Nitrogen Gas Atomization Research Articles

Experimental determination of the nuclei number in the deeply undercooled and rapidly solidified powder particles of high-alloyed steel

In metallurgical practice, solidification of metallic materials usually starts by heterogeneous nucleation. Homogeneous nucleation can occur in deeply undercooled melts. Gas atomization represents one of the methods which can be used to achieve deep melt undercooling. In the gas atomization process, a certain number of the melt droplets can even reach hypercooling, which may provide suitable conditions for homogeneous nucleation in such particles.The paper presents an original approach to the quantitative analysis of the number of crystallization nuclei originated in deeply undercooled melt by the process of homogeneous nucleation using the particles of rapidly solidified (RS) powder of high alloyed Cr-Mo-V tool steel of a ledeburite type prepared by nitrogen gas atomization. For microstructural analysis, RS powder particles were sieved into six standard-sized granulometric fractions ranging from 50 μm to 125 μm. The maximum volumetric density of homogeneously formed crystallization nuclei was identified using the methods quantitative stereology in the granulometric fraction of RS powder from 56 μm to 63 μm with the value approximately 1.5 × 106 mm−3.

The influence of cooling rate on the microstructure and phase fraction of gas atomized NiAl3 alloy powders during rapid solidification

NiAl3 alloy powders were produced by nitrogen gas atomization in this paper. The microstructure and phase compositions of different particle size powders were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and electron backscattered diffraction (EBSD). The atomized powders are composed of Ni2Al3, NiAl3 and Al-eutectic phases, and three main types of solidification microstructures were identified: dendrites, refined dendrites, and compounds. Phase fraction of three representative particles with diameter of 60, 140 and 220 μm were measured. Moreover, the Newtonian heat transfer formulation coupled with the classical heterogeneous nucleation was used for analyzing the influence of heat transfer coefficient and cooling rate on the thermal history of droplets. Therefore, this work highlights the experimental results matched well with the theoretical calculation and guides the further production of high NiAl3 phase Raney nickel catalytic powders.

Influence of Atomizing Gas and Cooling Rate on Solidification Characterization of Nickel-based Superalloy Powders

Nickel-based superalloy powders have been produced by high pressure argon gas and nitrogen gas atomization, separately. The microstructural characterization of nickel-based alloy powders has been performed by a scanning electron microscope equipped with an EDS microanalysis unit. Based on a Newtonian cooling model, the flight speed and the cooling rate of two kinds of alloy droplets were calculated. The results show that the droplet cooling rate, which depends on atomization medium and droplet size, has an effect on the solidification microstructure. For argon-gas atomized powders, the developed dendrite structure is predominant at a lower cooling rate and a mixed microstructure composed of dendrite structure and cellular structure is observed at a higher cooling rate. For nitrogen-gas atomized powders, the dendrite structure is predominant at a lower cooling rate and a full cellular structure can be observed at a higher cooling rate. According to calculation, the cooling rate of argon-gas atomized droplets is in a range from 1.0×105 K·s−1 to 4.24×106 K·s−1, while the cooling rate of nitrogen-gas atomized droplets is from 1.0×105 K·s−1 to 4.8×106 K·s−1. The cooling rate increases with decreasing of droplets diameter. Two kinds of atomizing gases have a slight influence on the cooling rate of droplets. The dendrite axis is rich in elements such as Cr, Co, W, Ni and Al while the inter-dendrite region is rich in Ti element.

Nucleation and microstructure development in Cr-Mo-V tool steel during gas atomization

Nucleation studies of undercooled metallic melts are of essential interest for the understanding of phase selection, growth kinetics and microstructure development during their rapid non-equilibrium solidification. The paper deals with the modelling of nucleation processes and microstructure development in the hypoeutectic tool steel Ch12MF4 with the chemical composition of 2.37% C, 12.06 % Cr, 1.2% Mo, 4.0% V and balance Fe [wt. %] in the process of nitrogen gas atomization. Based on the classical theory of homogeneous nucleation, the nucleation temperature of molten rapidly cooled spherical particles from this alloy with diameter from 40 μm to 600 μm in the gas atomization process is calculated using various estimations of parameters influencing the nucleation process – the Gibbs free energy difference between solid and liquid phases and the solid/liquid interfacial energy. Results of numerical calculations are compared with experimentally measured nucleation temperatures during levitation experiments and microstructures developed in rapidly solidified powder particles from the investigated alloy.

Osteoblasts MC3T3-E1 Response in 2D and 3D Cell Cultures Models to High Carbon Content CoCr Alloy Particles. Effect of Metallic Particles on Vimentin Expression

The study of the biocompability of the metallic materials is a priority treating to avoid the osteolysis and aseptic loosening of prosthesis. Wear debris is considered one of the main factors responsible for aseptic loosening of orthopedic endoprostheses. We examined the response of mouse osteoblasts MC3T3-E1 to high carbon cobalt-chrome (HCCoCr) particles obtained a) from wear-corrosion assays on a pin-on-disk tribometer using as pair an alumina ball and a disc of a HCCoCr alloy, and b) HCCoCr bulk particles obtained by nitrogen gas atomization from an alloy used in clinic for prostheses application. Mitochondrial activity and lactate dehydrogenase activity assayed in 2D and 3D osteoblasts cell culture models were used to evaluate the cellular response to size, shape, and chemical composition of the metallic particles. 2D cell model was used to study the direct interaction of cells with particles and 3D cell cultures was used to more closely mimic in vivo conditions. The results showed that vimentin was overexpressed in the 2D osteoblasts cultures in presence of metal particles. This might be related to the appearance of pseudotumor in the peri-prosthetic vicinity described in some implanted patients.

Assessing microstructures and mechanical resistances of as-atomized and as-extruded samples of Al-1wt%Fe-1wt%Ni alloy

Current applications of Al–Fe–Ni alloys include Alnico permanent magnets, industrial furnaces, and cladding of nuclear fuel plates. In spite of industrial interest, limited knowledge regarding to inter-relations between microstructure and mechanical resistance can be noted to date. Thus, the aim of the present contribution is, firstly, to analyze the microstructure features of α-Al phase (size and morphology) during atomization of the ternary Al-1wt%Fe-1wt%Ni alloy, including determination of cooling rates and hardness of the obtained powders. Secondly, the nature, size and distribution of intermetallic compounds (IMC), strength and ductility of hot consolidated bulks by extrusion from two different ranges of Al-Fe-Ni powder size (powder size between 75 and 106 μm and powder size up 106 μm and less 180 μm) are examined. The sequence of processes includes nitrogen gas atomization followed by compaction and hot extrusion consolidation at both 350 °C and 400 °C. The procedures to characterize the samples involve X-ray diffraction (XRD), optical microscopy, scanning electron microscopy (SEM), Vickers hardness and mechanical tensile tests. Al-rich cells prevailed for either smaller or larger Al-1wt%Fe-1wt%Ni atomized powders with formation of IMCs not only in the cell walls but also precipitated within the α-Al matrix. Strength and ductility of as-extruded samples are found to be consistent with their microstructures.

The critical role of heating rate in enabling the removal of surface oxide films during spark plasma sintering of Al-based bulk metallic glass powder

Metallic glass powder of Al86Ni6Y4.5Co2La1.5, prepared by nitrogen gas atomization, was consolidated into 10mm diameter bulk metallic glass (BMG) samples using spark plasma sintering (SPS). The use of a high heating rate is critical to ensure oxide-free, well-sintered clean interfaces between the powder particles and therefore desired mechanical properties, compared to oxide-enveloped, poorly-sintered interfaces that result from the use of a slow heating rate. The marked influence of heating rate is attributed to the surface-cleaning effect of SPS associated with the increased electrical discharge.

Semi-quantitative model of the microstructure development in the high-alloyed iron based alloy during atomization

The paper deals with the analysis of microstructure formation in the tool steel of ledeburite type Ch12MF4 with the chemical composition of 2.37% C, 12.06% Cr, 1.2% Mo and 4.0 % V [wt. (%)] in the process of nitrogen gas atomization. Three main types of solidification microstructures were observed in rapidly solidified powder particles: dendritic, compound and cellular. Based on the morphological features of microstructures observed in rapidly solidified particles and mathematical modeling of the thermal history of solidifying spherical droplets, the semi-quantitative model of the microstructure development in the Ch12MF4 steel during atomization was suggested. According to this model, it is supposed that the transition from dendritic to partially dendritic (compound) and nondendritic microstructures results from the thermally induced fragmentation of dendrites by the mechanism of their remelting, morphological changes of dendrite fragments and following spheroidization. The intensity of dendrite fragmentation in solidifying particles of different diameters is controlled mainly by the recalescence temperature and duration of quasi-isothermal period of solidification.

Influence of the thermal history of a particle during atomization on the morphology of carbides in a hypereutectic iron based alloy

Basic principles and consequences of the rapid solidification processing of melts have been successfully exploited in several progressive technologies of material production. In the paper, the solidification microstructures developed in the hypereutectic iron based alloy with the chemical composition of 3% C–3% Cr–12% V (wt.%) prepared by nitrogen gas atomization are presented and analysed. Several main types of solidification microstructures were identified in the rapidly solidified powder particles. According to the morphological features of carbide phases and computed thermal history of rapidly solidified particles, the microstructures were divided into two groups – microstructures morphologically non-affected during the post-recalescence period of solidification, and microstructures with morphological transitions occurring during the quasi-isothermal period of structure development. Based on the thermokinetic newtonian model of rapid solidification of a spherical droplet in the process of atomization, the thermal history of droplets with diameter from 20μm to 400μm rapidly solidified from different nucleation temperatures was studied. The thermo-physical conditions necessary for the development of variable microstructures in single rapidly solidified powder particles are predicted and discussed. The nucleation temperature, recalescence temperature and duration of quasi-isothermal plateau are supposed to be the most important parameters influencing the microstructure development in the rapidly solidified particles of the investigated alloy.

Preparation, characterization and microwave absorbing properties of FeNi alloy prepared by gas atomization method

In this work, Fe–50 wt%Ni alloy was synthesized by nitrogen gas atomization method. The phase composition, morphology, element composition and saturation magnetization of the alloys were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray fluorescence spectrometer (XRF) and vibrating sample magnetometer (VSM). The electromagnetic parameters and microwave absorbing properties of the microwave absorbing material made from silicone rubber and the as-prepared FeNi alloy were also studied using vector network analyzer (VNA) and transmission line theory. The results show that the particle size of most of the particles is less than 150 μm, and the average particle size is about 48 μm. The Fe–50 wt%Ni powder has a spherical shape, and the main phase composition is FCC (Fe, Ni) alloy. The as-atomized FeNi alloy possesses low coercivity and high specific saturation magnetization. As the frequency increases, the real part of relative permittivity and relative permeability of the Fe–50 wt%Ni/silicone rubber absorber decrease, while the imaginary part of relative permittivity increases. The Fe–50 wt% Ni alloy/silicone rubber absorbers with 0.8–4 mm thickness exhibit adjustable microwave absorbing properties in the range of 1–18 GHz.

Temperature induced transformation of metastable austenite in a hypereutectic iron-based rapidly solidified powder

Rapidly solidified powder of a hypereutectic iron-based alloy with the chemical composition of 3% C–3% Cr–12% V (wt.%) was prepared by nitrogen gas atomization. Phase identification of the rapidly solidified particles in the as-atomized state was performed by X-ray analysis and Mössbauer spectrometry. The main phase constituents present in the rapidly solidified particles were found to be metastable austenite and vanadium rich carbide phases of M 4C 3 type. Magnetic α-phase was identified as a minor constituent. Thermal stability of metastable austenite in the RS particles was analyzed during differential thermal analysis continuous heating experiment and by Mössbauer spectrometry and X-ray diffraction analysis after isothermal exposition at room temperature. The beginning of a fcc (austenite) → bcc (ferrite) + carbide transformation was detected during continuous heating experiment at 612 °C as exothermic reaction. No appreciable changes in the spectral lines after isothermal treatment were observed up to 500 °C. A fcc-to-bcc transformation started after tempering at 500 °C and was completed after tempering at 560 °C.

Amorphous and nanostructured Al 85Ni 5Y 6Co 2Fe 2 powder prepared by nitrogen gas-atomization

Al 85Ni 5Y 6Co 2Fe 2 fully amorphous and partially amorphous powders were prepared by nitrogen gas-atomization method. The microstructure and thermal stability as a function of powder size were investigated. A majority of the powders are below 45 μm, and the content of amorphous phase is as high as 97.3% in the <25 μm size range. With the increase of particle size, the phase constituent alters from amorphous to amorphous’ + fcc-Al, then fcc-Al + AlNiY + Al 3(Ni, Fe), and finally fcc-Al + Al 9Co 2 + Al 3Y + Al 3(Ni, Fe). All of the powders show three exothermic reactions in the DSC traces, and the <25 μm size range presents a wide supercooled liquid region of 22 K and a primary crystallization temperature of 580 K. The Al 85Ni 5Y 6Co 2Fe 2 alloy powders exhibit a high microhardness of near 348 Hv, suggesting its good potential for structural application.

Effect of niobium on the mechanical properties of powder-metallurgy processed high-speed steels

Abstract Powder-metallurgy processed high-speed steels containing up to 5 wt.% of Nb were fabricated by nitrogen gas atomization of elemental powders followed by hot isostatic pressing and hot working. Microstructural changes, particularly the carbide formation during the heat treatment and the resulting mechanical properties were investigated. After the heat treatment, the primary carbides were identified as MC and M6C-types. With Nb addition, the total volume of primary carbides increased from 16 to 23 vol.% and the volume fraction of MC-type carbide in the primary carbides also increased. The chemistry of the MC-type carbide changed from a V-rich to a V–Nb-rich carbide with increasing Nb content. Hardness significantly increased to a maximum of Rockwell C66 with 1 wt.% Nb and then slightly decreased with further Nb additions. Bend strength, on the other hand, gradually decreased with increasing Nb content. Fracture toughness is almost inversely proportional to the hardness and wear and softening resist...

Nitrogen versus helium: effects of the choice of the atomizing gas on the structures of Fe50Ni30Si10B10 and Fe32Ni36Ta7Si8B17 powders

Gas atomization can produce, besides a possible significant degree of undercooling, high cooling rates, whose extent depends on the size of the droplets, on their velocity with respect to the surrounding medium, on the thermo-physical properties of both the alloy and the gas, and of course on the operating conditions such as melt overheating and gas-to-metal flow ratio. In this respect it is well-known that the atomizing gas can play a significant role in determining both the powder size distribution and the kind and mix of phases which result from the solidification and cooling processes. The microstructures and solidification morphologies of powders obtained from nitrogen and helium sonic gas atomization of two iron-nickel base glass forming alloys, Fe 50 Ni 30 Si 10 B 10 and Fe 32 Ni 36 Ta 7 Si 8 B 17 , were investigated by means of light microscopy, X-ray diffraction (XRD) and differential thermal analysis (DTA). The Fe 32 Ni 36 Ta 7 Si 8 B 17 alloy exhibits a higher proneness to the development of amorphous phase than the Fe 50 Ni 30 Si 10 B 10 alloy, while the effect of the higher speed attainable by the stream of helium with respect to that of nitrogen, affords not only to obtain a larger amount of particles in the finer size ranges, but also to affect the relative amounts of phases within the different size fractions.

Grain refinement and stabilization in spray-formed AISI 1020 steel

An AISI 1020-grade mild steel, with a small amount of aluminum, was spray-formed by nitrogen gas-atomization and deposition. The spray-formed 1020 steel contained 0.05 mass% of nitrogen and 0.06 mass% of aluminum. Rolling the spray deposit at 1123 K for a thickness reduction of 70% and subsequent normalizing at low austenitic temperatures produced a fully dense steel having a refined ferrite grain size as small as 3 μm. This grain refinement resulted from the pinning of prior austenite grain boundaries by fine AlN particles which precipitated during the thermomechanical treatments. Room-temperature tensile properties (YS: 550 MPa, UTS: 630 MPa, elongation: 23%), exceeding those of conventional microalloyed high-strength low alloy steels, were achieved in the normalized state. The AlN-pinned fine-grained microstructure survived subsequent re-austenizing at temperatures as high as 1273 K. When tensile-tested in the austenite region at 1123 K, the microalloyed 1020 steel showed a large elongation exceeding 200%.

Thermal Sprayed Titanium Anode for Cathodic Protection of Reinforced Concrete Bridges

Stable operation of cobalt catalyzed thermal sprayed titanium anodes for cathodic protection (CP) of bridge reinforcing steel was maintained in accelerated tests for a period equivalent to 23 years service at Oregon Department of Transportation (Oregon DOT) bridge CP conditions with no evidence that operation would degrade with further aging. The cobalt catalyst dispersed into the concrete near the anodeconcrete interface with electrochemical aging to produce a more diffuse anode reaction zone. The titanium anode had a porous heterogeneous structure composed of α-titanium containing interstitial oxygen and nitrogen, and a fee phase thought to be Ti(O,N). Splat cooling rates were 10 to 150 K/s, and microstructures were produced by equilibrium processes at the splat solidification front. Nitrogen gas atomization during thermal spraying produced a coating with more uniform composition, less cracking, and lower resistivity than using air atomization.

Solidification behaviour of Pd–Rh droplets during spray atomization

Solidification microstructure in spray-atomized Pd–10 wt% Rh powders using high-pressure gas atomization was studied. The solidification cooling rate and the solidification front velocity were investigated using a transient heat-transfer finite element method. Two different atomization gases, nitrogen and helium, were considered in the modelling studies. On the basis of the results obtained, it was found that gas atomization using helium gas led to solidification cooling rates and solidification front velocities which were two times higher than those obtained using nitrogen gas. Moreover, the cooling rate and the solidification front velocity increased with decreasing powder size for both types of atomization gas. The numerically estimated solidification front velocity using finite element analysis for nitrogen gas atomization was found to be smaller than the analytically determined absolute stability velocity that is required to promote a segregation-free microstructure. This was noted to be consistent with the segregated microstructure that was experimentally observed in nitrogen gas atomized powders. In the case of helium gas atomization, however, the increased cooling rate and solidification front velocity are anticipated to promote the formation of a segregation-free microstructure in the gas-atomized powders.

Nitrogen-atomized, nickel-based, corrosion-resistant alloys

Nitrogen gas atomization has been used for many years to produce iron-based powder-metal materials such as stainless and tool steels. However, it is more typical to use argon atomization with nickel-based alloys because it avoids the formation of nitrides that, in some cases, can be detrimental to the mechanical properties of these materials. In this article, two nickel-based materials— alloy 625 and alloy 690—normally used for applications where corrosion resistance is of primary importance were evaluated in their nitrogen-atomized powder metal form. Nitrogen atomization uncovered attributes of these nickel alloys that are not present in their conventionally produced counterparts or in argon-atomized versions of the same compositions.

Synthesis of silicon nitride powder through nitrogen gas atomization

The feasibility of synthesizing silicon nitride powder utilizing reactive atomization processing was analysed. The range of times required for the flight time of particles, the cooling rate of the silicon melt, the reaction time of silicon and nitrogen, and the diffusion of nitrogen through silicon nitride layers were obtained and compared. The results of this study indicated that the production of silicon nitride powder through the reactive atomization process would be limited by diffusion of nitrogen through the nitride (ash) layer, assuming the nitride layer was coherent and the unreacted core model was a valid representation of the liquid silicon-silicon nitride system.

M80S20 metallic glass powder and its explosive consolidation

The factors influencing metallic glass forming ability (GFA) are discussed. An alloy of the general formula M80S20 which can easily be atomized into amorphous powder by conventional nitrogen gas-atomization techniques has been developed. The explosive consolidation and devitrification of M80S20 powder are described.