An overview of the High-Performance Corrosion-Resistant Materials (HPCRM) Program, which was cosponsored by the Defense Advanced Research Projects Agency (DARPA) Defense Sciences Office (DSO) and the U.S. Department of Energy (DOE) Office of Civilian and Radioactive Waste Management (OCRWM), is discussed. Programmatic investigations have included a broad range of topics: alloy design and composition, materials synthesis, thermal stability, corrosion resistance, environmental cracking, mechanical properties, damage tolerance, radiation effects, and important potential applications. Amorphous alloys identified as SAM2X5 (Fe 49.7 Cr 17.7 Mn 1.9 Mo 7.4 W 1.6 B 15.2 C 3.8 Si 2.4 ) and SAM1651 (Fe 48 Mo 14 Cr 15 Y 2 C 15 B 6 ) have been produced as meltspun ribbons (MSRs), dropcast ingots, and thermal-spray coatings. Chromium (Cr), molybdenum (Mo), and tungsten (W) additions provided corrosion resistance, while boron (B) enabled glass formation. Earlier electrochemical studies of MSRs and ingots of these amorphous alloys demonstrated outstanding passive film stability. More recently, thermal-spray coatings of these amorphous alloys have been made and subjected to long-term saltfog and immersion tests; good corrosion resistance has been observed during salt-fog testing. Corrosion rates were measured in situ with linear polarization, while the open-circuit corrosion potentials (OCPs) were simultaneously monitored; reasonably good performance was observed. The sensitivity of these measurements to electrolyte composition and temperature was determined. The high boron content of this particular amorphous metal makes this amorphous alloy an effective neutron absorber and suitable for criticality-control applications. In general, the corrosion resistance of such iron-based amorphous metals is maintained at operating temperatures up to the glass transition temperature. These materials are much harder than conventional stainless steel and Ni-based materials, and are proving to have excellent wear properties, sufficient to warrant their use in earth excavation, drilling, and tunnel-boring applications. Large areas have been successfully coated with these materials, with thicknesses of approximately 1 cm. The observed corrosion resistance may enable applications of importance in industries such as oil and gas production, refining, nuclear power generation, shipping, etc.

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Thick thermal barrier coatings with thicknesses on the order of a few millimeters are being developed for use in diesel engines with operating temperatures of about 800°C. In this environment, a coating will experience thermomechanical cycling due to differences in elastic and thermal expansion properties between the coating and the substrate. The inelastic constitutive behavior of the coating material results in both compressive and tensile stresses. To observe the effects of such stresses, specimens of plasma-sprayed 8%Y2O3-ZrO2 were fabricated to allow testing of the coating material independent of the substrate. Cyclic compression fatigue tests were conducted at room and high temperature (SOOT) to simulate the loading environment to which the coating materials will be exposed during service. At high temperature, the compressive fatigue strength of the coating material increased by nearly 100%. Fatigue tests in tension and combined tension/compression were conducted at room temperature to evaluate the effect of mean stress. It was observed that a varying mean stress had no significant impact on the fatigue lives of the coating material and the fatigue life was controlled by the maximum tensile stress of the cycle. Results from fatigue tests and SEM observations indicated that the damage accumulated during the tensile and the compressive portions of the fatigue cycle were independent of each other.

Thick thermal barrier coatings (TTBCs) of plasma-sprayed 8% Y,O,-ZrO, were fatigued in compression as part of a reliability and durability study to evaluate their potential use in high-performance diesel engines. Test specimens were designed to test the bulk ceramic uniaxially, independent of the substrate. A test machine was designed to alleviate the mechanical gripping and alignment difficulties associated with cyclically stressing brittle ceramics in compression. Higher fatigue limits, 375 vs 200 MPa, were observed at 800°C than at room temperature. Specimens tested at room temperature after high-temperature compressive cycling also had higher fatigue limits, indicating that the strengthening was permanent. At temperatures of SOO'C, the coatings showed evidence of low-temperature, pressure-induced sintering. The extent to which sintering occurred was determined by studying the change in the elastic modulus as a result of the application of varying temperatures and static stresses.

The passive film stability of several Fe-based amorphous metal formulations have been found to be comparable to that of high-performance Ni-based alloys, and superior to that of stainless steels, based on electrochemical measurements of the passive film breakdown potential and general corrosion rates. Chromium (Cr), molybdenum (Mo) and tungsten (W) provide corrosion resistance; boron (B) enables glass formation; and rare earths such as yttrium (Y) lower critical cooling rate (CCR). The high boron content of this particular amorphous metal also makes it an effective neutron absorber, and suitable for criticality control applications, as discussed in companion publications. Corrosion data for SAM2X5 (Fe49.7Cr17.7Mn1.9Mo7.4W1.6B15.2C3.8Si2.4) is discussed here.