George T. Gray

An analysis of the low temperature, low and high strain-rate deformation of Ti-6Al-4V

The deformation behavior of Ti−6Al−4V at temperatures between 76 and 495 K, strain rates between 0.001 and 3000 s−1, and compressive strains to 0.3 has been investigated. Measurements of yield stress as a function of test temperature, strain rate, and prestrain history are analyzed according to the model proposed by Kocks and Mecking. The mechanical threshold stress (flow stress at 0 K) is used as an internal state variable, and the contributions to the mechanical threshold stress from the various strengthening mechanisms present in this alloy are analyzed. Transmission electron microscopy (TEM) is used to correlate deformation substructure evolution with the constitutive behavior. The deformation substructure of Ti-6-4 is observed to consist of planar slip in the α grains at quasistatic strain rates. At high strain rates, deformation twinning is observed in addition to planar slip. Increasing the temperature to 495 K is seen to alter the deformation mode to more random slip; the effect of this on the proposed deformation model is discussed.

Constitutive behavior of tantalum and tantalum-tungsten alloys

The effects of strain rate, temperature, and tungsten alloying on the yield stress and the strainhardening behavior of tantalum were investigated. The yield and flow stresses of unalloyed Ta and tantalum-tungsten alloys were found to exhibit very high rate sensitivities, while the hardening rates in Ta and Ta-W alloys were found to be insensitive to strain rate and temperature at lower temperatures or at higher strain rates. This behavior is consistent with the observation that overcoming the intrinsic Peierls stress is shown to be the rate-controlling mechanism in these materials at low temperatures. The dependence of yield stress on temperature and strain rate was found to decrease, while the strain-hardening rate increased with tungsten alloying content. The mechanical threshold stress (MTS) model was adopted to model the stress-strain behavior of unalloyed Ta and the Ta-W alloys. Parameters for the constitutive relations for Ta and the Ta-W alloys were derived for the MTS model, the Johnson—Cook (JC), and the Zerilli-Armstrong (ZA) models. The results of this study substantiate the applicability of these models for describing the high strain-rate deformation of Ta and Ta-W alloys. The JC and ZA models, however, due to their use of a power strain-hardening law, were found to yield constitutive relations for Ta and Ta-W alloys that are strongly dependent on the range of strains for which the models were optimized.

Influence of temperature and strain rate on slip and twinning behavior of zr

The stress-strain behavior of pure Zr was studied systematically at various temperatures and strain rates. At 76 K, Zr deforms predominantly by twinning, whereas above room temperature (RT), slip is the controlling deformation mode. A transition in the rate-controlling deformation mode from slip to twinning has been observed to occur at intermediate temperatures during the course of plastic deformation. Above 373 K, slip dominates the entire course of deformation. The transition from slip to twinning in the stress-strain behavior is linked to differing strain-hardening rates and temperature sensitivities of the two deformation modes.

Structural interpretation of the nucleation and growth of deformation twins in Zr and Ti—II. Tem study of twin morphology and defect reactions during twinning

A TEM study of mechanical twinning in Ti and Zr was performed. A large number of [0001]-dislocations which are rarely seen in the matrix, and a new type of stacking fault were found within deformation twins in both materials. Matrix dislocations were observed to actively interact with the tips of twin embryos and step ledges of growing twin lamellae. Twin embryos bounded by straight coherent and inclined semi-coherent boundaries were observed. These results as well as the morphological variation of twin lamellae during deformation do not agree with twinning dislocation theories but support a step-wise nucleation and growth mechanism proposed for deformation twinning in h.c.p. structures. Extended discussions are provided to address unanswered questions in the literature concerning deformation twinning.

Influence of Temperature and Strain Rate on the Mechanical Behavior of Adiprene L-100

The effect of sample thickness, strain rate, and temperature on the mechanical response of Adiprene-L100 is presented. The compressive stress-train response of Adiprene-L100 was found to depend on both the applies train rate; 0.001 {le} {dot {var_epsilon}} {le} 7,000 s{sup {minus}1}, and the test temperature at high-rate; 77 {le} T {le} 298 K. Due to the slow, dispersive wave propagation in Adiprene-L100, thinner sample thicknesses are needed to assure uniform, uniaxial stress conditions within Hopkinson Bar samples; the optimal sample thickness being dependent on test temperature. Decreasing temperature from 298 to 77 K at 3,000 s{sup {minus}1} was found to increase the maximum flow stress in Adiprene-L100 from 10 to {approximately} 210 MPa.High-strain-rate (2000 s{sup -1}) compression measurements utilizing a specially-designed Split-Hopkinson-Pressure Bar have been obtained as a function of temperature from -55 to +50{degree}C for the plastic-bonded explosive PBX 9501. The PBX 9501 high-strain-rate data was found to exhibit similarities to other energetic, propellant, and polymer-composite materials as a function of strain rate and temperature. The high-rate response of the energetic was found to exhibit increased ultimate compressive fracture strength and elastic loading modulus with decreasing temperature. PBX 9501 exhibited nearly invariant fracture strains of {approximately}1.5 percent as a function of temperature at high-strain rate. The maximum compressive strength of PBX 9501 was measured to increase from {approximately}55 MPa at 50{degree}C to 150 MPa at -55{degree}C. Scanning electron microscopic observations of the fracture mode of PBX 9501 deformed at high-strain revealed transgranular cleavage fracture of the HMX crystals.

Structural interpretation of the nucleation and growth of deformation twins in Zr and Ti—I. Application of the coincidence site lattice (CSL) theory to twinning problems in h.c.p. structures

A step-wise nucleation and growth mechanism based on the coincidence site lattice (CSL) theory is proposed for deformation twinning in h.c.p. structures. Lattice transformation during twinning is accomplished by a coordinated movement of a large number of atoms between two lattice match planes from the matrix to twin positions rather than a layer by layer movement through twinning dislocations as proposed for twinning dislocation theories. The sidewise propagation and thickening of a twin lamella proceeds in a step-wise manner with lattice match planes being the coherent boundaries between the matrix and twins. This model predicts that twinning in h.c.p. lattices can occur at a high velocity close to the sound speed, which is impossible according to twinning dislocation theories. The proposed mechanism is also consistent with other observations such as lack of critical-resolved-shear-stress for twinning, emissary dislocations, and insensitivity to temperature. Dislocation reactions may be involved in twinning although they are, at high stresses, not required.

A METALLOGRAPHIC AND QUANTITATIVE ANALYSIS OF THE INFLUENCE OF STACKING FAULT ENERGY ON SHOCK- HARDENING IN Cu AND Cu-Al ALLOYS

Abstract This paper deals with the mechanical behavior of Cu and solid–solution Cu–Al alloys that were shock-deformed to 10 and 35 GPa. All the shock-deformed materials showed shock-strengthening that was greater at higher shock pressure and decreased with decreasing stacking fault energy (SFE) at both shock pressures. In the literature, shock-strengthening has been qualitatively ascribed to a greater dislocation density and the formation of deformation twins without addressing the question as to why shock-strengthening is lower in low SFE materials. This question is addressed in the present work by quantifying the twin contribution to the total post-shock strength. The twin contribution was found to increase with decreasing SFE suggesting that the contribution of dislocations concurrently decreases. The stored energy of as-shock-deformed materials was measured and found to decrease with decreasing SFE implying a lower net stored dislocation density in the lower SFE alloys. It is suggested that a lower net stored dislocation density in low SFE alloys results in the observed lower shock strengthening.

Dynamic deformation behavior of AlZnMgCu alloy matrix composites reinforced with 20 Vol.% SiC

Abstract The dynamic mechanical response and substructure evolution of underaged and overaged AlZnMgCu alloys with and without 20 vol.% SiC particles were investigated and compared to those following quasi-static compression. The hardening rates of the overaged composites and control alloys were found to be smaller than those of their underaged counterparts. Overaged composites and the control alloys showed more strain rate sensitive behavior than the underaged composites and alloys. The flow stress of the composites was found to decrease at total strains larger than 0.15 in the high-rate Hopkinson pressure bar tests. A more rapid decrease in stress in the underaged composites suggests that microstructural damage in the underaged condition is greater than that in the overaged condition tested at high strain rates. Cracks near the SiC/matrix interfaces were observed more frequently in the underaged Hopkinson pressure bar samples. The more frequent interface cracks in the underaged composites are thought to result from much slower relaxation of the stresses and strains built up at the interace due to much more difficult thermally activated deformation.

Influence of peak pressure and temperature on the structure/property response of shock- loaded Ta and Ta-10W

The deformation behavior and substructure evolution of unalloyed-Ta and Ta-10W under quasistatic conditions have been compared to their respective responses when shock prestrained to 20 GPa at 25 °C as well as to unalloyed-Ta shocked to 7 GPa at 25 °C, 200 °C, and 400 °C. The reload yield behavior of shock-prestrained Ta and Ta-10W did not exhibit enhanced shock hardening when compared to their respective quasistatic stress-strain response at an equivalent strain level. In addition, the reload yield behavior of Ta shock prestrained to 7 GPa at 200 °C or 400 °C was found to exhibit increased hardening compared to the shock prestraining at 25 °C. The quasistatic substructure evolution and shock-hardening responses of Ta and Ta-10W were investigatedvia transmission electron microscopy (TEM). The dislocation substructures in both materials and at each strain rate condition and temperature were similar and consisted primarily of long, straight, ( α/2) 〈111〉 type screw dislocations. The propensity for long, straight screw dislocations, irrespective of the loading condition, supports the theory of strong Peierls stress control on defect generation and defect storage. The substructure evolution and mechanical behavior of Ta and Ta-10W are discussed in terms of defect storage mechanisms and compared to the mechanisms operative in face-centered cubic (fcc) metals.

Reinforcement shape effects on the fracture behavior and ductility of particulate-reinforced 6061-Al matrix composites

Particle shape effects on the fracture and ductility of a spherical and an angular particulate-reinforced 6061-Al composite containing 20 pct vol Al2O3 were studied using scanning electron microscopy (SEM) fractography and modeled using the finite element method (FEM). The spherical particulate composite exhibited a slightly lower yield strength and work hardening rate but a considerably higher ductility than the angular counterpart. The SEM fractographic examination showed that during tensile deformation, the spherical composite failed through void nucleation and linking in the matrix near the reinforcement/matrix interface, whereas the angular composite failed through particle fracture and matrix ligament rupture. The FEM results indicate that the distinction between the failure modes for these two composites can be attributed to the differences in the development of internal stresses and strains within the composites due to particle shape.