Precipitation Of AlN Research Articles

Strain-free, single-phase metastable Ti0.38Al0.62N alloys with high hardness: metal-ion energy vs. momentum effects during film growth by hybrid high-power pulsed/dc magnetron cosputtering

A hybrid deposition process consisting of reactive high-power pulsed and dc magnetron cosputtering (HIPIMS and DCMS) from Ti and Al targets is used to grow Ti1−xAlxN alloys, with x~0.6, on Si(001) at 500°C. Two series of films are deposited in which the energy and momentum of metal ions incident at the growing film are individually varied. In both sets of experiments, a negative bias Vs ranging from 20 to 280V is applied to the substrate in synchronous, as determined by in-situ mass spectrometry, with the metal-ion-rich part of the HIPIMS pulse. Ion momentum is varied by switching the HIPIMS and dc power supplies to change the mass m and average charge of the primary metal ion. Al-HIPIMS/Ti-DCMS layers grown under Al+ (mAl=26.98amu) bombardment with 20≤Vs≤160V are single-phase NaCl-structure alloys, while films deposited with Vs>160V are two-phase, cubic plus wurtzite. The corresponding critical average metal-ion momentum transfer per deposited atom for phase separation is 〈pd⁎〉≥135 [eV-amu]1/2. In distinct contrast, layers deposited in the Ti-HIPIMS/Al-DCMS configuration with Ti+/Ti2+ (mTi=47.88amu) ion irradiation are two-phase even with the lowest bias, Vs=20V, for which 〈pd⁎〉>135 [eV-amu]1/2. Precipitation of wurtzite-structure AlN is primarily determined by the average metal-ion momentum transfer to the growing film, rather than by the deposited metal-ion energy. Ti-HIPIMS/Al-DCMS layers grown with Vs=20V are two-phase with compressive stress σ=−2GPa which increases to −6.2GPa at Vs=120V; hardness H values range from 17.5 to 27GPa and are directly correlated with σ. However, for Al-HIPIMS/Ti-DCMS, the relatively low mass and single charge of the Al+ ion permits tuning properties of metastable cubic Ti0.38Al0.62N by adjusting Vs to vary, for example, the hardness from 12 to 31GPa while maintaining σ~0.

3%Si鋼中のMnS，MnSeおよびAlNとの複合析出と粒成長抑制効果

The precipitation behavior of MnS, MnSe and AlN in 3% Si steel was investigated for understanding inhibition of grain growth under isothermal annealing. It became clear that the precipitation of AlN was greatly influenced by that of MnSe. The precipitates became fine when hot-rolled at low temperature, and therefore the grain size after the recrystallization annealing became small. Similarly, the grain size after the isothermal annealing became small when the precipitates became fine. When MnS or MnSe precipitates formed complex with AlN, Ostwald ripening was suppressed, as a result, grain growth was inhibited.

Study of aluminum nitride precipitation in Fe- 3%Si steel

For good performance of electrical steels it is necessary a high magnetic induction and a low power loss when submitted to cyclic magnetization. A fine dispersion of precipitates is a key requirement in the manufacturing process of Fe- 3%Si grain oriented electrical steel. In the production of high permeability grain oriented steel precipitate particles of copper and manganese sulphides and aluminium nitride delay normal grain growth during primary recrystallization, causing preferential growth of grains with Goss orientation during secondary recrystallization. The sulphides precipitate during the hot rolling process. The aluminium nitride particles are formed during hot rolling and the hot band annealing process. In this work AlN precipitation during hot deformation of a high permeability grain oriented 3%Si steel is examined. In the study, transfer bar samples were submitted to controlled heating, compression and cooling treatments in order to simulate a reversible hot rolling finishing. The samples were analyzed using the transmission electron microscope (TEM) in order to identify the precipitates and characterize size distribution. Precipitate extraction by dissolution method and analyses by inductively coupled plasma optical emission spectrometry (ICP-OES) were used to quantify the precipitation. The results allowed to describe the precipitation kinetics by a precipitation-time-temperature (PTT) diagram for AlN formation during hot rolling.

SHELL LIKE FRACTURES IN CONTINUOUSLY CAST SLABS

Open network cracking, transverse central and transverse corner cracks, with shell like shaped fracture surfaces, are analyzed in this work. Cracked slabs were analysed, made 6 low carbon steels heats – 2 heats showed high Al and N contents, 2 heats showed high N and 2 ones had high Al content. In fracture surfaces or in their close vicinity presence of Cu grains, AlN nitrides and sulphides (Mn,Fe) S was shown. The assumption, that the thermodynamic failure cause, producing shell like fractures in continuously cast slabs is of the same nature as in traditional large castings was verified. It is caused by: high Al and N content, precipitation of AlN and coarse austenite grains. In low carbon steels as a risk factor for shell like cracks, contents higher than the following limits can be assumed Al > 0,04 % and N > 0,008 % , or [Al] [N] > 3.10 -4 or Al/N > 5.

Influence of residual stresses and grain size on the spinodal decomposition of metastable Ti1 − xAlxN coatings

At elevated temperatures, the metastable Ti1−xAlxN solid solution decomposes in cubic AlN and cubic TiN. Within this work, the effect of residual stresses on the decomposition of sputtered Ti1−xAlxN coatings was investigated. Using different bias voltages, a series of Ti1−xAlxN coatings (x=0.63) with stresses ranging from +630 to −2500MPa was synthesized on Si (100) substrates. Vacuum annealing treatments and subsequent X-ray diffraction measurements showed that tensile stresses foster the formation of more volume consuming cubic TiN domains, while compressive stresses promote the formation of smaller cubic AlN domains. The influence of the grain size has been considered by investigations of free standing films using differential scanning calorimetry. Smaller grains lead to faster decomposition and earlier precipitation of wurtzite AlN.

Analysis of the Mechanical Properties of N-Added CMn Structural Steel by the Impulse Internal Friction Technique

The presence of substitutional solutes has a strong influence on the Snoek effect of interstitial atoms in body-centered cubic (bcc) Fe alloys, such as the structural CMn steel grades produced in hot strip mills. In the current study, bcc Fe-Mn-N alloys were analyzed using the impulse technique and the results were correlated to their mechanical properties and the AlN precipitation characteristics. The broadened Snoek peak related to the octahedral interstitial N could be resolved into three distinct Debye peaks related to the arrangement of Mn atoms surrounding the N atoms. The simulation results suggest that precipitation of AlN occurred mainly during coiling and that the precipitation kinetics was enhanced at higher coiling temperatures. Alloys isothermally held at high temperatures had a yield strength and tensile strength linearly proportional to the difference between the nominal N content and the interstitial N content. The observations point to the pronounced precipitation strengthening effect of AlN after high-temperature coiling simulations. The result also reveals that interstitial N increases both the yield strength and the tensile strength in steels coiled at low temperatures.

Precipitation of AlN and MnS in Low Carbon Aluminium-Killed Steel

The precipitation kinetics of AlN and MnS in low carbon aluminium-killed steel was calculated. Transmission electron microscopy (TEM), energy disperse spectroscopy (EDS) and phase analyses have been used to investigate the morphology, compositions and particle size distribution of AlN and MnS precipitates in three positions of the coil. The particles of AlN and MnS precipitates in the ferrite region after coiling and distributes along and adjacent to the ferrite grain boundaries. The shapes of AlN are plate-like, the precipitates size is about 10 to 60 nm; the shapes of MnS are spherical, the precipitates size is about 200 to 600 nm. The precipitation behavior of AlN is sensitive to the isothermal temperature and holding time, the precipitation quantity and particle size distribution of AlN in different positions of coil are unequal.

Application of computational modeling to the kinetics of precipitation of aluminum nitride in steels

In previous works the possibilities and limitations of the application of calculations in the Al-Fe-N system to describe the precipitation of AlN in steel, both in the solid state and during the solidification were discussed and some difficulties related to the extension of these calculations to more complex steel systems, due to limitations in the thermodynamic data were also presented. Presently, the precipitation kinetics of AlN in ferrite (BCC) and austenite (FCC) is discussed. The correct description of the precipitation of AlN in both phases is relevant to: (a) the precipitation at higher temperatures, in the austenite field, that occurs in some steels, (b) the concurrent precipitation of this nitride with the annealing treatment, when the steel is mostly ferritic, used in the processing of some types of deep drawing steels (c) the precipitation of this nitride in some silicon alloyed electric steels at relatively high temperatures, when these steels can have significant fractions of BCC and FCC in their microstructure. The precise knowledge of the precipitation-dissolution behavior of AlN in special in these two latter classes of steels is of great importance to their correct processing. In this work, a computational tool for simulating multiparticle precipitation kinetics of diffusion-controlled processes in multi-component and multi-phase alloy systems is employed in an attempt to describe these precipitation processes. The results are compared with experimental data on precipitation. The assumptions necessary for the application of the multi-particle modeling tool are discussed, agreements and discrepancies are identified and some possible reasons for these are indicated. Furthermore, the impact of the use of different sources of data on steel processing development is discussed and the need for further studies highlighted.

Loss of Ductility Caused by AlN Precipitation in Hadfield Steel

Two modified X120Mn12 Hadfield steels, differing in the amount of the alloying elements Al and N, are analyzed with respect to AlN precipitation and its effects on ductility. Charpy impact tests are performed, demonstrating the loss of ductility in the one grade containing a high density of AlN precipitates. The characterization of the precipitates is carried out by high-resolution scanning electron microscopy (HRSEM). Depending on chemical composition, primary and secondary AlN precipitates are detected on prior austenite grain boundaries and within the bulk volume. The experimental observations are confirmed by thermokinetic simulations, using the software package MatCalc (Vienna University of Technology, Vienna, Austria).

Influence of peritectic phase transformation on hot ductility of high aluminium TRIP steels containing Nb

Increasing Al from 0·05 to 1% in Nb containing transformation induced plasticity steel resulted in deepening and considerable widening of the hot ductility trough. Further increase in the Al level to 1·5% produced a trough similar to the low Al steel but having better ductility in the temperature range of 650–800°C. This improved ductility could be ascribed to its finer austenite grain size. Nb(CN) was able to precipitate readily in these steels and was important in influencing the hot ductility of the 0·05 and 1·5%Al steel in the temperature range of 750–1000°C, with ductility improving as the particle size increased with test temperature. No AlN was found in 0·05%Al containing steel, and there was no significant dendritic precipitation of AlN in 1·5%Al containing steel, although precipitation of AlN in plate form was readily observed. In 1%Al steel, copious dendritic precipitation of AlN was present at the γ grain boundaries, leading to rock candy fracture. The poor ductility shown in 1%Al containing steel is due to a combination of this dendritic precipitation, which took place only in a steel of peritectic carbon composition, and its coarse grain size. Both low and 1·5%Al containing steels had compositions outside the peritectic range. It is strongly advised that for this type of steel, the composition should be designed to fall outside the peritectic carbon range.

Decomposition pathways in age hardening of Ti-Al-N films

The ability to increase the thermal stability of protective coatings under work load gives rise to scientific and industrial interest in age hardening of complex nitride coating systems such as ceramic-like Ti1−xAlxN. However, the decomposition pathway of these systems from single-phase cubic to the thermodynamically stable binary nitrides (cubic TiN and wurtzite AlN), which are essential for age hardening, are not yet fully understood. In particular, the role of decomposition kinetics still requires more detailed investigation. In the present work, the combined effect of annealing time and temperature upon the nano-structural development of Ti0.46Al0.54N thin films is studied, with a thermal exposure of either 1 min or 120 min in 100 °C steps from 500 °C to 1400 °C. The impact of chemical changes at the atomic scale on the development of micro-strain and mechanical properties is studied by post-annealing investigations using X-ray diffraction, nanoindentation, 3D-atom probe tomography and high-resolution transmission electron microscopy. The results clearly demonstrate that the spinodal decomposition process, triggering the increase of micro-strain and hardness, although taking place throughout the entire volume, is enhanced at high diffusivity paths such as grain or column boundaries and followed within the grains. Ab initio calculations further show that the early stages of wurtzite AlN precipitation are connected with increased strain formation, which is in excellent agreement with experimental observations.

The hot ductility of Nb/V containing high Al, TWIP steels

The hot ductility of Nb/V containing high Al, twin induced plasticity (TWIP) steels has been examined over the temperature range 650–1150°C after melting and after ‘solution treatment’. Previous work had shown that the hot ductility is poor for the 1·5 mass-%Al, TWIP steel due to precipitation of AlN at the austenite grain boundaries, the depth of the trough being similar to that for an X65 grade pipeline steel but with the trough covering a much wider temperature range. Adding Nb and V made the ductility even worse due to the additional precipitation of NbCN and VN. Very low reduction of area values, 10–20% were obtained in the temperature range 700–900°C. Increasing the cooling rate to the test temperature resulted in even worse ductility. The ductility of these steels after ‘solution treatment’ is similar to that obtained after melting but when the cast was hot rolled followed by ‘solution treatment’ and cooling to the test temperature ductility improved due to grain refinement.

AlN Precipitation During Isothermal Annealing of Ultra Low Carbon Steel

AbstractThe precipitation of AlN is investigated in the austenite region of ultra low carbon steel. The evolution of the size and the morphology of AlN precipitates are studied by transmission electron microscopy (TEM) after isothermal annealing for different times at temperatures of 950 °C and 1050 °C. Various different morphologies are observed, including cuboids, large plates as well as irregular structures. In addition to the experimental analysis, thermo‐kinetic simulations are carried out with the software package MatCalc. The numerically calculated evolution of the mean radii as well as the time‐temperature‐precipitation (TTP) diagram for AlN precipitation in the present alloy show good agreement with experiment.

Hot ductility of TWIP steels

The hot ductility of twin induced plasticity (TWIP), 0·6 wt-%C steels containing 18–22 wt-%Mn with N levels in the range 0·005–0·023 wt-% and the Al additions either low (<0·05 wt-%) or high (1·5 wt-%) has been examined. Little change in ductility occurred in the temperature range 1100–650°C as the structure was always fully austenitic. Ductility was generally poor (<40%), reduction of area values, the best ductility at the higher Al level being given by the steel with the lowest N and S levels. Because the steel is fully austenitic, the ductility is solely dependent on that for unrecrystallised austenite. Therefore, to avoid transverse cracking the volume of second phase particles should be kept to a minimum, i.e. the N should be low to reduce the amount of AlN that can be precipitated out and the S level should be as low as possible to limit the amount of MnS inclusions. Metallographic and TEM studies were carried out and the poor ductility was found to be due to extensive precipitation of AlN at the austenite grain boundaries. Increasing the cooling rate from the melting point to the test temperature from 60 to 180°C min−1 or introducing an undercooling step both led to even worse ductility.

Effect of Hot Rolling Process on Microstructure and Properties of Low-Carbon Al-Killed Steels Produced Through TSCR Technology

Low-carbon Al-killed hot rolled strips for direct forming, cold rolling, and galvanizing applications are produced from the similar chemistry at Ezz Flat Steel (EFS) through thin slab casting and rolling (TSCR) technology. The desired mechanical and microstructural properties in hot bands for different applications are achieved through control of hot rolling parameters, which in turn control the precipitation and growth of AlN. Nitrogen in solid solution strongly influences the yield strength (YS), ductility, strain aging index (SAI), and other formability properties of steel. The equilibrium solubility of AlN in austenite at different temperatures and its isothermal precipitation have been studied. To achieve the formability properties for direct forming, soluble nitrogen is fixed as AlN by coiling the strip at higher temperatures. For stringent cold forming, boron was added below the stoichiometric ratio with nitrogen, which improved the formability properties dramatically. The requirements of hot band for processing into cold rolled and annealed deep drawing sheets are high SAI and fine-grain microstructure. Higher finish rolling and low coiling temperatures are used to achieve these. Fully processed cold rolled sheets from these hot strips at customer’s end have shown good formability properties. Coil break marks observed in some coils during uncoiling were found to be associated with yielding phenomenon. The spike height (difference between upper and lower yield stresses) and yield point elongation (YPE) were found to be the key material parameters for the break marks. Factors affecting these parameters have been studied and the coiling temperature optimized to overcome the problem.

Concurrent Precipitation of AlN and VN in Microalloyed Steel

AbstractThe precipitation kinetics of AlN and VN in microalloyed steel is investigated numerically with the thermo‐kinetic software MatCalc. To mimic the heterogeneous precipitation of AlN along austenite grain boundaries, a new model is utilized, which takes into account the fast short circuit diffusion along grain boundaries and sluggish bulk diffusion inside the grains. It is demonstrated that the precipitation of VN inside the grains can be more rapid than the precipitation of AlN at the grain boundaries. However, the thermodynamically more stable AlN can overgrow and dissolve the existing VN again. The computed results are compared to experimental results from literature and good agreement is observed.

Computational Analysis of Precipitation during Continuous Casting of Microalloyed Steel

AbstractIn this paper, the kinetics of TiN, V(C,N)) and AlN precipitation in microalloyed steel during continuous casting is investigated experimentally and theoretically. The precipitate phase fraction, mean radius, number density and composition are simulated with the thermo‐kinetic software MatCalc and compared with experimental results obtained from transmission electron microscopy analysis. A new methodology for modelling precipitation in cast steel is proposed, which consists of two parts: First, a Scheil – Gulliver simulation, which is carried out to obtain information on the amount of microsegregation during solidification. Then, based on this information, two precipitation kinetics simulations are performed: One with the chemical composition representative for the solute‐poor core of the secondary dendrite arms, the other with the composition of the residual liquid at a fraction of 5%, corresponding to the segregated solute‐rich interdendritic regions. The results of the computer simulations using the new methodology are in good agreement with experimental observation.

Kinetics of AlN precipitation in microalloyed steel

In this work, the thermodynamic information on AlN formation in steel and experimental data on AlN precipitation kinetics are reviewed. A revised expression for the Gibbs energy of AlN is presented with special emphasis on microalloyed steel. Using the software package MatCalc, computer simulations of AlN precipitation are performed and compared with independent experimental results from the literature. A new model for grain boundary precipitation is employed, which takes into account fast short-circuit diffusion along grain boundaries as well as slower bulk diffusion inside the grain, together with the classical treatment for randomly distributed precipitates with spherical diffusion fields. It is demonstrated that the precipitation of AlN can be modelled in a consistent way if precipitation at grain boundaries and dislocations is taken into account, dependent on chemical composition, grain size and annealing temperature. It is also demonstrated that, for consistent simulations, the influence of volumetric misfit stress must be taken into account for homogeneous precipitation of AlN in the bulk and heterogeneous precipitation at dislocations.

The microstructure of the diffusion zone of a gaseously nitrided Fe–1·5 wt-%Cr–1·5 wt-%Al alloy

Gaseous nitriding experiments of an Fe–1·5 wt-%Cr–1·5 wt-%Al (i.e. Fe–1·6 at.-%Cr–3·1 at.-%Al) alloy were carried out as a function of time at 853 K. The microstructure of the diffusion zone was characterised by microhardness, electron probe microanalysis (EPMA), X-ray diffraction analysis (XRD), scanning transmission electron microscopy (STEM) in combination with energy dispersive X-ray spectroscopy (EDX) and Auger electron spectroscopy (AES). Chromium and aluminium precipitate together as a mixed Cr1−xAlxN phase in the diffusion zone. The size of the (semi)coherent precipitates and the amount of excess nitrogen have a strong influence on the microstructure of the diffusion zone. Crack formation occurs after a certain nitriding time starting from the specimen surface and propagating along grain boundaries more or less perpendicularly to the surface towards larger depth. The grain boundary brittleness could be ascribed to the precipitation of excess nitrogen as nitrogen gas at the grain boundaries and to the segregation of Al at grain boundaries promoting precipitation of AlN at the grain boundaries. The residual stress–depth profile was determined and a simple model was proposed to explain the surprising initially occurrence of tensile stress parallel to the surface in the diffusion zone.

Precipitation Kinetics of Aluminium Nitride in Austenite in Microalloyed HSLA Steels

In this work, the thermodynamic information on aluminium nitride formation and experimental precipitation kinetics data are reviewed. A revised expression for the Gibbs energy of AlN is developed with special emphasis on microalloyed steel. Using the software package MatCalc, computer simulations of AlN precipitation kinetics are performed and compared to several independent experimental results from literature. To mimic the geometrical arrangement of AlN precipitates along austenite grain boundaries, a new model for precipitation at grain boundaries is used, which takes into account fast short-circuit diffusion along grain boundaries as well as the slower bulk diffusion of atoms from inside the grain to the grain boundaries. This is essential for the calculation of AlN precipitation in austenite where nucleation occurs predominantly on grain boundaries. By studying the AlN precipitation at grain boundaries numerically, and by comparison with experimental data, it is demonstrated that the precipitation kinetics of AlN differs significantly from the simulated precipitation kinetics of randomly distributed precipitates assuming spherical diffusion fields.