Search

This study investigated the phase transformation of the flaky shaped Ni–Mn–Ga powder particles with thickness around 1 μm prepared by vibration ball milling and post-annealing. The SEM, XRD, DSC and ac magnetic susceptibility measurement techniques were used to characterize the Ni–Mn–Ga powders. The structural transition of Heusler → disordered fcc occurred in the powders prepared by vibration ball milling (high milling energy) for 4 h, which was different from the structural transition of Heusler → disordered fct of the powders fabricated by planetary ball milling (low milling energy) for 4 h. The two different structures after ball milling should be due to the larger lattice distortion occurred in the vibration ball milling process than in the planetary ball milling process. The structural transition of disordered fcc → disordered bcc took place at ∼320 °C during heating the as-milled Ni–Mn–Ga powders, which was attributed to the elimination of lattice distortion caused by ball milling. The activation energy for this transition was 209 ± 8 kJ/mol. The Ni–Mn–Ga powder annealed at 800 °C mainly contained Heusler austenite phase at room temperature and showed a low volume of martensitic transformation upon cooling. The inhibition of martensitic transformation might be attributed to the reduction of grain size in the annealed Ni–Mn–Ga particles.

Strain-controlled multiaxial fatigue experiments were conducted on extruded AZ61A magnesium alloy using thin-walled tubular specimens in ambient air. The experiments included fully reversed tension–compression, cyclic torsion, proportional axial-torsion, and 90° out-of-phase axial-torsion. For the same equivalent strain amplitude, fatigue life under proportional loading was the highest and the nonproportional loading resulted in the shortest fatigue life. Detectable kinks were identified in the strain–life curves for all the loading paths. Fatigue experiments subjected to fully reversed strain-controlled torsion with a static axial load were also conducted. A positive static axial stress reduced the fatigue life and a compressive static axial stress was found to significantly enhance the fatigue life. Two critical plane multiaxial fatigue criteria were evaluated in terms of fatigue life predictions based on the experimental results. The Fatemi–Socie criterion correlated well with the fatigue life in the low-cycle fatigue regime which was characterized by shear cracking. The fatigue life predictions made by the Fatemi–Socie criterion did not agree well with the experimental results in the high-cycle fatigue regime. A modified Smith–Watson–Topper (SWT) criterion was found to be able to predict fatigue lives well for all the loading paths conducted in the current investigation.

Microstructural characterizations were performed on thin walled tubular samples of extruded Mg6Al1ZnA (AZ61A) magnesium alloy that had experienced room temperature fatigue testing. The results show that the initial basal texture and loading amplitude/direction affect the deformation mechanism and microstructural features of AZ61A. Twinning activates under compression when the stress is ∼120 MPa and the strain is ∼0.005. With the strain amplitude is higher than 0.005, twinning–detwinning is the dominant deformation mechanism and residual twins change the grain crystallographic orientation and weaken the texture.

Fully reversed strain-controlled tension–compression fatigue experiments were conducted on extruded Mg6Al1ZnA (AZ61A) magnesium alloy by employing thin-walled tubular specimens in ambient air. The strain–life fatigue curve displayed a detectable transition from lower cycle fatigue region to higher cycle fatigue region in the vicinity corresponding to a strain amplitude of 0.5%. When the strain amplitude was higher than 0.5%, shear cracking and significant twinning were observed. When the strain amplitude was lower than 0.5%, tensile cracking and little twinning were observed.

There is great interest in metallic multilayer thin films with the layer thickness on the order of nanometers due to their high hardness and strength. The yield strength deviates from Hall–Petch and Orowan predictions with layer thickness as the controlling factor, and other factors are in need to explain this phenomenon. In this work, the effect of dislocation source length on yield strength of multilayer system in the nanoscale regime is investigated using a three dimensional cellular automaton dislocation model. Different initial dislocation source lengths are assumed in the model for the metallic multilayer thin films with different layer thicknesses. The multilayer system studied here is composed of alternating A and B components with FCC structure, cube-on-cube orientation relationship, and the same layer thickness. The results show that the initial dislocation source length can provide an explanation for the above deviations.

Lattice block structure panels with of a core separating two plates, both consisting of 3.2 mm diameter struts, were produced by investment casting of Ti-6Al-2Sn-4Zr-2Mo. Individual struts exhibit a microstructure and mechanical properties in compression and bending comparable to the bulk cast alloy. The panels show a decrease in compressive strength (from 35 to 17 MPa) and an increase in compressive ductility as the testing temperature increases from 20 to 482 °C.

Transformation superplastic forming is an attractive alternative forming technique to microstructural superplastic forming, since it requires no special microstructures and, therefore, eliminates the limitation of superplastic forming capability to only expensive materials with stable high-temperature fine grains. Transformation superplasticity occurs through biasing the internal stress produced from an allotropic phase transformation by a small external stress. In this work, finite element modeling was implemented to study the transformation superplastic forming of domes from flat circular thin plate samples. The evolution and distribution of stress, strain, and dome thickness was analyzed in detail. The thickness distributions in the formed domes were compared with the theoretical predictions of two models, which assume different stress states in the domes. The appropriate stress state was identified through this comparison. Different gas pressure amplitudes were applied during forming to investigate the effect on the formed-dome apex height, when the forming time was fixed.

Ti-6Al-4V lattice block structure panels were fabricated using an aerospace-quality investment casting process. Testing in compression, bending, and impact show that high strength, ductility, and energy absorption are achieved for both individual struts and full panels, despite the intricacies involved with casting fine struts (1.6 or 3.2 mm in diameter) from a highly reactive, poor-fluidity liquid titanium alloy. The panel stress-strain curve calculated by finite-element modeling correlates well with experimental results, indicating that the occasional defects, which are common to aerospace grade castings and may be present in the struts and nodes, have little detrimental effect on the overall panel compressive properties.

Samples of unalloyed titanium and Ti-6Al-4V with a cast, coarse-grain structure were subjected to simultaneous mechanical loading and thermal cycling about their transformation range to assess their capability for transformation superplasticity. Under uniaxial tensile loading, high elongations to failure (511 pct for titanium, and 265 pct for Ti-6Al-4V) and an average strain-rate sensitivity exponent of unity are observed. Samples previously deformed superplastically to a strain of 100 pct show no significant degradation in room-temperature mechanical properties as compared to the undeformed state. Biaxial dome bulging tests confirm that transformation superplasticity is activated under thermal cycling and faster than creep deformation. The cast, coarse-grained titanium and Ti-6Al-4V have similar transformation-superplasticity characteristics as wrought or powder-metallurgy materials with finer grains. This may enable superplastic forming of titanium objects directly after the casting step, thus bypassing the complicated and costly thermomechanical processing steps needed to achieve fine-grain superplasticity.

A 3D dislocation cellular automaton model is employed to simulate yield and hardening in nanostructured metallic multilayer thin films during in-plane, biaxial tensile loading. The films consist of two types of alternating, single-crystalline FCC layers with (0 0 1) epitaxy, a mismatch in stress-free lattice parameter, but no elastic modulus mismatch. The simulations monitor the operation of interfacial and threading sources with lengths greater than the individual layer thickness. At larger layer thickness, strength increases with decreasing layer thickness, due to slip confinement to individual layers. At sufficiently small layer thickness, slip confinement is not possible, even during initial stages of plastic deformation. Consequently, strength is not controlled by layer thickness but rather by source length, as well as coherency stress and interfacial barrier strength. Here, strength may increase, decrease, or reach a plateau depending on how source length and barrier strength vary with layer thickness.

A three-dimensional dislocation cellular automaton model is developed to study the evolution of dislocation configurations in FCC single crystals. Crystallographic {111} slip planes with three-fold symmetry are discretized into equilateral triangular patches with sides along 110 directions. These patches slip provided there is a sufficient driving force associated with reduction in system energy. Perfect 110/{111} dislocations are considered. The resulting variables are the triangular patch size and dislocation core cut-off, measured relative to Burgers vector magnitude b. Three examples involving operation of a Frank–Read source are chosen to highlight the benefits and drawbacks of the method. A benefit to discretization is that dislocation evolution may be analysed via spatial averaging over collections of patches, so that the discrete versus continuum nature of the results may be studied. Further, dislocation reactions and cross slip are accommodated easily and, in principle, Monte-Carlo schemes can be integrated into the evolution formalism. A drawback is that collections of patches do not reflect a smooth variation in configuration so that artificial fluctuations in dislocation line length and direction can result. Overall the discrete nature of the method is attractive for incorporating the kinetics of thermally activated states and for simplifying the range of geometries and threshold criteria associated with dislocation reactions.

Mechanical and microstructural characterizations were performed on pure magnesium samples processed by different rolling routes and annealing conditions to investigate the effects of thickness reduction strain and annealing temperature and duration on mechanical behaviors. The results indicate that (a) rolling increases yield strength significantly by about 100% to 130% and decreases ductility by about 55% to 66%; (b) a rolling route with a lower thickness reduction strain per rolling pass corresponds to a higher strength increase; and (c) annealing does not affect the magnitudes of yield strength and ductility, but largely increases ultimate tensile strength and strain hardening rate compared with the as-rolled material without experiencing annealing.

Computational simulations were performed to investigate the dynamic mechanical behavior of pure polycrystalline magnesium under different high loading strain rates with the values of 800, 1000, 2000, and 3600 s −1 . The Johnson-Cook model was utilized in the simulations based on finite element modeling. The results showed that the simulations provided well-matched predictions of the material behavior such as the strain rate-time history, the stress-strain curve, and the temperature increase. Under high loading strain rates, the tested material experienced linear strain hardening at the early stage of plastic deformation, increased strain hardening at the intermediate plastic deformation region, and decreased strain hardening at the region before fracture. The strain hardening rates for the studied high loading strain rate cases do not vary much with the change of strain rates.

The primary goal of this project is to explore the fundamental deformation and failure mechanisms for magnesium with a hexagonal close packed (HCP) crystal structure. It is critical to perform this project for a number of reasons. First, magnesium is the lightest structural metal and its application in various structural components can save the final component weight. Second, the weight reduction from the usage of magnesium-based structural components in transportation vehicles such as automobiles and aircrafts can improve fuel efficiency and reduce the greenhouse gas emissions. Third, structural components often experience dynamic loading such as cyclic loading conditions. Fourth, magnesium with a HCP crystal structure generally has its special deformation responses under loading conditions. This project investigated magnesium based materials (magnesium single crystal, pure polycrystalline magnesium, and some magnesium alloys) under various loading conditions, and also explored some processing routes to manipulate the microstructure and mechanical properties of magnesium. The research results were published in a number of articles and also disseminated through presentations in various conferences such as TMS annual meetings, MRS meetings, the international Plasticity conferences, the Pacific Rim International Congress on Advanced Materials and Processing, and AeroMat. In addition to the contribution to the research/academic community, thismore » project is also beneficial to the general public. With the actual usage of magnesium in the passenger cars, the weight reduction and fuel consumption reduction will save the fuel bill of individual owners.« less

A closed-form solution is presented for the stress field in an infinite, homogeneous, isotropic elastic material, due to a uniform transformation strain in a cuboidal region. The dislocation-based method used here offers an alternate approach and furnishes the stress state directly, as compared to the displacement gradient furnished by Chiu [2]. Both solutions are shown to produce numerically equivalent values of stress. The present results are available as a numerical (MathCad) program to compute the stress state for abitrary cuboidal geometries

Superplastic forming is an appealing and affordable technique for manufacturing titanium-based structural parts. In this work, circular samples with uniform initial thickness were formed superplastically using finite element modeling (FEM) and the formed domes are analyzed to investigate their thickness distributions. The results indicate that the dome thickness is not uniform and decreases from the apex to the periphery for each formed dome. The increase of the formed dome height results in the increased thinning of the samples, and the increase of the ratio between the thickness at the dome apex and that at the dome periphery. The predicted thickness distribution based on the Enikeev and Kruglov (E-K) model has a good agreement with the FEM result for the formed domes with a large apex height, while it deviates from the FEM result for the formed domes with a small apex height. A possible reason is the neglecting of the body force from the sample weight in the analytical E-K model. The obtained knowledge will help guide the initial thickness design for the plate samples to realize the expected final thickness distributions.

The peak/plateau strength of multilayer thin films is analyzed in terms of the stress to bow out a dislocation loop from an interface. Comparison of approximate analytic models to experimental data suggests that the bow out is “constrained” by nearby interfaces, at least for e-NbN/Mo and e-Ni/Cu films. Estimates of the interfacial pinning distance to form the bow out are ˜20b for e-NbN/Mo and ˜70b for e-Ni/Cu around peak/plateau strength (b is the magnitude of Burgers vector).

This paper examines the effect of plastic deformation on the strength and transformation heat and temperatures of the superelastic Nitinol samples. Two types of compression tests and differential scanning calorimetry tests were conducted. The first type of compression test deformed the samples to final fracture, and the second type included two loading –unloading cycles with or without plastic deformation. The mechanically tested samples were analyzed using differential scanning calorimetry.

In nanostructured metallic multilayers, hardness and strength are greatly enhanced compared to their microstructured counterparts. As layer thickness is decreased, three different regions are frequently observed: the first region shows Hall-Petch behavior; the second region shows an even greater dependence on layer thickness; and the third region exhibits a plateau or softening of hardness and strength. The second and third regions are studied using our discrete dislocation simulation method. This method includes the effects of stress due to lattice mismatch, misfit dislocation substructure, and applied stress on multilayer strength. To do so, we study the propagation of existing threading and interfacial dislocations as the applied stress is increased to the macroyield point. Our results show that in region 2, dislocation propagation is confined to individual layers initially. This “confined layer slip” builds up interfacial content and redistributes stress so that ultimately, the structure can no longer confine slip. The associated macroyield stress in this region depends strongly on layer thickness. In region 3, layers are so thin that confined layer slip is not possible and the macroyield stress reaches a plateau that is independent of layer thickness.

Nanoscale polycrystalline metals typically exhibit increasing hardness with decreasing grain size down to a critical value on the order of 5 to 30 nm. Below this, a plateau or decrease is often observed. Similar observations are made for nanoscale multilayer thin films. There, TEM observations and modeling suggest that the hardness peak may be associated with the inability of interfaces to contain dislocations within individual nanoscale layers. This manuscript pursues the same concept for nanoscale polycrystalline metals via an analytic study of dislocation nucleation and motion within a regular 2D hexagonal array of grains. The model predicts a hardness peak and loss of dislocation confinement in the 5 to 30 nm grain size regime, but only if the nature of dislocation interaction with grain boundaries changes in the nanoscale regime.

Magnesium single crystal samples are compressed at room temperature under quasistatic (∼0.001 s−1) loading in a universal testing machine and dynamic (430, 1000, and 1200 s−1) loading in a split Hopkinson pressure bar system. Stress-strain curves show that (a) the fracture strain slightly increases with the strain rate; and (b) the maximum strength and strain hardening rate increase significantly when the testing changes from quasistatic to dynamic, although they do not vary much when the strain rate for dynamic testing varies in the range of 430–1200 s−1. The operation of the secondary pyramidal slip system is the dominating deformation mechanism, which leads to a fracture surface with an angle of ∼42° with respect to the loading axial direction. A theoretical material model based on Johnson-Cook law is also derived. The model includes the strain hardening and strain rate hardening terms, and provides the stress-strain relations matching with the experimental results. Finite element simulations for the s...