Arsenic loss during metallurgical processing of arsenical bronze

The chemical composition of ancient copper-based metal changes over time due to repetitive recycling and mixing of old metal. Prehistoric copper usually contains impurities from the copper ores themselves, and some have been used as evidence of anthropomorphically induced chemical change. Research into these changes has historically relied upon the assumption of element loss linearity, which is highly misleading and in fact varies with a multitude of factors. To illustrate the complexity of such losses for prehistoric alloys, we have selected arsenical bronze (Cu-As alloys) for study. The mass loss of several Cu-As-alloys under reducing atmosphere was measured by DTA/TGA. From our comparison of the experimental results to thermodynamic calculations and literature data, it was unclear whether weight losses were solely caused by the elemental loss of arsenic. However, a prolonged time temperature-cycling run demonstrated that mainly arsenic volatilizes; hence, the non-linear mass loss from the alloy can be directly attributed to arsenic

Die Masteralloy-Route als attraktive Legierungsvariante für sinterhärtende Stähle

Die Möglichkeit, die Eigenschaften von PM-Stählen durch eine Sinterhärtung zu verbessern, hängt von der Menge und der Art der verwendeten Legierungselemente ab. Elemente wie Cr und Mn zeigen eine signifikant stärkere Wirkung auf die Härtbarkeit als das bei PM-Stähle übliche Ni, Cu und Mo. Ihre hohe Sauerstoffaffinität stellt jedoch eine Herausforderung beim Sintern dar. Diese Elemente müssen somit mit anderen Elementen mit geringerer Sauerstoffempfindlichkeit kombiniert werden; z. B. Fe. Dies wird traditionell durch die Verwendung von vorlegierten Pulvern erreicht. Aber auch das Zumischen eines Masterlegierungspulvers zu reinem Eisenpulver stellt eine Alternative dar. Das Masteralloy-Konzept bietet gegenüber der Vorlegierungsroute den Vorteil, dass es eine sehr hohe Flexibilität bei der Auswahl der endgültigen Zusammensetzung des Stahls bietet. Den verschiedenen Arten von Basispulvern können unterschiedliche Mengen an Master-Legierungen beigemischt werden, wodurch sich ein breites Portfolio an Materialeigenschaften ergibt. In dieser Arbeit werden kostengünstige Fe-Mn-Si-Cr-Masterlegierungspulver mit verschiedenen Basispulvern (reines Eisen, Mo-Vorlegierungen und Cr-Vorlegierungen) kombiniert. Die mechanischen Eigenschaften und Mikrostrukturen wurden anhand von Proben ermittelt, die bei 1120 ℃ und 1250 ℃ gesintert wurden, und zwar sowohl im gesinterten Zustand als auch nach einer anschließenden Wärmebehandlung, die durch Gasabschrecken (N2) von 900 ℃ (Abkühlgeschwindigkeit von etwa 5 ℃/s) erreicht wurde. Die Ergebnisse geben einen hervorragenden Überblick über die verschiedenen Eigenschaften, die erreicht werden können, wenn der Masteralloy-Ansatz verwendet wird, um eine maßgeschneidertes Eigenschaftsprofil zu erzielen. Selbst beim Sintern bei 1120 ℃ führen Zusätze von Masterlegierung zu niedrig vorlegierten Basislegierungen zu einer bemerkenswerten Erhöhung der Härte, ohne die Schlagzähigkeit nachteilig zu beeinflussen.The ability to improve the properties of PM steels by sintering hardening depends on the number and type of alloying elements used. Elements such as Cr and Mn show a significantly greater effect on hardenability than the Ni, Cu, and Mo, which are common in PM steels. However, their high oxygen affinity presents a challenge in sintering. Thus, these elements must be combined with other elements of lower oxygen sensitivity, e.g. Fe. This is traditionally achieved through the use of pre-alloyed powders. But adding a master alloy powder to pure iron powder is also an alternative. The master alloy concept has the advantage over the pre-alloy route that it offers very high flexibility in selecting the final composition of the steel. The different types of base powders can be mixed with different amounts of master alloys, resulting in a broad portfolio of material properties. In this work, inexpensive Fe-Mn-Si-Cr master alloy powders are combined with various base powders (pure iron, Mo and Cr prealloyed steels). The mechanical properties and microstructures were determined from samples sintered at 1120 ℃ and 1250 ℃, both in the sintered state and after a subsequent heat treatment reached by gas quenching (N2) of 900 ℃ (cooling rate of approximately 5 ℃/s). The results provide an excellent overview of the various properties that can be achieved when using the master alloy approach to achieve a tailored property profile. Even when sintered at 1120 ℃, additions of master alloy to low pre-alloyed base alloys result in a notable increase in hardness without adversely affecting impact strength

Interstitial chemistry in sintering of metallic materials – often overlooked but decisive

Sintering is a process by which particulate materials – loose or compacted – are transformed into a body that may be fully dense or may still contain pores, but in any case has structural integrity and load-bearing capacity. The physical mechanisms – especially the transport processes – that are responsible for these changes have been studied since the 1950s. However, the chemical part of sintering also is of decisive importance in particular for metallic systems, especially concerning the interstitial elements O and C. Any metal powder that has ever been exposed to air bears oxygen on the surface, and this oxygen has to be removed in the early stages of sintering to enable the physical transport processes to become effective. In the present work, various chemical reactions involving oxygen and/or carbon are described, and it is shown how the alloying system selected as well as the starting powder grade affect these reactions and the properties of the final products

Effects of H2 Atmospheres on Sintering of Low Alloy Steels Containing Oxygen-Sensitive Masteralloys

Processing of novel sintered steels with compositions including oxygen-sensitive elements requires deep understanding of the chemistry of sintering. The use of H2 atmospheres alleviates the oxygen transference from the base powder to the oxygen-sensitive particles. However, in H2, methane formation at 700–1200°C causes dramatic homogeneous decarburization of the part that affects both mechanical behavior and dimensional stability. The intensity and the critical temperatures of this effect depend strongly on the alloying elements, being significantly enhanced in presence of Si. When combining the alloying elements as Fe-Mn-Si masteralloys, methane formation is enhanced around 760°C due to the high Mn content (40 wt.%) in the masteralloys. Nevertheless, the benefits of H2 towards oxide reduction can still be advantageously used if diluting it in the form of N2-H2 atmospheres, or if limiting the use of H2 to temperatures below 500°C. Thus, decarburization due to methane formation can be successfully controlled.European Union’s SeventhFramework Programme FP7/2007–20136356441

Master Alloy Compositions for Tailoring Liquid Phases in Lean Steels

Optimized Cu-Ni-Si master alloys (MA) have been used to tailor the properties of lean PM steels by means of a liquid phase. The degree of intersolubility between MA and iron base powder has been used as a key factor for controlling the amount and time interval when liquid is available, the distribution of the liquid and even the impact on dimensional behavior. Two MA compositions -with dissimilar dissolutive character in contact with iron- are used to clarify the effect of solubility on microstructure development and dimensional response: a Cu-based MA able to dissolve up to 2 wt.% of iron is compared to a Ni-based MA which favors higher degrees of iron dissolution, 35 wt.%. Solid-liquid interactions taking place at high temperatures (iron dissolution and wettability) are characterized through the use of thermodynamic calculations and monitored wetting experiments. Microstructure development and dimensional behavior is evaluated through interrupted sintering trials in a quench dilatometer under different sintering conditions. The results show how critical aspects for the steel performance (densification, dimensional behavior, microstructure, etc.) can be specifically tailored (during the design step) through the proper characterization of the degree of interaction between the phases

"Internal Gettering” – Metallothermic Reduction Processes In The Early Stage Of Sintering

One of the aspects of modern material systems for high strength sintered steel parts is the presence of alloy elements with widely different oxygen affinity. Compared to the base iron, the “new” alloy elements Cr, Mn and Si form oxides with much higher stability. It is shown here that in case of powder mixes this leads to oxygen transfer from the base iron particles to the alloy elements during heating up to sintering temperature, i.e. metallothermic reduction of the iron surfaces. With prealloyed powders, the surface oxides, which are originally mostly iron oxide, are transformed into alloy element oxides during heating unless the iron oxides can be reduced at low temperature with H2. In any case, the heterogeneity of the oxygen affinity is a parameter that has to be considered when defining alloy systems for sintered steels

Surface phenomena during the early stages of sintering in steels modified with Fe–Mn–Si–C master alloys

The characteristics of the metallic powder surface play a critical role in the development of strong bonds between particles during sintering, especially when introducing elements with a high affinity for oxygen. In this study, Mn and Si have been combined in a Fe–Mn–Si–C master alloy powder in order to reduce their chemical activity and prevent oxidation during the heating stage of the sintering process. However, when this master alloy powder is mixed with an iron base powder, differences in chemical activity between both components can lead to an oxygen transfer from the iron base powder to the surface of the master alloy particles. The present research is focused on studying the evolution of the master alloy particle surface during the early stages of sintering. Surface characterization by X-ray Photoelectron Spectroscopy (XPS) shows that the master alloy powder surface is mostly covered by a thin easily reducible iron oxide layer (~ 1 nm). Mn–Si particulate oxides are found as inclusions in specific areas of the surface. Evolution of oxides during sintering was studied on green compacts containing iron powder, graphite and Fe–Mn–Si–C master alloy powder that were heat treated in vacuum (10− 6 mbar) at different temperatures (from 400, 600, 800 to 1000 °C) and analyzed by means of XPS. Vacuum sintering provides the necessary conditions to remove manganese and silicon oxides from the powder surface in the range of temperatures between 600 °C and 1000 °C. When sintering in vacuum, since the gaseous products from reduction processes are continuously eliminated, oxidation of master alloy particles due to oxygen transfer through the atmosphere is minimized

Interparticle Neck Connections in Innovative Insulated Iron Powder Compounds

Goal of the present paper is the analysis of the interparticle neck connections in a system made of insulated iron powder compounds with different additions of an Al-Mg-Si-Cu alloy (0.25, 0.5 and 0.75 wt%). The introduction of the aluminium alloy powder has been made in order to improve the mechanical properties, evaluated as the transverse rupture strength, without decreasing the agnetic properties (evaluated in terms of iron loss and coercivity force). The fracture analysis of investigated systems puts into evidence the breaking of interparticle neck connections. Heat treatment (at the temperature of 500C) contributes to the early stages of interparticle neck developments. The chosen aluminium alloy presents a sort of pre-sintering behaviour at 500C, with the possibility of mass-transport processes around the insulated iron powder compounds. The air heat treatment applied aims at providing an increase in the mechanical behaviour of the material, with a final good rigidity after the cooling process. Fracture surfaces and transverse rupture strength values show that, at 500 MPa, the strength and the area related to the inter-particle necks can be correlated to the occasional broken insulated point-to-point surfaces that hinder the development of inter-particles necks

Processing of a new high entropy alloy: AlCrFeMoNiTi

This work, a new composition of high-entropy alloys (HEAs) was designed. The composition was carefully tailored with the aim to obtain a solid solution with a BCC crystalline structure to be an alternative binder in cermets. Thus, the composition of the HEA has been designed taking into account various criteria which has fulfilled a metallic binder of a Ti(C,N) cermet:(1) high hardness and oxidation resistance and (2) good wetting behaviour with Ti(C,N) particles because the processing of cermets is by LPS. The design of the alloy has been performed using theoretical calculations of physicochemical properties of the elements involved and the calculation of phase diagram by Thermocalc. The designed alloy has been processed by casting and powder metallurgy (PM) to study the influence of the processing route on the phases formed and on the resulting properties. The powders were produced by gas atomisation and then consolidated by hot pressing. Special theme block on high entropy alloys, guest edited by Paula Alvaredo Olmos, Universidad Carlos III de Madrid, Spain, and Sheng Guo, Chalmers University, Gothenburg, Sweden