Kinetics and microstructure of laser chemical vapor deposition of titanium nitride

Titanium nitride (TiN) films were deposited onto Ti–6Al–4V substrates by laser chemical vapor deposition using a cw CO₂ laser and TiCl₄, N₂ and H₂ reactant gases. Laser-induced fluorescence (LIF) and pyrometry determined relative titanium gas phase atomic number density and deposition temperature, respectively. Auger electron spectroscopy found substoichiometric films, caused by diffusion of nitrogen through TiN grain boundaries to the titanium alloy substrate. The morphology is a polyhedral structure with crystallite sizes ranging from 10 to 1000 nm. The activation energy was calculated to be 122 ± 9 kJ mol⁻¹ using growth rates measured by film height and 117 ± 23 kJ mol⁻¹ using growth rates measured by LIF signals. Above N₂ and H₂ levels of 1.25% and below TiCl₄ input of 4.5%, the growth rate has a half-order dependence on nitrogen and a linear dependence on hydrogen. The rate-determining steps of TiN growth are discussed.     

A corrosion study of nanocrystalline copper thin films

Thin nanocrystalline, compact films, based on the copper–nitrogen system, up to 2.5 μm thickness and 3.5% nitrogen, were deposited by magnetron sputtering at different partial pressure ratios of N₂ and Ar, without formation of Cu_xN compounds, the nitrogen concentration influencing grain size (down to 30 nm) and film homogeneity. Electrochemical corrosion properties were investigated using polarization curves and electrochemical impedance spectroscopy in 0.5 M NaCl aqueous solution, and compared with pure bulk copper; morphology was examined by scanning electron microscopy. Significant variations in corrosion currents between samples were attributed to grain size and structural defects on the grain boundaries.     

First-principles assessment of hydrogen absorption into FeAl and Fe3Si: Towards prevention of steel embrittlement

We characterize the ability of two potential surface alloys, FeAl and Fe₃Si, to prevent H incorporation into steel, with a view toward inhibiting steel embrittlement. Periodic density functional theory calculations within the generalized gradient approximation are used to evaluate H dissolution energetics and the kinetics of H diffusion into and through FeAl and Fe₃Si. We predict increased dissolution endothermicities and diffusion barriers in both alloys compared to bulk Fe. Fe₃Si is predicted to be the most effective at inhibiting H incorporation, with a 1.91 eV [0.97 eV] surface-to-subsurface diffusion barrier on the (1 1 0) surface [(1 0 0 surface)] and a 0.79 eV endothermicity to bulk dissolution, compared to a 1.02 eV [0.38 eV] barrier and 0.20 eV dissolution energy in pure Fe [37]. We therefore propose that a thin layer of Fe₃Si may provide protection against H embrittlement of the underlying steel.     

Structural evolution and grain growth kinetics of the Fe–28Al elemental powder during mechanical alloying and annealing

The structural evolution and grain growth kinetics of the Fe–28Al (28 at.%) elemental powder during mechanical alloying and annealing were studied. Moreover, the alloying mechanism during milling the powder was also discussed. During mechanical alloying the Fe–28Al elemental powder, the solid state solution named Fe(Al) was formed. The lattice parameter of Fe(Al) increases and the grain size of Fe(Al) decreases with increasing milling time. The Fe and Al particles were first deformed, and then, the composite particles of the concentric circle-like layers were generated. Finally, the composite particles were substituted by the homogeneous Fe(Al) particles. The continuous diffusion mixing mechanism is followed, mainly by the diffusion of Al atoms into Fe. During annealing the milled Fe–28Al powder, the order transformation from Fe(Al) to DO₃-Fe₃Al and the grain growth of DO₃-Fe₃Al occurred. The grain growth kinetic constant, K = 1.58 × 10⁻⁹ exp(−540.48 × 10³/RT) m²/s.     

Accelerating disorder–order transitions of FePt by preforming a metastable AgPt phase

Many approaches had been reported to successfully reduce the transition temperature of FePt from A1 to L1₀ phase, though without detailed knowledge. In this work, we deposited the metastable AgPt layer adjacent to the Fe layer and addressed the importance of vacancies in the disorder–order transition of FePt at reduced temperatures on the basis of a kinetic diffusion model. The decomposition of the metastable AgPt phase, creating excess vacancies during the post-deposition annealing process, accelerated the intermixing between Fe and Pt and the nucleation of L1₀ FePt. The evolution of phase transformation from AgPt–Fe to L1₀ FePt–Ag was monitored by in situ high temperature X-ray diffractometry and was also validated by first-principles calculations. The intermixing between Fe and Pt and the nucleation of L1₀ FePt after annealing at 230 °C were directly observed by transmission electron microscopy and grazing incidence X-ray diffractometry, respectively. With the assistance of the decomposition of AgPt, we obtained a (0 0 1)-dominated L1₀ FePt film with an out-of-plane coercivity as large as 13.3 kOe after annealing at a temperature as low as 350 °C. The principles of the proposed method can be applied for versatile disorder–order phase transitions.     

Nanocrystalline Fe–C alloys produced by ball milling of iron and graphite

A series of nanocrystalline Fe–C alloys with different carbon concentrations (x_tot) up to 19.4 at.% (4.90 wt.%) are prepared by ball milling. The microstructures of these alloys are characterized by transmission electron microscopy and X-ray diffraction, and partitioning of carbon between grain boundaries and grain interiors is determined by atom probe tomography. It is found that the segregation of carbon to grain boundaries of α-ferrite can significantly reduce its grain size to a few nanometers. When the grain boundaries of ferrite are saturated with carbon, a metastable thermodynamic equilibrium between the matrix and the grain boundaries is approached, inducing a decreasing grain size with increasing x_tot. Eventually the size reaches a lower limit of about 6 nm in alloys with x_tot > 6.19 at.% (1.40 wt.%); a further increase in x_tot leads to the precipitation of carbon as Fe₃C. The observed presence of an amorphous structure in 19.4 at.% C (4.90 wt.%) alloy is ascribed to a deformation-driven amorphization of Fe₃C by severe plastic deformation. By measuring the temperature dependence of the grain size for an alloy with 1.77 at.% C additional evidence is provided for a metastable equilibrium reached in the nanocrystalline alloy.     

Physical properties of ultrasonic sprayed nanosized indium doped SnO2 films

Here, we report on the preparation and characterization of nanosized indium doped tin oxide films (TO:In). The films are grown by ultrasonic spray pyrolysis deposition (USPD) onto glass. The structural, optical, electrical and morphological properties of SnO₂ (TO) films are investigated. The as-deposited films SnO₂ have preferred orientation along the (2 0 0) plane and are polycrystalline with a tetragonal crystal structure. Following this direction, the average grain size, obtained from XRD patterns, decreases with the rate doping. It ranges from 64 to 17 nm. In UV spectrum, the transmittance increases followed by a slight decay within visible range. Optical band gap, E_g, is about 4.1 eV. The samples reveal a high resistivity which varies in the range 10⁴–10⁷ Ω cm. Activation energies of shallow levels, as obtained from Arrhenius plots, vary from 85 meV to 165 meV. SEM and AFM analysis demonstrate nanostructure morphology.     

Structure and phase formation in Cu–Mn alloy thin films deposited at room temperature

Transmission electron microscopy characterization of Cu–Mn alloy thin films deposited by DC magnetron sputtering is applied to reveal the formation of phases throughout the composition range. Pure Cu and Mn films exhibit face-centred cubic (fcc) Cu and α-Mn phases, respectively. At room temperature the low Mn content films have fcc structure (γ-phase). Mn can substitute Cu in the fcc Cu lattice up to ～35 at.% Mn. The lattice parameter of fcc Cu–Mn alloy films follows a linear relationship of a₀ = a_Cu + 0.322c (in Å), where a_Cu = 3.615 Å is the lattice parameter of Cu and c is the Mn atomic concentration. At high Mn content, above 50 at.% Mn, a homogeneous one-phase structure is observed, possessing the short-range order of α-Mn. The incorporation of Cu into Mn suggests that this structure changes from crystalline α-Mn to disordered structure as the Cu content increases. A narrow two-phase region exists between 35 and 45 at.% Mn. A grain size minimum of 2–3 nm was observed in the 35–65 at.% Mn region.     

Microstructure and magnetic properties of FeMoBCu alloys: Influence of B content

Fe_91–xMo₈Cu₁B_x (x = 12, 15, 17, 20) amorphous and nanocrystalline alloys were studied to examine the influence of B content on their microstructure and magnetic behaviour. Changes in the magnetic properties provoked by microstructural evolution upon thermal treatments of as-cast samples were also analyzed. Nanocrystallization kinetics can be described by an isokinetic approach except for the 20 at.% B content alloy. The Curie temperature of the amorphous as-cast samples increases with the alloy’s B content. Mössbauer results suggest the presence of Mo atoms in the nanocrystals. Crystalline volume fraction and mean grain size of the nanocrystals at the end of the nanocrystallization process are higher for the lowest B content alloy. The 20 at.% B content alloy develops a boride phase just after the early stages of the nanocrystallization process, which provokes a magnetic hardening in this alloy. The softest magnetic behaviour of the studied compositions corresponds to nanocrystallized 17 at.% B content alloy.     

In situ TEM study of microplasticity and Bauschinger effect in nanocrystalline metals

In situ transmission electron microscopy straining experiments with concurrent macroscopic stress–strain measurements were performed to study the effect of microstructural heterogeneity on the deformation behavior of nanocrystalline metal films. In microstructurally heterogeneous gold films (mean grain size d_m = 70 nm) comprising randomly oriented grains, dislocation activity is confined to relatively larger grains, with smaller grains deforming elastically, even at applied strains approaching 1.2%. This extended microplasticity leads to build-up of internal stresses, inducing a large Bauschinger effect during unloading. Microstructurally heterogeneous aluminum films (d_m = 140 nm) also show similar behavior. In contrast, microstructurally homogeneous aluminum films comprising mainly two grain families, both favorably oriented for dislocation glide, show limited microplastic deformation and minimal Bauschinger effect despite having a comparable mean grain size (d_m = 120 nm). A simple model is proposed to describe these observations. Overall, our results emphasize the need to consider both microstructural size and heterogeneity in modeling the mechanical behavior of nanocrystalline metals.     

Crystallization of amorphous ceria solid solutions

Next-generation micro-solid oxide fuel cells for portable devices require nanocrystalline thin film electrolytes in order to allow fuel cell fabrication on chips at low operating temperatures and with high fuel cell power outputs. In this study amorphous gadolinia-doped ceria (Ce_0.8Gd_0.2O_1.9−x) thin film electrolytes were fabricated by spray pyrolysis and their crystallization to nanocrystalline microstructures was investigated by means of X-ray diffraction and transmission electron microscopy. At temperatures higher than 500 °C the amorphous films crystallize to a biphasic ceramic that is amorphous and nanocrystalline. The driving force for the crystallization is the reduction of the free enthalpy resulting from the transformation of amorphous into crystalline material. Self-limited grain growth kinetics prevail for the nanocrystalline grains where stable microstructures are established after short dwell times. A transition to classical curvature-driven grain growth kinetics occurs when the fully crystalline state is reached for average grain sizes larger than 140 nm and annealing temperatures higher than 1100 °C.     

The formation of nanograin structures and accelerated room-temperature theta precipitation in a severely deformed Al–4 wt.% Cu alloy

The grain size achievable and long-term stability of a severely deformed aluminium copper alloy have been investigated when copper is used in solution to inhibit recovery. It is shown that copper is more effective than magnesium in inhibiting dynamic recovery. A grain width of only ～70 nm was obtained in an Al–4 wt.% Cu alloy, after processing by equal-channel angular extrusion to a strain of ε_eff = 10, resulting in a lamellar nanograin structure. However, post-processing, the severely deformed solid solution was found to be unstable at room temperature and copious precipitation of θ occurred at grain boundaries within the deformed state, leading to recovery of the deformation structure and a loss of strength. The solute level fell to equilibrium within ～9 months. The precipitation kinetics were shown to occur at many orders of magnitude higher than can be predicted by classical nucleation and growth theory. The reasons for this discrepancy are discussed.     

Influence of transformation temperature on microtexture formation associated with α precipitation at β grain boundaries in a β metastable titanium alloy

The influence of transformation temperature on microtexture development associated with α precipitation at β/β grain boundaries (GB) in the near-β Ti17 alloy was studied using electron backscatter diffraction and considering isothermal treatments. For the alloy studied and the temperature range considered, decreasing the transformation temperature decreased the local microtexture strength within each prior β grain because of a larger number of α_WGB colonies (standing for α Widmanstätten GB) formed per β grain, each colony increasing by one the number of α orientations inside each prior β grain. This larger number of α_WGB colonies was a consequence of faster formation along β/β GB of their precursors, the allotriomorphic α_GB grains (standing for α-GB) at lower transformation temperatures, as evidenced by detailed examination of the first stages of α_GB formation. α_GB crystallographic orientations frequently followed a variant selection (VS) criterion based on the alignment of (0 1 1)β//(0 0 0 1)α_GB//(0 1 1)β. From a statistically relevant number of observations, VS was found to be more frequent at a lower transformation duration and a lower temperature, but the effect was not significant enough to influence the final α microtexture, considered at the scale of one prior β grain. α_GB grains that followed the VS criterion emitted two α_WGB colonies on either side of the β/β GB more frequently than those with no particular orientation.     

Role of interface morphology in the exchange-spring behavior of FePt/Fe perpendicular bilayers

The role of interface morphology in the magnetic behavior of FePt/Fe exchange-coupled systems has been considered. Hard/soft bilayers with different interface morphologies were obtained by depositing Fe thin layers on FePt epitaxial layers with different morphologies, but all with a high degree of L1₀ ordering and high coercivity. It was found that the hard/soft coupling can be tuned by changing the FePt morphology, allowing tailoring of the hysteresis loops and modification of the magnetic regime at a fixed Fe thickness (from Rigid Magnet to Exchange-Spring magnet). Remarkably, all bilayer series showed a drastic reduction in coercivity with increasing Fe thickness to 3.5 nm (μ₀H_C ? 1.1 T).     

Structure and magnetoresistive properties of current-perpendicular-to-plane pseudo-spin valves using polycrystalline Co2Fe-based Heusler alloy films

We report current-perpendicular-to-plane giant magnetoresistance (CPP–GMR) of pseudo-spin valves (PSVs) with polycrystalline Co₂Fe(Al_0.5Si_0.5) (CFAS) and Co₂Fe(Ga_0.5Ge_0.5) (CFGG) Heusler alloy films. Strongly [0 1 1] textured polycrystalline Heusler alloy films grew on the Ta/Ru/Ag underlayer. Relatively large CPP–GMR values of ΔRA up to 4 mΩ μm² and ΔR/R up to 10% were obtained with 5 nm thick Heusler alloy films and Ag spacer layer by annealing CFAS PSV at 450 °C and CFGG PSV at 350 °C. Transmission electron microscopy revealed a flat and sharp interface between the [0 1 1] textured CFAS layers and the [1 1 1] textured Ag spacer layer. Annealing above an optimal temperature for each PSV led to reductions in MR values as a result of the thickening of the spacer layer induced by the Ag diffusion from the outer Ag layers.     

Size effects in the superelastic response of Ni54Fe19Ga27 shape memory alloy pillars with a two stage martensitic transformation

The superelastic behavior of Ni₅₄Fe₁₉Ga₂₇ shape memory alloy (SMA) single crystalline pillars was studied under compression as a function of pillar diameter. Multiple pillars with diameters between 10 μm and 200 nm were cut on a single crystalline bulk sample oriented along the [1 1 0] direction as the compression axis and that had undergone fully reversible two stage martensitic transformation, i.e. L2₁ austenite to 10M/14M modulated martensite and then to L1_o martensite. The results revealed an increase in the critical stress for stress-induced martensitic transformation and the yield strength of martensite with decreasing pillar size. The stress hysteresis also increased with the reduction in pillar size and the superelastic response started to diminish below 500 nm pillar diameter. Two-stage martensitic transformation was suppressed for pillar sizes of 1 μm and below, which were shown to exhibit a direct austenite to L1_o transformation. Such a change in the transformation pathway, i.e. from a two stage to one stage transformation, was also observed in bulk single crystals with increasing temperature. We demonstrated the absence of two stage transformation in bulk at high temperatures. This finding suggests that decreasing the sample size and increasing the temperature have similar effects on the superelastic response of NiFeGa SMAs that had undergone two-stage transformation and indicates that a reduction in pillar diameter decreases the transformation temperature due to the difficulty of martensite nucleation on small scales. The damping coefficients of the pillars were also calculated and the results highlighted that damping capacities higher than those of bulk metallic alloys can be achieved using submicron sized pillars.     

The effect of annealing on the microstructure and tensile properties of a β/γ′ Ni–Al–Fe alloy

The evolution of the microstructure of a (β/γ ′) Ni–32 at.% Al–5 at.% Fe alloy during annealing has been studied by electron microscopy and X-ray diffraction. Annealing at 800°C and 1100°C causes a reverse martensitic transformation, L1₀→B2 (β), and a B2→L1₂ (γ ′) phase transformation. The lower annealing temperature leads to a higher volume fraction of the γ ′-phase but a smaller size of the γ ′-particles. The kinetic laws of the coarsening and of the increase in the volume fraction of the γ ′-phase are discussed. The orientation relationships between the β and γ ′ phases appeared to be mainly of Nishiyama–Wassermann and Bain types after 800°C annealing, while Kurdjumov–Sachs and Bain orientation relationships were predominant in the alloys annealed at 1100°C. A strong correlation between the volume fraction of the γ ′-phase and the tensile characteristics of the alloy has been established.     

Segregation engineering enables nanoscale martensite to austenite phase transformation at grain boundaries: A pathway to ductile martensite

In an Fe–9 at.% Mn maraging alloy annealed at 450 °C reversed allotriomorphic austenite nanolayers appear on former Mn decorated lath martensite boundaries. The austenite films are 5–15 nm thick and form soft layers among the hard martensite crystals. We document the nanoscale segregation and associated martensite to austenite transformation mechanism using transmission electron microscopy and atom probe tomography. The phenomena are discussed in terms of the adsorption isotherm (interface segregation) in conjunction with classical heterogeneous nucleation theory (phase transformation) and a phase field model that predicts the kinetics of phase transformation at segregation decorated grain boundaries. The analysis shows that strong interface segregation of austenite stabilizing elements (here Mn) and the release of elastic stresses from the host martensite can generally promote phase transformation at martensite grain boundaries. The phenomenon enables the design of ductile and tough martensite.     

Plasma nitridation of a novel Ni–10.8 wt.%Cr nanocomposite

A Ni–10.8Cr nanocomposite (by wt.%), consisting of nanocrystalline Ni matrix (mean grain size: 60 nm) and dispersed Cr nanoparticles (mean particle size: 42 nm), has been synthesized by nanocomposite electrodeposition. The unique structure causes the nanocomposite to form a double-layered nitrided zone during plasma nitridation at 560 °C for 10 h. The outer layer (∼50 μm thick) precipitates nanometer-sized CrN (<100 nm), which increased in size but decreased in number with increasing nitridation depth (following Böhm–Kahlweit’s mode). The inner layer (∼5 μm thick) exhibits larger-coarsened nitride precipitates (100–200 nm) which almost link together. The greatly enhanced nitriding kinetics in the nanocomposite compared to a compositionally similar but microstructurally different Ni–10Cr alloy (mean grain size: 30 μm) is mainly associated with the fact that the numerous grain boundaries dramatically increase the nitrogen permeability, according to the treatment using a classical Wagner’s approach. The nanohardness profile in relation to the microstructure of the nitrided zone in the nanocomposite has also been investigated.     

Expanding microwave plasma process for thin molybdenum films nitriding: Nitrogen diffusion and structure investigations

Transition metal nitrides exhibit very interesting properties for mechanical and catalytic applications. Thin nitride layers are expected to prevent metal films from oxidizing under working conditions. Molybdenum thin films of about 200 nm thick and deposited on Si (100) wafers are processed in pure N₂, (Ar–35%N₂) and (Ar–25%N₂–30%H₂) expanding microwave plasma at 673 K. Secondary neutral mass spectrometry (SNMS) and Raman spectroscopy are both used to make correlations between the composition and the structure of the as-formed compounds. The low nitrogen diffusion up to a depth of about 40 nm is correlated to the formation of well crystallized MoO₂ of monoclinic structure acting as a barrier of diffusion for nitrogen, in molybdenum films exposed to pure N₂ and (Ar–35%N₂) plasma. The large nitrogen diffusion into the film exposed to ternary (Ar–25%N₂–30%H₂) plasma is correlated to the reduction of MoO₂ oxides by hydrogen species such as atomic hydrogen, NH_x<3… contained in ternary plasma. The formation of Mo–N phases with defects could take place in molybdenum films processed at 673 K compared to the results obtained at 873 K.