Microstructural phase evaluation of high-nitrogen Fe–Cr–Mn alloy powders synthesized by the mechanical alloying process

In this study, the formation of Fe18Cr8MnxN alloys by mechanical alloying (MA) of the elemental powder mixtures was investigated by running the milling process under nitrogen and argon gas atmospheres. The effect of the milling atmosphere on the microstructure and phase contents of the as-milled powders was evaluated by X-ray diffraction and transmission electron microscopy. The thermal behavior of the alloyed powders was also studied by differential scanning calorimetry. The results revealed that in the samples milled under nitrogen, three different phases, namely ferrite (α), austenite (γ), and a considerable amount of amorphous phase are present in the microstructure. In contrast, in the samples milled under argon, the structure contains the dominant crystalline ferrite phase. By progression of MA under the nitrogen atmosphere, the ferrite-to-austenite phase transformation occurs; meanwhile, the quantity and stability of the amorphous phase increase, becoming the dominant phase after 72 h and approaching 83.7 wt% within 144 h. The quantitative results also showed that by increasing the milling time, grain refinement occurs more significantly under the nitrogen atmosphere. It was realized that the infused nitrogen atoms enhance the grain refinement phenomenon and act as the main cause of the amorphization and α-to-γ phase transformation during MA. It was also found out that the dissolved nitrogen atoms suppress the crystallization of the amorphous phase during the heating cycle, thereby improving the thermal stability of the amorphous phase.     

Can the degree of crystallinity of ball-milled Mg2Ni intermetallic compound decide its electrochemical characteristics?

ABSTRACT

Nanostructured Mg₂Ni intermetallic compounds were synthesized by high-energy ball milling. Effect of milling time on structure and surface morphology of milled powders were studied using x-ray diffraction and scanning electron microscopy. Crystallite size and degree of crystallinity were confirmed by using transmission electron microscopy and selected area electron diffraction analysis. The particle size of 20 h milled electrode material is 230 nm and it reduced to 40 nm when the milling time is increased to 30 h. Further increase in the milling time reduces the particles size drastically and starts agglomerating. In order to understand the effect of milling time on reaction rates, differential thermal analysis was performed. Activation energy of the milled powders was calculated using Kissinger analysis. 30 h milled powder exhibits lower activation energy than others. Cyclic voltammetry, electrochemical impedance spectroscopy, and charge–discharge studies were done on the prepared electrode materials. 30 h milled electrode material delivers maximum discharge capacity with a superior capacity retention after 20 cycles at 20 mAg^?1.     

Microstructural examination of two experimental high-strength bainitic low-alloy steels containing silicon

Abstract

A detailed microstructural characterization of two silicon-containing low-alloy steels, Fe–0·2C–2Si–3Mn and Fe–0·4C–2Si–4Ni (nominal wt-%), isothermally transformed in the bainitic temperature range (~ 400–250°C), has been carried out using principally electron microscopy, X-ray diffraction, and dilatometry. Upper bainite in these silicon-containing steels consists of bainitic ferrite laths and interwoven thin films of retained austenite instead of cementite. Coarser granular regions of retained austenite may also be obtained. The bainitic ferrite laths (or plates) in lower bainitic structures contain intralath carbides, but the interlath morphology of retained austenite still occurs. The variations in these microstructures with isothermal transformation temperature, and the thermal stability of the retained austenite phase is described and discussed.

MST/526     

Effect of Trivalent Ion Substitution on the Physical Properties of M-Type Hexagonal Ferrites

Aluminum-substituted barium hexagonal ferrite particles BaAl _x Fe_12-x O₁₉ with 0 ≤ x ≤ 3.5 have been prepared by solid state reaction method. The qualitative phase analysis of studied powder samples and the morphology of powders after milling were determined using the x-ray diffraction method and scanning electron microscopy, respectively. The barium hexagonal ferrite phase appeared to be the main component of the samples. The crystal size of BaFe₁₂O₁₉ phase is above 25 nm. The scanning electron microscopy images showed irregular shape and size of powder particles. According to the analytical method findings, the type of crystal lattice was confirmed to be hexagonal and the parameters of unit cell volume and x-ray density were determined. It is shown that such parameters decrease with increasing Al substitution from 699.019 to 696.702 Å³ and 5.258 to 4.828 gm/Cm³, respectively. The values of lattice parameters, grain size, microstrain, and dislocation density of all samples were calculated. The c/a value obtained from the x-ray indicates that notable changes of the atomic lattice anisotropy were induced by the Al-substitution and preheat treatments. Characteristics such as the interchain distance and interplanar distance parameter, which were obtained in the analytical method calculations, decrease with increasing Al substitution, in addition to the fact that they are related to the binding energy.     

Effect of milling time on the structure,micro-hardness,and thermal behavior of amorphous/nanocrystalline TiNiCu shape memory alloys developed by mechanical alloying

In the present paper, the effect of milling process on the chemical composition, structure, microhardness, and thermal behavior of Ti–41Ni–9Cu compounds developed by mechanical alloying was evaluated. The structural characteristic of the alloyed powders was evaluated by X-ray diffraction (XRD). The chemical composition homogeneity and the powder morphology and size were studied by scanning electron microscopy coupled with electron dispersive X-ray spectroscopy. Moreover, the Vickers micro-indentation hardness of the powders milled for different milling times was determined. Finally, the thermal behavior of the as-milled powders was studied by differential scanning calorimetery. According to the results, at the initial stages of milling (typically 0–12 h), the structure consisted of a Ni solid solution and amorphous phase, and by the milling evolution, nanocrystalline martensite (B19′) and austenite (B2) phases were initially formed from the initial materials and then from the amorphous phase. It was found that by the milling development, the composition uniformity is increased, the inter-layer thickness is reduced, and the powders microhardness is initially increased, then reduced, and afterward re-increased. It was also realized that the thermal behavior of the alloyed powders and the structure of heat treated samples is considerably affected by the milling time.     

Solidification microstructures selection of Fe–Cr–Ni and Fe–Ni alloys

The transition of solidified phases in Fe–Cr–Ni and Fe–Ni alloys was investigated from low to high growth rate ranges using a Bridgman type furnace, laser resolidification and casting into a substrate from superheated or undercooled melt. The ferrite–austenite regular eutectic growth, which is difficult to find in typical production conditions of stainless steels, was confirmed under low growth rate conditions. The transition velocity between eutectic and ferrite cell growth had a good agreement predicted by the phase selection criterion. Which of either ferrite or austenite is easier to form in the high growth range was discussed from the point of nucleation and growth. Metastable austenite formation in stable primary ferrite composition was mainly a result of growth competition between ferrite and austenite. For a binary Fe–Ni system, a planar metastable austenite in the steady state, simultaneous growth such as eutectic and banded growth between ferrite and austenite in an initial transient region are confirmed.     

A new resource-saving,low chromium and low nickel duplex stainless steel 15Cr–xAl–2Ni–yMn

A new family of resource-saving, low Cr and low Ni duplex stainless steels, with compositions of 15Cr–xAl–2Ni–yMn (x = 1.2–2.8, y = 8–12, wt.%) has been developed by examining the effect of Al and Mn on microstructure, mechanical property and corrosion property. The results show that 15Cr–1.2Al–2.0Ni–8Mn and 15Cr–2.0Al–2.0Ni–10Mn alloys have a balanced ferrite–austenite relation and that 15Cr–2.8Al–2.0Ni–12Mn alloy has a primary ferrite phase structure. The ferrite volume fraction increases with the solution treatment temperature and Al content while decreases with Mn content. No precipitate was found after solution-treated at 750 °C for 30 min. 15Cr–1.2Al–2.0Ni–8Mn alloy has a strong strain hardening effect, and 15Cr–2.0Al–2.0Ni–10Mn alloy has a good TRIP effect. Both of the 15Cr–1.2Al–2.0Ni–8Mn and 15Cr–2.0Al–2.0Ni–10Mn alloys have excellent impact toughness at low temperature with the impact energy higher than 125 J at −40 °C. The pitting corrosions always occur in austenite phase. Among the designed alloys, 15Cr–1.2Al–2.0Ni–8Mn and 15Cr–2.0Al–2.0Ni–10Mn are found to be excellent alloys with a proper phase proportion and a better combination of superior mechanical property and good pitting corrosion resistance.     

Solidification microstructures selection of Fe-Cr-Ni and Fe-Ni alloys

The transition of solidified phases in Fe–Cr–Ni and Fe–Ni alloys was investigated from low to high growth rate ranges using a Bridgman type furnace, laser resolidification and casting into a substrate from superheated or undercooled melt. The ferrite-austenite regular eutectic growth, which is difficult to find in typical production conditions of stainless steels, was confirmed under low growth rate conditions. The transition velocity between eutectic and ferrite cell growth had a good agreement predicted by the phase selection criterion. Which of either ferrite or austenite is easier to form in the high growth range was discussed from the point of nucleation and growth. Metastable austenite formation in stable primary ferrite composition was mainly a result of growth competition between ferrite and austenite. For a binary Fe–Ni system, a planar metastable austenite in the steady state, simultaneous growth such as eutectic and banded growth between ferrite and austenite in an initial transient region are confirmed.     

Synthesis of the Supermalloy Powders by Mechanical Alloying

The mixture of the Ni, Fe and Mo elemental powders with the nominal composition of the Supermalloy was milled in a planetary mill under Ar atmosphere. Several milling times have been used ranging from 4 to 16 h. A heat treatment of 30 min, 1, 2 and 4 h at temperature of 350°C has been performed in vacuum in order to improve the alloying process and remove the internal stresses. The formation of the Fe-Ni-Mo alloys by mechanical alloying was evidenced by X-ray diffraction. The nanocrystalline Supermalloy powders have been obtained after 16 h milling and after 8 h milling followed by 4 h annealing. A typical grain size of 11 ± 2 nm have been obtained after 16 h milling. The chemical homogeneity composition and the morphology of the powder particles have been studied by X-ray microanalysis and scanning electron microscopy respectively.     

Thermoelectric properties of chromium disilicide prepared by mechanical alloying

CrSi and Cr_1?x Fe _x Si particles embedded in a CrSi₂ matrix have been prepared by hot pressing from CrSi_1.9, CrSi₂, and CrSi_2.1 powders produced by ball milling using either WC or stainless steel milling media. The samples were characterized by powder X-ray diffraction, scanning, and transmission electron microscopy and electron microprobe analysis. The final crystallite size of CrSi₂ obtained from the XRD patterns is about 40 and 80 nm for SS- and WC-milled powders, respectively, whereas the size of the second phase inclusions in the hot pressed samples is about 1–5 μm. The temperature dependence of the electrical resistivity, Seebeck coefficient, thermal conductivity, and figure of merit (ZT) were analyzed in the temperature range from 300 to 800 K. While the ball-milling process results in a lower electrical resistivity and thermal conductivity due to the presence of the inclusions and the refinement of the matrix microstructure, respectively, the Seebeck coefficient is negatively affected by the formation of the inclusions which leads to a modest improvement of ZT.     

Heating-induced transformations of multicomponent alloys prepared by mechanochemical synthesis

Multicomponent (Cr, Fe, Co, Ni, Al, Ti, and Nb) powder alloys prepared by milling five- to seven-component equiatomic mixtures of elemental powders in a Fritsch (P-7) planetary mill have been characterized by differential thermal analysis and X-ray diffraction. The results demonstrate that, if the starting mixture contains Al, the BCC solid solution formed as a result of the milling undergoes heating-induced CsCl-type ordering (β-phase). If not only Al but also Ti are present in the starting mixture, further heating causes the β-phase to convert to an L2₁ phase with the composition (Ni,Co)TiAl. The elements Cr and Fe form a tetragonal σ-phase. The presence of Nb in the starting mixture suppresses the formation of the σ- phase and favors the formation of a hexagonal Laves phase of complex composition: (Fe,Co)CrNb.     

Effect of Ni addition on formation of amorphous and nanocrystalline phase during mechanical alloying of Al-25 at.%Fe-(5,10) at.%Ni powders

Al-Fe-Ni ternary powder mixtures containing 25 at.%Fe-5 at.%Ni and 25 at.%Fe-10 at.%Ni were mechanically alloyed by a high-energy planetary ball mill. Structural evolution of these powders during milling was investigated by X-ray diffraction technique and transmission electron microscopy. Almost complete amorphous phase in Al₇₀Fe₂₅Ni₅ system is observed at the early milling stage. The amorphous phase transforms into metallic compound Al₅(Fe,Ni)₂ and then the compound changes to ordered Al(Fe,Ni) phase. The last milling products in Al₇₀Fe₂₅Ni₅ system are amorphous phase plus nanocrystalline of the disordered Al(Fe,Ni) phase changed from the ordered Al(Fe,Ni) phase. During milling of Al₆₅Fe₂₅Ni₁₀ system, α-Al and α-Fe solid solutions formed at the early stage change to the ordered Al(Fe,Ni) compound and at last the ordered phase changes to the disordered Al(Fe,Ni) phase. Ten percent of Ni addition promotes retardation of the formation of the amorphous phase.     

Microstructural characterization of ball-milled metal matrix nanocomposites (Cr,Ni, Ti)-25 wt% (Al2O3np,SiCnp)

The microstructure of high-temperature metals such as Ti, Ni, and Cr can be modified using ceramic nanoparticles to form metal matrix nanocomposites (MMNCs). Such materials are generally prepared via powder metallurgy routes. In this study, 25?wt% SiC_np and Al₂O_3np were separately ball milled as a reinforcement of Ti, Cr, and Ni matrices to investigate their effects on the phase formation and morphology of the MMNCs. The x-ray diffraction (XRD), scanning electron microscopy (SEM), and field emission scanning electron microscopy (FESEM) results indicated that the alumina–metal system could not be thermodynamically stable in a high-energy ball mill, while the SiC reinforcement could be retained and milled with the metals even after 24?h. It was further observed that the distribution of nanoparticles was not affected by the type of metal, ceramic, and milling time. Finally, it was determined that the nanoparticles significantly reduced the average particle size of composite powders.     

Solidification structure of C2.08Cr25.43Si1.19Mn0.43Fe70.87 powders fabricated by high pressure gas atomization

Powders of hypoeutectic high chromium white cast iron (C_2.08Cr_25.43Si_1.19Mn_0.43Fe_70.87) were produced by high pressure gas atomization. The microstructure of the powders was characterized using light microscopy, scanning electron microscopy and X-ray diffraction. The results showed that the as-atomized powders were mainly composed of austenite and M₇C₃ (M = Fe, Cr) type carbide, and became ferrite and carbide after annealing. With the decrease of the powder diameter, the number of austenite grains, primary dendrite length and second dendrite arm spacing were decreased. The relationship between cooling rate and microstructure was also determined.     

Analysis of the microstructure of Cr–Ni surface layers deposited on Fe3Al by TIG

A series of Cr–Ni alloys were overlaid on a Fe₃Al surface by tungsten inert gas arc welding (TIG) technology. The microstructure of the Cr–Ni surface layers were analysed by means of optical metallography, scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results indicated that when the appropriate TIG parameters were used and Cr25–Ni13 and Cr25–Ni20 alloys were used for the overlaid materials, the Cr–Ni surface layers were crack-free. The matrix of the surface layer was austenite (A), pro-eutectoid ferrite (PF), acicular ferrite (AF), carbide-free bainite (CFB) and lath martensite (LM), distributed on the austenitic grain boundaries as well as inside the grains. The phase constituents of the Cr25–Ni13 surface layer were γ-Fe, Fe₃Al, FeAl, NiAl, an Fe–C compound and an Fe–C–Cr compound. The microhardness of the fusion zone was lower than that of the Fe₃Al base metal and Cr25–Ni13 surface layer.     

Mechanochemical synthesis and heating-induced transformations of a high-entropy Cr-Fe-Co-Ni-Al-Ti alloy

An amorphous-crystalline two-phase (amorphous phase + BCC solid solution) powder alloy has been produced by mechanochemical synthesis (MS): by grinding an equiatomic mixture of Cr, Fe, Co, Ni, Al, and Ti metals in a Fritsch (P-7) ball mill at a powder-to-ball weight ratio of 1: 8. Using X-ray diffraction, X-ray microanalysis, and scanning electron microscopy, we have determined the sequence of reaction steps during milling of the mixture. In the early stages of milling (2 h), we observed the formation of an ordered phase (B2), Al + Ni → NiAl, and the Co_HCP → Co_FCC polymorphic transformation. Milling for 3 h led to the formation of a BCC solid solution. Further milling produced an amorphous phase (AP). In the range 6–25 h of milling, the percentage of the AP increased and that of the BCC solid solution decreased. The phase transformations induced by heating the alloy to 1200°C after MS have identified using differential thermal analysis and X-ray diffraction: \(MS (FCC + AP)\xrightarrow{{450^ \circ C}}(B2)\xrightarrow{{650^ \circ C}}(L2_1 + BCC)\xrightarrow{{850^ \circ C}}L2_1 + \sigma - phase\) (FeCr structure). Prolonged milling has been shown to stabilize the metastable BCC solid solution at temperatures of ?650°C.     

Comparative Studies on the Structure and Magnetic Properties of Ni–Zn Ferrite Powders Prepared by Glycine-Nitrate Auto-combustion Process and Solid State Reaction Method

Ni–Zn ferrite compositions (Ni_1?x Zn _x Fe₂O₄) are well known due to their remarkable soft magnetic properties, which potentially have a broad range of applications in many areas. In this study, Ni–Zn ferrite with the chemical formula of Ni_0.64Zn_0.36Fe₂O₄ was prepared by the glycine-nitrate autocombustion process (GNP) and solid state reaction method (SSRM). In order to achieve a desirable particle size, the SSRM powders were milled for 3 h at a milling rate of 200 rpm. The structure and magnetic properties of the ferrite powders, which were synthesized by both methods, were characterized and their properties were compared. The results indicate that a significant amount (～?90 wt.%) of nanocrystalline Ni_0.64Zn_0.36Fe₂O₄ ferrite with the average crystallite size of 47 nm, particle size of 200 nm, saturation magnetization of 73 emu/g and coercivity of 54 Oe has been formed by means of the glycine-nitrate process. The results also show that not only the saturation magnetization of the GNP ferrite powder is relatively similar to that of the milled SSRM powders, but also it is synthesized at a much shorter duration than that of the solid state reaction method.     

Nitriding of Fe–18Cr–11Mn powders using mechanical alloying method through aerating nitrogen circularly

Mechanical alloying (MA) method was applied to nitride Fe–18Cr–11Mn stainless steel powders through aerating nitrogen circularly. Both the MA process and increasing nitrogen pressure circularly caused the expansion of nitrogen solubility in Fe. The microstructure of powders was affected by the milling time. The phase transformation of α-Fe to γ-Fe occurred during the MA process. The grain size of powder decreased, while the internal lattice strain increased by increasing the milling time and cycles of aerating nitrogen. The as-milled powder could obtain a fully austenitic structure after sintering and subsequent water quenching. The sintered samples had better density and higher microhardness when the powders milled for more time. The formation mechanism of nitriding of stainless steel powders using MA method in nitrogen was presented.     

Processing of Cu-Fe and Cu-Fe-SiC nanocomposites by mechanical alloying

The Cu-Fe and Cu-Fe-SiC nanocomposite powders were synthesized by a two step mechanical alloying process. A supersaturated solid-solution of Cu-20 wt% Fe was prepared by ball milling of elemental powders up to 5 and 20 h and subsequently the SiC powder was added during additional 5 h milling. The dissolution of Fe into Cu matrix and the morphology of powder particles were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. It was found that the iron peaks in the XRD patterns vanish at the early stages of mechanical alloying process but the dissolution of Fe needs more milling time. Moreover, the crystallite size of the matrix decreases with increasing milling time and the crystallite size reaches a plateau with continued milling. In this regard, the addition of SiC was found to be beneficial in postponing the saturation in crystallite size refinement. Moreover, the effect of SiC on the particle size was found to be significant only if it is added at the right time. It was also found that the silicon carbide and iron particles are present after consolidation and are on the order of nanometer sizes.     

Electrocatalytic properties of mechanically alloyed Co-20wt%Ni-10wt%Mo and Co-70wt%Ni-10wt%Mo alloy powders

Mechanically alloyed Co-20wt%Ni-10wt%Mo and Co-70wt%Ni-10wt%Mo (nominal compositions) alloy powders were produced by milling of pure elemental powders. Mechanically alloyed powders were characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. MA powder specimens were tested electrollitically in a 30% KOH aqueous solution at 298 K. X-ray diffraction analysis and transmission electron microscopy of milled powders showed the presence of two phases, an fcc solid solution and intermetallic compounds of Ni or Co with Mo. These phases showed a nanometric size. The linear sweep voltammograms confirmed also the presence of two phases in both mechanically alloyed alloy powders. The Co-20wt%Ni-10wt%Mo alloy powders showed the best electrocatalytic activity for hydrogen evolution reaction.