Specific features of the oxidation of complex titanium alloys in the as-cast and recrystallized states

We study the heat resistance of complex titanium alloys with 4.5–11 wt.% Al in the as-cast and recrystallized states at a temperature of 800°C for 50 h. It is shown that the procedure of alloying has the same influence on the as-cast and recrystallized metals, and the initial structure affects the kinetics of oxidation of the latter. In this case, the ground recrystallized structure promotes the formation of oxide films with high adhesive strength guaranteeing high levels of the heat resistance of the alloy for long periods of holding. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 44, No. 3, pp. 112–117, May–June, 2008.     

Determining cyclic corrosion cracking resistance for titanium alloys with allowance for electrochemical conditions at the fatigue corrosion crack tip

Published data are examined on how various factors affect fatigue crack growth rates. Basic diagrams have been constructed for the cyclic cracking resistance in Ti-6AI-4V and Ti-6AI-6V-2Sn alloys in air, distilled water, and 3.5% NaCl for use in working-life calculations. Appropriate heat treatment can produce two microstructures in a titanium alloy, one of which has the largest cyclic cracking resistance, while in the second, the cracks grow at the lowest rate. The cyclic corrosion cracking resistance for a titanium alloy should be determined in relation to the state of stress and strain and to the electrochemical conditions at the corrosion fatigue crack tip, while the variations in fatigue crack growth rate for a given stress intensity factor in a corrosive medium are due to differing electrochemical conditions at the crack tip during the testing on different specimens. Basic diagrams can be derived for titanium alloys by using a physically sound methodology developed previously for steels, which is based on invariant diagrams for cyclic cracking resistance in air and in the corresponding medium, which can be constructed in relation to extremal working and electrochemical conditions at corrosion-fatigue crack tips.Translated from Problemy Prochnosti, No. 12, pp. 3–11, December, 1993.     

Periodically Laser Patterned Fe?B?Si Amorphous Ribbons: Phase Evolution and Mechanical Behavior

The properties of amorphous alloys are significantly influenced by structural relaxation and partial/full crystallization induced by thermal annealing of the alloy. In this paper, the phase evolution and mechanical behavior of laser‐patterned Fe? B? Si amorphous alloys are reported. The laser patterning was employed to cause localized thermal effects on the surface of amorphous ribbons. The laser irradiation with a lower fluence (12 J · cm^?2) caused significant embrittlement of the alloy due to the structural relaxation. The partial crystallization of an amorphous alloy into α‐Fe(Si) was also observed with laser irradiation using higher laser fluences (15 and 17 J · cm^?2). The embrittlement effect due to laser‐irradiation‐induced crystallization was more severe than that due to structural relaxation.     

Effect of prestrain on the fatigue properties of Ti834

The effects of cold dwell on titanium alloys have been widely studied in recent years, with many alloys showing detrimental mechanical properties when dwell periods are introduced at peak load. Failures in such situations are often characterised by the formation of quasi-cleavage facets, and models have been suggested to explain the mechanism by which they occur. This paper seeks to investigate how different mechanical test regimes influence facet formation. It is shown in the titanium alloy Ti834 that facet formation occurs readily under stress relaxation and creep loading, but is less influenced by cyclic strain control loading. The implications of this type of strain accumulation process on the mechanical properties of Ti834 are discussed, with close correlation between suggested models and experimental data.     

Effects of heat treatment on mechanical properties of high-chromium nickel-base superalloy IN 939

Abstract

The effects of initial heat treatment on the tensile and creep properties of alloy IN 939 are described and explained in terms of microstructural and microcompositional variations. It is demonstrated that a substantial increase in room temperature properties and in short-term high-temperature properties can be obtained by a modified three stage, 14 h heat treatment. Long-term elevated-temperature creep properties are found to be insensitive to initial heat treatment, since in this case the evolution of the microstructure is largely determined by test conditions. A comparison is made between IN 939 and IN 738, and it is shown that the very long-term high-temperature creep properties of the two alloys are closely similar. The selection of heat treatments to obtain optimum properties over the entire range of temperature and stress conditions encountered within a turbine blade is also discussed.

MST/434     

Effect of hot water and heat treatment on the apatite-forming ability of titania films formed on titanium metal via anodic oxidation in acetic acid solutions

Titanium and its alloys have been widely used for orthopedic implants because of their good biocompatibility. We have previously shown that the crystalline titania layers formed on the surface of titanium metal via anodic oxidation can induce apatite formation in simulated body fluid, whereas amorphous titania layers do not possess apatite-forming ability. In this study, hot water and heat treatments were applied to transform the titania layers from an amorphous structure into a crystalline structure after titanium metal had been anodized in acetic acid solution. The apatite-forming ability of titania layers subjected to the above treatments in simulated body fluid was investigated. The XRD and SEM results indicated hot water and/or heat treatment could greatly transform the crystal structure of titania layers from an amorphous structure into anatase, or a mixture of anatase and rutile. The abundance of Ti–OH groups formed by hot water treatment could contribute to apatite formation on the surface of titanium metals, and subsequent heat treatment would enhance the bond strength between the apatite layers and the titanium substrates. Thus, bioactive titanium metals could be prepared via anodic oxidation and subsequent hot water and heat treatment that would be suitable for applications under load-bearing conditions.     

Nucleation and growth kinetics of reaction-product formation for copper–titanium and silver–titanium alloys on alumina

A nucleation and growth mechanism is proposed for the formation of the reaction product at the interface between polycrystalline alumina and liquid-metal alloy drops containing titanium. The reaction product had been previously identified to be an oxide of titanium. The growth of reaction product islands was clearly observed at the alumina–metal interface using optical microscopy after dissolving the metal droplets with acid. The fractional coverage was quantified as a function of time and, by assuming Avrami-type reaction kinetics, surface reaction rate constants, k, were calculated for copper–titanium and silver–titanium alloys on alumina. Reaction rate constants between 1.4 × 10^-4 and 18 × 10^-4 s^-2 were obtained for copper–titanium alloys on alumina. The k values for silver–titanium alloys were found to be an order of magnitude lower (2.5 × 10^-6 and 7.2 × 10^-6 s^-2) then the k values obtained for copper–titanium alloys on alumina. This revised version was published online in November 2006 with corrections to the Cover Date.     

Magnetothermal Analysis of Nanocrystallization of Amorphous and Relaxations of Nanocrystalline Materials

For a few years it has been realized that nanocrystalline phases can be formed during crystallization of amorphous alloys annealed isothermally below the crystallization temperature of usual heating experiments. Data of this transformation monitored by the measurement of magnetic susceptibility are presented. A method using a magnetic balance with electronic stabilisation and combined computer facilities is applied. Constant heating and cooling rates as well as isothermal heat treatments are used. Magnetic measurements are able to detect the onset of the transformation of amorphous Ni-P alloys much earlier than was possible with differential scanning calorimetry. The transformation kinetics can be analyzed by means of the Avrami plot based on the Johnson-Mehl-Avrami equation.The kinetics of solid state reactions in the nanostructured material can be investigated similarly. Formation of a Ni-phase in a nanostructured Hf-Ni alloy could be detected in a very early stage, where calorimetric methods are not sensitive. Segregation phenomena could be detected from the experiments even after long time. The sensitivity of the applied method is not dependent on the heating rate as the sensitivity of scanning calorimetry is     

Prediction of the behaviour of copper alloy components under complex loadings by electro-thermomechanical coupled simulations

ABSTRACT

Electronic devices must be served with electric power for different reasons. A robust and reliable electric connectivity is often realised by electric connectors. For its leading properties, precipitation hardened copper alloys are widely used for designing connectors with high level mechanical or conductance properties. However, copper alloys show a characteristic stress relaxation under mechanical or thermal loads. Finite element analysis is a standard method to design and optimise components with respect to reliability and performance. Hence, a material model considering the characteristic of the mechanical properties and allowing for the simulation of time and temperature dependent elasto-viscoplastic material behavior was developed at the Fraunhofer IWM. The parameters of the model were determined using tensile and relaxation test data of a C19010 alloy. The material model is applied to electro-thermomechanical coupled finite element simulations of connectors with different load histories. The goal of the simulations is the analysis of the impact of stress relaxation on the mechanical properties of systems over time. From the numerical results with the new model it is shown, how stress relaxation influences the connector clamping forces or contact pressure, respectively, in dependence with time or temperature. The simulation results documents that stress relaxation has to be taken into account in finite element simulations during the designing process of electrical devices.

This paper is part of a Thematic Issue on Copper and its Alloys.     

The effect of minor additions of titanium on the fracture toughness of Fe-12 at% Ni alloys at−196° C

The reasons for the improvement in the fracture toughness of an Fe-12 at% Ni base alloy at –196° C by the addition of small amounts of Ti were investigated employing transmission electron microscopy (TEM), scanning electron microscopy (SEM) and Auger electron spectroscopy. Ti additions ranging from 0.18 to 0.99 at% and heat treatments of 2 h at 550, 685 and 820° C respectively followed by a water quench were considered, since previous work by Witzke and Stephens had shown that maximumK _IC occurred for an Fe-12 at% Ni-0.18 at% Ti alloy heat treated at 685° C. It was found here thatK _IC at –196° C for all the alloys and heat treatments correlated with the fraction of ductile fracture compared to intergranular and cleavage fracture, the latter modes being predominant in the Fe-12 at% Ni base alloy without Ti additions. Cubic and rectangular shaped inclusions were noted in the SEM fractographs of the alloys with the Ti additions. A fine precipitate was observed by TEM for the Fe-12 at% Ni-0.18 at% Ti alloy heat treated at 550° C; this precipitate was not observed for the 685 and 820° C heat treatments of the same alloy. Auger mappings of the fracture surfaces indicated a weak to moderate association of the interstitials C, N and O with Ti, the degree of which depended on the particular interstitial and the heat treatment temperature. It was concluded that the increase inK _IC due to the initial 0.18 at% addition of Ti was due to a scavenging of interstitials which normally segregate at the grain boundaries and to the refinement of the microstructure. The subsequent decrease inK _IC with further Ti additions was attributed to the increase in flow stress and slight lowering of the fracture stress resulting from these additions. It was further inferred that the Ti additions and heat treatment on the flow and fracture stresses may be due in a large part to their influence on the amount, size and distribution of the precipitate which was observed.     

Application of Nonlinear Superposition to Creep and Relaxation of Commercial Die-Casting Aluminum Alloys

Die-cast aluminum alloys are heavily used in small engines, where they are subjected to long-term stresses at elevated temperatures. The resulting time-dependent material responses can result in inefficient engine operation and failure. A method to analytically determine the stress relaxation response directly from creep tests and to accurately interpolate between experimental time-history curves would be of great value. Constant strain, stress relaxation tests and constant load, creep tests were conducted on aluminum die-casting alloys: B-390, eutectic Al–Si and a 17% Si–Al alloy. A nonlinear superposition integral was used to (i) interpolate between empirical primary inelastic creep-strain and stress-relaxation time histories and (ii) to determine the stress relaxation response from corresponding creep data. Using isochronal stress-strain curves, prediction of the creep response at an intermediate stress level from empirical creep curves at higher and lower stresses resulted in a correlation (R) of 0.98. Similarly for relaxation, correlations of 0.98 were obtained for the prediction of an intermediate strain level curve from higher and lower empirical relaxation curves. The theoretical prediction of stress relaxation from empirical creep curves fell within 10% of experimental data.This paper has not been submitted elsewhere in identical or similar form, nor will it be during the first three months after its submission to Mechanics of Time-Dependent Materials     

拉-拉疲劳载荷下钛合金湿喷丸的残余应力松弛及再次喷丸工艺EI北大核心CSCD

采用湿法喷丸强化工艺(wet shot-peening)对TC4钛合金表面进行处理,研究高、低周的拉-拉疲劳过程中合金残余应力松弛规律,探讨再次喷丸工艺(re-shot-peening,RSP)对疲劳寿命的影响。结果表明:在拉应力载荷状态下,残余压应力依然发生松弛现象。疲劳载荷水平对喷丸TC4钛合金残余压应力场(CRSF)的松弛速率、松弛程度和松弛范围具有重要影响。高周疲劳(HCF)过程中残余应力松弛主要发生在近表层0~30μm,松弛速率较慢。低周疲劳(LCF)过程中残余应力松弛发生在0~80μm,范围更大,速率更快。RSP周期对于TC4钛合金的疲劳寿命也具有较大影响。在25%和50%初始喷丸疲劳寿命进行RSP处理会显著提高疲劳寿命,而在75%初始喷丸疲劳寿命处进行RSP处理对于疲劳寿命基本没有影响。此外,RSP的强化效果与疲劳载荷水平相关,对于高周疲劳寿命提高明显。     

A new method for stress relieving amorphous alloys to improve magnetic properties

An analysis of the kinetics of stress relief annealing in amorphous alloys, with particular emphasis on the effect of annealing on the resulting magnetic properties, is made. Based on this analysis, an optimal procedure for annealing amorphous alloy magnetic cores is suggested. It involves a fast heatup to a high temperature for a short time. The expected benefits of this procedure are decreased embrittlement, lower strain induced anisotropy, and lower residual stresses. The conventional technique for stress relieving magnetic cores proved unsuitable for the application of the new annealing treatment. A new technique was devised in which the ribbon is dynamically annealed as it is wound onto the core. The magnetic properties of the resulting toroids are superior to those produced by the conventional technique.     

Further trials of a strain hardening index of fatigue damage

Previous cyclic-strain, smooth-specimen fatigue tests of α–β titanium alloys displayed an anomolous endurance enhancement for some of the alloy conditions. This could be explained by associating resistance to fatigue damage directly with the stress-normalized plastic strain hardening rate at the point of maximum cyclic tensile stress. Since this rate also controls the extent of stress-relaxation-induced tensile creep strain in each cycle, it was thought that fatigue damage might be associated with it. To test this hypothesis, data with varied load hold time, and over a full range of cyclic life, is reported here for some of the previously reported alloys of Ti-6A1-4V, as well as for an A36 steel plate. Notch fatigue tests of the A36, combined with those of Yoder et al. for the titanium alloys, are compared to the smooth specimen data. Results tend to support the damage-inhibiting role of the plastic strain hardening rate, but not of the creep strain portion of each cycle. Notch fatigue data agrees with smooth specimen trends if Neuber's rule is used to characterize the stress concentration factor, particularly with the A36 steel. As with Yoder's notch fatigue results, smooth specimen LCF life, though quite different in the range less than 10³ cycles, tends to converge near the endurance limit, thus mitigating adverse effects of alloy conditions which favor resistance to fatigue crack propagation in α-β titanium alloys.     

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

Abstract

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 d ₀ 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 d ₀ 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.     

Omega phase formation in RMI (38-6-44) beta titanium alloy

The stabilized beta titanium alloy RMI (38-6-44) was designed to limit the formation of omega-phase during quenching and subsequent ageing. It has been shown in the present paper, however, that both athermal and thermal omega do form and that the growth kinetics of the thermal omega follows aD=Ct ^0.45 relationship. As expected from the alloy additions, the maximum volume fraction of omega phase observed is considerably less than in other beta titanium alloys and the severe embrittlement due to omega phase formation is not a serious problem in this alloy. This is an important consideration for engineering applications in the service temperature range below 900?F (480?C).     

Cyclic oxidation of copper doped Ti-48Al-2Cr-2Nb

The widespread use of titanium aluminide alloys will require effective joining techniques for primary fabrication and repair. One such technique is Transient Liquid Phase (TLP) bonding, which has been used to join titanium aluminide alloys. A successful TLP bonding process uses a copper-containing composite interlayer and thus introduces a small amount of copper into the alloy. Although even relatively small alloying additions can be detrimental to the oxidation resistance of titanium aluminide alloys, the amount of copper added to the alloy during the TLP bonding process has previously been shown to be neutral or beneficial to the isothermal oxidation resistance of the alloy. In this paper, a small amount of copper introduced during TLP bonding is shown to have no detrimental effect on the cyclic oxidation of Ti-48 at% Al-2 at% Cr-2 at% Nb.     

Nanopore formation on the surface oxide of commercially pure titanium grade 4 using a pulsed anodization method in sulfuric acid

Titanium and its alloys form a thin amorphous protective surface oxide when exposed to an oxygen environment. The properties of this oxide layer are thought to be responsible for titanium and its alloys biocompatibility, chemical inertness, and corrosion resistance. Surface oxide crystallinity and pore size are regarded to be two of the more important properties in establishing successful osseointegration. Anodization is an electrochemical method of surface modification used for colorization marking and improved bioactivity on orthopedic and dental titanium implants. Research on titanium anodization using sulphuric acid has been reported in the literature as being primarily conducted in molarity levels 3 M and less using either galvanostatic or potentiostatic methods. A wide range of pore diameters ranging from a few nanometers up to 10 μm have been shown to form in sulfuric acid electrolytes using the potentiostatic and galvanostatic methods. Nano sized pores have been shown to be beneficial for bone cell attachment and proliferation. The purpose of the present research was to investigate oxide crystallinity and pore formation during titanium anodization using a pulsed DC waveform in a series of sulfuric acid electrolytes ranging from 0.5 to 12 M. Anodizing titanium in increasing sulfuric acid molarities showed a trend of increasing transformations of the amorphous natural forming oxide to the crystalline phases of anatase and rutile. The pulsed DC waveform was shown to produce pores with a size range from ≤0.01 to 1 μm². The pore size distributions produced may be beneficial for bone cell attachment and proliferation.     

Thermo-metallo-mechanical modelling of heat treatment induced residual stress in Ti–6Al–4V alloy

Residual stress fields dynamically fluctuate throughout the manufacturing process of metallic components and are caused by local misfit of a thermal, mechanical or metallurgical nature. Recent advances have been made in the area of microstructure and residual stress prediction; yet few have considered dual-phase titanium alloys. The aim of the work presented was to carry out a review of the existing state-of-the-art in residual stress modelling with an intended application to industrial heat treatment of Ti–6Al–4V alloy. Four areas were evaluated: thermal, mechanical and metallurgical sub-models, and model validation via residual stress measurement. Recommendations for future research include further investigation of transformation induced plasticity and stress relaxation behaviour in Ti–6Al–4V.

This review was submitted as part of the 2019 Materials Literature Review Prize of the Institute of Materials, Minerals and Mining run by the Editorial Board of MST. Sponsorship of the prize by TWI Ltd is gratefully acknowledged.     

REVIEW Bioactive metals: preparation and properties

Some ceramics, such as Bioglass, sintered hydroxyapatite, and glass-ceramic A-W, spontaneously form a bone-like apatite layer on their surface in the living body, and bond to bone through the apatite layer. These materials are called bioactive ceramics, and are clinically important for use as bone-repairing materials. However, they cannot be used at high-load sites, such as is found in femoral and tibial bones, because their fracture toughness values are not as high as that of human cortical bone. Titanium metal and its alloys have high fracture toughness, and form a sodium titanate layer on its surface when soaked in a 5 M-NaOH solution at 60 degrees C for 24 h, followed by a heat treatment at 600 degrees C for 1 h. On moving toward the metal interior, the sodium titanate layer gradually changes into the pure metal within a distance of 1 microm from the surface. The mechanical strength of the titanium metal or a titanium alloy is not adversely affected by these chemical and thermal treatments. The titanium metal and its alloys resulting from the above treatment can release Na+ ions from its surface into a surrounding body fluid via an ion exchange reaction with H3O+ ions, resulting in many Ti-OH groups forming on its surface. These Ti-OH groups initially combine with Ca2+ ions to form amorphous calcium titanate in the body environment, and later the calcium titanate combines with phosphate ions to form amorphous calcium phosphate. The amorphous calcium phosphate eventually transforms into bone-like apatite, and by this process the titanium metals are soon tightly bonded to the surrounding living bone through the bone-like apatite layer. The treated metals have already been subjected to clinical trials for applications in artificial total hip joints. Metallic tantalum has also been found to bond to living bone after it has been subjected to the NaOH and heat treatment to form a sodium tantalate layer on its surface.