Dynamic fracture of tungsten heavy alloys was induced by two different test techniques. The first was spall (e.g., 1-D strain fracture). The second was transverse impact, as occurs in a yawed penetrator. Spall failure is stress driven, and spall stress corresponds to the threshold for void formation, which is 2.6 GPa for a 91% WNiCo alloy and 2.1–2.5 GPa for a 95% WNiFe alloy. Yaw-induced fracture, on the other hand, is strain driven. Surface flaws can provide fracture sites. At the meso scale, grain cleavage is mainly responsible for transverse fracture. Grain fracture also appears to play a critical role in the initiation of spall fracture.
Tungsten heavy alloys;
The mechanisms of tensile failure in four 5083 aluminum sheet materials are evaluated under conditions of interest for superplastic and quick-plastic forming. Two mechanisms are shown to control failure of the AA5083 materials under uniaxial tension at elevated temperatures: cavitation and flow localization (i.e., necking). Conditions for which failure is controlled by cavitation correspond to those under which deformation is primarily by grain-boundary-sliding creep. Conditions for which failure is controlled by flow localization correspond to those under which deformation is primarily by solutedrag creep. A geometric parameter, Q, is used to determine whether final failure is controlled by cavitation or by flow localization. Differences in elongations to failure between the different AA5083 materials at high temperatures and slow strain rates are the result of differences in cavitation behaviors. The rate of cavitation growth with strain is nearly constant between the AA5083 materials for identical testing conditions, but materials with less tensile ductility evidence initial cavitation development at lower strain levels. The rate of cavitation growth with strain is shown to depend on the governing deformation mechanism; grain-boundary-sliding creep produces a faster cavitation growth rate than does solute-drag creep. A correlation is found between the early development of cavitation and the intermetallic particle-size population densities of the AA5083 materials. Fine filaments, oriented along the tensile axis, are observed on fracture surfaces and within surface cavities of specimens deformed primarily under grain-boundary-sliding creep. As deformation transitions to control by solute-drag creep, the density of these filaments dramatically decreases.
The plastic deformation of seven 5083 commercial aluminum materials, produced from five different alloy heats, are evaluated under conditions of interest for superplastic and quick-plastic forming. Two mechanisms are shown to govern plastic deformation in AA5083 over the strain rates, strains, and temperatures of interest for these forming technologies: grain-boundary-sliding (GBS) creep and solutedrag (SD) creep. Quantitative analysis of stress transients following rate changes clearly differentiates between GBS and SD creep and offers conclusive proof that SD creep dominates deformation at fast strain rates and low temperature. Furthermore, stress transients following strain-rate changes under SD creep are observed to decay exponentially with strain. A new graphical construction is proposed for the analysis and prediction of creep transients. This construction predicts the relative size of creep transients under SD creep from the relative size of changes in an applied strain rate or stress. This construction reveals the relative size of creep transients under SD creep to be independent of temperature; temperature dependence resides in the “steady-state” creep behavior to which transients are related.
In this experimental investigation, the tensile creep behavior of commercial-purity molybdenum sheet at temperatures between 1300°C and 1700°C is critically evaluated, based upon experimental creep testing and microstructural characterizations. The high-temperature properties of molybdenum are of interest because there are many applications in which molybdenum and molybdenum alloys are used at elevated temperatures. Understanding of the creep mechanisms and the constitutive relations between stress and strain at elevated temperatures is needed in order to determine if molybdenum is an appropriate choice for a given high-temperature design application and to accurately predict its creep life. The creep behavior of two commercially-available grades of molybdenum was determined using short-term creep tests (1/2 to 14 hours) at slow to moderate true-strain rates of 10⁻⁶ to 10⁻⁴ s⁻¹ and temperatures between 1300°C and 1700°C. High-temperature, uniaxial tensile testing was used to produce data defining the relationship between tensile creep strain-rate and steady-state flow stress at four temperatures: 1340°C, 1440°C, 1540°C, 1640°C. Microstructural changes that occurred during creep testing were evaluated and compared to changes resulting from elevated temperature exposure alone. Mechanisms for dynamic abnormal grain growth that occurred during creep testing and the causes of the microstructural changes that occurred as a function of temperature are discussed.
A method for growing large single crystals of metals is disclosed. A polycrystalline form of a metal specimen is initially heated in a non-oxidizing environment. A minimum plastic strain is then applied to the heated metal specimen to initiate the growth of a selected grain within the heated metal specimen. Additional plastic strain is subsequently applied to the heated metal specimen to propagate the growth of the selected grain to become a large single crystal.
Solute-drag (SD) creep in Class I alloys is characterized by several features. Among these is the presence of “inverse” creep transients, which are unique to these solid-solution alloys and the SD creep mechanism. Creep transients in commercial AA5083 materials under SD creep are analyzed using a model based on a graphical construct previously proposed. It is observed that transient behavior can be represented in a general fashion which predicts the decay in relative transient size as a function of strain. Experimental data for SD creep are presented using the proposed graphical construct to determine the dependence of dislocation glide speed on stress and the dependence of equilibrium mobile dislocation density on stress. It is observed that the high stress exponents of the commercial AA5083 materials under SD creep, relative to low-impurity, binary Al–Mg materials, are primarily the result of an increased dependence of dislocation glide speed on stress.
Al2O3/Y-TZP particulate composites and particulate laminates with varying compositions and ratios of layer thickness were fabricated by tapecasting, lamination, and sintering. Tensile strain-rate-change (SRC) tests were conducted on the particulate composites and particulate laminates at a temperature of 1350 °C and compared to previous results where tests were conducted in compression. Stress exponents for particulate composites and laminates were measured to be approximately two in both tension and compression. The observed similarity of SRC data suggests that a common deformation mechanism exists in tension and compression. Elongation-to-failure tests were also conducted at 1350 °C at a constant true-strain rate of 10−4 s−1. It was found that the elongation-to-failures of particulate laminates are lower than for particulate composites with similar overall compositions because of interlayer constraint in the particulate laminates which induces cavitation in the harder layer. The increase in flow stress from dynamic grain growth was used to determine that flow stress depends on grain size to approximately the 1.5 power. Elongations for fine grained particulate composites produced by pressureless sintering were similar to those described in the literature for hot-pressed particulate composites of similar composition, but with slightly coarser grain sizes.
Not available. This is the link to a similar paper from 2004 in Acta Materials, September 6, 2004:
Al2O3/Y-TZP particulate laminates with varying compositions and ratios of layer thickness were fabricated by tapecasting, lamination, and sintering. The resulting particulate laminates were tested in compression at a temperature of 1350 °C over strain rates from 1.00 × 10−5 to 3.16 × 10−4 s−1. Microstructural changes during testing were observed to be minor. Stress exponents were measured to be approximately two and are consistent with previous data for particulate composites. Using parameters determined from particulate composites, the behaviors of the particulate laminate composites are accurately predicted using a constrained isostrain model without additional fitting parameters.
High-temperature deformation; Compression test; Laminates
The aging behaviors of three Al–Mg–Zn alloys have been investigated under conditions similar to the paint–bake cycle currently used in automotive manufacturing. The three alloys contain Mg in atomic concentrations from one to two times those of Zn. Natural aging at 25 °C after solutionizing is found to produce a linear increase in hardness with logarithmic time for times of up to 1 year. Hardnesses in naturally and artificially aged conditions are found to increase with Mg content. Artificial aging at 175 °C for 30 min, which simulates the automotive paint–bake cycle, produces increases in hardness of 15–36% over the solution-treated conditions. Peak hardness from artificial aging at 175 °C is produced in all alloys after approximately 8 h. Natural aging for 10 days prior to artificial aging at 175 °C does not produce significant changes in hardness compared with artificial aging alone. Natural aging for 1 year after simulated paint–bake aging increases hardnesses by 41–78% over those after simulated paint–bake aging alone. The precipitation strengthening mechanism in these alloys is consistent with η′ formation. Increases in hardness and strength with increasing Mg content are consistent with increased solid–solution strengthening, which is retained even after artificial aging.