Mechanically induced reactive synthesis of TiN via magnetically controlled ball milling of titanium under nitrogen gas was investigated using X-ray diffraction, advanced electron microscopy and Raman ...spectroscopy. Ball milling of titanium powder with nitrogen gas was performed in Uni-Ball-Mill, with external temperature of the vial and initial pressures of the nitrogen gas monitored, while the milled samples were taken out periodically, both before and after detection of an exothermic ignition point. Before ignition, nitrogen-enriched Ti, small proportions of TiN and very minor amounts of Ti
2
N are formed, in addition to the heavily deformed Ti. Raman spectroscopy revealed the pre-ignition products to include off-stoichiometric nitrides (TiN
x
) and oxynitride skin (TiO
x
N
y
). The formation of the new TiN and Ti
2
N products before ignition was attributed to the diffusion of highly polarized active N atoms into the mechanically activated clean surfaces of Ti, followed by local reaction. This local reaction is likely promoted by numerous cycles of induced local temperature rise and rapid quenching, large surface area and accumulation of deformation defects. After the exothermic ignition, there was rapid nucleation of new TiN crystals, and simultaneous growth of the pre-existing TiN and the newly formed TiN crystals. This understanding explains the reaction pathways leading to the formation of small proportions of TiN and very minor amounts of Ti
2
N before ignition and the thin-plated TiN after ignition.
The influence of microstructure and composition on permeation of hydrogen in 1.2 and 0.5 wt.% Mn X70 pipeline steels after different processing was investigated using an electrochemical permeation ...technique.
For 1.2 wt.% Mn (standard Mn) steel, the microstructure of normalised transfer bar was coarse equiaxed ferrite grains. This sample exhibited the highest diffusivity, followed by transfer bar, with a mixed ferrite–bainitic ferrite microstructure; and hot rolled strip, with fine elongated ferrite grains.
The 0.5 wt.% Mn (medium Mn) strip displayed lower diffusivity than the 1.2 wt.% Mn strip, due to hydrogen trapping by finer ferrite grains and a higher density of carbonitride precipitates. Moreover, the medium Mn strip exhibited a uniform microstructure and consequently similar diffusion coefficients for the edge and centreline regions, whereas the finer grains of the edge region of the standard Mn strip resulted in a lower diffusivity than the centreline region.
► Permeation of hydrogen in 1.2 and 0.5 wt.% Mn X70 pipeline steel was examined. ► The normalised transfer bar having coarse grains showed the highest diffusivity. ► The hot rolled strips showed intermediate values of diffusivity. ► The 0.5 wt.% Mn strip showed the lowest diffusivity due to precipitates. ► Edge and centreline regions of 0.5 wt.% Mn strip exhibited similar diffusivities.
► Increasing the hydrogen content decreases the fracture toughness (JQ) linearly. ► X70 strip exhibits higher JQ than MX70 strip before and after hydrogen charging. ► The normalized X70 transfer bar ...shows much lower JQ than the hot rolled strips. ► The fracture morphology changed from ductile to brittle with increasing hydrogen. ► Simulated CGHAZ structures show low JQ which decreases with increasing hydrogen.
The influence of composition and microstructure on susceptibility to hydrogen induced cracking (HIC) was investigated in high strength pipeline steels, with Mn contents of 1.2% (standard, X70), and 0.5% (medium, MX70). The HIC resistance of the simulated coarse grained heat affected zone microstructures and normalized X70 transfer bar was also investigated. Notched and fatigue pre-cracked samples were charged with hydrogen prior to three point bend tests.
The conditional fracture toughness JQ was determined. The results are discussed in relation to grain size, microstructure, composition and the type and distribution of non-metallic inclusions and precipitates.
The mechanism of mechanically induced self-propagating reaction (MSR) of titanium with graphite to produce TiC during reactive ball milling of elemental powders was investigated using high resolution ...electron microscopy, and Raman and X-ray photoelectron spectroscopies. Prior to the exothermic ignition, off-stoichiometric Ti(1+x)C(1−x) nuclei are formed by reaction at the interface between the severely deformed Ti particles and graphite milling debris. After ignition, both rapid growth of existing nuclei, and nucleation and growth of additional off-stoichiometric Ti(1+x)C(1−x) occurs. Local product morphologies after the ignition varied, depending local restrictions on the rate of diffusion of carbon into the TiC as it grows.
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This paper presents a novel method of reversible absorption and desorption of hydrogen in metallic powder. This method is based on electrical discharges forming hydrogen plasma, which in ionized ...state reacts rapidly with metals and alloys to form solid state solutions or metal hydride phases. By selection of proper electric discharge condition, absorption or desorption condition can be created.
In our research, we have tested this method on Zr metallic powder. Fast synthesis of zirconium hydride, and reversed decomposition of this compound, was tested using Electrical Discharge Assisted Mechanical Milling (EDAMM) technique in hydrogen plasma environment.
ZrH1.8 phase was directly synthesized from zirconium powder within 10 min. Subsequently, hydrogen desorption from pre-prepared ZrH1.8 powder was performed in desorption plasma condition at a low pressure of hydrogen gas. After 30 min up to 80% of hydrogen was extracted from ZrH1.8.
All phase and structural changes were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Carbon-Hydrogen-Nitrogen (CHN) combustion micro-analysis.
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•Novel method of plasma assisted mechanical milling solid-state hydrogen synthesis.•Both hydrogen absorption and desorption are possible with the single method.•Extremely fast hydrogen absorption.•Low pressure and low average temperature of the process.
A magneto ball mill was used to mill titanium and carbon elemental powder mixtures with compositions of Ti
50C
50 and Ti
60C
40 under a helium atmosphere. Previous studies on the milling of titanium ...and carbon powder mixtures have reported a sudden increase in the temperature of the milling vial, which occurs after a specific milling interval, referred to as
t
ig Z.G. Liu, J.T. Guo, L.L. Ye, G.S. Li, Z.Q. Hu, Appl. Phys. Lett. 65 (1994) 2666–2668; G.B. Schaffer, J.S. Forrester, J. Mater. Sci. 32 (1997) 3157–3162; N.Q. Wu, S. Lin, J.M. Wu, Z.Z. Li, Mater. Sci. Technol. 14 (1998) 287–291; Z. Xinkun, Z. Kunyu, C. Baochang, L. Qiushi, Z. Xiuqin, C. Tieli, S. Yunsheng, Mater. Sci. Eng. C 16 (2001) 103–105; C. Deidda, S. Doppiu, M. Monagheddu, G. Cocco, J. Metastable Nanocryst. Mater. 15/16 (2003) 215–220. This sudden temperature increase has been found to correspond to the formation of TiC via a rapid, highly exothermic reaction. In these cases, XRD analysis did not detect TiC in powder sampled before
t
ig. These results, combined with those from studies suggested that the milling period prior to
t
ig represents an incubation period during which the powders become mechanically activated and that no reaction between the starting powders occurs during this time L. Takacs, J. Solid State Chem. 125 (1996) 75–84; B.K. Yen, T. Aizawa, J. Kihara, J. Am. Ceram. Soc. 81 (1998) 1953–1956; M. Puttaswamy, Y. Chen, B. Jar, J.S. Williams, Mater. Sci. Forum 312–314 (1999) 79–84; G.B. Schaffer, P.G. McCormick, Metall. Transact. A 23A (1992) 1285–1290; M. Mingliang, L. Xinkuan, X. Shenqui, C. Donglang, Z. Jingen, J. Mater. Process. Technol. 116 (2001) 124–127.
In the current investigation a sudden increase in the temperature of the milling vial was also observed after a specific milling duration of
t
ig. XRD analysis of powder sampled before
t
ig did not detect TiC, whilst for powder sampled after
t
ig, XRD analysis indicated that the powder had almost completely transformed into TiC. However, Raman spectroscopy showed the formation of non-stoichiometric TiC in Ti
50C
50 and Ti
60C
40 powders sampled before
t
ig. It is believed that the current, and many previous XRD analyses did not detect the formation of TiC prior to
t
ig either because the TiC grain size was too small and/or the phase represented too small a volume fraction of the powder samples to be detected by the XRD method employed. These Raman spectroscopy results suggest that a significant component of the heat generated at
t
ig may be due to a combination of growth of TiC accompanied by consumption of unreacted Ti and carbon, and/or recrystallisation of the TiC formed prior to
t
ig, rather than the direct formation of TiC.
Metal carbonitride powders and coatings are well known as superhard materials which are widely applied in many engineering products. Of this group of materials hafnium carbonitride properties are one ...of the least known due to difficulties with preparation. In this study, the electric discharge assisted mechanical milling (EDAMM) method involving a nitrogen plasma together with mechanical milling was used to synthetise hafnium carbonitride powders within minutes. It was found that difficulties in conventional manufacturing methods, resulting in the formation of only thin layers instead of bulk materials can be overcome by applying the new technique, and HfCxNy powder could be successfully engineered with controlled amount of nitrogen uptake.
In this investigation we studied the effects of AC and DC discharges on nitrogen solubility in HfC powder. Microstructure and phase evolution were determined using XRD and SEM, while the nitrogen content in the grains was evaluated by EDS and bulk nitrogen content by CHN analysis. Maximum solubilities of 3.42 wt% and 2.95 wt%N under DC an AC discharge currents, respectively, were obtained. Reaction paths varied depending on processing conditions with HfCxNy formed under DC discharge processing and mixed products containing HfC, several HfCxNy phases and HfN like phase formed after AC processing. After nitrogenation the most highly nitrided powders were subjected to 1 min additional EDAMM processing using high power discharges to allow the formation of large, sintered particles of sufficient size to perform hardness measurements on. Vickers harnesses of fully sintered large particles after 5 and 10 min of EDAMM were 1808 and 2169HV, respectively.
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•HfCN powders were for the first time successfully synthesized by EDAMM.•Under DC plasma mode HfC fully transforms into HfCxNy compound.•AC mode results in partial transformation to HfC1-xNx and HfN.•EDAMM achieves maximum nitrogen solubility of 3.42 wt% N after 10 min under DC mode.•EDAMM enabled micro-sintering of the ceramic powders for microhardness measurement.
Mechanical milling is an effective technique for the preparation of fine metallic and ceramic powders and can also be used to drive a wide range of chemical reactions. Milling devices include ...planetary machines, attritors and vibrational mills; products include amorphous, nanocrystalline and quasicrystalline materials, supersaturated solid solutions, reduced minerals, high-surface-area catalysts and reactive chemicals. During milling, solid-solid, solid-liquid and solid-gas reactions are initiated through repeated deformation and fracture of powder particles. A separate materials synthesis and processing technique involves reacting a material in a gas atmosphere under an electrical discharge. Here we show that the application of low-current, high-voltage electrical impulses during milling can result in both faster reactions and new synthesis and processing routes. We demonstrate the effects of glow (cold) and spark (hot) discharge milling on particle fracture for brittle, low-conductivity materials and ductile metals. Glow discharge milling was found to promote solid-gas reactions whereas spark discharge milling promotes fast fracturing, recrystallization, mineral reduction and solid-solid reactions.
In this study, synthesis of titanium diboride from elemental powders of Ti and B by electric discharge assisted mechanical milling technique was investigated. This recently developed technique has ...the following advantages: rapid reaction rate, controlled reaction, direct reaction between Ti and B without adding another element into the system and cost effectiveness. TiB2 samples were prepared using an in-house built reactor with an ac high voltage transformer, generating impulses within kV/mA range. The structures of reaction products were characterized by X-ray diffractometry, and powder morphologies by SEM. X-ray diffraction studies showed that the milling product was TiB2 with small fraction of TiB.
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