Materials Science Forum Vol. 941

Abstract: Titanium aluminides based on the L1₀ ordered g-phase are promising structural light-weight materials for applications in aircraft engines. Typical compositions for γ-TiAl alloys lie in the range Ti-(44-48)Al (at.-%). For high creep resistance, a two-phase microstructure based on lamellar (α₂+γ)-colonies is desirable that may be tuned towards better ductility by introducing pure γ-grains (near lamellar or duplex microstructure).γ-TiAl alloys are often alloyed with niobium for increased oxidation resistance and improved mechanical properties. HEXRD and TEM studies of the alloy Ti-42Al-8.5Nb revealed that the orthorhombic O-phase forms during annealing at 500-650°C. This orthorhombic phase has been known in Nb-rich, Al-lean, α₂-based Ti-aluminides since the late 1980ies (Nb> 12.5 at.-%, Al< 31 at.-%) but the finding in γ-based alloys is new.TEM imaging showed that the O-phase is located within α₂ lamellae of lamellar (α₂+γ)-colonies. O-phase domains and α₂ phase form small columnar crystallites based in the α₂/γ interface. The columnar crystallites grow parallel to the [0001] direction of the α₂ phase and appear as facets when observed along this direction. The evolution of domains and facets with annealing time and the chemical homogeneity of the phases are investigated.The results of STEM imaging show that O-phase domains form during annealing at 550 °C for 8hours or 168 hours. After 168 hours of annealing Nb segregations are observed by EDX mapping within O-phase domains. In comparison, no segregation of niobium is detected after 8 hours of annealing.

Abstract: The effect of α₂ precipitation on the creep and tensile properties was investigated for bimodal and lamellar microstructures in two Ga-added near-α Ti alloys with Al equivalences of 10.6 and 11.5. Fine α₂ phase formed in the α phase of both alloys. The volume fraction of the α₂ phase for the Al equivalences of 10.6 and 11.5 is equivalent to 57.6 % and 73.3 %, respectively, in the binary Ti-Al system at 600 °C. Creep tests were carried out under a constant stress of 310 MPa at 600 °C and tensile tests were performed at room temperature. Lamellar microstructure showed lower minimum creep strain rates than bimodal microstructure for both alloys. The increase in Al equivalence increased creep life by a factor of 1.6 and decreased the minimum creep strain rate from 6.51 × 10^-8 s^-1 to 3.99 × 10^-8 s^-1 in bimodal microstructure. In addition, the increase in Al equivalence decreased room temperature tensile elongation although both alloys contained a similar volume fraction of equiaxed α in a bimodal microstructure.

Abstract: In order to analyze aging behavior of an Al-8.0Zn-1.8Mg-2.0Cu alloy, the microstructure of the alloy subjected to T6 and T76 states are investigated by transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM). Based on the precipitate observations, precipitate size distributions and average precipitate size are extracted from bright-field TEM images projected along 〈110〉_Al orientation with the aid of an imaging analysis. The results indicate that the main precipitates are GPI zone, GPII zone and η' phase in the T6 alloy while η' phase and η phase in the T76 alloy. The bright-field TEM observations reveal that the matrix precipitates for the T6 alloy have small size and dispersive distribution while that for the T76 alloy has big size and sparse distribution. Both have discontinuously distributed grain boundary precipitates. Quantitative structural information including precipitate size distribution and average precipitate size has been calculated by an image analysis based on the bright-field TEM images projected along 〈110〉_Al orientation. The results show that the T6 alloy has a narrower precipitate size range than the T76 alloy and thus the T6 alloy possesses a smaller average precipitate size than the T76 alloy.

Abstract: The hardness of alloys after phase coarsening is analyzed through the linkage between the multiparticle diffusion simulation and the finite element method simulation. Our analysis demonstrates that considerable degradation of the hardness of alloys occurs due to phase coarsening, which is related to the precipitate sizes and their distribution within the alloy. The microstructural evolution alters the manners of the nucleation, multiplication and movement of dislocations within the material, and further leads to different degradation of macroscopic hardness of alloys.

Abstract: Ni-based single crystal superalloy turbine blades have excellent mechanical strength and resistance to corrosion and oxidation due to a uniformly distributed gamma prime phase in a gamma matrix. However, defect grains have been often found on the surface of turbine blades after manufacturing, which can be potential sites of crack initiation. In this study, several different types of surface defect grains formed in third generation Ni-based single crystal turbine blades, such as stray grains, freckle chain grains, equiax grains, and a new grain formed in surface scale, had been investigated. The grain boundary regions were observed by high resolution electron microscopy. Although the formation mechanism of each grain defect is different, secondary phases, such as rhenium-rich particles, have been always found in each grain boundary. In addition, depending on the existence of the secondary phases as well as the size of defect grains, different microstructures were observed even in the same defect grain boundary. Finally, the observed results suggest that if there is any boundary region in a turbine blade, secondary phases, such as Re-rich particles, can be found.

Abstract: In this paper, we report microstructure and mechanical properties evolution of the CoCrFeNiMn-type high entropy alloy, containing small amounts of Al and C, during cold rolling and subsequent annealing at 700-1100°C. In the initial as-cast condition the alloy has coarse-grained single face-centered cubic (fcc) phase structure. Cold rolling and annealing substantially refine fcc grains; in addition M23C6 type carbides appear. After annealing at relatively low temperatures (≤900°C), these particles are arranged in characteristic arrays aligned with rolling directions. The specific microstructure of the thermomechanically processed alloy is suggested to be the reason of the balanced combination of tensile strength and ductility.

Abstract: National Institute for Fusion Science (NIFS) launched in 2014 a research program for developing Dispersion Strengthened (DS) Cu alloys for application to the heat sink materials of divertors of fusion reactors, using newly installed ball-milling, encapsulation, and Hot Isostatic Pressing (HIP) facilities. A unique feature of these facilities is that the entire process can be performed without exposing the materials to air, enabling precise impurity control. Cu-Al, Cu-Zr and Cu-Y alloys have been produced in this program. Various technological advancement has been made for the fabrication, such as suppression of powder adhesion to the wall of containers during the ball milling, and encapsulation technology including development of small volume tubular capsules.

Abstract: In order to meet the requirements of lightweight and replace steel with the aluminum for a component on the high speed rail, the forging process of a complex-shaped aluminum alloy component was researched and the parameters were optimized with the DEFORM-3D finite element simulation technology. The qualified products were finally obtained instead of the original steel castings by reducing weight of 65%. It is noted that the parts with complicated shape and non-symmetry, metal flow uneven during forging process that lead to incomplete forming, higher forging pressure problems. In this paper, such problems were analyzed couple with numerical simulation method based on a certain forming pressure. Moreover, the model and slot was reasonably designed. In addition, the size of blank was constantly optimized to change the metal flows direction and cavity filling mode. Finally, the forgings with good surface quality and mechanical properties were obtained by production test, and can be used as reference for this kind of forging components.

Abstract: In this study, a Mg-0.3at.%Y alloy was provided for a severe plastic deformation by high pressure torsion (HPT) and subsequent annealing. After the HPT by 5 rotations, nanocrystalline structures with a mean grain size of 0.23 μm having deformed characteristics were obtained. Fully recrystallized microstructures with mean grain sizes ranging from 0.66 μm to 32.7 μm were obtained by subsequent annealing at various temperatures. Room temperature tensile tests revealed that ultrafine grained (UFG; grain sizes smaller than 1 μm) specimen exhibited very high yield strength over 250 MPa but limited ductility. In contrast, good balance of strength and ductility was realized in fine grained specimens with grain sizes around 2~5 μm. Particularly, the yield strength and total tensile elongation of a specimen with a mean grain size of 2.13 μm were 184 MPa and 37.1%, respectively, which were much higher than those of pure Mg having a similar grain size. The significant effects of grain size and Y addition on the mechanical properties were discussed.

Abstract: Quench trials were performed on AA6005A and AA6016 alloys to assess the sensitivity of their tensile properties as well as bendability to quench after solution heat treatment. Results indicate that the tensile properties in T4 and in the paint-baked state (2% pre-strain + 185 °C/20 min) are hardly affected by quench rate as long as the exit temperature (T_exit) is sufficiently low. The bendability however, appears to be more sensitive to quench rate, and the sensitivity depends on the chemical composition of the alloy. The alloy with a higher excess Si content exhibits higher sensitivity to natural aging which in turn affects the bending and hemming performance of the material. Therefore, it is not only the quench rate which affects the bendability but also the temperature of the material at the end of quench. DSC analysis revealed how cluster formation proceeding the solution heat-treatment (SH) and quench provokes the quench sensitivity.