On the temperature dependence of the critical resolved shear stress of the γ′-strengthened superalloy NIMONIC PE16

The temperature dependence of γ^′-strengthening of the commercial nickel-base superalloy NIMONIC PE16 has been investigated by measuring the critical resolved shear stress in the temperature range 373–1173 K and observing the resulting slip line patterns.     

Dependence of α-phase size on flow stress during dynamic recrystallization steady state in Ti60 alloy

The dependence of a-phase size on flow stress was characterized by a proposed kinetic model during dynamic recrystallization(DRX) steady state in Ti60 alloy. According to the isothermal compression tests, the influence of deformation parameters on the steady-state flow stress was analyzed and the constitutive equation was established to predict the steady-state flow stress under different deformation temperatures and strain rates. A power-law relationship between the DRX average grain size and steadystate flow stress with an exponent of —2 is obtained from the dynamic balance during DRX steady state. The effect of deformation parameters on a-phase size was observed through the microstructure after deformation, and the applicability of the model for Ti60 alloy was verified by the comparison between predicted and experimental data.     

Dislocation induced line-broadening in cold-worked Pb-Bi binary alloy system in the α-phase using X-ray powder profile analysis

An X-ray diffraction line profile analysis on cold-worked Pb-Bi alloys in the α- phase is presented. The anisotropic broadening of X-ray diffraction lines has been interpreted in terms of dislocation induced strain broadening only. Dislocations are found to be predominantly of screw type from a modified Williamson–Hall analysis. Due to the almost symmetric shape of the X-ray line profiles, a restrictedly random dislocation distribution was assumed in the modified Warren–Averbach analysis. The dislocation densities are of the order of 10¹¹ cm⁻² and decrease for annealed specimen. The dislocation arrangement parameter (μ) decreases from more than 1.0 for cold-worked samples to less than 1.0 for annealed samples. This indicates a rather short ranged strain fields in annealed samples compared to cold-worked samples. Relative change in the dislocation arrangement parameter increases with solute concentration.     

Flow stress analysis for determining the critical condition of dynamic ferrite transformation in 6Ni–0.1C steel

In order to clarify the occurrence of dynamic ferrite transformation in a 6Ni–0.1C steel, the stress–strain behavior in uniaxial compression was analyzed for a wide range of temperatures and strain rates. Significant softening of flow stress for austenite was observed at lower temperatures at a constant strain rate, which seemed to correspond with the occurrence of dynamic transformation to ferrite. Analysis of the maximum stress in the stress–strain curves indicated that dynamic ferrite transformation occurred above a certain value of the Zener–Hollomon parameter (Z). The critical deformation condition (Z_C) for the occurrence of dynamic transformation was determined. Increasing the amount of softening resulted in an increase in the fraction of ferrite, and the maximum flow stress came close to the flow stress of ferrite. Microstructural observations revealed that the specimens exhibiting softening consisted of ferrite grains with typical characteristics of deformation microstructure, such as a change in crystal orientation within the ferrite grain, inhomogeneity in ferrite morphology and dislocation substructures inside the grains. All these characteristics confirmed the occurrence of ferrite transformation during deformation, i.e. dynamic ferrite transformation.     

Microstructure characterisation and stress analysis of the Ti48Al2Ag coating on the near-α titanium alloy

The microstructure and phase composition of the protective Ti48Al2Ag coating produced on Timetal 834 by magnetron sputtering have been examined by scanning and analytical transmission electron microscopy (SEM, TEM). TEM investigations revealed that Ti48Al2Ag coating consists of two sublayers: outer columnar γ-TiAl and amorphous Ti₅Al₃O₂. Energy-dispersive synchrotron radiation diffraction was applied for stress analysis. The results show that there are tensile residual stresses present within the Timetal 834 substrate and compressive residual stresses within the γ-TiAl sublayer.     

Spinodal decomposition of the γ-phase upon quenching in the Ti–Al–Nb ternary alloy system

The γ-TiAl with L1₀ crystal structure shows extensive solubility for Nb at elevated temperatures. Recently (Rios et al., Acta materialia 2009; 57:6243), we have demonstrated that the high-Nb γ-TiAl phase becomes unstable upon rapid cooling into a nano-scale two-phase microstructure. In this paper, using detailed compositional and microstructural analyses, we have demonstrated that this phase goes through a spinodal decomposition that results in the compositionally distinct phases identified as a lower-Nb γ-phase and the h-phase, which is rich in Nb and forms by the ordering of this element in the γ-phase.     

Non-isothermal oxidation of ceramic nanocomposites using the example of Ti–Si–C–N powder: Kinetic analysis method

A method of kinetic analysis applicable to non-isothermal oxidation processes of ceramic nanocomposites is presented using Ti–Si–C–N powder as the substrate. The nanoparticle size and phase composition were determined using high-resolution transmission electron microscopy and X-ray diffraction (XRD). Thermogravimetric measurements were carried out for powder samples in dry air in the temperature range 298–1770 K. The following heating rates were applied: 3, 5, 10, 20 K min⁻¹. Mass spectrometry was used to analyze gaseous oxidation products and solid products were identified by the XRD technique. The Coats–Redfern equation was applied for the kinetic analysis. For each stage of the oxidation kinetic models, the best accuracy was achieved using a series of criteria, and then the A and E parameters of the Arrhenius equations were estimated. Both linear regression and artificial neural networks were applied in testing kinetic models.     

Two mechanisms for the normal and inverse behaviors of the critical strain for the Portevin–Le Chatelier effect

The critical strain of the Portevin–Le Chatelier (PLC) effect marks the boundary between stable and unstable flow. In this paper, tension tests were conducted at different temperatures ranging from 173 to 333 K. The PLC effect is present at temperatures of 223–328 K. Normal behavior of the critical strain is observed at temperatures of 223–298 K, while inverse behavior is observed at 298–333 K. By comparing the stress–strain curves at different temperatures, the curves for 173 and 333 K are identified as the lower and upper envelope curves, respectively. Before the critical strain, the stress follows the lower envelope curve at low temperatures, whereas it follows the upper envelope curve at high temperatures. The subsequent serrations, which are upward at low temperatures but downward at high temperatures, oscillate between the two envelope curves. Furthermore, different dislocation states for the lower and upper envelope curves are proposed. The lower envelope curve implies that few dislocations are pinned by solute, while the upper envelope curve implies that some dislocations are pinned by the solute prior to escape. The dominant factor of the critical strain is the diffusibility of the solute at low temperatures and the pinning strength at high temperatures. Finally, based on dynamic strain aging, two critical mechanisms in relation to the first pinning at low temperatures, and the first unpinning at high temperatures, are proposed and are highly consistent with the experiment.     

Experimental and finite-element analysis of the anisotropic response of high-purity α-titanium in bending

In this paper, we present results of four-point bending tests performed on beams of high-purity α-titanium material. These tests have been performed at room temperature for different beam configurations and loading orientations with respect to the orthotropy axes of the material. Digital image correlation was used to determine local strains in the deformed beams. Experimental results compare very well with the predictions of finite-element simulations obtained using the elastic/plastic model developed by Nixon et al. (2010) [12]. Specifically, we compare local deformations and the cross-sections of each beam for all loading configurations. We show that the model predicts with great accuracy the tension–compression asymmetry and the evolving anisotropy of the material. The experimentally observed upward shift of the neutral axis, as well as the rigidity of the response along the hard to deform c-axes are very well described by the proposed model.     

Comparison of mechanical properties in welded joint for CO_2 arc welding by using the micro－shear test and impact test

ＣｏｍｐａｒｉｓｏｎｏｆｍｅｃｈａｎｉｃａｌｐｒｏｐｅｒｔｉｅｓｉｎｗｅｌｄｅｄｊｏｉｎｔｆｏｒＣＯ_２ａｒｃｗｅｌｄｉｎｇｂｙｕｓｉｎｇｔｈｅｍｉｃｒｏ－ｓｈｅａｒｔｅｓｔａｎｄｉｍｐａｃｔｔｅｓｔ￥ＺｈｏｕＬｉｘｉａ；ＷａｎｇＳｈｉｙｕａｎａｎｄ?..     

Microstructure and shear strength of the brazed joint of Ti（C,N）-based cermet to steel

Firm joins were obtained between Ti(C,N)-based cermet and steel with Ag-Cu-Zn-Ni filler metal by vacuum brazing. The effects of technological parameters such as brazing temperature, holding time, and filler thickness on the shear strength of the joints were investigated. The microstructure of welded area and the reaction products of the filler metal were examined by scanning electron microscopy (SEM), metallographic microscope (OM), energy-dispersive X-ray analysis (EDS), and X-ray diffraction (XRD). The brazing temperature of 870°C, holding time of 15 min, and filler thickness of 0.4 mm are a set of optimum technological parameters, under which the maximum shear strength of the joints, 176.5 MPa, is achieved. The results of microstructure show that the wettability of the filler metal on Ti(C,N)-based cermet and steel is well. A mutual solution layer and a diffusion layer exist between the welding base materials and the filler metal.     

Influence of hydrogen on the deformation morphology of vanadium (1 0 0) micropillars in the α-phase of the vanadium–hydrogen system

1. Download : Download high-res image (67KB)
2. Download : Download full-size image

     

Implications from the Poliak–Jonas criterion for the construction of flow stress models incorporating dynamic recrystallization

Dynamic recrystallization (DRX) processes are widely used in industrial hot working operations, not only to keep the forming forces low, but also to control the microstructure and final properties of the workpiece. According to Poliak and Jonas, the onset of DRX can be detected from an inflection point in the strain hardening rate as a function of flow stress. Various models are available that predict the evolution of flow stress from incipient plastic flow to steady-state deformation in the presence of DRX, but their consistency with the criterion of Poliak and Jonas has not been investigated. This work analyzes the conditions that a flow stress model incorporating DRX has to fulfill to be consistent with the criterion of Poliak and Jonas. As the most important inconsistency, it is found that a model might suffer from insufficient differentiability at the critical point. For all models that use a classical JMAK equation for the DRX kinetics, it is shown that the Avrami exponent must exceed a value of 3. If the Avrami exponent is at most 3, a kink may develop in the strain hardening rate, and the second derivative criterion is violated. For DRX kinetics based on nucleation and growth rates that are functions of time, criteria are derived that ensure consistency with the criterion of Poliak and Jonas. DRX kinetics that are consistent with the second derivative criterion are put forward, drawing upon kinetics proposed by Cahn for transformations that originate at grain boundaries. Finally, a minimal model that is consistent with the second derivative criterion is formulated.     

Prediction model for flow stress during isothermal compression in α+β phase field of TC4 alloy

Isothermal compression of TC4 alloy was performed on a Thermecmaster-Z simulator at the deformation temperatures ranging from 1093 to 1243 K, the strain rates ranging from 0.001 to 10.000 s~(-1) and a maximum strain of0.8. The experimental results show that the flow stress increases with the decrease in the deformation temperature and the increase in the strain rate. The apparent activation energy for deformation is much lower at lower strain rates than that at higher strain rates. The flow stress model considering strain compensation was established. The average relative error between the calculated flow stress and experimental results is about 7.69%, indicating that the present model could be used to accurately predict the flow stress during high temperature in α+β phase field of TC4 alloy.     

Influence of the residual stress state of coatings on the wear behavior in external turning of AISI 4140 and Ti–6Al–4V

The residual stress state of coatings influences tool life and performance in machining significantly. Due to thermo-mechanical loads during the cutting process the coatings require tailored properties. Beside the coating structure the two most important properties are the residual stress state and the chemical composition of the coating. Therefore, the influence of these two properties is investigated in this study, comparing the cutting performance in continuous and interrupted cutting of 42CrMo4 (AISI 4140) and Ti–6Al–4V. It is shown that the residual stress states of the coating close to the surface and close to the substrate are important for the wear behavior. High compressive residual stresses near the substrate combined with a material-optimized composition increase the resistance against chipping. Flank wear resistance increases with high compressive residual stresses near the surface and decreasing stresses towards the substrate.