Correlation between texture and corrosion properties of magnesium coatings produced by PVD

Physical vapour deposition with energetic ions is an established technology for creating functional surfaces where changing morphologies are observed with increasing energy deposition. In this presentation, magnetron sputtering (MS) is compared with ion beam sputtering (IBS) and vacuum arc deposition (VAD) for corrosion resistant Mg coatings. With increasing average energy flux along the three methods, a transition from a columnar growth regime towards a layer-by-layer growth at increased energies was observed, while a basal texture with the c-axis normal to the surface was found in all cases. However, the full width at half maximum (FWHM) of the corresponding Mg(002) rocking curves showed a pronounced minimum of 3° for IBS deposited films, apparently caused by the reflected high energy primary Ar⁺ ions. For pure Mg films, no larger differences in the corrosion potential and the corrosion rates were measured.     

Experimental tests and numerical simulation studies on nano-indentation of TiN film deposited on N-implanted aluminum

Nitrogen was implanted into aluminum substrate prior to magnetron sputtering of TiN films to introduce a modified layer between the TiN film and Al substrate. In another sample, a Ti interlayer was produced on the untreated substrate by means of magnetron sputtering. From the load-displacement curves obtained by the nano-indentation tests, the ring-like cracks appeared at 38 mN on the TiN/N⁺-implanted aluminum sample, 25 mN on the TiN/Ti/aluminum sample, and 18 mN on the TiN/untreated aluminum sample. The finite element method (FEM) was used to analyze the stress distribution at the interface of the various samples. The decline in the film base tensile stress and shear stress revealed the advantages of the pre-implanted substrate. All the numerical simulation results are consistent with the increased loading capacity obtained from the load-displacement curves.     

反应溅射非晶态氧化铝薄膜SCIEI

研究了平面磁控直流反应溅射法沉积氧化铝薄膜的过程。结果表明氧分压大小对反应溅射过程有决定性作用,当氧分压增大或减小过程中存在两个阈值。氧分压小于阈值为金属Al溅射区,大于阈值为氧化铝溅射区。在阈值附近总气压、溅射电压和沉积速率发生突变,溅射特性(V-J)曲线有不同规律。沉积的氧化铝膜为非晶态,高温下可晶化,本文还讨论了反应溅射的机理。     

Reactive sputtering of TiOxNy coatings by the reactive gas pulsing process: Part II: The role of the duty cycle

The reactive gas pulsing process (RGPP) was used to deposit titanium oxynitride thin films by dc reactive magnetron sputtering. A titanium target was sputtered in a reactive atmosphere composed of Ar + O₂ + N₂. Argon and nitrogen gases were continuously introduced into the sputtering chamber whereas oxygen was injected with a well-controlled pulsing flow rate following a rectangular and periodic signal. A constant pulsing period T = 45 s was used for every deposition and the duty cycle α = t_ON/T was systematically changed from 0 to 100%. The operating conditions were investigated taking into account the poisoning phenomena of the target surface by oxygen and nitrogen. Kinetics of poisoning were followed from measurements of the total sputtering pressure and titanium target potential during the depositions. Deposition rate and optical transmittance of titanium oxynitride coatings were also analysed and correlated with the process parameters. Pulsing the oxygen flow rate with rectangular patterns and using suitable duty cycles, RGPP method allows working according to reversible nitrided-oxidised target conditions and leads to the deposition of a wide range of TiO_xN_y thin films, from metallic TiN to insulating TiO₂ compounds.     

Kinetics and critical conditions for initiation of dynamic recrystallization during hot compression deformation of AISI 321 austenitic stainless steel

Dynamic recrystallization behavior of AISI 321 austenitic stainless steel were studied using hot compression tests over the range of temperatures from 900 °C to 1200 °C and strain rates from 0.001 s^-1 to 1 s^-1. The critical strain and stress for initiation of dynamic recrystallization were determined by plotting strain hardening rate vs. stress curves and a constitutive equation describing the flow stress at strains lower than peak strain. Also, the strain at maximum flow softening was obtained and the effect of deformation conditions (Z parameter) on the critical strain and stress were analyzed. Finally, the volume fraction of dynamic recrystallization was calculated at different deformation conditions using these critical values. Results showed that the model used for predicting the kinetics of dynamic recrystallization has a great consistency with the data, in the form of θ-ε curves, directly acquired from experimental flow curves.     

Generation of isochronous stress-strain curves with a nonlinear least square fitting method for modified 9Cr-1Mo steel

This study deals with the generation of isochronous stress-strain curves for modified 9Cr-1Mo steel. Material property data (s_y, D, and m) from tensile tests and strain-time curves from creep tests were obtained at 550 °C. On the basis of the experimental data, the creep curves were characterized by Garofalo’s creep model. The three parameters of P ₁, P ₂ and P ₃ in Garofalo’s model could be optimized properly by a nonlinear least square fitting analysis. The three parameters showed a good stress dependency with a linear relation. However, the P ₃ parameter, which represents the steady state creep rate, exhibited two-slope behavior with different stress exponents at a transient stress of about 250 MPa. The results verify that the long-term creep curves for G91 steel can be modeled by Garofalo’s model with only several short-term creep data. When the modeled creep curves are used, long-term isochronous curves of up to 10⁵ h can be successfully constructed.     

Sensitivity analysis of history dependent material mechanical models for numerical simulation of welding process

Abstract

Yield stress of 6013-T6 aluminium alloy was tested on Gleeble 1500D thermal–mechanical system at predesigned temperatures during different typical thermal cycles, in order to accurately reflect the influence of weld thermal history on material properties. The typical thermal cycles were referred to the temperature field simulation results of real welding process. The changes of yield stress were obtained directly from the stress–strain curves generated by the tensile tests. The tests were more accurate than previous publications, where only the yield stresses at room temperature after thermal history were tested or calculated from microstructure evolution model. Experimental results showed that the changes of yield stress during the cooling stage of typical thermal cycles followed one set of curves. These yield stress–temperature curves were different from those during the heating stage. Temperature and temperature history dependent material model M2 and M3 were established based on the experimental results. M2 model was perfectly plastic model while work hardening effect was considered in M3 model. Compared with conventional temperature dependent material model M1, the distributions of longitudinal residual stress and strain obtained with temperature and temperature history dependent models fit better with published results. Yield stress of the material at the weld zone decreased a lot after having experienced weld thermal history and longitudinal compressive plastic strain at the weld zone recovered to some extent during the cooling stage in M2 and M3 models. These were the main causes for lower peak longitudinal residual tensile stress in M2 and M3 models.     

Electrical properties of sputtered ZnO films with nitrogen and phosphorous Co-doping

Acceptor (N)-acceptor (P) co-doped ZnO films were deposited at N₂O pressures varied between 5m Torr and 50 m Torr on Si (001) substrates using radio frequency sputtering. N₂O plasma in a growth chamber and a sputtering target containing Zn₃P₂ were used as the sources for N and P, respectively. Electrical measurements revealed that all the films had an n-type conduction in the as-grown state for all N₂O pressures. However, for the films grown at an N₂O pressure of 10 m Torr, annealing at 800°C resulted in flipping conduction behavior from n-to p-type. Low temperature photoluminescence results confirmed the presence of an acceptor-related emission at 3.303 eV for the p-type ZnO films only.     

On surface temperatures during high power pulsed magnetron sputtering using a hot target

A study of temperature of a magnetron target was performed for the case of high power pulsed magnetron sputtering (HPPMS) of titanium thin films, using a water-cooled target and a hot target. Temporal evolution and spatial distribution of surface temperature were investigated. Temperature measurements were made by an infrared camera for target diameter of 100 mm, pulse repetition frequencies of 1 and 10 kHz, and discharge average pulse currents of from 2.5 to 35 A. For the case of hot target, surface temperature of the erosion zone increased up to 1750 °C and melting occurred. Temporal evolution of temperature after the end of deposition revealed phase change in solid-state from β-Ti to α-Ti at 882 °C and, for the case of high average pulse currents, also solidification at 1670 °C. The solid state phase transformation plateau was used to determine an emissivity of Ti target for the present case, and therefore precisely calibrate infrared camera measurements. The target melting was analysed in detail. The dependencies of maximum temperature on average pulse current and on average target power density for the case of hot target revealed the existence of heat losses other than radiation (i.e. enhanced sputtering, sublimation, electron emission and evaporation) at temperatures above 1500 °C, which correlates with higher erosion and deposition rates shown in another work.     

Flow stress evaluation of zinc copper and carbon steel tubes by hydraulic bulge tests considering their anisotropy

The object of this paper is to evaluate the stress–strain characteristics of annealed C26800 zinc copper tubes and AISI 1215 carbon steel tubes considering their anisotropic effects by hydraulic bulge tests and tensile tests. In this analytical model, Hill's orthogonal anisotropic theory was adopted for deriving the effective stresses and effective strains under a biaxial stress state. The tube thickness at the pole, bulge height and the internal forming pressure were measured simultaneously during the bulge tests. The effective stress–effective strain relations could be determined by those measured values and this analytical model. The flow stress curves of C26800 copper and AISI 1215 carbon steel tubes obtained by this approach were compared with those obtained by the tensile test with consideration of material's anisotropy. The finite element method was also adopted to conduct the simulations of hydraulic bulge forming with the flow stress curves obtained by the bulge tests and tensile tests. The analytical forming pressures versus bulge heights were compared with the experimental results to validate the flow stress modeling proposed in this paper.     

溅射气压对AlN薄膜纳米结构和纳米力学性能的影响

研究在不同气压下,运用射频磁控溅射法在Si（100）上沉积氮化铝（AlN）薄膜,并使用XRD、SEM、AFM、XPS和纳米压痕等表征手段研究薄膜的性质。XRD结果表明,在低压下有利于沉积c轴取向的薄膜,而在高气压下有利于（100）面的生长。SEM和AFM结果表明,随着气压的升高,沉积速率和表面粗糙度均减小而表面粗糙度则增加。XPS结果表明,降低气压有利于减少薄膜中的氧含量,从而使制备的薄膜成分更接近其化学计量比。通过测试AlN薄膜的纳米力学性能表明,在0.30 Pa下制备的薄膜具有最大的硬度和弹性模量。     

Influence of Applied Pressure on Tensile Behaviour and Microstructure of Squeeze Cast Mg Alloy AM50 with Ca Addition

The development of alternative manufacturing processes is essential for the success in applying Ca-containing magnesium alloys for automotive applications due to their relatively poor die castability. Squeeze casting with its inherent advantages has been demonstrated capable of minimizing the formation of casting defects in Mg-Al-Ca alloys. In this study, the effect of applied pressures on tensile behavior and microstructure of squeeze cast Mg-5wt.%Al-1%wt.%Ca alloy (AMX501) was investigated with the applied pressure varying from 3 to 90 MPa. The results of tensile testing indicate that the tensile properties of AMX501 alloy including ultimate tensile strength, yield strength, and elongation (E _f) increase from 153.7, 80 MPa and 3.26% to 183.7, 90.5, and 5.42% with increasing applied pressure levels from 3 to 90 MPa, respectively. The analysis of true stress versus strain curves shows that an increase in applied pressure levels result in high straining hardening rates during the plastic deformation of the alloy. Microstructural analysis and density measurements indicate that, as the applied pressure increases, the porosity levels of the alloy decrease considerably, despite of almost no significant reduction in grain sizes of the squeeze cast alloys due to their high aspect ratio of cylindrical castings. Hence, the improvement in tensile properties should be primarily attributed to casting densification resulting from applied pressures. The scanning electron microscopy observation on fractured surfaces reveals that the fracture modes of the squeeze cast alloys transit to ductile from brittle with increasing applied pressures.     

Control of morphology (ZrN crystallite size and SiNx layer thickness) in Zr-Si-N nanocomposite thin films

DC reactive magnetron sputtering was used for the deposition of Zr-Si-N thin films. Four series of samples have been deposited at various substrate temperatures T_S: 300 K, 510 K, 710 K and 910 K. Depending on T_S, different N₂ partial pressures p_N2 were required to obtain nearly stoichiometric ZrN films. Si content (C_Si) was varied in each series by changing the power applied on the Si target, whereas the power on the Zr target was kept constant. The microstructure of the coatings was examined by XRD and in cross-section by transmission electron microscopy (TEM). Depending on T_S and p_N2, the deposition rate showed significant variations from 0.04 to 0.18 nm/s. The correlation between film morphology (preferential orientation of crystallites, grain size, column dimensions, thickness of the SiN_x layer covering ZrN crystallites) and the deposition conditions (power applied on Si target, temperature, nitrogen partial pressure and deposition rate) provides useful information for optimizing the deposition process.     

Equation of state and elastic constants of alkali metals at negative pressures

The equation of state and the elastic constants of solids at T = 0 K at negative pressures have been considered by the example of alkali metals. The calculations for Na, K, Rb, and Cs have been performed using the pseudopotential model. The “ideal” tensile strength of these metals is evaluated. The second-and third-order elastic constants are calculated at different pressures, and the mechanical stability of the crystal lattice of alkali metals is analyzed at negative pressures up to the spinodal curve.     

Determination of the flow stress of five AHSS sheet materials (DP 600, DP 780, DP 780-CR,DP 780-HY and TRIP 780) using the uniaxial tensile and the biaxial Viscous Pressure Bulge (VPB) tests

Room temperature uniaxial tensile and biaxial Viscous Pressure Bulge (VPB) tests were conducted for five Advanced High Strength Steels (AHSS) sheet materials, and the resulting flow stress curves were compared. Strain ratios (R-values) were also determined in the tensile test and used to correct the biaxial flow stress curves for anisotropy. The pressure vs. dome height raw data in the VPB test was extrapolated to the burst pressure to obtain the flow stress curve until fracture. Results of this work show that the flow stress data can be obtained to higher strain values under biaxial state of stress. Moreover, it was observed that some materials behave differently if subjected to different state of stress. These two conclusions, and the fact that the state of stress in actual stamping processes is almost always biaxial, suggest that the bulge test is a more suitable test for obtaining the flow stress of AHSS sheet materials for use as an input to Finite Element (FE) simulation models.     

Development of x-ray mask fabrication process using w-sputtering

Properties of DC magnetron sputter-deposited W and WNx absorber films were investigated for x-ray mask applications. Low stress film (5 x 10⁸ dyne/cm² tensile stress) is difficult to obtain with pure Ar gas as the film stress changes from highly compressive to highly tensile with pressure change. The variation of stress with pressure is significantly reduced with N₂/Ar mixture gas, and a reasonable tensile stress and stress stability were obtained with 5% N₂ at 3.5 mTorr. Film density decreases with increasing sputtering pressure and N₂/Ar + N₂ ratio. XRD patterns for films deposited at 3.5 mTorr show crystalline α-W structure for Ar sputtered film but amorphous structure for 5% N₂-sputtered film. Surface smoothness is very good at 5% N₂ but further increase of N₂/Ar+N₂ ratio results in a surface roughening and this is also confirmed by TEM analysis. At this sputtering condition (5% N₂, 3.5 mTorr), film stress stability during air-exposure and annealing was also superior, suggesting a optimum process condition for W-based absorber films.     

Wire based additive layer manufacturing: Comparison of microstructure and mechanical properties of Ti-6Al-4V components fabricated by laser-beam deposition and shaped metal deposition

The microstructure and the mechanical properties of Ti-6Al-4V components, fabricated by two different wire based additive layer manufacturing techniques, namely laser-beam deposition and shaped metal deposition, are presented. Both techniques resulted in dense components with lamellar α/β microstructure. Large ultimate tensile strength values between 900 and 1000 MPa were observed. The strain at failure strongly depends on the orientation, where highest values up to 19% were obtained in direction of the building direction. Heat treatment increased the highest strain at failure up to 22%. The fatigue limit was observed to be higher than 770 MPa.     

High pressure EIGA preparation and 3D printing capability of Ti—6Al—4V powder

Ti—6Al—4V alloy powder was processed by electrode induction melting gas atomization (EIGA) at high gas pressure (5.5–7.0 MPa). The effects of atomizing gas pressure on the powder characteristics and the microstructure, along with the mechanical properties of the as-fabricated block by laser melting deposition (LMD), were investigated. The results indicate that the diameters of powders are distributed in a wide range of sizes from 1 to 400 μm, and the median powder size (d₅₀) decreases with increasing gas pressure. The powders with a size fraction of 100–150 μm obtained at gas pressures of 6.0 and 6.5 MPa have better flowability. The oxygen content is consistent with the change trend of gas pressure within a low range of 0.06%–0.20%. Specimens fabricated by LMD are mainly composed of α+β grains with a fine lamellar Widmanstatten structures and have the ultimate tensile strength (UTS) and yield strength of approximately 1100 and 1000 MPa, respectively. Furthermore, the atomized powders have a favorable 3D printing capability, and the mechanical properties of Ti—6Al—4V alloys manufactured by LMD typically exceed those of their cast or wrought counterparts.     

Plastic instability in polycrystalline metals after low temperature irradiation

The dose dependence of plastic instability behavior has been investigated for polycrystalline metals after neutron irradiation at low temperatures (<200 °C). The analyzed materials consist of 10 body-centered cubic (bcc), 7 face-centered cubic (fcc), and 2 hexagonal close packed (hcp) metals. It was found that the metals after irradiation showed necking at yield when the yield stress exceeded the true plastic instability stress, σ_IS, for the unirradiated material. It was also shown that σ_IS was almost independent of dose below a critical dose. The critical dose is called the dose to plastic instability at yield, D_C, because at higher doses the material shows necking at yield. The D_C values ranged from 0.002 to 0.2 dpa for bcc and hcp metals, except for a high purity iron, that had a D_C value of 6 dpa; whereas the fcc metals gave generally high values ranging from 0.1 to 40 dpa. It is attempted to explain the dose independence of the plastic instability stress by a straightforward shifting of tensile curves by the appropriate strain corresponding to the radiation-induced increase in yield stress. The dose independence of strain-hardening behavior suggests that radiation-induced defects and deformation-produced dislocations give similar net strain-hardening effects.