Stress Of Ti Research Articles

Effect of the substrate treatment on the microstructure of CVD Ti(C,N)/α-Al2O3/TiN hard coatings

Although the use of protective Ti(C,N)/Al2O3 coatings produced by chemical vapour deposition (CVD) on cemented carbide (WC-Co) substrates is the state of the art in high-speed metal cutting, the effect of the substrate treatment on the microstructure of such coatings is not fully understood yet. In this study, the influence of the original substrate grain size (coarse-grained, fine-grained) and the substrate treatment (as-sintered, wet blasted, ground or polished) on the grain size, preferred orientation of crystallites and residual stress in Ti(C,N)/Al2O3 coatings was systematically investigated. The microstructure analyses carried out using scanning electron microscopy with electron backscatter diffraction and symmetrical X-ray diffraction (XRD) revealed that the utilization of fine-grained substrates and the substrate treatment reduce the grain size in both layers, and increase the preferred orientations 〈211〉 and 〈0001〉 of fcc-Ti(C,N) and α-Al2O3 crystallites, respectively. In α-Al2O3, the microstructure changes are mainly connected with the morphology of the bonding layer, as it follows the morphology of the fcc-Ti(C,N) facets. The residual stress analyses done using glancing angle X-ray diffraction (GAXRD) disclosed a reduction of the tensile thermal stress in α-Al2O3 layers that were deposited on coarse-grained substrates. The residual stress in fcc-Ti(C,N) was substantially less reduced in the whole fcc-Ti(C,N)/α-Al2O3 stacks than in the reference fcc-Ti(C,N) monolayers. Vice versa, fcc-Ti(C,N) in the full stacks contained less crystal structure defects and lower microstrain than in the reference fcc-Ti(C,N) monolayers, because they were exposed to higher temperatures.

Temperature dependence of electroplastic effect on reducing the ultimate stress in Ti–6Al–2Zr–1Mo–1V alloy during tension

To investigate the electroplastic effect of Ti–6Al–2Zr–1Mo–1V alloy (TA15 alloy) at different temperatures, the uniaxial tension tests assisted with the pulsed current and temperature matched tension tests without current were performed at the temperature ranging from 200 °C to 800 °C in this study. The results demonstrated that the electroplastic effect of TA15 alloy became appreciable as the temperature approached 600 °C, namely that the pulsed current contributed about 12% reduction in ultimate stress with respect to that of temperature matched test when the temperature was higher than 600 °C. The microstructure observation showed that the pulsed current reduced the ultimate stress by enhancing dislocation recovery and α→β phase transformation. These electroplastic effects for reducing the ultimate stress were related on temperature. When the temperature was higher than 700 °C, the effect of pulsed current on ultimate stress drop changed from promoting dislocation motion to accelerating the nucleation and growth of softer β phase by accelerating the diffusion of Mo, V elements enriched in the β phase. The electroplastic mechanism was considered as the ‘local heat spot’. The higher energy surrounding the defects could reduce a resistance to move an ion core for titanium, which may have a significant effect on dislocation motion and element diffusion promoted by pulsed current.

Unexpected de-twinning of strongly-textured Ti mediated by local stress

• An in-situ study coupling EBSD was carried out on twinning behaviors of Ti in Ti-Al layered metal. • The unexpected de-twinning of { 10 1 ¯ 2 } 〈 10 1 ¯ 1 ¯ 〉 extension twins was observed. • The local Schmid factor (LSF) that considers local stress, and the displacement gradient tensors of twinning provided valuable insights into the twinning behaviors of Ti in Ti-Al layered metal. The local stress tailored by a layered structure is believed to have a crucial influence on the mechanical properties of layered metals, but the correlation between local stress and deformation mechanism is still ambiguous. In the present work, a quantitative analysis of the twinning behavior of Ti in Ti-Al layered metal was conducted by means of electron backscatter diffraction (EBSD) coupled with in-situ tensile tests. Upon the uniaxial tensile loading, an unconventional de-twinning of { 10 1 ¯ 2 } 〈 10 1 ¯ 1 ¯ 〉 extension twin in Ti was observed instead of the expected { 11 2 ¯ 2 } 〈 11 2 ¯ 3 ¯ 〉 contraction twinning. This observation could be attributed to the multiaxial local stress in Ti induced by the layered structure and to differences in the local deformation accommodation among twinning modes. The local Schmid factor (LSF) that considers local stress, and the displacement gradient tensors of twinning provided insights into the twinning behaviors of Ti in Ti-Al layered metal.

The Effect of the Metal Phase on the Compressive and Tensile Stresses Reduction in the Superhard Nitride Coatings

The influence of the compressive and tensile stresses forming in the nanostructured Ti–Al–N coatings during deposition on their physical-mechanical properties was studied. The modifying influence of metal components (Ni and Cu) introduction into Ti–Al–N coatings, which do not interact with nitrogen and have limited solubility with the nitride phase, was also under research. Coatings were deposited on WC–(6 wt.%)Co carbide cutting inserts with an arc-PVD method using a cathodic vacuum arc evaporation apparatus. The introduction of Ni and Cu to the composition leads to the reduction of nitride phases grain size in both investigated coatings from 120 to 10–12 nm for Ti–Al–Cu–N and to 15–18 nm for Ti–Al–Ni–N. Thus, the hardness increases from 29 to 43 and 51 GPa for the mentioned above coatings, respectively. Meanwhile, Ti–Al–Cu–N and Ti–Al–Ni–N coatings are characterized by tensile stresses about 0.12–0.32 MPa against the much higher value of compressive stresses in Ti–Al–N coatings (4.29–5.31 GPa). The modification of Ti–Al–N coatings also leads to the changing of their destruction mechanism during the scratch-test. The critical loads characterizing the emergence of the first cracks in the coatings and complete abrasion of the coating (Lc1 and Lc3) were determined. They had the value of 20; 22 N (Lc1) and 64; 57 N (Lc3) for Ti–Al–Ni–N; Ti–Al–Cu–N coatings, respectively. The Lc1 parameter for Ti–Al–N coatings was much lower and was equal to 11 N. Along with those, Ti–Al–N coatings destructed according to the adhesion mechanism when the critical load was 35 N. In addition, the decreasing level of compressive stresses in Ti–Al–Cu–N and Ti–Al–Ni–N coatings as compared to that in the Ti–Al–N coating, their crack resistance during multi-cycle shock-dynamic impact test was significantly higher. The results can indicate that high hardness and crack resistance of the coatings is to a greater extent determined by coatings nanostructuring, not the stresses value. In addition, it confirms the possibility to obtain coatings with low stresses value while maintaining their superhardness.

Numerical Modeling of Ti Deformation for the Development of a Titanium and Stainless Steel Transition Joint

Finite element analysis (FEA) was used to model the joining of titanium grade 2 (Ti) to AISI 321 stainless steel (SS) transition joint of lap configuration with grooves at the interface on SS side. The hot forming of Ti for filling the grooves without defects was simulated. FEA involving large plastic flow with sticking friction condition was initially validated using compression test on cylindrical specimen at 900 °C. The barreled shape and a no-deformation zone in the sample predicted by FEA matched with those of the compression experiments. For the joining process, FEA computed the distribution of strain and hydrostatic stress in Ti and the minimum ram load required for a defect-free joint. The hot forming parameters for Ti to fill the grooves without defects and any geometrical distortion of the die were found to be 0.001 s−1 at 900 °C. Using these conditions a defect-free Ti-SS joint was experimentally produced.

Evolution of residual stress in Ti–Al–Ta–N coatings on hard metal milling inserts

Coated WC–Co hard metal milling inserts applied in milling application show thermal fatigue induced by interrupted tool-workpiece contact and wear as the two main damage mechanisms. Depending on the magnitudes of thermal and mechanical loads, either wear or thermal fatigue in form of combcracks may be dominant and determine the insert's lifetime. The present work illustrates the evolution of residual stress in an arc-evaporated Ti–Al–Ta–N coating for two different milling test setups, in one of which wear acted as the dominant damage mechanism. In the other test setup thermal fatigue fostered the formation of combcracks. Earlier work revealed a location on the tool's rake face, referred to as region of interest, with a significant buildup of tensile residual stresses in the WC phase of the substrate using synchrotron facilities. The residual stress state in the coating was determined in this region of interest by a cover method on the tools' rake faces after a defined number of cuts by X-ray diffraction using in-house facilities. In the wear dominated test setup, compressive residual stresses remained present until the end of tool life in coatings. Tensile residual stresses were found in coatings on inserts in which thermal fatigue was dominant.

Thermal relaxation of residual stress in laser shock peened Ti–6Al–4V alloy

Laser shock peening (LSP) induced residual stresses in Ti–6Al–4V, and their thermal relaxation due to short-term exposure at elevated temperatures are investigated by an integrated modeling/simulation and experimental approach. A rate and temperature-dependent plasticity model in the form of Johnson–Cook (JC) has been employed to represent the nonlinear constitutive behavior under both LSP and thermal loads. By comparing the simulation results with experimental data, model parameters for Ti–6Al–4V are first calibrated and subsequently applied in analyzing the thermal stability of the residual stress in LSP-treated Ti–6Al–4V. The analysis shows that the magnitude of stress relaxation increases with the increase of applied temperature due to material softening. Most of stress relaxation occurs before 10min to 20min exposure in this study, and stress distribution becomes more uniform after thermal exposure. An analytical model based on the Zener–Wert–Avrami formula is then developed based on the simulation results. The activation enthalpy of the relaxation process for laser shock peened Ti–6Al–4V is determined to be in the range of 0.71eV to 1.37eV.

Constitutive analysis of compressive deformation behavior of ELI-grade Ti–6Al–4V with different microstructures

In this study, a constitutive analysis of the flow responses of Ti–6Al–4V under various strain rates \( \dot{\varepsilon } \) was conducted by separately quantifying the hardening and softening effects of microstructure, interstitial solute and deformation heating on the total stress. For this purpose, a series of compression tests on an extra-low interstitial grade alloy with equiaxed, lamellar, or bimodal microstructures was performed at \( 10^{ - 3} \le \dot{\varepsilon } \le 10\;{\text{s}}^{ - 1} \) until the metal fractured, and the results were compared to those of the commercial grade alloy. The thermal stress σ* increased with an increasing interstitial solute concentration; the athermal stress increased in the order of equiaxed, lamellar, and bimodal microstructures. Load–unload–reload tests revealed that the flow softening at a relatively high \( \dot{\varepsilon } \) was likely caused by deformation heating rather than by microstructure change; thus flow softening was attributed to a decrease in σ*. Finally, a mechanical threshold stress model was extended to capture those observations; the modified model can provide a reasonable prediction of flow stress in Ti–6Al–4V with different microstructures and interstitial solute concentrations.

Comparison of residual stresses in Ti–6Al–4V and Ti–6Al–2Sn–4Zr–2Mo linear friction welds

In this paper, the levels of residual stress in the vicinity of linear friction welds in Ti–6Al–4V (Ti-64), a conventional α–β titanium alloy, and Ti–6Al–2Sn–4Zr–2Mo (Ti-6242), a near α titanium alloy with higher temperature capability, are mapped and contrasted. The alloys have significantly different high temperature properties and the aim of this work was to investigate how this might affect their propensity to accumulate weld residual stresses and their response to post-weld heat treatment. Measurements are reported using high energy synchrotron X-ray diffraction and the results are compared to those made destructively using the contour method. The strain free lattice plane d0 variation across the weld has been evaluated using the biaxial sin2Ψ technique with laboratory X-rays. It was found that failure to account for the d0 variation across the weld line would have led to large errors in the peak tensile stresses. Contour method measurements show fairly good correlation with the diffraction results, although the stresses are underestimated. Possible reasons for the discrepancy are discussed. The peak tensile residual stresses introduced by the welding process were found to be greater for Ti-6242 (∼750 MPa) than for Ti-64 (∼650 MPa). Consistent with the higher temperature capability of the alloy, higher temperature post-weld heat treatments have been found to be necessary to relieve the stresses in the near α titanium alloy compared to the α+β titanium alloy.

Prediction of flow stress in Ti–6Al–4V alloy with an equiaxed α + β microstructure by artificial neural networks

Flow stress during hot deformation depends mainly on the strain, strain rate and temperature, and shows a complex and nonlinear relationship with them. A number of semi-empirical models were reported by others to predict the flow stress during hot deformation. This work attempts to develop a back-propagation neural network model to predict the flow stress of Ti–6Al–4V alloy for any given processing conditions. The network was successfully trained across different phase regimes (α + β to β phase) and various deformation domains. This model can predict the mean flow stress within an average error of ∼5.6% from the experimental values, using strain, strain rate and temperature as inputs. This model seems to have an edge over existing constitutive model, like hyperbolic sine equation, and has a great potential to be employed in industries.

Neural network modelling of flow stress in Ti–6Al–4V alloy with equiaxed and Widmanstätten microstructures

In the present study, artificial neural networks (ANNs) were used to model flow stress in Ti–6Al–4V alloy with equiaxed and Widmanstätten microstructures as initial microstructures. Continuous compression tests were performed on a Gleeble 3500 thermomechanical simulator over a wide range of temperatures (700–1100°C) with strain rates of 0˙001–100 s–1 and true strains of 0˙1–0˙6. These tests have been focused on obtaining flow stress data under varying conditions of strain, strain rate, temperature, and initial microstructure to train ANN model. The feed forward neural network consisted of two hidden layers with a sigmoid activation function and backpropagation training algorithm was used. The architecture of the network includes four input parameters: strain rate ϵ, Temperature T, true strain ϵ and initial microstructure and one output parameter: the flow stress. The initial microstructure was considered qualitatively. The ANN model was successfully trained across (α+β) to β phase regimes and across different deformation domains for both of the microstructures. Results show that the ANN model can correctly reproduce the flow stress in the sampled data and it can predict well with the nonsampled data. A graphical user interface was designed for easy use of the model.

Finite element analysis of thermal stress in magnetron sputtered Ti coating

Abstract The thermal, shear and radial stresses generated in the Ti coating deposited on glass and Si substrates were investigated by finite element analysis (ANSYS). The four-node structural and quadratic element PLANE 42 with axisymmetric option were used to model the Ti coating on glass and Si substrates. The influence of deposition temperature, substrate and coating properties on the generation of thermal stress in Ti is analyzed. It is found that the thermal stress of Ti coating exhibits a linear relationship with deposition temperature and Young's modulus of the coating, but it exhibit an inverse relationship with the coating thickness. The results of simulated thermal stress are in accordance with the analytical method. The radial stress and shear stress distribution of the coating–substrate combination are calculated. It is observed that high tensile shear stress of Ti coating on glass substrate reduces its adhesive strength but high-compressive shear stress of Ti on Si substrate improves its adhesive strength.

Deformation Behavior in the Isothermal Compression of Hydrogenated Ti–5.6Al–4.8Sn–2.0Zr–1.0Mo Alloy

The isothermal compression of hydrogenated Ti–5.6Al–4.8Sn–2.0Zr–1.0Mo alloy has been carried out. The experimental result shows that the additional hydrogen significantly decreases the flow stress of Ti–5.6Al–4.8Sn–2.0Zr–1.0Mo alloy. The minimum peak stress at deformation temperature of 830–900 °C corresponds to the hydrogen content of 0.4 wt.%, alternatively the appropriate hydrogen content raises the true strain rate up to one order of magnitude in comparison with the received Ti–5.6Al–4.8Sn–2.0Zr–1.0Mo alloy. X-ray diffraction examination shows the appropriate hydrogen content accelerates the $${\upbeta}$$ phase transformation so as to improve the workability of this alloy. However, the hydride phase appears when the hydrogen content is about 0.734 wt.%, which increases of the flow stress in comparison to the flow stress of this alloy with hydrogen content of 0.4 wt.%.

Low temperature relaxation of residual stress in Ti–6Al–4V

Abstract Isothermal tests were performed on Ti–6Al–4V to observe time-dependent changes in the lattice parameter of the α (hexagonal close-packed) and β (body-centered cubic) phases. Synchrotron-based experiments show contraction in β at temperatures below 550 °C, and expansion at higher temperatures. Contraction results from annealing of preexisting residual stresses, while expansion results from V diffusion when the α → β phase transformation becomes kinetically active.

Twinning behavior in the Ti–5 at.% Al single crystals during cyclic loading along [0 0 0 1

Cyclic deformation behavior of Ti–5 at.% Al single crystals subjected to pull–push cyclic load along [0 0 0 1] crystallographic orientation was studied. A higher cyclic stress response was displayed in the Ti–5Al single crystal oriented for [0 0 0 1] than that oriented for single prism slip. Optical microscopy and transmission electron microscopy examinations show that twinning is a dominant plastic deformation mode in the single crystals during cycling. Trace analysis of prepolished surfaces was used to identify the twin systems primarily responsible for deformation. The major twin type observed was {1 0 1 ¯ 2}, {1 1 2 ¯ 2}, {1 0 1 ¯ 1} and {1 1 2 ¯ 1}. 〈 c + a〉 slip was observed in the neighboring region of twins in the fatigued specimens. The activation of multiple twinning systems contributed to the higher cyclic saturation stress in Ti–5Al single crystals oriented for [0 0 0 1].

Mechanical and tribological properties of titanium–aluminium–nitride films deposited by reactive close-field unbalanced magnetron sputtering

Thin films of titanium–aluminium–nitride (Ti 1− x Al x N), with 0 ≤ x ≤ 1.0, were deposited onto Si(1 0 0) and hardened M42 tool steel substrates at room temperature by reactive close-field unbalanced magnetron sputtering at a bias voltage of −50 V in an Ar–N 2 gas mixture. These films were characterized and analyzed using X-ray photoelectron spectroscopy (XPS), optical interference method, nanoindentation measurements, scanning electron spectroscopy (SEM), scratch tester, cyclic impact tester, reciprocating wear tester, and Raman spectroscopy. It was found that the films (typically 1.0 μm in thickness) deposited under aluminium-free conditions (TiN) had a high compressive stress of ∼3.0 GPa. The compressive stress in Ti 1− x Al x N films decreased significantly with an increase in the Al content ( x) and reached its lowest value of ∼0.3 GPa at x = 0.41. Passing this point, the compression of films rose again. These results can be ascribed to the decrease in the lattice parameter caused by incorporating small Al atoms in the lattice sites and structural changes (the development of an amorphous AlN network in the range of about x = 0.3–0.41) in Ti 1− x Al x N films as the incorporation of aluminium was increased. By scratch and dynamic impact tests, it was also found that the films at x = 0.41 (hardness of ∼31 GPa and elastic modulus of ∼312 GPa) exhibited the best adhesion and cohesive strength. For the wear evaluation of the films, three distinct processes have been evidenced. (i) The TiN films ( x = 0) exhibited a mild wear behaviour, resulting in lower friction coefficients (∼0.3–0.5) and wear rates (∼3 × 10 −6 mm 3/Nm); (ii) the films at x = 0.09 showed the presence of micro-scratches penetrating the substrate; and (iii) the films with higher x values (>0.09) revealed the presence of the interfacial layer in the wear track. Raman scattering results showed that the wear debris in TiN films composed of a mixture of nanocrystalline anatase and rutile TiO 2 at applied loads of 2 and 5 N, whereas nanocrystalline rutile TiO 2 was observed at an applied load of 20 N. For films with x > 0.09, only the rutile TiO 2 was observed. These different adhesive properties and wear performance for TiN, Ti–Al–N, and AlN films are explained on the basis of microstructure, mechanical properties and tribochemical wear mechanisms.

Determination of the habit plane characteristics in the β–α′ phase transformation induced by stress in Ti–5Al–2Sn–4Zr–4Mo–2Cr–1Fe

The characteristics of the habit planes of the α″ orthorhombic martensite plates, induced by stress within a commercial sheet of Ti–5Al–2Sn–4Zr–4Mo–2Cr–1Fe have been investigated. First, the orientations of several grains in BCC phase which presented at least one α″ orthorhombic martensite plate, were determined by EBSD. The Miller indexes of habit planes within a parent grain were deduced from the intersection lines of the martensite plates on two perpendicular sides of the sample and from the orientation of the parent grain. These data are compared to the characteristics of the habit planes, calculated from the phenomenological theory of the martensitic transformation proposed by Wechsler, Liebermann and Read. The different results and their dispersions are analysed in this contribution.

Stress analyses and in-situ fracture observation of wear protective multilayer coatings in contact loading

A model was developed to describe stress distributions in hard multilayer coatings under applied contact load, simulating coating application for wear protection. Elastic deformations were considered and an algorithm was suggested to calculate normal and shear stress across the coating thickness for various numbers of layers and layer thickness. The model was applied to analyze contact stress in Ti–TiN multilayer coatings produced by vacuum arc deposition. Calculated locations of maximum shear stress agreed well with locations of actual coating failures, which were studied in-situ using a scanning electron microscope (SEM). A three-point bending test stage was designed for SEM observations of deformation and crack development under a cylindrical indenter. Results indicated that the highest shear stress occurs under the substrate/coating interface and that this region may plastically deform causing coating adhesive and cohesive failure. When using a multilayer coating design with 10 pairs, the peak stress was moved into the coating volume, reducing normal and shear stresses in the substrate by as much as 40 and 22%, respectively. A good correlation between computational and experimental studies verified the model applicability for optimizing the design of hard multilayer coatings in tribological contacts.

Effect of thermal cycling on martensitic transformation temperatures in Ti–Ni–Cu shape memory alloys

Changes in martensitic transformation temperatures during thermal cycling in Ti–Ni–Cu shape memory alloys have been investigated by means of electrical resistivity measurements, thermal cycling tests under constant load and transmission electron microscopy. During thermal cycling without applied stress, the B2→B19′ transformation temperature Ms decreased, while the B2→B19 transformation start temperature Ms′ kept almost constant. During thermal cycling with applied stress, in solution treated Ti–45Ni–5Cu alloy, changes in Ms depended on the amount of applied stress. That is, Ms decreased when the applied stress was 39.2 MPa, while its value kept almost constant when a stress of 117.2 MPa was applied. It was also found that Ms′ increased during thermal cycling in the solution treated Ti–35Ni–15Cu and Ti–30Ni–20Cu alloys, irrespective of the amount of applied stress. All changes in Ms and Ms′ during thermal cycling with applied stress in Ti–Ni–Cu alloys were explained well by a combination of the thermal cycling effect and the structural refinement effect.

Residual Stress in Ti(C,N) Coatings on HSS Substrate

Residual Stress in Ti(C,N) Coatings on HSS Substrate