David E. Alman

Searching for Next Single-Phase High-Entropy Alloy Compositions

There has been considerable technological interest in high-entropy alloys (HEAs) since the initial publications on the topic appeared in 2004. However, only several of the alloys investigated are truly single-phase solid solution compositions. These include the FCC alloys CoCrFeNi and CoCrFeMnNi based on 3d transition metals elements and BCC alloys NbMoTaW, NbMoTaVW, and HfNbTaTiZr based on refractory metals. The search for new single-phase HEAs compositions has been hindered by a lack of an effective scientific strategy for alloy design. This report shows that the chemical interactions and atomic diffusivities predicted from ab initio molecular dynamics simulations which are closely related to primary crystallization during solidification can be used to assist in identifying single phase high-entropy solid solution compositions. Further, combining these simulations with phase diagram calculations via the CALPHAD method and inspection of existing phase diagrams is an effective strategy to accelerate the discovery of new single-phase HEAs. This methodology was used to predict new single-phase HEA compositions. These are FCC alloys comprised of CoFeMnNi, CuNiPdPt and CuNiPdPtRh, and HCP alloys of CoOsReRu.

The abrasive wear of sintered titanium matrix–ceramic particle reinforced composites

Particulate (TiC, TiB2 or Si3N4) reinforced Ti composites were produced by vacuum sintering (at 1400°C for 2 h). Ti+TiC composites could be sintered to high fractional densities (>93%), even at high TiC loadings (e.g., 40 volume percent (vol%)). No reactions were observed to occur between the Ti and TiC. By contrast, the Ti and TiB2 and Ti and Si3N4 reacted to form composites consisting of Ti, TiB and TiB2 and α-Ti(N), Ti5Si3, Ti3Si, and Ti2N, respectively. As a consequence, Ti was consumed and/or the reaction products intrinsically generated porosity during sintering. These composites were more difficult to consolidate via solid state sintering, particularly at higher volume fractions. Despite the porosity, the composites were more wear resistant (pin-on-drum abrasive wear against 100 μm garnet particles) than unreinforced Ti, with the exception of the Ti+2.5 vol% TiB2 and Ti+≤10 vol% TiC composites. The ranking of microhardness and abrasion wear resistance of the composites was as follows: (hardest, most wear resistant) Ti+Si3N4 (i.e., α-Ti(N), +Ti5Si3, Ti3Si, and Ti2N)≫Ti+TiB2≫Ti+TiC (softest, least wear resistant). The microhardness coupled with the apparent strength of the chemical interface that developed between the constituent composite phases was responsible for the observed wear behavior.

Abrasive wear of intermetallic-based alloys and composites

In this study, the abrasive wear behavior of Fe3Al, TiAl, Ti3Al, Al3Ti, NiAl, Ni3Al and MoSi2, and composites based on these compounds, were assessed and compared to the behavior of selected metals, alloys and ceramics. Under the wear conditions used for these tests, the softer intermetallic compounds (e.g. TiAl and Fe3Al) behaved in a manner similar to the metals and alloys, whereas, the harder intermetallic compound (i.e. MoSi2) behaved more like a ceramic. The influence of Al atomic fraction, superlattice structure and ternary alloying additions on the wear behavior of Fe3Al was investigated. Controlling the Al content and third element additions affected wear resistance more than superlattice structure. Composite strengthening was also explored as a method for improving wear resistance. The addition of hard second phase particles (i.e. TiB2 to NiAl and SiC to MoSi2) was also very effective improving wear resistance. Surprisingly, the addition of softer Nb particles did not significantly degrade the wear resistance of a MoSi2 matrix, even at Nb additions of 40%.

Abrasive wear behavior of NiAl and NiAl-TiB2 composites

Abstract Abrasive wear of NiAl and NiAl with 10, 20, and 40 vol.% TiB 2 has been investigated using particles of different types and sizes. The addition of TiB 2 as a particulate reinforcement to NiAl increases the hardness of the composite with respect to NiAl, and reduces the wear rate at all volume fractions on garnet and Al 2 O 3 abrasives. Abrasion on SiC resulted in a minimum of the wear rate for the composite with 20% TiB 2 for most conditions. The composite with 40% TiB 2 consistently exhibited wear rates higher than the other composites when abraded on SiC. The only instance when the NiAl–40% TiB 2 composite had a lower wear rate was when it was abraded on 16 and 37 μm SiC particles. The NiAl–TiB 2 composite serves as a model system for studying the effect of reinforcement volume fraction on composite wear behavior and is discussed in terms of a composite wear model developed by Axen and Jacobson.

Influence of Silicon and Aluminum Additions on the Oxidation Resistance of a Lean-Chromium Stainless Steel

The effect of Si and Al additions on the oxidation of austenitic stainless steels with a baseline composition of Fe–16Cr–16Ni–2Mn–1Mo (wt.%) has been studied. The combined Si and Al content of the alloys did not exceed 5 wt.%. Cyclic-oxidation tests were carried out in air at 700 and 800°C for a duration of 1000 hr. For comparison, conventional 18Cr–8Ni type-304 stainless steel specimens were also tested. The results showed that at 700°C, alloys containing Al and Si, and alloys with only Si additions showed weight gains about one half that of the conventional type-304 alloy. At 800°C, alloys that contained both Al and Si additions showed weight gains approximately two times greater than the type-304 alloy. However, alloys containing only Si additions showed weight gains four times less than the 304 stainless. Further, alloys with only Si additions preoxidized at 800°C, showed zero weight gain in subsequent testing for 1000 hr at 700°C. Clearly, the oxide-scale formation and rate-controlling mechanisms in the alloys with combined Si and Al additions at 800°C were different than the alloys with Si only. ESCA, SEM, and a bromide-etching technique were used to analyze the chemistry of the oxide films and the oxide–base-metal interface, in order to study the different oxide film-formation mechanisms in these alloys.

Abrasive wear behavior of a brittle matrix (MoSi2) composite reinforced with a ductile phase (Nb)

The toughness of a variety of brittle ceramic and intermetallic matrices has been improved through the incorporation of ductile metallic reinforcements. In these composites resistance to catastrophic failure of the matrix is derived through a combination of mechanisms, including matrix crack bridging, matrix crack defection and rupturing of the ductile phase. The degree to which these mechanisms operate is a function of composite microstructure. In general, the ductile phase is softer than the matrix phase. This may have unique implications when the materials are subjected to a wear environment, whether intentional or not. Hence, it is important to understand the wear behavior of these new materials. MoSi2–Nb was selected as a model composite system, in part because of the wide body of open literature regarding this system. The influences of abrasive wear environment and the composite microstructure (Nb reinforcement size, shape and volume fraction) on the wear resistance of the composites are reported.

Abrasive wear of Si3N4MoSi2 composites

Molybdenum disilicide (MoSi2) particles have been added to silicon nitride (Si3N4) to form ceramic matrix-intermetallic composites. Benefits associated with the addition of the MoSi2 to Si3N4 include higher strength, higher fracture toughness, no loss in oxidation resistance, and lower electrical resistivity. However, since the hardness of MoSi2 is approximately half that of Si3N4, a significant decrease in the specific wear rate of the Si3N4MoSi2 composites is expected to result from the incorporation of the MoSi2 in the Si3N4. In this study, it was found, however, that the wear resistance of Si3N4 improves slightly or is unaffected by additions of small volume fractions (≤20%) of MoSi2 particles, during two-body abrasion by SiC particles. At higher volume fractions, the Si3N4MoSi2 composites wear at rates no greater than 1.5 times that of monolithic Si3N4. The volume wear of the composites was dependent on composite hardness, fracture toughness and microstructural features (i.e. MoSi2 particle size). Given equivalent hardness and fracture toughness for a composite pair at the same volume fraction, however, the composite with the smaller MoSi2 particle exhibited the lower wear rate. The abrasive wear behavior of the Si3N4MoSi2 composites can be described in terms of the inverse rule of mixtures for composites.

Solid particle erosion behavior of an Si3N4-MoSi2 composite at room and elevated temperatures

The solid particle erosion behavior at room and elevated temperatures (180, 500, 700 and 900 C) of an Si[sub 3]N[sub 4]-MoSi[sub 2] composite was studied. Alumina particles entrained in a stream of nitrogen gas impacted the target material at a velocity of 40 m/s. Impingement angles of either 60, 75 or 90[degree] were used. It was found that the erosion rate for the Si[sub 3]N[sub 4]-MoSi[sub 2] composite (measured at room temperature) was a maximum at the 90[degree] incident angle, erosion behavior typical of brittle materials. The erosion rate of the composite at a 75[degree] impingement angle increased slightly with increasing test temperature up to 700 C (i.e. from 4.1 to 4.9 mm[sup 3]/g). At 900 C, the measured erosion rate decreased to 2.9 mm[sup 3]/g. The erosion behavior of the Si[sub 3]N[sub 4]-MoSi[sub 22048mposite was compared to that of commercially available Si[sub 3]N[sub 4], WC-6%Co, 304 SS, IN-800 (Ni-Fe-Cr alloy) and Stellite-6B (Co-Cr-W-Mo alloy).

Effect of manganese addition on reactive evaporation of chromium in Ni-Cr alloys

Chromium is used as an alloy addition in stainless steels and nickel-chromium alloys to form protective chromium oxide scales. Chromium oxide undergoes reactive evaporation in high-temperature exposures in the presence of oxygen and/or water vapor. Deposition of gaseous chromium species onto solid oxide fuel-cell electrodes can reduce the efficiency of the fuel cell. Manganese additions to the alloy can reduce the activity of chromium in the oxide, either from solid solution replacement of chromium with manganese (at low levels of manganese) or from the formation of manganese-chromium spinels (at high levels of manganese). This reduction in chromium activity leads to a predicted reduction in chromium evaporation factors as much as 35 at 800 °C and 55 at 700 °C. Quantifying the effects of manganese additions on chromium evaporation should aid alloy development of metallic interconnects and balance-of-plant alloys.

Wear Evaluation of High Interstitial Stainless Steels

A new series of high nitrogen-carbon manganese stainless steel alloys are studied for their wear resistance. High nitrogen and carbon concentrations were obtained by melting elemental iron-chromium-manganese (several with minor alloy additions of nickel, silicon, and molybdenum) in a nitrogen atmosphere and adding elemental graphite. The improvement in material properties (hardness and strength) with increasing nitrogen and carbon interstitial concentration was consistent with previously reported improvements in similar material properties alloyed with nitrogen only. Wear tests included: scratch, pin-on-disk, sand-rubber-wheel, impeller, and jet erosion. Additions of interstitial nitrogen and carbon as well as interstitial nitrogen and carbide precipitates were found to greatly improve material properties. In general, with increasing nitrogen and carbon concentrations, strength, hardness, and wear resistance increased.