Engineering Alloys Research Articles

(Corrosion Division H. H. Uhlig Award) Non-Faradaic Release of Copper during Corrosion of Engineering Alloys

The phenomenon of “deposition corrosion” involving dissolution and replating of copper on aluminum alloys during corrosion has been known for decades. While the reduction of Cu ions onto aluminum alloys during corrosion was easily understood from an electrochemical point of view, how Cu ions were generated in the first place by a corrosion process occurring hundreds of millivolts below the copper oxidation process lacked a satisfying explanation. The advent of high brightness, high resolution electron sources in commercial scanning electron microscopes in the late-1980s and early-1990s enabled critical clues to be found as to how copper parted from the alloy. High resolution scanning electron imaging of corroded 2024-T3 surfaces suggested physical emission Cu nanoparticles by a non-faradaic process involving coarsening of dealloyed intermetallic particles. Subsequent electrochemical and surface analytical measurements validated the hypothesis that Cu nanoparticles emitted from the alloy were not constrained to be at the alloy corrosion potential and could be directly oxidized in aerated electrolyte conditions to form Cu ions. Further investigation demonstrated non-faradaic emission of Cu from several different intermetallic compounds, and dilute Al-Cu solid solutions with as little as 0.2wt.% Cu. Recent evidence of Cu release during corrosion of magnesium alloys and Cu-bearing stainless steels suggests that non-faradaic release of Cu from reactive engineering alloys is a corrosion characteristic to be expected when alloying with copper. In this presentation, the electrochemical framework for non-faradaic release of Cu will be descried along with key supporting experimental evidence. Examples of non-faradaic release from a number of different materials will be shown to underscore the ubiquity of this fascinating and technologically relevant phenomenon.

On the applicability of boundary condition based tensile creep model in predicting long-term creep strengths and lifetimes of engineering alloys

Creep behaviour of metal alloys for critical high temperature engineering applications is usually studied by uniaxial tensile creep tests. The stress and temperature dependence of steady-state or minimum creep rate observed in such tests is rationalised at present on the basis of either Norton or MBD equation. But, the stress exponent determined on such basis depend strongly and often irregularly on test temperature and the creep activation energy varies with stress or stress range. Hence, these parameters cannot be used for long-term predictions. Here, it is shown that these difficulties can be removed if a new creep model, which incorporates tensile strength, is used to rationalise the creep data. The stress exponent determined then does not depend on temperature although it depends on stress range, and the creep activation energy does not depend on stress. Thus, there is no relationship between stress exponent value and creep mechanism. Consequently, the new tensile creep model can be used in combination with Monkman-Grant relationship to predict the long-term tensile creep strengths and lifetimes at different temperatures using creep parameters determined from short-term tensile creep tests. Both the deficiencies of the Norton and MBD equations and the predictive quality of the new tensile creep model are demonstrated here using uniaxial tensile creep data and tensile strength data measured for two totally different types of creep resistant engineering alloys: 9Cr-1Mo steel and Ni base superalloy Ni-16Cr-8.5Co-3.5Al-3.5Ti-2.6W-1.8Mo-0.9Nb.

A Review of Applications and Research of Shape Memory Alloys in Civil Engineering

This article presents a timely review of thermomechanical behaviours, recent structural applications, and researches of shape memory alloys (SMAs) in civil engineering. SMAs are used in a variety of fields, such as mechanical, aeronautical and medical fields. Other fields of knowledge have been researching these materials, attracted by their superb shape memory effect and pseudoelasticity under mechanical and thermal conditions. This article not only reports a bibliographic review on the constitutive models and thermodynamic properties for SMAs, but also describes the attributes of SMAs that make them ideally suitable to structural control, intelligent structure and structural damage repair.

Processing and characterization of crack-free aluminum 6061 using high-temperature heating in laser powder bed fusion additive manufacturing

During solidification of many so-called high-performance engineering alloys, such as 6000 and 7000 series aluminum alloys, which are also unweldable autogenously, volumetric solidification shrinkage and thermal contraction produces voids and cracks. During additive manufacturing processing, these defects can span the length of columnar grains, as well as intergranular regions. In this research, laser powder bed fusion (LPBF) of aluminum alloy (AA) 6061 used powder bed heating at 500 °C in combination with other experimentally determined processing parameters to produce crack-free components. In addition, melt-pool banding, which is a normal solidification feature in LPBF, was eliminated, illustrating solidification process modification as a consequence of powder bed heating. Corresponding microindentation hardness and tensile testing of the as-fabricated AA6061 components indicated an average Vickers hardness of HV 54, and tensile yield, ultimate strength, and elongation values of 60 MPa, 130 MPa, and 15%, respectively. These mechanical properties and those of heat treated parts showed values comparable to annealed and T6 heat treated wrought products, respectively. X-ray diffraction and optical microscopy revealed columnar grain growth in the build direction with the as-fabricated, powder-bed heated product microstructure characterized by [100] textured, elongated grains (∼ 25 μm wide by 400 μm in length), and both intragranular and intergranular, noncoherent Al-Si-O precipitates which did not contribute significantly to the mechanical properties. The results of this study are indicative that powder bed heating may be used to assist with successful fabrication of AA6061 and other alloy systems susceptible to additive manufacturing solidification cracking.

Solidification behavior and Laves phase dissolution during homogenization heat treatment of Inconel 718 superalloy

The solidification behavior and microstructural evolutions during homogenization heat treatment of Inconel 718 superalloy were studied. The microstructure of the as-cast alloy was characterized by optical micrographs, scanning electron microscopy (SEM) images, and elemental analysis based on energy dispersive spectroscopy (EDS). The optimum etching techniques, especially electroetching, were characterized to reveal the dendritic nickel matrix, the microsegregation between dendrite arms, and the austenite/Laves eutectic structure. The elemental analysis of different microstructural features, especially the variation of the chemical composition of austenite, was successfully related to the pseudo-binary solidification constitution diagram. Moreover, the extension of the nonequilibrium eutectic isotherm was evaluated to unravel the solidification path of this important engineering alloy. The dissolution of the Laves phase and its kinetics during exposure at elevated temperatures with the purpose of homogenization were also discussed.

Influencing the Mechanical Properties of Engine and Vehicle Constructions Alloy with Laser Surface Heating

The self-hardening aluminum alloys are interesting for producers of casting, because these materials have the required properties without using heat treatment. This casts production result in the following advantages: energy saving, reduction of the costs, no permanent deformation, largest casting dimensions possible, and so on. Nevertheless, it is well known that improvement of mechanical properties and structure of aluminum alloys can often significantly increase the lifetime of casting and reduce costs for fuel and reduction of environmental loading. The basic and energy saving method for improvement of mechanical properties of self-hardened alloys is surface hardening. Surface hardening improves properties such as: corrosion resistance, wear resistance and hardness. In regards this facts great interest in reducing the cost of components makes a great opportunity for use laser surface processing. Therefore this article deals with using laser surface heating and evaluations its influence onto mechanical properties of self-hardening ENAC-71100, i.e. AlZn10Si8Mg cast alloy. This alloy is especially used for engine and vehicle constructions, hydraulic unit and mound so the surface properties are important for application in industries. The results show that laser surface heating have influence on properties, but well defined conditions of heating are very important.

Improving the corrosion properties of amorphous Ni-P thin films using different additives

Ni-P amorphous coatings are considered as important engineering alloys. In this paper, the effects of various additives on the properties of Ni-P thin films were investigated, including saccharine, glycine, pyridinium propyl sulfonate, coumarin, sodium citrate, and cerium sulfate. Potentiodynamic polarization and electrochemical impedance spectroscopy tests, scanning electron microscopy, X-ray fluorescence and diffraction, and atomic force microscopy were employed to study the properties of the samples. It was found that a proper concentration of these additives, except saccharine, noticeably improved the corrosion resistance, especially at high potentials (about 25% increase in instantaneous corrosion efficiency and about 300% in corrosion efficiency at high potentials) and decreased the surface roughness (by about 10 to 55%) of Ni-P thin films. Thinner, more uniform, and less porous coatings were formed in the presence of additives. Except saccharine, all the additives enhanced the P content of Ni-P films, which thus kept their amorphous structure; saccharine highly suppressed the incorporation of P inside the Ni lattice and a mixed amorphous-crystalline structure was stabilized.

Laser welding of Cu-Al-Be shape memory alloys: Microstructure and mechanical properties

Cu-based shape memory alloys are potential substitutes of NiTi shape memory alloys, owing to their lower production costs and recent increase in mechanical properties arising from better control of the microstructure. The development of joining technologies for advanced materials is key to expand to the potential applications of any engineering alloy. In this work, laser welding of single crystal-like Cu-Al-Be shape memory alloys was performed. The microstructure and tensile properties were evaluated to understand the effect of laser welding on the microstructural and mechanical features of the welded joints. Cycling tests performed for a wide range of temperatures revealed that the joints possess extraordinary superelastic recovery after joining, with potential applications in damping devices due to the significant amount of energy that can be absorbed during superelastic deformation.

Predicting the 3D fatigue crack growth rate of small cracks using multimodal data via Bayesian networks: In-situ experiments and crystal plasticity simulations

Small crack propagation accounts for most of the fatigue life of engineering structures subject to high cycle fatigue loading conditions. Determining the fatigue crack growth rate of small cracks propagating into polycrystalline engineering alloys is critical to improving fatigue life predictions, thus lowering cost and increasing safety. In this work, cycle-by-cycle data of a small crack propagating in a beta metastable titanium alloy is available via phase and diffraction contrast tomography. Crystal plasticity simulations are used to supplement experimental data regarding the micromechanical fields ahead of the crack tip. Experimental and numerical results are combined into a multimodal dataset and sampled utilizing a non-local data mining procedure. Furthermore, to capture the propensity of body-centered cubic metals to deform according to the pencil-glide model, a non-local driving force is postulated. The proposed driving force serves as the basis to construct a data-driven probabilistic crack propagation framework using Bayesian networks as building blocks. The spatial correlation between the postulated driving force and experimental observations is obtained by analyzing the results of the proposed framework. Results show that the above correlation increases proportionally to the distance from the crack front until the edge of the plastic zone. Moreover, the predictions of the propagation framework show good agreement with experimental observations. Finally, we studied the interaction of a small crack with grain boundaries (GBs) utilizing various slip transmission criteria, revealing the tendency of a crack to cross a GB by propagating along the slip directions minimizing the residual Burgers vector within the GB.

Understanding the Interdependencies Between Composition, Microstructure, and Continuum Variables and Their Influence on the Fracture Toughness of α/β-Processed Ti-6Al-4V

The fracture toughness of a material depends upon the material’s composition and microstructure, as well as other material properties operating at the continuum level. The interrelationships between these variables are complex, and thus difficult to interpret, especially in multi-component, multi-phase ductile engineering alloys such as α/β-processed Ti-6Al-4V (nominal composition, wt pct). Neural networks have been used to elucidate how variables such as composition and microstructure influence the fracture toughness directly (i.e., via a crack initiation or propagation mechanism)—and independent of the influence of the same variables influence on the yield strength and plasticity of the material. The variables included in the models and analysis include (i) alloy composition, specifically, Al, V, O, and Fe; (ii) materials microstructure, including phase fractions and average sizes of key microstructural features; (iii) the yield strength and reduction in area obtained from uniaxial tensile tests; and (iv) an assessment of the degree to which plane strain conditions were satisfied by including a factor related to the plane strain thickness. Once trained, virtual experiments have been conducted which permit the determination of each variable’s functional dependency on the resulting fracture toughness. Given that the database includes both K 1 C and K Q values, as well as the in-plane component of the stress state of the crack tip, it is possible to quantitatively assess the effect of sample thickness on K Q and the degree to which the K Q and K 1 C values may vary. These interpretations drawn by comparing multiple neural networks have a significant impact on the general understanding of how the microstructure influences the fracture toughness in ductile materials, as well as an ability to predict the fracture toughness of α/β-processed Ti-6Al-4V.

Application of Aluminum and Aluminum Alloys in Engineering

Application of Aluminum and Aluminum Alloys in Engineering

Predicting creep strengths and lifetimes of creep resistant engineering alloys

The physical basis for predicting the long-term creep strengths and lifetimes at application temperatures using creep parameters determined from short-term creep tests is investigated for complex creep resistant engineering alloys. It is shown that the seemingly unpredictable stress and temperature dependence of minimum creep rate of such alloys can be rationalised using an approach based on the new power law creep equation that incorporate the tensile strength. This is demonstrated using the tensile and creep data measured for two completely different types of alloys: steel 11Cr-2W-0.4Mo-1Cu-Nb-V and Ni base superalloy 15Cr-28Co-4Mo-2.5Ti-3Al. For both alloys, the stress exponent n determined does not depend on temperature and activation energy of creep does not depend on stress. Consequently, it becomes possible to use the new power law creep equation in combination with the Monkman-Grant relationship to predict the long term creep rupture strengths and lifetimes and microstructure stability of the two alloys from short term creep test data. The implications of the results for creep mechanism identification and future microstructure analysis are discussed.

In-Situ Imaging of Liquid Phase Separation in Molten Alloys Using Cold Neutrons

Understanding the liquid phases and solidification behaviors of multicomponent alloy systems becomes difficult as modern engineering alloys grow more complex, especially with the discovery of high-entropy alloys (HEAs) in 2004. Information about their liquid state behavior is scarce, and potentially quite complex due to the presence of perhaps five or more elements in equimolar ratios. These alloys are showing promise as high strength materials, many composed of solid-solution phases containing equiatomic CoCrCu, which itself does not form a ternary solid solution. Instead, this compound solidifies into highly phase separated regions, and the liquid phase separation that occurs in the alloy also leads to phase separation in systems in which Co, Cr, and Cu are present. The present study demonstrates that in-situ neutron imaging of the liquid phase separation in CoCrCu can be observed. The neutron imaging of the solidification process may resolve questions about phase separation that occurs in these alloys and those that contain Cu. These results show that neutron imaging can be utilized as a characterization technique for solidification research with the potential for imaging the liquid phases of more complex alloys, such as the HEAs which have very little published data about their liquid phases. This imaging technique could potentially allow for observation of immiscible liquid phases becoming miscible at specific temperatures, which cannot be observed with ex-situ analysis of solidified structures.

Long-time storage effects on shape memory alloy wires

A rich literature studies shape memory alloy (SMA) wires for their potential use in dampers devices able to reduce wind, rain, and traffic-induced oscillations of stayed cables. Restrainers for bridges and improved aseismic devices also exploit alloy components. Thus, SMAs should be regarded as materials susceptible to storage in the yard. In this paper, the authors discuss the evolution of SMA macroscopic behavior as caused by a long-time storage of the product as acquired. The study discriminates between wires of different diameters, because the flat cycles shown by thin wires (i.e., diameter \(\le 0.5\,\hbox {mm}\)) and the non-classical S-shaped cycles of thick wires (of diameter 2.46 mm in this paper) answer differently to environmental modifications. The hysteretic behavior of some specimens of wires, of diameter 2.46 mm, is here investigated to mark the unpredictability of the consequence of a long-time storage, which could prevent from the practical exploitation of such alloys in civil engineering.

Applications of aluminium alloys in civil engineering

Although aluminium is a long known structural material, its use is not in accordance with the benefits achieved by its implementation. There are several reasons for such an adverse state. Those that stand out are the late development of a regulative framework for the design of structures, and still the need for improvement, lack of knowledge on application examples and not stressed enough potential areas of use. However, positive trends are present and aluminium alloys are competitive, especially if their positive properties can be utilized and negative properties diminished though purpose oriented design approach. Given examples present good application utilization of aluminium alloys characteristics, and shown new application research can provide directions for further expansion of competitiveness. Structural uses are grouped in suitable areas of application and put into local and global context.

Microstructural statistics for fatigue crack initiation in polycrystalline nickel-base superalloys

In advanced engineering alloys where inclusions and pores are minimized during processing, the initiation of cracks due to cyclic loading shifts to intrinsic microstructural features. Criteria for the identification of crack initiation sites, defined using elastic-plastic loading parameters and twin boundary length, have been developed and applied to experimental datasets following cyclic loading. The criteria successfully quantify the incidence of experimentally observed cracks. Statistical microstructural volume elements are defined using a convergence approach for two nickel-base superalloys, IN100 and René 88DT. The material element that captures the fatigue crack-initiating features in René 88DT is smaller than IN100 due to a combination of smaller grain size and higher twin density.

Materials and corrosion trends in offshore and subsea oil and gas production

The ever-growing energy demand requires the exploration and the safe, profitable exploitation of unconventional reserves. The extreme environments of some of these unique prospects challenge the boundaries of traditional engineering alloys, as well as our understanding of the underlying degradation mechanisms that could lead to a failure. Despite their complexity, high-pressure and high-temperature, deep and ultra-deep, pre-salt, and Arctic reservoirs represent the most important source of innovation regarding materials technology, design methodologies, and corrosion control strategies. This paper provides an overview of trends in materials and corrosion research and development, with focus on subsea production but applicable to the entire industry. Emphasis is given to environmentally assisted cracking of high strength alloys and advanced characterization techniques based on in situ electrochemical nanoindentation and cantilever bending testing for the study of microstructure-environment interactions.

Criticality in Bulk Metallic Glass Constituent Elements

Bulk metallic glasses (BMGs), which readily form amorphous phases during solidification, are increasingly being used in first applications of watch components, electronic casings, and sporting goods. The compositions of BMGs typically include four to six elements. Various political and geological factors have recently led to supply disruptions for several metals, including some present in BMG compositions. In this work, we assess the “criticality” of 22 technologically interesting BMG compositions, compare the results with those for three common engineering alloy groups, and derive recommendations for BMG composition choices from a criticality perspective. The criticality of BMGs is found to be generally much higher compared with those for the established engineering alloys. Therefore, criticality concerns should also be considered in the choice between existing and developing novel BMGs.

Diffusional-displacive transformation enables formation of long-period stacking order in magnesium

Mg is the most important lightweight engineering alloy enabling future weight-reduced and fuel-saving engineering solutions. Yet, Mg is soft. Long-period stacking ordered (LPSO) structures in Mg alloys have unique crystal structures, characterized by both complex chemical and stacking order. They are essential for strengthening of Mg alloys. The formation mechanism of these LPSO structures is still under discussion. Here we report that Y/Zn enriched Guinier-Preston (GP) zones observed in a lean Mg-Y-Zn model alloy are precursors of early stage LPSO structures. We provide evidence of a new type of phase transformation mechanism which comprises the diffusional formation of Y/Zn enriched GP zones and their subsequent shear transformation into LPSO building blocks. The mechanism constitutes a new type of coupled diffusional-displacive phase formation sequence which may also be applicable to other alloy systems.

Understanding multi-element alloy passivation in acidic solutions using operando methods

The kinetics of passivation and dissolution of many corrosion resistant engineering alloys remain incompletely characterized. Clarification of such kinetics and the unique role of alloying elements were investigated upon Ni-22% Cr and Ni-22% Cr-6% Mo, wt%, in an acidic chloride-free sulfate environment using simultaneous electrochemistry, on-line Inductively Coupled Plasma-Mass Spectroscopy (ICP-MS), and Single Frequency-Electrochemical Impedance Spectroscopy (SF-EIS). The combination of methods enabled in-operando tracking of the total current densities for (i) oxidation, (ii) cation ejection by multiple paths, and (iii) oxide film growth during non-steady state passivation. An improved and physically accurate interpretation of passivation processes is presented and clarifies and alters the frequently made assumption that Cr2O3 is the dominant oxide on Ni-Cr alloys throughout oxidation.