An update to the ductility potential of Al-Si and all other cast aluminum alloys is provided. Through analysis of extreme data in the literature, it is demonstrated that the highest elongation values in cast aluminum alloys are quite similar to those in wrought aluminum alloys. Through meticulous attention to all details of liquid metal quality throughout the entire production system, castings with exceptional quality can be produced.

New proposals include the precipitation of new phases and intermetallics in light alloys occur on bifilms rather than on grain boundaries (confusion is understandable because, commonly, it seems that as many as 50 per cent of grain boundaries contain bifilms). The formation and growth on bifilms result from the saving of plastic work to accommodate the volume and shape change of the new phase, which can be orders of magnitude higher than the saving of interfacial energy on grain boundaries. The bifilm, if previously closed, is forced to open by this process, called ‘Precipitation Cleavage’. The term ‘cleavage’ refers to the cleaving open of the bifilm, dispers- ing the volume change elastically over a relatively extensive area. This is the mechanism explain- ing the sensitisation heat treatments for (i) embrittlement, (ii) invasive corrosion as in stress corro- sion cracking, and (iii) invasion of hydrogen into opened bifilms as ‘sinks’, leading to surface blis- tering and hydrogen embrittlement of the matrix. Direct visual evidence is provided by (i) surface blisters during hydrogen charging, and (ii) fracture surfaces displaying quasi cleavage facets, and fisheyes observed in steels and more recently also in light alloys.

This account is an exploration of concepts exploring the widespread damage to liquid metals caused by poor current liquid metal handling and casting technology. The defects introduced in the liquid state are suggested to affect many properties of our engineering metals, especially tensile elongation and Charpy toughness, but also time-dependent degradation processes, which can result in failure by fracture, and which can be significantly aided by hydrogen, leading to hydrogen embrittlement (HE), and invasive corrosion, leading to stress corrosion cracking (SCC). The new phenomenon of ‘precipitation cleavage’ is introduced, explaining the sensitization of alloys by certain heat treatments. Direct visual evidence for precipitation cleavage is provided by the previously unexplained phenomenon of ‘fisheyes’ observed frequently on the fracture surfaces of steels, and more recently also in light alloys.

The precipitation of intermetallic compounds (IMCs) in an aluminium alloy Al-7Si-0.4Mg (A356) was studied, making additions of Fe, Sr, and Mg. The experiment avoided metallographic techniques, using a novel experimental approach in which the outside surface of the cast metal was studied. Precipitation of IMCs was observed to occur on the surface of oxide bifilms in suspension in the liquid metal, occasionally as a Sr-rich IMC growth from one side and a Fe-rich IMC growth from the other, corroborating the structure of the bifilm as having two wetted outer sides. The bifilms onto which heavy IMCs had precipitated were observed by nanotomography to sediment rapidly, indicating a potential method of cleaning secondary metals from both bifilms and metals like Fe.

In the secondary metals refining processes, vacuum arc remelting (VAR) and electroslag remelting (ESR), the consumable electrode is commonly produced by vacuum induction melting (VIM) which employs the regrettably primitive casting technique of simply pouring into the open top of the mold. Despite the vacuum, the resulting oxidizing conditions and the immensely powerful turbulence accompanying the top-pouring of the electrode is now known to create a substantial density of serious cracks. The cracks in the cast electrode are bifilms (double oxide films), which in turn are proposed to be responsible for the major faults of the VAR ingot, including undetectable, horizontal macroscopic cracks, white spots (clean and dirty varieties) and in-fallen crown. The remedial action to solve all these issues at a stroke is the provision of a counter-gravity cast electrode, cast in air or vacuum, or provision of any similar electrode substantially free from bifilm defects. The ESR process is also described, explaining the reasons for its significantly reduced sensitivity to the top-poured VIM electrode, but indicating that with an improved electrode, this already nearly reliable process has the potential for perfect reliability. The target of this critical overview is an assessment of the potential of these secondary refining processes to produce, for the first time, effectively defect-free metals, metals we can trust.

The author describes his view of how the metal casting process is often carried out turbulently, generating doubled-over oxide films (bifilms) in most metals (with interesting exceptions). The double films act as cracks, whose oxide films resist bonding, enabling the crack population to survive plastic working such as forging and rolling. Experimental evidence reveals their action in initiating tensile and fatigue failures, and reducing ductility and creep resistance. The bifilm cracks are mainly located at grain boundaries, where they encourage the precipitation of inclusions and second phases as a result of the savings of strain energy (far exceeding the normal driving force from interfacial energies). The precipitates force open the bifilms rather than otherwise plastically deforming the matrix to accommodate the changes of size and shape. It is reasonable to suppose that the opening of bifilms at grain boundaries would increase hydrogen permeation and embrittlement, and permit the ingress of corrodents leading to etch pitting and stress corrosion cracking. It seems the pre-existing population of microcracks is a phenomenon explaining the initiation of many of our failure mechanisms in metals. It is strongly recommended to reduce the microcrack population by the application of improved melting and casting technology.

The fatigue of engineered components involves more than the fatigue of metals as studied in laboratories. The miniscule laboratory test pieces cannot represent the pre-existing macroscopic crack defects in real engineering components. This brief study illustrates five examples in which major cracks are pre-existing as a result of the presence of bifilm defects. The pre-existing defects account for up to 90 per cent of the failure of so-called fatigue failure. The presence of pre-existing bifilm defects is of overwhelming importance. It is, with regret, suggested that the attempts at the elimination of so-called fatigue failures by only studying fatigue is misguided. The so-called fatigue failures of engineering components can be understood and addressed by realizing the major contributions of bifilms.

ABSTRACT Current techniques for the casting of ingots involve much mixing with air, ensuring a high population of oxide inclusions in the steel. The inclusions are in the form of folded, doubled-over oxides, which act as cracks for the initiation of many varieties of failure. Casting in vacuum reduces the thickness of the oxides, reducing the total oxygen in the steel, but does not appear to reduce the crack population. Vacuum casting is therefore mainly an expensive diversion from the point of view of avoiding failure by fracture. Techniques for reducing air entrainment are presented, including contact pour. The control of bifilm populations in steels is predicted to revolutionize the elimination of many of our current metallurgical failure processes.

There is significant evidence from light metals that turbulence in casting leads to bifilm defects; enfolded, doubled-over oxide films which act like cracks in the liquid, and are inherited as cracks by the solid. This population of introduced cracks is now known to significantly influence the tensile failure behaviour of light alloys. There is evidence that analogous defects exist in steels. This paper examines the possibility that bifilms may control the hot ductility of TWIP and TRIP steels, and therefore the problems of straightening during continuous casting. Techniques for overcoming these problems are indicated.

The population of preexisting microcracks originally proposed by Griffith to explain the initiation of fracture is identified as bifilms, introduced by the consolidation processes for engineering metals, but most importantly during the casting processes for metals. Other theoretical mechanisms for the initiation of cracks (such as, for instance, vacancy condensation or dislocation pile-up) appear to be unimportant or nonexistent. The elimination of bifilms is predicted to eliminate fatigue failure and reduce creep. The precipitation of inclusions on bifilms particularly during heat treatment in the solid state appears to prize open the bifilms, encouraging their role as pathways for gases and corrodents to penetrate the metal and for gases to pressurize bifilm cracks. In this way, new explanations of stress corrosion cracking and hydrogen embrittlement are proposed. All these embrittlement phenomena in metals appear to be solvable by improved casting techniques. A new era of metallurgy and engineering achievement seems possible.

Ablation casting is a relatively new casting technique which can improve the mechanical properties of castings. In this paper, the microstructure and mechanical properties of plain carbon steel castings produced by conventional casting and ablation casting techniques were compared. It was found that ablation increased the ultimate strength from 638 to 1094 MPa and tensile elongation increased from approximately 13 to 16% as a result of severe microstructural changes from ferrite and pearlite to acicular ferrite, bainite, and martensite. Ablation increased the core hardness of the casting from 182 to 467 HB and the surface hardness from 205 to 508 HB.

The fundamental process of the consolidation of metals ensures bifilms are created and are widespread in engineering metals. The bifilm is a crack resulting from the impingement of two oxide films. The impingement of oxides on powder particles during the consolidation of powders to make powder metallurgical products is easily understood, but results in relatively harmless and controlled dispersions of cracks. Bifilms formed during the casting of liquid metals are quite different, ranging in sizes from submicrometer to fractions of meters. The many phenomena associated with their creation and their morphological changes with temperature and time are presented. The behaviors of the light metals and the higher temperature metals, particularly irons, steels, and Ni-based alloys, are contrasted. Techniques for the avoidance of bifilm formation are outlined. The macroscopic bifilm cracks, fundamental to current vacuum arc remelted steels, are a special danger, which are highlighted.

Most of the major defects in Al alloy castings are the result of entrainment processes. The entrainment of the surface of the liquid creates bifilm defects, and the entrainment of air creates bubbles and bubble trails.Occasionally the entrainment of foreign inclusions, such as sand inclusions, can also be a problem. Bifilms form the initiators of gas porosity, shrinkage porosity, hot tears, and cracks.Since bifilms can be controlled by improved melting and casting techniques, all these defects are controllable. In addition, bifilms control the mechanical properties of castings, particularly tensile elongation, toughness, and fatigue. The other important effects caused by bifilms such as invasive corrosion behavior including pitting, stress corrosion cracking, and possibly hydrogen embrittlement, are beyond the scope of this review.

The concept of the bifilm is now nearly 20 years old. It has been clearly shown to control structural defects in castings such as porosity and hot tearing, plus, in solid wrought products, mechanical properties such as ductility and fatigue. It appears to explain for the first time the structures of Al–Si alloys and cast irons. Furthermore, although there are a number of proposed mechanisms for crack initiation in metals, it seems most probable that these are invalid. It follows that there is probably no intrinsic crack failure mechanism for most engineering metals; metals should never crack. The only crack-initiating feature appears to be the bifilm, introduced accidentally during casting. By appropriate casting techniques, the introduction of bifilms can be avoided. For this reason, failure by fatigue and creep may be eliminated, together with invasive corrosion such as pitting, stress corrosion cracking and possibly other failure modes.

The Ten Rules are a checklist of the conditions necessary for the production of successful castings, particularly dealing with the metal quality, with a view to achieving a casting with minimal, preferably zero, faults. The first five Rules specify the conditions to avoid entrainment defects, particularly bifilms and bubbles. The remaining Rules deal with the provision of feeding, avoidance of convection, chemical segregation, and residual stress, and the provision of pickup locations for machining.

There appear to be two main growth mechanisms for graphite in cast iron i) Coupled eutectic growth forms of gray irons which are classical growth modes of simultaneous parallel growth of graphite and austenite, not reliant on a bifilm mechanism. These are necessarily fine structures as a result of their control by the rate of diffusion of carbon in the liquid. These structures are well understood. (ii) Uncoupled eutectic mechanisms which appear to be much less well understood, including (a) growth of graphite on silica bifilm substrates floating freely in the melt, forming such structures as A-type graphite flakes. This prediction appears to have now been confirmed by direct observation. The transition to (b) nodular morphology occurs by Mg eliminating the silica bifilms by an exchange reaction. In this way the substrates for flake growth are instantly removed, and graphite can now wrap completely around nuclei, thereby growing as a nodule. Graphite structures in heavy sections such as chunky graphite may now be understandable in terms of the reorganisation by flotation of bifilms and/or nuclei.

The control of the metallurgy of steels is now highly developed. This is in contrast to the casting techniques for steels. Although continuous casting is generally conducted well, current ingot casting techniques are poor, unnecessarily introducing masses of oxides. For some steel compositions, double oxide films, bifilms, are entrained. This mechanism, occurring naturally during pouring, but till now generally overlooked, appears to be capable of explaining most of the features of steel defects in all their forms. For most Ni alloys and some steels it appears capable of generating a dense population of cracks, greatly impairing subsequent mechanical working and even final properties. In other steels the effects are much less severe. The techniques to avoid this damage to liquid steels and Ni alloys are described, including contact pouring, and naturally pressurized filling system designs. An ultimate system is counter gravity casting. For remelting processes, the risks of unreliability because of cracks intrinsic to VIM and VAR are discussed for both shop floor production and laboratory research. The potential crack-free properties of ESR when correctly made are recommended. Even so, remelting processes might constitute an unnecessary luxury if steels and Ni alloys were cast to avoid the entrainment of oxides.

Background: During the production of engineering metals there is always a consolidation stage at which smaller particles of metal are consolidated to create macroscopic pieces for engineering applications. Powder metallurgy is exemplary. Such processes involve the impingement of oxides, creating unbonded double films, called bifilms, acting as cracks. Such consolidation cracks appear to be ubiquitous throughout metallurgy and engineering. They appear to be the Griffith cracks required for failure by cracking, being responsible for initiating failure by cracking. Interestingly, they appear also to be necessary for invasive corrosion processes such as pitting, filiform corrosion, and possibly stress corrosion cracking and hydrogen embrittlement. Methods: Possible alternative sources of Griffith cracks have been researched. These include nucleation of pores and/or cracks in the liquid, solidification defects, and lattice defects in the solid, such as vacancy condensates and dislocation pile ups. Results: No alternative sources of Griffith cracks have been discovered. The only defect capable of the initiation of cracks or invasive corrosion appears to be the bifilm. Conclusions: Although consolidation by the casting of liquid metal into ingots and other shapes has traditionally been carried out poorly, resulting in dense bifilm populations, explaining the unreliability of traditional cast materials, the casting route is now capable of consolidating engineering products such as ingots and shaped castings with unequalled perfection. In principle, for the first time, it seems that the metals incapable of failure by either cracking or invasive corrosion are now possible. Keywords: Bifilms, cracks, consolidation, liquid metals, corrosion.

During their processing from ores, all metals go through a particulate stage. These separate pieces have to be consolidated usually with heat and pressure to form useful engineering forms. In the case of metals which undergo a melting stage, the pouring and stirring actions which are commonly employed disintegrate the liquid into splashes and droplets which appear to mutually assimilate, creating a consolidated bulk liquid. However, in every case, whether consolidation takes place in the solid or liquid states, the consolidation mechanism naturally incorporates an oxide-to-oxide interface (although occasionally other surface films such as nitrides and pure carbon are involved). The concept of the meeting of oxides is of course assumed to be trivial and therefore almost universally overlooked. However, the consequences seem to be far from trivial. The creation of opposed, double (never single), unbonded oxide films, called ‘bifilms’ by the author, acts as cracks. They can survive extensive plastic working and seem to be prolific throughout metallurgy. They appear to exert significant control over the failure properties of metals, by both cracking and corrosion. They are proposed to constitute the Griffith cracks required for the failure of metals by fracture and fatigue. Bifilms may now be eliminated from some of our liquid processing routes, enabling for the first time the production of crack-resistant and corrosion-resistant metals.