Alloys Part Research Articles

Re-examination of martensitic stabilization in Cu-based shape memory alloys Part II. Key factors determining occurrence of martensitic stabilization in Cu-based alloys

Present proposed mechanisms for ageing-induced martensitic stabilization cannot explain why the amount of martensites losing reverse transformation ability (RTA) positively depended on Ms temperatures in directly-quenched CuZnAl alloys owing to dynamic ageing during the cooling process of quenching and subsequent slow heating. We proposed and confirmed that the number of vacancies at martensites boundaries (Nvmb) was the key factor controlling the occurrence of martensitic stabilization and its resulting RTA or loss in shape memory effect, not the number of vacancies inside the martensites and their long-range diffusion. All reported results can be rationalized by the positive dependence of original Nvmb on the Ms temperature.

Re-examination of martensitic stabilization in Cu-based shape memory alloys Part I. Identification of occurrence stage and degree of martensitic stabilization in CuZnAl alloys

Previous work did not identify occurrence stage and degree of martensitic stabilization in Cu-based shape memory alloys due to lower heating rate. By flash heating into liquid metals, we systematically clarified contribution of static ageing and the ones of dynamic ageing during process of quenching and heating to the martensitic stabilization in directly-quenched CuZnAl alloys. We discovered that the dynamic ageing during the cooling process of quenching could instantaneously stabilize a few already formed martensites totally although it did not stabilize the overwhelming majority of martensites. The dynamic ageing during the process of subsequent slow heating was responsible for the partial and total stabilization of overwhelming majority of martensites. Both the amounts of martensites stabilized totally by the two dynamic ageing positively depended on Ms temperature. The conventional up-quenching into oil or air actually made more martensites stabilized due to the dynamic ageing during the process of slow heating. The increase in As temperature owing to the static ageing also positively depended on the Ms temperature. These dependences cannot be explained by two well-known mechanisms for the martensitic stabilization.

Fabrication, Structure and Mechanical and Ultrasonic Properties of Medical Ti6Al4V Alloys Part I: Microstructure and Mechanical Properties of Ti6Al4V Alloys Suitable for Ultrasonic Scalpel.

Ti6Al4V alloy has been considered as a key component used in ultrasonic scalpels. In this series of papers, the fabrication, structure, and mechanical and ultrasonic properties of medical Ti6Al4V alloys suitable for ultrasonic scalpel are studied systemically. These alloys with low elastic modulus and present a typical bimodal microstructure with relatively high β phase content (~40%) and lamellar α thickness of ≤ 0.9 µm. In the first paper, the relationship between the microstructure and mechanical properties of hot-rolled Ti6Al4V alloys treated by heating treatment is discussed. In the second paper, the dependence of the ultrasonic properties on the microstructure of the heat-treated Ti6Al4V alloys is reported. With increasing solid solution temperature, the content and size of the primary α phase decrease. In contrast, the content and size of the lamellar α phase increase. Additionally, the β phase content first increases and then decreases. The microstructure of Ti6Al4V alloys could be slightly changed by aging treatment. When the solid solution treatment temperature increases to 980 °C from 960 °C, the average size of the lamellar α phase in the alloys increases by 1.1 µm. This results in a decrease in the average yield strength (93 MPa). The elastic modulus of alloys is mainly controlled by the β phase content. The microstructure of alloys by solution-treatment at 960 °C shows the highest β phase content and lowest average elastic modulus of 99.69 GPa, resulting in the minimum resonant frequency (55.06 kHz) and the highest average amplitude (21.48 µm) of the alloys at the length of 41.25 mm.

Fabrication, Structure, and Mechanical and Ultrasonic Properties of Medical Ti6Al4V Alloys Part II: Relationship between Microstructure and Mechanical Properties and Ultrasonic Properties of Ultrasonic Scalpel.

In this study, the ultrasonic resonance parameters of Ti6Al4V alloys under different heat treatments are measured by an impedance analyzer. The amplitude of the specimens is measured experimentally by means of optical microscope and image analysis software. These results show that the ultrasonic properties of Ti6Al4V alloys are closely related to β phase content and elastic modulus of the alloys. The highest volume fraction of the β phase appears in the specimen treated by solid solution treatment at 960 °C is 40.2%. These alloys present the lowest average elastic modulus (~99.69 GPa) and the minimum resonant frequency (55.06 kHz) and the highest average amplitude (21.48 µm) when the testing sample length is 41.25 mm. These findings can be used to guide the design of medical Ti6Al4V alloys for ultrasonic scalpels.

Modeling mechanical properties and plastic strain for hot forming-quenching AA6061 aluminum alloy parts

The hot forming-quenching integrated process (HFQ) has proved to be an effective forming method, achieving good formability and excellent mechanical properties simultaneously for aluminum alloys parts. However, the relationship between the mechanical properties and plastic deformation in the part formed by the HFQ process remains unclear. An analytical model of mechanical properties and plastic strain for an HFQ-formed 6061 aluminum alloys part was established by curve fitting of experimental data. The mechanical properties were evaluated by tensile and micro-hardness tests, and the plastic strain distribution in the HFQ process was analyzed by an automatic strain measuring system. The tensile strength and Vickers hardness showed a linear function for the HFQ-formed 6061 aluminum alloys part, and the relationship between Vickers hardness and plastic strain also presented a linear mathematical function model. The proposed analytical model reveals the mechanical properties of the HFQ-formed 6061 aluminum alloys part quickly and accurately in a non-destructive way. Furthermore, the microstructural characteristics of the HFQ-formed aluminum alloys parts were observed by scanning electron microscopy and transmission electron microscopy to clarify the strengthening mechanism. Disperse and dense Mg2Si precipitates significantly increased with increasing plastic strain during hot forming. The results showed that the improvement in strength was mainly attributable to the increase in precipitates caused by dislocation tangle and lattice distortion after HFQ forming.

Studies on Pitting Corrosion of Al-Cu-Li Alloys Part III: Passivation Kinetics of AA2098-T851 Based on the Point Defect Model.

In this paper, the passivation kinetics of AA2098–T851 was investigated by a fundamental theoretical interpretation of experimental results based on the mixed potential model (MPM). The steady state passive layer formed on the AA2098–T851 in NaHCO3 solution in a CO2 atmosphere upon potentiostatic stepping in the anodic direction followed by stepping in the opposite direction was explored. Potentials were selected in a way that both anodic passive dissolution of the metal and hydrogen evolution reaction (HER) occur, thereby requiring the MPM for interpretation. Optimization of the MPM on the experimental electrochemical impedance spectroscopy (EIS) data measured after each potentiostatic step revealed the important role of the migration of Al interstitials in determining the kinetics of passive layer formation and dissolution. More importantly, it is shown that the inequalities of the kinetics of formation and dissolution of the passive layer as observed in opposite potential stepping directions lead to the irreversibility of the passivation process. Finally, by considering the Butler–Volmer (B–V) equation for the cathodic reaction (HER) in the MPM, and assuming the quantum mechanical tunneling of the charge carriers across the barrier layer of the passive film, it was shown that the HER was primarily controlled by the slow electrochemical discharge of protons at the barrier layer/solution (outer layer) interface.

Studies on Pitting Corrosion of Al-Cu-Li Alloys Part II: Breakdown Potential and Pit Initiation.

Prediction of the accumulated pitting corrosion damage in aluminum-lithium (Al-Li) is of great importance due to the wide application of these alloys in the aerospace industry. The Point Defect Model (PDM) is arguably one of the most well-developed techniques for evaluating the electrochemical behavior of passive metals. In this paper, the passivity breakdown and pitting corrosion performance of AA 2098-T851 was investigated using the PDM with the potentiodynamic polarization (PDP) technique in NaCl solutions at different scan rates, Cl− concentrations and pH. Both the PDM predictions and experiments reveal linear relationships between the critical breakdown potential (Ec) of the alloy and various independent variables, such as and pH. Optimization of the PDM of the near-normally distributed Ec as measured in at least 20 replicate experiments under each set of conditions, allowing for the estimation of some of the critical parameters on barrier layer generation and dissolution, such as the critical areal concentration of condensed cation vacancies (ξ) at the metal/barrier layer interface and the mean diffusivity of the cation vacancy in the barrier layer (D). With these values obtained—using PDM optimization—in one set of conditions, the Ec distribution can be predicted for any other set of conditions (combinations of , pH and T). The PDM predictions and experimental observations in this work are in close agreement.

Studies on Pitting Corrosion of Al-Cu-Li Alloys Part I: Effect of Li Addition by Microstructural, Electrochemical, In-situ, and Pit Depth Analysis.

To analyze the effect of lithium and microstructure on the pitting corrosion behavior of aluminum alloys, three types of aluminum alloys were studied via scanning electron microscopy, transmission electron microscopy, electrochemical polarization, and by immersion tests coupled with in-situ observation of pitting and statistical analysis of pit depths measured by surface profilometry. It was found that, with increasing lithium content, the resistance to pitting corrosion was enhanced and the passive range was enlarged. In-situ observation revealed that the development of pitting corrosion exhibited three stages, including an initial slow nucleation stage (Stage I), a fast development stage (Stage II), and a stabilized growth stage (Stage III). Higher lithium content contributed to shorter time periods of Stages I and II, resulting in faster pitting evolution and a higher number of pits. However, the pits were generally shallower for the specimen with the highest lithium content, which is in agreement with the results of the electrochemical analysis.

Characterization of active hybrid structures made of fiber reinforced composites and shape memory alloys—part A: characterization of the load transfer

Active hybrid composites consisting of fiber reinforced polymers (FRP) and shape memory alloys (SMA) can be used for actuation purposes. With this they can provide substantial savings concerning weight, space and cost or also enable new functions. A critical point for the use of the complete actuation performance of active SMA wires for a large number of cycles is the load transfer between these wires and the FRP structure. Also the loosening of the wire from the FRP structure has to be prevented. In this paper we will have a close look to the load transfer. There solutions for the testing of this load transfer as well as its improvement still do not satisfy. We show an improved experimental set up for a pull-out test, which are used most commonly, with optical stress measurement via photo elasticity for an exact measurement of the transferred load leading to primary failure of the interface. This enables a better forecast of the transferred load of the specimen. We also compare our results with those of typical pull-out tests. From these results we estimate the shear stress at first failure and compare these to theoretical values. Furthermore, we present new experimental data and compare different possibilities to improve the load transfer.

Viscosity of Industrially Important Zn–Al Alloys Part II: Alloys with Higher Contents of Al and Si

The viscosity of Zn–Al alloys melts, with industrial interest, was measured for temperatures between 693 K and 915 K, with an oscillating cup viscometer, and estimated expanded uncertainties between 3 and 5 %, depending on the alloy. The influence of minor components, such as Si, Mg and Ce + La, on the viscosity of the alloys is discussed. An increase in the amount of Mg triggers complex melt/solidification processes while the addition of Ce and La renders alloys viscosity almost temperature independent. Furthermore, increases in Al and Si contents decrease melts viscosity and lead to an Arrhenius type behavior. This paper complements a previous study describing the viscosity of Zn–Al alloys with quasi-eutectic compositions.

An investigation of ductility-dip cracking in the base metal heat-affected zone of wrought nickel base alloys—part II: correlation of PVR and STF results

Ductility-dip cracking (DDC) in the base metal heat-affected zone (HAZ) of NiCr15Fe-type alloys was studied using the programmable-deformation-crack (PVR) and the strain-to-fracture (STF) test. This paper concentrates on the correlation of the different cracking criteria used in both tests for DDC susceptibility assessment. Full temperature-strain curves were developed for three base metal NiCr15Fe-alloy variants using the STF test. The results were compared to a previous material ranking obtained by PVR testing. To determine a local cracking criterion for base metal DDC in the PVR test, a finite element analysis (FEM) was conducted on the thermomechanical aspects of crack initiation during PVR testing. The local temperature and strain conditions associated with DDC in the PVR base metal HAZ were observed and discussed with regard to the comparability and transferability of the cracking criteria used in the PVR and STF test.

Fabrication, microstructure and corrosive behavior of different metallographic tin-leaded bronze alloys part II: Chemical corrosive behavior and patina of tin-leaded bronze alloys

This paper presents the chemical corrosion behavior and surface morphologies including cracks, pits and corrosive-depth of “patina” in the different metallographic Cu–Sn–Pb bronze alloys by the trace amounts of S2−, SO42−, NO3−, Cl− and CO32− aggressive environments. Typical patinas have been formed as brochantite patina (Cu4SO4·(OH)6) by SO42− solution with 46–144 μm corrosion depth, atacamite patina (Cu2(OH)3Cl) by Cl− solution through many subsequent “dissolution-ion pairing-precipitation” steps with 15–70 μm corrosion depth, gerhardite patina (Cu(NO3) (OH)3) by NO3- solution with 0.3 μm corroded layer, gerhardite or lead nitrate (Pb(NO3)2) or malachite (Cu2(OH)2CO3) patina by CO32−/NO3- solution as 390 μm depth of corrosive layer, atacamite Cu2Cl(OH)3 along with cuprite (Cu2O) and cessiterite (SnO2) patina with 11–390 μm multilayered corrosive crust by CO32−/Cl− solution, and a mixed black patina of brochantite, brendtite (SnS2), roxbyite (Cu7S4) by S2− solution with 34–256 μm corrosive depth. Among all environments, the deterioration rate is evaluated as order of S2− > CO32−/Cl− > Cl− > CO32−/NO3- > SO42− > NO3−. The minor bronze disease is observed in highly tin-contained alloys (>19%), while the amount of lead in alloy exhibits no specific role in corrosion. Based on these results, the phenomenological models of corrosion behavior due to an internal oxidation, ionic migration and de-alloying of “a protective barrier” are proposed.

Precipitation of Non-Spherical Particles in Aluminum Alloys Part I: Generalization of the Kampmann–Wagner Numerical Model

Particles precipitated during aging treatments often have non-spherical shapes, e.g., needles or plates, while in the classical Kampmann–Wagner Numerical (KWN) precipitation model, it is assumed that the particles are of spherical shape. This model is here generalized resulting in two correction factors accounting for the effects induced by the particles’ non-spherical shape on their growth kinetics. The first one is for the correction of the growth rate. It is derived from the approximate solution of the diffusion problem on spheroidal coordinate and verified by the three-dimensional numerical solutions for cuboid particles. The second factor is for the energetic correction due to the particle surface curvature. It is derived from chemical potential equality (or Gibbs energy minimization principle) at equilibrium for non-spherical particles and provides a correction factor for the Gibbs–Thomson effect. In the accompanying paper, the two correction factors are implemented into a multi-component KWN modeling framework, and the resulting improvements on the model’s predictive power are demonstrated.

Precipitation of Non-spherical Particles in Aluminum Alloys Part II: Numerical Simulation and Experimental Characterization During Aging Treatment of an Al-Mg-Si Alloy

This is the second part of the investigation on the precipitation of needle-shaped particles. In part I, two particle aspect ratio-dependent correction factors are introduced to describe the effects of non-spherical precipitate shape on precipitate growth kinetics. In this part II, the two factors are integrated into a CALPHAD-coupled multi-component Kampmann–Wagner numerical model to predict precipitation kinetics of needle-shaped metastable particles during aging treatment of an Al-Mg-Si alloy. The predictions are compared with transmission electron microscopy observations on precipitate number density, volume fraction, and size distribution. Improved agreement is reported, and in particular, the shape of the predicted particle size distribution density function becomes more realistic.

Aluminothermic production of titanium alloys (Part 2): Impact of activated rutile on process sustainability

The aluminothermic process provides a cost-reduced production method for titanium and titanium alloys by reduction of TiO2 with subsequent refining by electroslag remelting The aluminothermy involves high heating rates, high temperatures and short reactions times combined with a self-propagating behaviour of the reaction. By co-reduction of TiO2 and oxides of alloying elements such as vanadium pentoxide, direct synthesis of a titanium alloy is possible. The use of rutile ore concentrates causes a further reduction of process steps. In order to charge rutile ore complex thermodynamic calculations are required taking enthalpy input of various bycomponents into account. The aluminothermic reduction is conventionally enhanced by a highly heatproviding reaction based on the reduction of KClO4. In order to minimize the use of chlorine-based products extensive studies are made to investigate the feasibility of using mechanically activated rutile as input material for the aluminothermic process. Due to the mechanical activation the intrinsic enthalpy of the reaction is increased thus facilitates a process with reduced amount of KClO4. A major challenge represents the determination of a compromise between low activation duration and reduced KClO4 amount. In order to define the process window parameters like intrinsic chemical energy (enthalpy of the reaction mixture), equilibrium temperature and physical properties (particle size and mixing degree) were optimized. After adjusting the process parameters it is possible to save up to 42 % KClO4 for the ATR reaction with 2h activated input material. This reduction of KClO4 material affects a decrease of the produced gaseous compounds and the subsequent off-gas cleaning system.

Aluminothermic Production of Titanium alloys (part 1): Synthesis of TiO2 as input material

This article reports on the hydrometallurgical production of synthetic anatase from ilmenite. Mechanical activation followed by pressure leaching facilitates the leaching of ilmenite and the separation of titanium/iron by means of the synchronous hydrolysis of anatase. At 95% TiO2, the produced synthetic anatase fulfills the requirements for the aluminothermic production of titanium alloys.

Molybdate-Based Conversion Coatings for Aluminum Alloys Part I: Coating Formation

In order to prevent the corrosion of aluminum alloys, a chromate conversion coating (CCC) is applied to the surface. However, chromate is a carcinogen. Thus, the development of chromate-free and environmentally-friendly replacement coatings is currently being pursued. In this study, a molybdate conversion coating (MoCC) was developed on aluminum alloy AA2024-T6. The coating ennobled the corrosion potential of AA2024-T6 and protected the underlying aluminum alloy via anodic inhibition. Scanning electron microscopy revealed the surface morphology to consist of a mud cracked pattern that is similar to what would be seen on a sample coated with a chromium conversion coating. A dense underlying protective layer was observed when examined using optical and scanning electron microscopes.

Influence of Embedded Nanocontainers on the Efficiency of Active Anticorrosive Coatings for Aluminum Alloys Part II: Influence of Nanocontainer Position

The present work contributes to the coating design of active anticorrosive coatings for the aluminum alloy, AA2024-T3. Part II is a continuation of Part I: Influence of Nanocontainer Concentration and describes further surprising aspects of the design of nanocontainer based active anticorrosive coatings, which influence their performance. The studied coating system consists of a passive sol-gel (SiO(x)/ZrO(x)) matrix and inhibitor (2-mercaptobenzothiazole) loaded mesoporous silica nanocontainers (MBT@NCs), which are dispersed only in half of the coating volume. Varying position and concentration of MBT@NCs the synergetic effect of inhibitor amount and path length on the metal surface were analyzed, considering the balance between optimum barrier properties, active protection and adhesion. The impact of MBT@NC position on passive and active corrosion resistance was investigated by electrochemical impedance spectroscopy and scanning vibrating electrode technique. Increasing the distance between MBT@NCs and metal surface led to better barrier properties but worse active corrosion inhibition. These findings improve the understanding of the factors influencing the overall performance of active anticorrosive coatings and enable the development of a coating system with optimum anticorrosion efficiency.

Influence of Embedded Nanocontainers on the Efficiency of Active Anticorrosive Coatings for Aluminum Alloys Part I: Influence of Nanocontainer Concentration

This work presents an effective anticorrosive coating for the industrially important aluminum alloy, AA2024-T3. The protective coating was designed by dispersing mesoporous silica nanocontainers, loaded with the nontoxic corrosion inhibitor, 2-mercaptobenzothiazole, in a hybrid sol-gel (SiOx/ZrOx) layer. The concentration of the embedded nanocontainers was varied (0.04-1.7 wt %) to ascertain the optimum conditions for anticorrosion performance. Attaining high efficiency was found to be a compromise between delivering sufficient corrosion inhibitor and preserving the coating barrier properties. The impact of nanocontainer concentration on the thickness and adhesion of freshly cured coatings was also investigated. The barrier properties of the intact coatings were assessed by electrochemical impedance spectroscopy. The active corrosion inhibition was evaluated during a simulated corrosion process by the scanning vibrating electrode technique. This study has led to a better understanding of the factors influencing the anticorrosion performance and properties of active anticorrosive coatings with embedded nanocontainers.

Avoiding or Promoting Graphite in Carbon-Rich Chromium-Containing CoNiFer Cast Alloys—Part 2: Microstructures of the Elaborated Alloys

Nine M-xCr-yC ternary alloys, three cobalt based, three nickel based and three iron based, were elaborated by foundry, from chemical compositions previously selected by the mean of thermodynamic calculations. They were metallographically characterized, using electron microscopy, image analysis, and X-ray diffraction. The as-cast microstructures are in rather good agreement with the ones predicted at 500 and/or 600°C, despite that the elaboration conditions did not meet any thermodynamic equilibrium criteria. Indeed, the obtained carbides and graphite fractions were close to the calculated ones, and the new chromium contents previously chosen effectively led to the expected microstructure modifications, notably almost total suppression of graphite in the nickel alloys and obtaining large fractions of carbides in the cobalt alloys. This allowed specifying the hardness evolution resulting, for these alloys, from the presence or absence of the soft graphite phase, and from the lowering or the enhancement of the carbides presence.