H. G. Jiang

On the applicability of the x-ray diffraction line profile analysis in extracting grain size and microstrain in nanocrystalline materials

Measurements of x-ray diffraction (XRD) profiles have been performed on commercially pure Fe and Al powders, cryomilled Fe–3 wt.% Al powders, cold pressed (CP) pure Fe and Al, hot pressed (HP) and hot isostatically pressed (HIP) Fe–3 wt.% Al. Scherrer equation (SE), integral breadth analysis (IBA), and single-line approximation (SLA) methods have been employed to extract grain size and microstrain. The results demonstrate that, in the case of the cryomilled nanocrystalline Fe–3 wt.% Al powders, all these XRD techniques yielded reasonable, consistent grain size results. However, discrepancies were found in cold pressed (CP-Fe), hot pressed (HP-Fe–3 wt.% Al), and hot isostatically pressed (HIP-Fe–3 wt.% Al) samples. TEM imaging revealed the presence of a certain density of dislocations inside the grains in the HP-Fe–3 wt.% Al and HIP-Fe–3 wt.% Al, which is thought to be partly or fully responsible for the observed discrepancies.

Thermal Spraying of Nanocrystalline Ni Coatings

The present paper describes the synthesis, characterization, and grain growth behavior of nanocrystalline Ni coatings generated using a novel synthesis approach, namely high velocity oxy-fuel (HVOF) thermal spraying. In the present investigation, the feedstock powders were prepared by mechanical milling in a methanol environment which yielded agglomerates with a flake-shaped geometry and an average grain size of less than 100 nm. The milled powders were then introduced into the HVOF spray system in order to investigate the feasibility of generating a coating with grain sizes in the nanocrystalline range (e.g., <100 nm). Scanning electron microscopy and transmission electron microscopy were used to study the morphology of the nanometric particles and the microstructure of the milled powders and the as-sprayed coatings. Transmission electron microscopy analysis performed on cross sections of the coating revealed a mixture of fine nanocrystalline grains and elongated coarse grains.

Grain growth behavior of nanocrystalline inconel 718 and Ni powders and coatings

Nanocrystalline Inconel 718 and Ni powders were prepared using two approaches: methanol and cryogenic attritor milling. High velocity oxy-fuel (HVOF) spraying of milled Inconel 718 powders was then utilized to produce coatings with a nanocrystalline grain size. Isothermal heat treatments were carried out to study the thermal stability of the methanol milled and cryomilled powders, as well as the HVOF-derived coatings. All nanocrystalline Inconel 718 powders and coatings studied herein exhibited significant thermal stability against grain growth by maintaining a grain size around 100 nm following annealing at 1273 K for 60 min. In the case of the cryomilled nanocrystalline Ni powders, isothermal grain growth behavior was studied, from which the parameters required for the prediction of the microstructural evolution during a non-isothermal annealing were acquired. The theoretical simulation of grain growth behavior of nanocrystalline Ni during non-isothermal annealing conditions yields results that are in good agreement with the experimental results.

Synthesis of nanocrystalline M50 steel powders by cryomilling

Abstract The present paper reports on a study of the synthesis of nanocrystalline high speed tool steel M50 powders (4.5% Mo, 4.0% Cr, 1.0% V, 0.8% C, balance Fe) by cryogenic high energy ball milling (cryomilling). Pre-alloyed M50 steel is spray atomized, and subsequently cryomilled in liquid nitrogen for 25 hours. Elemental Al powder is added prior to cryomilling to promote the formation of nanoscale Al2O3 and AlN dispersoids to improve the thermal stability of the nanocrystalline M50 steel. High resolution transmission electron microscopy (HRTEM) reveals the formation of various carbides (V8C7, Fe3C, and FeC), oxides (Al2O3, MoO3, and V3O7), and a nitride phase (AlN) during cryomilling. Following one hour of heat treatment at 1373 K (0.77 Tm), an average grain size of 70 nm was retained for the M50 steel powders.

Grain size stability of nanocrystalline cryomilled Fe-3wt.%Al alloy

Abstract Nanocrystalline Fe-3wt.%Al alloy powders are synthesized using cryogenic mechanical alloying in liquid N. Uptake of O by the powders is significantly reduced by controlling the atmosphere and pressure during milling. The initial 18 nm grain size of the cryomilled powder is found to experience grain growth at temperatures above 500°C (0.42 T m ), as determined by X-ray diffraction. Cryomilled pure Fe, in contrast, is found to be significantly less stable. Transmission electron microscopy indicates that the grain growth in the Fe-3wt.%Al is abnormal, while lattice parameter measurements indicate limited dissolution of Al in the Fe matrix.

The effect of Ni on the cryogenic attritor milling of Metglas Fe78B13Si9

Abstract The addition of 17 at.% of elemental Ni powders to the cryogenic attritor milling of Metglas Fe78B13Si9 slowed the mechanical crystallization of the α-Fe and Fe2B phases. Transmission electron microscopy (TEM) observation and energy dispersive spectroscopy (EDS) analysis indicated that no more than 3.60 at.% of Ni dissolved into the Metglas, which was well within the equilibrium solubility limit of Ni in α-Fe. It is proposed that the addition of Ni impede mechanical crystallization during attritor milling by inhibiting bending and wear-like processes which could otherwise cause crystallization.

Thermal Spray Processing of Nanocrystalline Materials

Technological advancements in many sectors of modern society depend strongly on the materials science and engineering community’s ability to conceive of novel materials with attractive combinations of physical and mechanical properties. For instance, in the aerospace industry, the ever increasing demand to manufacture lighter aircraft that can travel at higher speeds and can withstand a higher payload capacity has fueled the development of high strength/low density materials with improved damage tolerance and enhanced temperature capabilities. Driven in part by this critical need, research in materials science and engineering has shifted towards the study and application of non-equilibrium processes. The significant departure from thermodynamic equilibrium associated with these types of processes allows material scientists and engineers to develop materials with unusual combinations of microstructure and physical attributes.

Synthesis and characterization of bulk nanocrystalline M50 steel by cryomilling

Abstract Nanocrystalline high speed tool steel M50 (4.5%Mo, 4.0%Cr, 1.0%V, 0.8%C, balance Fe) was synthesized by cryogenic high energy ball milling (cryomilling) for 25 hours. Elemental Al powder is added prior to cryomilling in order to promote the formation of nanoscale Al2O3 and AlN dispersoids in an effort to improve the thermal stability of the nanocrystalline M50 steel. High resolution transmission electron microscopy (HRTEM) reveals the formation of various carbides (V8C7, Fe3C, and FeC), oxides (Al2O3, MoO3, and V3O7), and a nitride phase (AIN) during cryomilling. The nanocrystalline M50-5 wt.% Al powders are consolidated using hot isostatic pressing (HIP) at 1223 K and 200 MPa for 2 hours. X-ray diffraction analysis performed on the consolidated M50-5 wt.% Al compact yields an average calculated grain size of 23 nm. Furthermore, TEM dark field imaging indicates that an average grain size of 17 ± 11 nm is retained for the bulk nanocrystalline M50-5 wt.% Al consolidated by HIP.

Structural Evolution of Fe Rich Fe-Al Alloys During Ball Milling and Subsequent Heat Treatment

X-ray diffraction (XRD) and differential scanning calorimetry (DSC) have been utilized to investigate the structural evolution of Fe rich Fe-Al alloys during ball milling. It is found that b.c.c. solid solutions can be formed either through ball milling alone or through ball milling together with heat treatment. Thermal diagrams of the milled Fe-Al powders reveal exothermic peaks corresponding to the formation of {alpha}-Fe(Al) solid solution (in both Fe-4wt.%Al and Fe-10wt.%Al) and the formation of FeAl intermetallic compound (in Fe-10wt.%Al). The transformation kinetics of {alpha}-Fe(Al) solid solution in Fe-4wt.%Al were found to follow the Johnson-Mehl-Avrami equation.