Jun Sun

Nanostructured high-strength molybdenum alloys with unprecedented tensile ductility

The high-temperature stability and mechanical properties of refractory molybdenum alloys are highly desirable for a wide range of critical applications. However, a long-standing problem for these alloys is that they suffer from low ductility and limited formability. Here we report a nanostructuring strategy that achieves Mo alloys with yield strength over 800 MPa and tensile elongation as large as ~ 40% at room temperature. The processing route involves a molecular-level liquid-liquid mixing/doping technique that leads to an optimal microstructure of submicrometre grains with nanometric oxide particles uniformly distributed in the grain interior. Our approach can be readily adapted to large-scale industrial production of ductile Mo alloys that can be extensively processed and shaped at low temperatures. The architecture engineered into such multicomponent alloys offers a general pathway for manufacturing dispersion-strengthened materials with both high strength and ductility.

Atomic structure and structural stability of Sc75Fe25 nanoglasses.

Nanoglasses are solids consisting of nanometer-sized glassy regions connected by interfaces having a reduced density. We studied the structure of Sc(75)Fe(25) nanoglasses by electron microscopy, positron annihilation spectroscopy, and small-/wide-angle X-ray scattering. The positron annihilation spectroscopy measurements showed that the as-prepared nanoglasses consisted of 65 vol% glassy and 35 vol% interfacial regions. By applying temperature annealing to the nanoglasses and measuring in situ small-angle X-ray scattering, we observed that the width of the interfacial regions increased exponentially as a function of the annealing temperature. A quantitative fit to the small-angle X-ray scattering data using a Debye-Bueche random phase model gave a correlation length that is related to the sizes of the interfacial regions in the nanoglass. The correlation length was found to increase exponentially from 1.3 to 1.7 nm when the sample temperature was increased from 25 to 230 °C. Using simple approximations, we correlate this to an increase in the width of interfacial regions from 0.8 to 1.2 nm, while the volume fraction of interfacial regions increased from 31 to 44%. Using micro-compression measurements, we investigated the deformation behavior of ribbon glass and the corresponding nanoglass. While the nanoglass exhibited a remarkable plasticity even in the annealed state owing to the glass-glass interfaces, the corresponding ribbon glass was brittle. As this difference seems not limited to Sc(75)Fe(25) glasses, the reported result suggest that nanoglasses open the way to glasses with high ductility resulting from the nanometer sized microstructure.

In situ study of the initiation of hydrogen bubbles at the aluminium metal/oxide interface

The presence of excess hydrogen at the interface between a metal substrate and a protective oxide can cause blistering and spallation of the scale. However, it remains unclear how nanoscale bubbles manage to reach the critical size in the first place. Here, we perform in situ environmental transmission electron microscopy experiments of the aluminium metal/oxide interface under hydrogen exposure. It is found that once the interface is weakened by hydrogen segregation, surface diffusion of Al atoms initiates the formation of faceted cavities on the metal side, driven by Wulff reconstruction. The morphology and growth rate of these cavities are highly sensitive to the crystallographic orientation of the aluminium substrate. Once the cavities grow to a critical size, the internal gas pressure can become great enough to blister the oxide layer. Our findings have implications for understanding hydrogen damage of interfaces.

Formation of Zr-based bulk metallic glasses from low purity materials by scandium addition

Zr55Al10Cu30Ni5 bulk metallic glass (BMG) is formed by using low purity sponge zirconium, instead of high purity zirconium, and other high purity raw materials with a small amount of scandium addition. The results show that glass forming ability and thermal stability of the Zr55Al10Cu30Ni5 alloy are improved with scandium addition. Compressive fracture strength is similar to that of BMG alloy produced with high purity raw materials. However, plasticity of BMGs with sponge zirconium and scandium addition deteriorates.

Evaluation of interfacial crack growth in bimaterial metallic joints loaded by symmetric three-point bending

Abstract Bimetallic three-point bend specimens (TPB) were produced from explosive clad alloy aluminium LY12/pure aluminium Al by electron beam welding of homogeneous Al and homogeneous LY12 to the respective surfaces. Cracks were introduced at the interface in the specimens by milling and spark-erosion. Similar bimetallic specimens of explosive clad LY12/steel were also prepared. Finite element analyses were undertaken to evaluate theJ-integral and the deformation ahead of the bimetallic interfacial crack tip field of the TPB specimens loaded normally. The calculations showed that, even for symmetric TPB loading of the specimens, a sharpening–blunting deformation existed on the profile of the interfacial crack tip for bimetals with a mismatch in mechanical properties. J-resistance curves for two kinds of bimetal interface were obtained from symmetric TPB tests. The results demonstrated that the resistance to crack propagation along the bimetallic interfaces was always lower than those of the respective counterpart metals.

Fatigue crack propagation normal to a plasticity mismatched bimaterial interface

Plasticity mismatched bimaterial specimens were made by explosive cladding with stainless steel and mild steel. Fatigue crack propagation normal to a bimetallic interface was experimentally investigated under a four-point bending condition. The results showed that material plasticity mismatch either promoted the crack growth as crack advanced to the interface from higher strength material side or retarded it as crack grew to the interface from lower strength material side. However, the cracks are always subjected to the interface shielding because of the interaction of the interface with the crack tip plastic zone for both cases above. For the present four-point bending specimens, the cracks can penetrate the next layer as the crack approaches the interface from either the higher or the lower yield strength side as long as the load levels are high enough. The effects of specimen geometry and dimensions on the fatigue crack deflection near to the interface are also discussed.

Effect of thermomechanical processing on anisotropy of cleavage fracture stress in microalloyed linepipe steel

Abstract The anisotropy of cleavage fracture stress and its relationship to thermomechanical processing (TMP) and microstructure has been determined by extensive study on a Nb, Ti microalloyed line pipe steel. 127xa0mm-thick pieces cut from a commercially cast slab were reduced to a 25xa0mm-thick plate by three different rolling schedules. The final microstructures of the rolled plate were various combinations of polygonal ferrite, bainite and deformed ferrite, having a range of grain sizes. Four-point bend tests (L–T, L–S, T–L orientations), compact tension tests (S–L orientation) and tensile tests (L, T, S orientations) were carried out at −196°C for each rolling schedule. An elastic–plastic finite element method was used to calculate the stress and strain distributions ahead of the notches for the various orientations. By measuring the distance of the cleavage fracture initiation site from the root of the notch, the cleavage fracture stress was determined for each orientation. The results show a significant variation in cleavage fracture stress with orientation and TMP treatment. The anisotropy of cleavage fracture stress increases with decreasing finish-rolling temperature (FRT), the S–L orientation generally giving the minimum value. The TMP treatment which produces unrecrystallized austenite at the FRT has the best combination of high cleavage fracture stress and low anisotropy.

Temperature-induced ductile-to-brittle transition of bulk metallic glasses

Uniaxial tensile and uniaxial compressive tests for Zr-based bulk metallic glasses (BMGs) were conducted at room and cryogenic temperatures, respectively. It was observed that both the change of macroscopic fracture mode from ductile shear fracture to brittle normal tensile fracture and microscopic fracture feature from micron-scaled vein patterns to nano-scaled dimples with decreasing test temperatures were identified, indicating a significant ductile-to-brittle transition (DBT) behavior. The mechanism of DBT behavior was revealed by the competition between the intrinsic critical shear strength τ0 and critical tensile strength σ0 at different temperatures.

Cyclic deformation leads to defect healing and strengthening of small-volume metal crystals

Significance Producing strong and defect-free materials is an important objective in developing many new materials. Thermal treatments aimed at defect elimination often lead to undesirable levels of strength and other properties. Although monotonic loading can reduce or even eliminate dislocations in submicroscale single crystals, such “mechanical healing” causes severe plastic deformation and significant shape changes. Inspired by observing an easier pullout of a partly buried object after shaking it first, we demonstrate that “cyclic healing” of the small-volume single crystals can indeed be achieved through repeated low-amplitude straining. The cyclic healing method points to versatile avenues for tailoring the defect structure and strengthening of nanoscale metal crystals without the need for thermal annealing or severe plastic deformation. When microscopic and macroscopic specimens of metals are subjected to cyclic loading, the creation, interaction, and accumulation of defects lead to damage, cracking, and failure. Here we demonstrate that when aluminum single crystals of submicrometer dimensions are subjected to low-amplitude cyclic deformation at room temperature, the density of preexisting dislocation lines and loops can be dramatically reduced with virtually no change of the overall sample geometry and essentially no permanent plastic strain. This “cyclic healing” of the metal crystal leads to significant strengthening through dramatic reductions in dislocation density, in distinct contrast to conventional cyclic strain hardening mechanisms arising from increases in dislocation density and interactions among defects in microcrystalline and macrocrystalline metals and alloys. Our real-time, in situ transmission electron microscopy observations of tensile tests reveal that pinned dislocation lines undergo shakedown during cyclic straining, with the extent of dislocation unpinning dependent on the amplitude, sequence, and number of strain cycles. Those unpinned mobile dislocations moving close enough to the free surface of the thin specimens as a result of such repeated straining are then further attracted to the surface by image forces that facilitate their egress from the crystal. These results point to a versatile pathway for controlled mechanical annealing and defect engineering in submicrometer-sized metal crystals, thereby obviating the need for thermal annealing or significant plastic deformation that could cause change in shape and/or dimensions of the specimen.

Crack propagation resistance along strength mismatched bimetallic interface

Bimetallic three-point-bending specimens and four-point-bending fatigue test specimens were produced from strength mismatched stainless steel/low carbon steel bi-material. Both the J resistance curves and fatigue crack growth behavior were investigated for the bi- and bulk materials. The results showed that a crack initiated easily at the interface, and crack growth resistance along interface was inferior to that of the corresponding bulk materials under either static or dynamic loading conditions.