Three dimensional Monte Carlo simulation of grain growth during GTA welding of titanium

The work reported here represents significant advancement in the modeling of grain structure evolution in metallic systems. Utilizing computed temperature fields from a well tested heat transfer and fluid flow model, the evolution of grain structure was simulated for the first time using a three dimensional (3D) Monte Carlo model of grain growth in the heat affected zone of commercially pure titanium welds. The computed weld geometry and the simulated mean prior β grain size for different heat inputs agreed well with the corresponding experimental results when turbulence in the weld pool was considered. The grain sizes at various locations equidistant from the fusion line were different, indicating that the results of previous 2D calculations need to be reexamined. The computed grain size distribution agreed well with the corresponding experimental data. The agreement indicates significant promise for understanding grain growth in the entire heat-affected zone using a comprehensive phenomenological model.     

Effect of preoxidation of titanium on the titanium-ceramic bonding

To improve the titanium-ceramic bonding is a challenged theme because the bonding plays an important role in long-term performance of dental prosthetic implants. This study focused on the pre-heat treatment effect on the surface characteristics of titanium and the bond strength of the porcelain-titanium system. Phase, morphology, and oxide layer thickness of the pre-heat-treated titanium were evaluated. In particular, the bond strength of porcelain fused to pre-treated titanium was measured by using three-point bending test. Statistical analyses were made using one-way analysis of variance (ANOVA). The results revealed that with an increase in treatment temperature, the intensity of diffraction peaks of rutile-type titania formed on pre-treated titanium surface increased. With regard to the thickness of the oxide layer, a monotonous change with increasing treatment temperature was observed. The bond strength of the porcelain-titanium system was 27.6 MPa for untreated titanium, which is above the minimum value of 25 MPa specified in ISO 9693 standard for the three-point bending test. When titanium subjected to pre-heat-treatment at 600-850 °C, the values became the range of 23.7-34.5 MPa, indicating significant differences (p < 0.05). The highest value resided in the optimal pre-treatment temperature of 750 °C at which specimens also presented greater amounts of remained porcelains on the titanium side among all tested groups after bending test. On the basis of the data, the pre-heat treatment in air for titanium substrates might be a simple and effective method for strengthening the titanium-ceramic bonding.     

Grain refinement of cast titanium alloys via trace boron addition

The grain size of as-cast Ti–6Al–4V is reduced by about an order of magnitude from 1700 to 200 μm with an addition of 0.1 wt.% boron. A much weaker dependence of reduction in grain size is obtained for boron additions from >0.1% to 1.0%. Similar trends were observed in boron-modified as-cast Ti–6Al–2Sn–4Zr–2Mo–0.1Si.     

On the possibility of DIGM-assisted abnormal grain growth

The possibility of promoting abnormal grain growth via diffusion induced grain boundary migration during alloying or dealloying is discussed.     

Comparative effect of grain size and texture on the corrosion behaviour of commercially pure titanium processed by equal channel angular pressing

The effect of grain size and texture on the corrosion properties of commercially pure titanium was investigated. Equal channel angular pressing (ECAP) was used to produce different grain size and various crystallographic orientations. Electrochemical impedance spectroscopy was employed to measure the corresponding surfaces’ general corrosion resistance. Samples with the (0 0 0 2) planes parallel to the surface were found to offer the highest corrosion resistance, regardless of their grain size.     

Effect of palladium on the corrosion behavior of titanium

The effects of palladium (Pd) additions on the localized and uniform corrosion of titanium (Ti) were examined by comparing the corrosion behavior of Ti Grade 2 (UNS R50400) to that of Pd-bearing Ti Grade 7 (UNS R52400). Pd additions were found to increase the pitting (E_pit) and repassivation (E_rp) potentials such that E_pit for Ti Grade 2 was significantly lower than E_rp for Grade 7 in chloride (Cl⁻) solutions. The effect of Pd on Ti can be explained through the effects Pd has on the hydrogen evolution reaction. Though Pd additions did significantly affect the localized corrosion resistance of Ti, Pd did not appear to influence the passive corrosion rate nor did it mitigate the deleterious effects of fluoride (F⁻). Fluoride was found to dramatically increase the measured corrosion current above a critical concentration of about 0.5 mM.     

Schlegel description of grain form evolution in grain growth

The paths of evolution of topological grain forms during grain growth are described in terms of number of faces, edges per face and face arrangements, as depicted by Schlegel diagrams and the topological events that change them. This “Schlegel tree” describes transitions to higher face classes by grain encounters at corners and to lower face classes by grain-pair separation at three-edged faces. Transitions within face classes are described through rearrangements that occur to neighboring grains during these events. The process is further described by probabilities of the different paths in terms of numbers of edges, corners and three-edged faces at which face gain and loss events occur. Schlegel data from separated grains and three-dimensional Monte Carlo and front-tracking simulations show good comparison. Grain form frequencies increase with increasing number of transition paths into them from other forms. The highest frequency forms have few or no three-edged faces, while those with the most three-edged faces are present the least. These observations suggest that three-edged faces are catalysts for topological change, and forms with higher frequencies of these have shorter residence times before transitioning to lower classes.     

Analysis of grain growth process in melt spun Fe-B alloys under the initial saturated grain boundary segregation condition

A grain growth process in the melt spun low-solid-solubility Fe-B alloys was analyzed under the initial saturated grain boundary (GB) segregation condition. Applying melt spinning technique, single-phase supersaturated nanograins were prepared. Grain growth behavior of the single-phase supersaturated nanograins was investigated by performing isothermal annealing at 700 °C. Combined with the effect of GB segregation on the initial GB excess amount, the thermo-kinetic model [Chen et al., Acta Mater. 57 (2009) 1466] was extended to describe the initial GB segregation condition of nanoscale Fe-B alloys. In comparison of pure kinetic model, pure thermodynamic model and the extended thermo-kinetic model, an initial saturated GB segregation condition was determined. The controlled-mechanism of grain growth under initial saturated GB segregation condition was proposed using two characteristic annealing times (t₁ and t₂), which included a mainly kinetic-controlled process (t ≤ t₁), a transition from kinetic-mechanism to thermodynamic-mechanism (t₁ < t < t₂) and pure thermodynamic-controlled process (t ≥ t₂).     

The effects of spark anodizing treatment of pure titanium metals and titanium alloys on corrosion characteristics

This study evaluated the surface characteristics of oxide films on commercially pure titanium metals (CP-Ti; Grade 2 and Grade 3) and titanium alloy (Ti6Al4V and Ti6Al7Nb) samples formed by an anodic oxidation treatment, and investigated the effects of anodization on the corrosion characteristics. FE-SEM, XRD, and Raman spectroscopy were used to evaluate the micromorphology and crystalline structure of the oxide films. The corrosion resistance of the sample groups was evaluated using open-circuit potential and cyclic polarization tests. After anodic oxidation up to dielectric breakdown with the same electric current, 150-200 nm-sized pores were distributed homogeneously on pure titanium metal samples, partially occluded pores were observed on the Ti6Al4V alloy, and there was an inhomogeneous size and distribution of pores on the Ti6Al7Nb alloy. The titanium dioxide films formed through anodic oxidation contained a phase mixture of anatase and rutile. The cyclic polarization tests showed that all the tested sample groups were not susceptible to localized corrosion. The as-received and anodically oxidized CP-Ti grade 3 groups showed a higher corrosion resistance than the other groups. The mean E_corr values of the anodically oxidized sample groups, except for the anodized Ti6Al7Nb alloy, showed higher values than those of the respective as-received sample groups. In particular, the Ti6AL7Nb alloy showed a statistically higher E_corr value in the anodized group than in the as-received group (p < 0.05).     

On the development of grain growth resistant tantalum alloys

During exposure of pure Ta to temperatures up to 1800 °C pronounced grain growth combined with embrittlement becomes a major problem. Doping with elements which form nanometer to submicron sized oxide or silicide particles is an appropriate way to prevent, or at least to hamper, uncontrolled grain growth. In the present paper the effects of doping with varying combinations of Si and Y on microstructure and mechanical properties of cold-worked and annealed Ta have been investigated. For these purposes methods including small-angle neutron scattering and transmission electron microscopy as well as light-optical microscopy have been applied. Ta samples doped with Si show a higher hardness and strength than those doped with Y or made from pure Ta powder. However, the grain growth behaviour of all alloy variants is rather similar. The particle size distributions of doped Ta change significantly with varying annealing treatments as coarsening and dissolution of the prevailing particles (oxides and silicides) take place. Therefore, particles do not play a significant role in grain growth kinetics at temperatures as high as 1800 °C. However, this loss in retarding force is partially compensated for by an increased solution drag stemming from elements in solid solution.     

Effect of friction pressure on joining phenomena of friction welds between pure titanium and pure copper

Abstract

This paper describes the effect of friction pressure on the joining phenomena of friction welds between pure titanium (P-Ti) and pure copper (OFC). When the joint was made at a friction pressure of 60 MPa or lower, the OFC side had deformation from the contact of both weld faying surfaces. Then the joint had generated sparkle from the P-Ti side with increased friction time. The peripheral portion of the P-Ti side had large deformation and was intensely upset when the joint had sparkle. On the other hand, the joint did not have sparkle on the P-Ti side when it was made at a friction pressure of 75 MPa or higher. The deformation of the OFC side was large and was intensely upset, although the P-Ti side was hardly deformed. The joining phenomena had dissimilarity because the difference of the yield stress for each material depended on the temperature in the friction process.     

Analysis of controlled-mechanism of grain growth in undercooled Fe-Cu alloy

An analysis of controlled-mechanism of grain growth in the undercooled Fe-4 at.% Cu immiscible alloy was presented. Grain growth behavior of the single-phase supersaturated granular grains prepared in Fe-Cu immiscible alloy melt was investigated by performing isothermal annealings at 500-800 °C. The thermo-kinetic model [Chen et al., Acta Mater. 57 (2009) 1466] applicable for nano-scale materials was extended to the system of micro-scale undercooled Fe-4 at.% Cu alloy. In comparison of pure kinetic model, pure thermodynamic model and the extended thermo-kinetic model, two characteristic annealing time (t₁ and t₂) were determined. The controlled-mechanism of grain growth in undercooled Fe-Cu alloy was proposed, including a mainly kinetic-controlled process (t ≤ t₁), a transition from kinetic-mechanism to thermodynamic-mechanism (t₁ < t < t₂) and purely thermodynamic-controlled process (t ≥ t₂).     

Kinetic studies of dimensional aspects in thin film grain growth

It has been shown that grain growth in succinonitrile thin films is accurately described by the Mullins–Von Neumann grain growth law. However, there is a small fraction of grains showing significant departures from ideal behavior (Palmer et al., Scripta metall. mater., 1994, 30, 633; Palmer et al., Metall. Mater. Trans. A, 1995, 26A, 1061). These grains are examined to see if they show departures from ideal behavior (the Uniform Boundary Model). It has been found that the vast majority of this small fraction of grains shows one of four distinct characteristics: they possess low-angle boundaries, they are small, they possess short boundaries, or they exhibit discontinuous boundary motion. With the exception of the low-angle boundaries all effects are due to the three-dimensional nature of the film. These characteristics have been correlated with grain growth behavior: small grains tend to shrink faster than ideal grains, grains possessing short boundaries tend to stagnate, those undergoing discontinuous boundary motion show irregular behavior. Through these experiments real films have been examined in the context of two-dimensional models.     

工业纯铝焊接热影响区晶粒生长的模拟

结合焊接工艺试验和热模拟试验,运用蒙特卡罗(MC)技术模拟了工业纯铝L1060焊接热影响区(HAZ)晶粒生长过程。首先应用试验和统计学的方法确定了工业纯铝L1060晶粒生长的动力学模型,然后计算了焊接热循环,同时基于蒙特卡罗技术编制了模拟程序,最后在不同焊接热输入情况下,模拟了晶粒生长过程。模拟结果与试验结果非常接近,为评定焊接工艺和材料焊接性提供了依据。     

Effect of second-phase particle morphology on grain growth kinetics

Second-phase particles are often employed to inhibit grain growth in polycrystalline metals and ceramics. In this work, we studied the effect of second-phase particle morphology on the effectiveness of inhibiting grain boundary migration using the phase-field method. We employed a multi-order parameter phase-field model in combination with an efficient memory allocation strategy which allows large-scale and coalescence-free grain growth simulations. We analyzed the dependence of pinning forces on the particle size and shape, and performed computer simulations of grain growth in the presence of second-phase particles with different sizes and varying aspect ratios. We also discuss the relationship between the pinned grain size and size distributions.     

A Quantitative Study on the Effect of Heat Treatment on the Microstructure and Orientation of Titanium Hydride in Electrochemically Hydrogenated Pure Titanium

The electrochemical hydrogen charging of pure titanium and its alloys has been investigated previously, while how a subsequent annealing treatment affects the type of hydride and its orientation relationship with matrix is not clear. In the present study, a quantitative study on the microstructure and orientation of titanium hydrides during electrochemical hydrogen charging and subsequent annealing treatment was carried out using scanning electron microscopy, transmission electron microscopy and electron backscatter diffraction. The results show that δ-hydride is the main in both the electrochemically hydrogenated sample and the subsequent annealing treated sample. After electrochemical hydrogen charging for 48 h, the surface is mainly composed of dense δ-hydride with a thickness of approximately 42 μm, the orientation relationship between α-matrix and δ-hydride follows only the orientation relationship of OR2, {0001}_α//{1$\overline{1}$1}_δ, $\langle 1\overline{2}10\rangle_{\alpha }$//$\langle 110\rangle_{\delta }$ and an interface plane $\{ 10\overline{1}3\}_{\alpha }$//$\{ 1\overline{1}0\}_{\delta }$. Besides OR2, a part of hydrides show an orientation relationship of OR1 with the matrix after annealing, {0001}_α//{001}_δ, $\langle 1\overline{2}10\rangle_{\alpha }$//$\langle 110\rangle_{\delta }$ and an interface plane of $\{ 10\overline{1}0\}_{\alpha }$//$\{ 1\overline{1}0\}_{\delta }$. It is further found that the relative frequency of OR1and OR2 is closely related to annealing duration. Under an argon atmosphere at 450 °C, the frequencies of OR1 and OR2 are nearly balance with an annealing time of 12 h, while OR1 becomes to be the predominant one with a relative frequency of 96.5% after annealing for 96 h. The mechanism for the evolution of orientation relationship of hydrides with annealing time was discussed.