cast into cylindrical molds for friction and wear tests, hot extrusion and tensile tests. The distribution of the nonmetals in the cast ingots was homogeneous. Friction and wear tests were done on a pin (52100 bearing steel) and dish type machine without lubrication. It was found that composites containing ∼10 wt pct or more of SiC, TiC, Si₃N₄, Al₂O₃, glass, solid waste slag, and silica sand wear less than the pure matrix alloy, but have slightly higher average coefficients of friction. Wear in composites containing soft particles, especially MgO and boron nitride was higher than the pure matrix alloy. The average coefficient of friction of all the composites was in the range of 0.35 to 0.58. Increasing the sliding velocity reduced this range to ∼ 0.4 to 0.45. The longitudinal tensile properties of the extruded composites (with the exception of loss of ductility in some cases) are comparable to that of the matrix alloys. Improvements in strength or ductility were noted. For example, addition of 15 wt pct of 3 μm size Al₂O₃ particles raised the yield and ultimate strength of the Al-4 pct Cu-0.75 pct Mg alloy matrix from 227 to 302 MPa, and 356 to 403 MPa, respectively. The corresponding percent elongation decreased from 25.8 to 12.5. The fact that the various composites can be readily cast and hot formed suggests a variety of engineering applications. AKIRA SATO, formerly Visiting Scientist at Massachusetts Institute of Technology, Cambridge.     

The effect of Sr and Fe additions on the microstructure and mechanical properties of a direct squeeze cast Al-7Si-0.3Mg alloy

This article describes the results of an investigation into the microstructure and mechanical properties of a gravity die cast and direct squeeze cast LM25 alloy (Al-7Si-0.3Mg-0.3Fe). The direct squeeze cast LM25 alloy has superior mechanical properties compared to the gravity die cast LM25 alloy, especially with regard to ductility, which is increased from ∼1.7 pct for the gravity die cast LM25 alloy to ∼8.0 pct for the direct squeeze cast LM25 alloy in the T6 heat-treated condition. This increase in ductility is due to (1) the removal of porosity, (2) a decrease in Si particle size, and (3) a refinement of the Fe-Si-aluminide particles. High cooling rates in direct squeeze casting result in quench modification of the Si particles, such that chemical modification with Sr or Na may not be required. In addition, direct squeeze casting is more tolerant of Fe impurities in the alloy, due to the formation of smaller Fe-Si-aluminide particles than those in gravity die cast material. The direct squeeze cast LM25+Fe alloy (Al-7Si-0.3Mg-1.0Fe) has a ductility of ∼6.5 pct, compared to that of ∼0.5 pct for the gravity die cast LM25 + Fe alloy in the T6 heat-treated condition. This increase in tolerance to Fe impurities can lead to a substantial reduction in manufacturing costs due to (1) reduced raw-material costs, (2) reduced die sticking, and (3) improved die life.     

The anisotropy of deformation and fracture in a directionally solidified Ni/Ni3Al-Ni3Cb lamellar eutectic alloy

The orientation dependence of deformation and fracture modes was investigated for a directionally-solidified Ni−Ni₃Al−Ni₃Cb lamellar eutectic alloy (Ni-20 wt pct Cb-2.5 wt pct al-6.0 wt pct Cr) using optical and transmission microscopy to examine tensile and compression specimens tested at temperatures below the softening point of the δ (Ni₃Cb) reinforcing phase (∼1050 K). In this temperature range there is a large difference between longitudinal and transverse tensile ductibility (>5 pct longitudinalvs<1 pct transverse). No single preferred fracture path (such as interfacial delamination) could be found to account for the low transverse tensile ductility. Analysis of the δ twinning geometry, however, indicated that the twinning strains for twins of the type {211}, which operate copiously in longitudinal tension, are negative in most transverse orientations, with Schmid factors being very low (<0.013) in the limited range of transverse orientations where {211} twin strains are positive. Examination of transverse tension test specimens broken at 1033 K confirm the absence of {211} twins, with only limited {011} twinning being found in selected grains, leading to the conclusion that the relatively low transverse tensile ductility of the eutectic results from the very limited number of deformation systems which operate in the δ reinforcing phase below the softening temperature.