Search

Multi-principal element alloys (MPEAs) have attracted rapidly growing attention from both research institutions and industry due to their unique microstructures and outstanding physical and chemical properties. However, the fabrication of MPEAs with desired microstructures and properties using conventional manufacturing techniques (e.g., casting) is still challenging. With the recent emergence of additive manufacturing (AM) techniques, the fabrication of MPEAs with locally tailorable microstructures and excellent mechanical properties has become possible. Therefore, it is of paramount importance to understand the key aspects of the AM processes that influence the microstructural features of AM fabricated MPEAs including porosity, anisotropy, and heterogeneity, as well as the corresponding impact on the properties. As such, this review will first present the state-of-the-art in existing AM techniques to process MPEAs. This is followed by a discussion of the microstructural features, mechanisms of microstructural evolution, and the mechanical properties of the AM fabricated MPEAs. Finally, the current challenges and future research directions are summarized with the aim to promote the further development and implementation of AM for processing MPEAs for future industrial applications.

Body-centred cubic magnesium-lithium-aluminium-base alloys are the lightest of all the structural alloys, with recently developed alloy compositions showing a unique multi-dimensional property profile. By hitherto unrecognised mechanisms, such alloys also exhibit exceptional immediate strengthening after solution treatment and water quenching, but strength eventually decreases during prolonged low temperature ageing. We show that such phenomena are due to the precipitation of semi-coherent D03-Mg3Al nanoparticles during rapid cooling followed by gradual coarsening and subsequent loss of coherency. Physical explanation of these phenomena allowed the creation of an exceptionally low-density alloy that is also structurally stable by controlling the lattice mismatch and volume fraction of the Mg3Al nanoparticles. The outcome is one of highest specific-strength engineering alloys ever developed., Solution treatment and quenching can strengthen magnesium-lithium-aluminium alloys, but this strength decreases with ageing. Here, the authors show this is due to semi-coherent nanoparticle precipitation followed by coarsening, and control the lattice mismatch to stabilise the microstructure.

The growth behavior of the primary β-Co3Sn2 phase in (Co67Sn33)100−x Nb x (x = 0, 0.5, 0.8, 1.0) hypereutectic alloys at different melt undercoolings was investigated systematically. The growth pattern of the β-Co3Sn2 phase at low undercooling changes with the Nb content from fractal seaweed (x = 0, 0.5) into dendrite (x = 0.8) and then returns to fractal seaweed (x = 1.0) as a response to the changes in interface energy anisotropy and interface kinetic anisotropy. As undercooling increases, the dendritic growth of the β-Co3Sn2 phase in (Co67Sn33)99.2Nb0.8 alloy gives way to fractal seaweed growth at an undercooling of 32 K (−241 °C). At larger undercooling, the fractal seaweed growth is further replaced by compact seaweed growth, which occurred in the other three alloys investigated. The growth velocity of the β-Co3Sn2 phase slightly increases at low and intermediate undercooling but clearly decreases at larger undercooling due to the Nb addition. The growth velocity sharply increases as the growth pattern of the Co3Sn2 phase transits from fractal seaweed into compact seaweed.

M23X6 (M-transition metal, X-nonmetal) phases widely exist in alloy steels and some other multicomponent alloys, but were seldom solidified from binary alloys. In the present work, the alloy melt with the nominal composition of Co23B6 phase, Co79.3B20.7, was undercooled to different degrees, and metastable Co23B6 phase was solidified at undercooling beyond 60 K owing to its lower energy barrier to nucleation than the equilibrium Co3B phase. The Co23B6 phase formed at high temperature can be retained to room temperature if the cooling rate is larger than 25 K min−1. This phase was confirmed to be the classic Cr23C6-type structure (space group \( {\text{Fm}}\bar{3}{{\rm m}} \)) with a lattice parameter, measured by Rietveld refinement method, of a = 10.4912(1) A.

The limit in structural refinement of lamellar bands (LBs) generated during accumulative roll bonding (ARB) of commercially pure nickel was investigated by transmission electron microscopy and transmission Kikuchi diffraction. A typical LB consists of an internal cellular substructure of low angle boundaries (LABs) bounded by two high angle boundaries (HABs) that are aligned parallel to the rolling plane. At low true strains (e 4.8; 6 to 10 ARB cycles), a homogenous distribution of well-defined, highly elongated LBs of average thickness 75 nm was generated throughout the entire thickness of the material. The thickness of these LBs decreased with increasing number of ARB cycles and reached a saturation thickness of ~75 nm after 6 to 8 cycles. A theoretical framework for the limit to LB refinement during ARB is presented based on the refinement rate due to the stored energy of deformation balanced by the growth rate caused by adiabatic heating. The analysis takes into account the unique features of LB structures and processing parameters.

The deformation heterogeneities that form in the vicinity of prior hot band grain boundaries in a 75 pct cold-rolled interstitial-free steel have been investigated by 3D electron backscatter diffraction. Grain boundary-affected regions occupy a large fraction of the overall material volume. The coexistence of several features, such as steep orientation gradients up to 5 deg/μm, high-angle boundary networks, and thin, elongated grain boundary fragments, has confirmed the highly complex nature of these regions. Most notably, these thin boundary fragments were found to be significantly misoriented from any of the deformed grains immediately adjacent to the boundary. Overall, grain boundary regions adopt the so-called ‘deformation banding’ mode of deformations on both the micro (e.g., steep gradients)- and nano (e.g., thin fragments)-length scales. Grain boundary structures comprise the essential features to act as preferred sites for recrystallization. The discovery of numerous thin grain boundary fragments in the deformation microstructure provides a plausible explanation for the origin of recrystallized grains with orientations other than those found within the adjoining deformed grains in the vicinity of grain boundaries; this phenomenon has been commonly observed in texture data for many years but remained unexplained.

In this work, an in situ bulk metallic glass composite in the Mg-Ni-Gd ternary system with high Mg content (>80 at. pct) that contains a ductile Mg-rich crystalline phase was produced by copper mold casting. The stability and devitrification process of the amorphous matrix in the composite structure have been studied using scanning electron microscopy, differential scanning calorimetry, and X-ray diffraction. The precipitation of crystalline phases from the amorphous matrix and the volume fraction of these phases were found to be dependent on both cooling rate and melt composition.

In this investigation, the three-dimensional nature of microband boundaries was investigated in a Goss-oriented Nickel single crystal. The sample was 30 pct deformed by plane strain compression and then characterized by several advanced techniques including three-dimensional electron backscatter diffraction (3D-EBSD) and transmission electron microscopy (TEM). In the longitudinal section of the sample, microbands were found to exhibit their classical strip-like appearance. However, the microband interfaces contain numerous curved features despite the average inclination of the interfaces closely matching the potential {111} slip planes. The microband boundary irregularities are argued to be the outcome of microband interactions, which are also linked to the orientation spread of deformation structures. Based on the detailed TEM investigation of the dislocation structures associated with intersecting microbands, it is argued that these interactions result in the disintegration and/or dissolution of microband boundary segments. A correlation between microband and cell boundaries was also found.

Offering a unique suite of mechanical, physical, and chemical properties, bulk metallic glasses (BMGs) show significant promise as engineering materials. Unfortunately, most BMGs exhibit low tensile ductility at ambient temperature that limits their use as structural (load-bearing) materials. To overcome this problem, BMG composites (BMGCs) containing a second phase are being developed for improving ductility by controlling the mechanics of shear band nucleation and growth in the glassy matrix, which is the primary mode of failure in these materials. This review describes some recent developments in BMGCs and discusses the influence of the type of second phase on mechanical behavior.

Multifunctional titanium alloys, termed Gum Metal™, are β-phase Ti alloys first developed in 2003. These alloys exhibit many interesting properties including, for example, low rate of work-hardening and superplasticity during cold deformation. The original report described a new plastic deformation mechanism not involving major dislocation activity to explain such deformation behavior. In the current study, a comparable Ti-36.8Nb-2.7Zr-2.0Ta-0.44O (wt pct) alloy to the original investigators was produced by powder sintering, hot forging, solution treatment, and cold swaging with the aim at investigating the microstructural development during swaging. XRD and TEM showed that the forged/solution-treated alloy was β-phase with a small amount of ω-phase. After cold swaging by up to 96 pct area reduction, TEM/HRTEM revealed the existence of dislocations, deformation twins, ω-phase, nanodisturbances, and lattice bending, with EBSD showing the grains to be highly elongated in the swaging direction, fragmented, and distorted. Most notably, swaging also generated a strong 〈110〉 fiber texture, even after moderate strains. The foregoing structural analysis provides substantial evidence that dislocations are present in the alloy after cold swaging. The major support of dislocation glide processes acting as the dominant plastic deformation mode in the swaged alloy is the strong 〈110〉 fiber texture that develops, which is a characteristic feature of all cold-drawn/swaged body centered cubic metals and alloys.

It is widely accepted that morality does not demand that we do our very best, but our most significant moral traditions do not easily accommodate this intuition. I will argue that the underlying problem is not specific to any particular tradition. Rather, it will be difficult for any moral theory to account for binary moral concepts like permissible/impermissible while also accounting for scalar moral concepts like better/worse. If only the best is considered permissible, morality will seem either unreasonably demanding or implausibly minimal. But if we draw a line of duty below the optimal, then we must explain how the act that is worse is nonetheless permissible. Some have tried to explain this by appealing to non-moral considerations, and others have appealed to agent-relative moral considerations. I argue that no such approach will work. We should instead exploit the distinction between reasons for performing an act and reasons for holding someone accountable for an act’s performance. This approach will also help to clear up a confusion regarding the notion of a moral demand.

A novel methodology for predicting specific compositions for glass-forming alloys based on efficiently packed atomic cluster selection, liquidus lines, and ab initio calculations is presented. This model has shown applicable adaptation to many known metallic and ceramic oxide glass-forming systems and has led to the discovery of soon to be reported Ag- and Zn-based bulk metallic glasses (BMGs). As a model system, glass formation in the Cu-Mg-Ca ternary system has been assessed using this alloy design methodology, which has led to the discovery of a number of Cu-based BMGs with compositions ranging from Cu-33 to 55 at. pct, Mg-18 to 45 at. pct, and Ca-18 to 36 at. pct. Included in this work are the calculated values of associated cluster binding energies and correlations between physical and thermal properties of these glassy compositions, which show significant physical evidence to support the likely existence of such clusters.

The three-dimensional morphology of crystalline plates that form in as-cast Mg65+x (Cu0.667Y0.333)30–x Zn5 (x = 6, 12, 14, and 16) bulk metallic glass (BMG) composites was investigated by focused ion beam (FIB) tomography. The size, shape, and distribution of the plates were found to be dependent both on alloy composition and cooling rate of the melt, whereby rapid cooling and lower x values generated a lower volume fraction of plates due to a decreased propensity for crystallization. Using FIB tomography, it was demonstrated that these plates may nucleate at micron-sized cuboidal, spherical, and irregularly shaped particles that form first during the casting process. The plates subsequently grow in preferential directions during cooling of the alloy to below the glass transition temperature to ultimately generate a multivariant, interwoven crystalline structure throughout the amorphous matrix. This complex structure is argued to contribute to the improved toughness of the alloy by hindering the propagation of gross shear bands and promoting the formation of multiple shear bands.

The effect of various casting parameters and thermophysical properties of bulk metallic glasses (BMGs) on critical section size, defined as the maximum casting size that generates a through-section amorphous alloy, has been analyzed using a simple heat-transfer model. It was found that the interfacial heat transfer between the mold and the casting has a strong influence on the critical section thickness of a BMG. It is argued herein that the critical cooling rate rather than the critical casting size is a more robust indicator of glass-forming ability (GFA). Further, with respect to the critical cooling rate and heat-transfer effects during casting, a distinct difference was found between the critical casting thickness in rectangular-shaped castings and the critical casting diameter in rod-shaped castings, and a relationship was derived for relating these parameters.

The crystallographic nature of the early stages of recrystallization within a volume of deformation microstructure in cold-rolled extra-low-carbon (ELC) steel was investigated by three-dimensional (3-D) electron backscatter diffraction (EBSD). The investigation revealed several pertinent restoration events not easily observed on two-dimensional (2-D) polished sections, including the following: (1) demonstration that each nucleus is formed with an orientation contained within its local deformation substructure, (2) observation of a single nucleus containing both a dislocation-containing core and protruding dislocation-free high-angle grain-boundary (HAGB) segments, and (3) preferential growth of certain HAGB segments into the surrounding highly misoriented regions of the deformation substructure. The technique, therefore, provided the unambiguous observation of the nucleation of grains with orientations within the spread of the local deformation substructure followed by the preferred growth of certain boundary segments; this is consistent with the concept of oriented nucleation followed by growth controlled by orientation-pinning effects.

Great progress has been made in understanding the atomic structure of metallic glasses, but there is still no clear connection between atomic structure and glass-forming ability. Here we give new insights into perhaps the most important question in the field of amorphous metals: how can glass-forming ability be predicted from atomic structure? We give a new approach to modelling metallic glass atomic structures by solving three long-standing problems: we discover a new family of structural defects that discourage glass formation; we impose efficient local packing around all atoms simultaneously; and we enforce structural self-consistency. Fewer than a dozen binary structures satisfy these constraints, but extra degrees of freedom in structures with three or more different atom sizes significantly expand the number of relatively stable, ‘bulk' metallic glasses. The present work gives a new approach towards achieving the long-sought goal of a predictive capability for bulk metallic glasses., The atomic structure of metallic glasses is related to their properties such as the ability of materials to form glasses. Here, the authors develop a new approach to model the atomic structure of metallic glasses, to enable a better connection between atomic structure and glass properties.

The fracture behaviour of Mg65Cu25Y10 bulk metallic glass (BMG) during room-temperature three-point bending was investigated. The BMG was initially produced by casting into a wedge-shaped mold which generated an amorphous structure below the ∼4 mm thickness zone of the wedge. Three-point bend testing was then carried out on the BMG with the fracture angles and salient features of the fracture surfaces examined by scanning electron microscopy. Observations indicate that this type of deformation mode results in fracture via crack propagation from both surfaces of the samples where the tensile and compressive stresses are greatest. The direction of crack propagation was also found to deviate considerably from 45° to the length direction of sample. A scanning electron microscopy (SEM) study of the fracture surfaces indicated that deformation banding was a feature of crack propagation within compressive zone whereas the tensile zone generated a featureless surface characteristic of brittle failure. The mechanism of failure of the present alloy is discussed on the basis of the observed features on the fracture surfaces and the direction of propagation of cracks during failure and compared with the failure mechanism of samples fractured under both simple tension and compression.

The recrystallized grain size following cold rolling and annealing of an Al alloy (AA2014) and a particulate reinforced AA2014 composite was investigated. The composite contained 20 vol% alumina particulates of average diameter 15 μm. The recrystallized grain size in the composite was finer than in the alloy, for a given set of conditions; this was most notable for material strained less than 50% cold-work. This behaviour was attributable to a higher nucleation efficiency in the matrix adjacent to coarse alumina particles in the composite. A model was presented for the composite to predict the recrystallized grain size as a function of strain, with respect to the size distribution and number density of alumina particles. This model predicted the strain dependence of the recrystallized grain size and, in particular, the grain size insensitivity to strain at moderate-to-high levels of cold-work.

The solidification behavior of AISI 304 stainless steel strip was studied using a melt/substrate contact apparatus, whereby a copper substrate embedded in a moving paddle is rapidly immersed into a steel melt to produce thin (∼1-mm gage) as-cast coupons. For cases where other casting conditions were kept constant, the effect of substrate topography and melt superheat on the development of microstructure and texture during solidification was studied using electron backscatter diffraction (EBSD) and optical microscopy. It was found that nucleation and growth of grains during solidification were influenced both by substrate topography and melt superheat. A ridged substrate produced a high density of randomly oriented grains at the chill surface with the preferred growth of 〈001〉-oriented grains perpendicular to the substrate wall producing a coarse columnar grain structure exhibiting a strong 〈001〉 fiber texture at the strip center. In contrast, a smooth substrate resulted in a lower nucleation density to produce a very coarse-grained columnar microstructure with moderate and essentially constant 〈001〉 fiber texture throughout the strip thickness. By the manipulation of casting parameters, it is possible to produce strip-cast austenitic stainless steel with a particular microstructure and texture.

Melt/substrate contacting experiments, designed to approximate conditions encountered during strip casting, were carried out to produce as-cast ferritic stainless steel strip. The results show that inoculation of the melt to produce TiN particles, together with casting onto a smooth substrate, results in the optimum conditions for nucleation and subsequent growth of an exceedingly high volume fraction of ferrite grains with 〈001〉 oriented within a few degrees of the normal direction (ND) of the strip surface. It is argued that, during casting, TiN particles either nucleate or deposit onto the substrate with 〈001〉 parallel to the ND, and since these particles exhibit crystallographic features similar to δ-ferrite, subsequent epitaxial growth inherits the initial particle orientation. Such oriented nucleation of ferrite from a smooth substrate results in the optimum heat-transfer conditions for further growth of dendrites with 〈001〉 perpendicular to the substrate, thus producing the intense through-thickness 〈001〉//ND fiber texture in the as-cast strip. The potential for producing grain-oriented silicon iron by direct strip casting is outlined.

The author argues that theory has no place in consumer law, but that consumer law itself takes the place of theory. Theories attempt to explain what something is and how and why it works while laws are themselves a set of created rules which act to regulate behavior. The truest explanation of consumer laws is found by examining the motivation and not the content of laws. Rather than a theoretical approach, the author advocates consumer law on a practical level, as a tool to be used to protect consumers from overreaching behavior and provide a means for them to protect themselves. After defining consumer law, the author goes on to examine those statutory attributes which would best achieve this goal, as well as illustrating current inadequacies in consumer law. Additionally he focuses on the importance of fairness and mercy as two of the foundations of consumer law.