Materials Science Forum Vol. 879

Abstract: Third-Generation advanced high strength steels are being developed with the goal of reducing the body-in-white weight while simultaneously increasing passenger safety. This requires not only the expected increase in strength and elongation, but also improved local formability. Optimizing elongation and formability were often contradictory goals in dual-phase steel developments. Recent results have shown that so-called "quench and partitioning" (Q&P) concepts can satisfy both requirements [1]. Many Q&P-concepts have been studied at thyssenkrupp Steel Europe. Thorough investigation of the microstructure has revealed relationships between features such as the amount, morphology and chemical stability of the retained austenite and the obtained mechanical properties. An evaluation of the lattice strain by means of electron-back-scattering-diffraction has also yielded a correlation to the obtained formability. The aim of this work is to present the interconnection between these microstructural features and propose hypotheses for the explanation of how these features influence the macroscopically observed properties.

Abstract: With the introduction of TiAl in aircraft jet engines, there is an increasing demand for the evaluation of novel processing routes for gamma titanium aluminides such as additive manufacturing (AM). A Ti-47Al-2Cr-2Nb powder material has been used as feedstock for laser fabrication of 3D samples by means of “Selective Laser Melting” (SLM) and “Direct Metal Deposition” (DMD). A number of processing parameters including laser power, laser scan rate, powder feed rate, have been varied to evaluate their effects on the material soundness. Optimised conditions can significantly reduce the crack sensitivity for this relatively low ductility material. In particular, crack-free experimental conditions have been identified by using additional heating strategies, thus limiting built-up residual stresses during fast cooling. The different samples have been examined using optical and scanning electron microscopy in the as-built condition. The non-equilibrium cooling conditions generate ultra-fine and metastable structures exhibiting high microhardness values. A range of post-heat treatments have been performed to relieve the residual stresses and to tailor more uniform microstructures. Conventional heat treatments in the α+γ two-phase domain or in the α single phase domain have been successfully used to fully restore homogeneous microstructures either duplex or fully lamellar. A comparison is made with microstructures of both laser treated materials and of conventionally processed materials.

Abstract: The cooling temperature field, microstructure and properties of an Al-12.7Si-0.7Mg alloy extrusion after an end-quenching experiment at 540 ^oC were investigated by means of an end-quenching test, hardness test, electrical conductivity determination and TEM observations. The results showed that the cooling rate of the alloy extrusion decreases sharply as the distance from the end-quenching spray point increased, when the distance from the end-quenching spray point is within 70 mm. Additionally, the cooling rate of the alloy extrusion decreased slowly along the length of the alloy extrusion when the distance from the end-quenching spray point exceeded 90 mm. The cooling rate of the alloy extrusion at distances of 20 mm and 32.5 mm from the end-quenching spray point exceeded 34 ^oC/s and 24 ^oC/s, respectively. Both hardness and electrical conductivity of the alloy extrusion within 32.5 mm from the end-quenching spray point were comparable to those of the alloy extrusion of off-line quenching at over 520 °C. There was no obvious precipitate in the alloy extrusion matrix 20 mm from the end-quenching spray point, and there were few fine and heterogeneous short rod-like precipitates in the alloy extrusion matrix 32.5 mm from the end-quenching spray point. However, there were many precipitates with lengths of approximately 10 nm in the alloy extrusion matrix 45 mm from the end-quenching spray point, and the lengths of the precipitates increased to approximately 20 nm when the distance from the end-quenching spray point exceeded 57.7 mm. The critical cooling rate of the 4-mm thick Al-12.7Si-0.7Mg alloy extrusion should exceed 24 ^oC /s.

Abstract: By means of creep properties measurement and microstructure observations, the effect of microstructure evolution on creep strength of the single crystal Ni-based alloy with different orientations is investigated. Results show that, during creep at 1040 oC/137 MPa, the γ′ phase in [001]-oriented alloy is transformed into mesh-like N-type rafted structure, the one in [011]-oriented alloy is transformed into the stripe-like rafted structure along [001] direction, the one in [111]-oriented alloy is transformed into the mesh-like rafted structure along (010) plane. The γ′ phase with different configurations results in the different creep resistance of alloys, the sequence of creep resistance of alloys with different orientations are determined to be σ_[001] > σ_[111] > σ_[011]. But after pre-compressed treated along [100] direction of [011] oriented alloy, the γ′ phase in [011] oriented alloy is transformed into the stripe-like rafted structure along [100] direction, which makes the creep life of the alloy enhancing from 11 h to 174 h at 850 oC/400 MPa, and from 74 h to 165 h at 1040 oC/137 MPa. This is attributed to the pre-treatment eliminating the gable channels to form the “labyrinth” structure of γ′ phase, which may increase the resistance of dislocation motion.

Abstract: The formation of nanocrystalline structures and mechanical properties were studied in a nitrogen-bearing 304-type stainless steel subjected to severe plastic deformation (SPD). The steel samples were processed at ambient temperature using three different methods, i.e., caliber rolling, multidirectional forging and high pressure torsion. All these techniques resulted in pronounced grain refinement. The microstructures consisting of austenite/ferrite crystallites with transverse dimensions of 50 and 30 nm evolved in the rolled and forged samples, respectively. The austenite fractions comprised approximately 0.4. In contrast, the microstructure consisted mainly of austenite with an average grain size of about 25 nm evolved after high pressure torsion. All samples of the stainless steel subjected to severe plastic deformation demonstrated significant strengthening. The ultimate tensile strengths of 2065 MPa and 1950 MPa, were obtained after rolling and high pressure torsion, respectively. The ultimate tensile strength of samples subjected to multidirectional forging was 1540 MPa.

Abstract: The microstructure and creep properties of a P911-type steel normalized at 1060°C and then subjected to one-step tempering at 760°C for 3 h or two-step tempering at 300°C for 3 h + 760°C for 3 h were examined. The transmission electron microscope (TEM) observations showed that the tempered martensite lath structure (TMLS) with a lath thickness of 340 nm evolved after both tempering regimes. High dislocation densities of 3×10¹⁴ or 5×10¹⁴ m^-2 retained after one-and two-step tempering respectively. M₂₃C₆ carbides with a mean size of 120 nm and V-rich MX carbonitrides having a “wing” shape with an average length of about 40 nm precipitated on high-and low-angle boundaries and within ferritic matrix, respectively. A number of Nb-rich M(C,N) carbonitrides with a mean size of 20 nm precipitated on dislocations during low temperature tempering. The creep tests were carried out under constant load condition at 650°С at applied stresses of 100 and 118 MPa. Analysis of creep rate versus time curves showed that the use of two-step tempering decreases the minimum creep rate providing an increase in the creep strength in long-term conditions.

Abstract: The athermal ω phase transformation, magnetic susceptibility and deformation behavior of Zr-xNb alloys (x = 10 and 14) for use in medical devices subjected to magnetic resonance imaging (MRI) were investigated using electrical resistivity measurements, transmission electron microscopy observations and compression tests. The alloy with x = 10 exhibited a positive temperature coefficient in the electrical resistivity curve and the presence of an athermal ω phase at room temperature. On the other hand, the alloy with x = 14 showed an anomalous negative temperature coefficient (NTC) in the resistivity curve. Similar NTCs also appear in β-Ti alloys, which is interpreted as the growth of an athermal ω phase and the appearance of lattice modulation. The ω phase and diffuse satellites, which are possibly related to lattice modulation, were confirmed in the Zr-14Nb alloy at room temperature. The volume fraction of the athermal ω phase and the appearance of lattice modulation are related to the operating deformation mode and Young’s modulus. Thus, controlling the ω phase transformation in Zr-Nb alloys is key to developing medical devices that can be used in MRI.

Abstract: The effect of Ni or Zr addition to Ti-Cu alloy was studied on the microstructure evolution during mechanical milling regarding to dependence of the amorphous transformation on the various composition elements. The microstructure of initial crystalline alloys and the remained phases after few hours of milling were investigated. The milling process lasted to the full amorphization of the powders. The results show that amorphous Ti₄₈Cu₄₂Ni₁₀ and Ti₄₈Cu₄₂Zr₁₀ powders are obtained after 13 h and 14 h of milling.

Abstract: Since 2004, Mitsubishi has been pursuing a 1,700°C gas turbine as part of the Japanese National Project [1][2]. One of the most important key technologies for the target is thermal barrier coatings (TBCs) which are capable of improving cooling efficiency of hot parts. With increasing the turbine inlet temperature, TBCs surface temperature is also rising up. In addition, the temperature gradient through TBCs thickness must steepen as a result of keeping metal temperature. Both have a significant effect on durability such as spallation and erosion of TBCs. To evaluate these issues, thermal cycle test and hot erosion test were introduced. After the screening of those component tests, the advanced TBCs coated in the first unit of M501J were verified at our pilot plant called T-point. Sound condition for row 1 blades and vanes had been confirmed after over 3 year operation.

Abstract: Already, we developed a high porosity alumina foam. However, alumina has high thermal conductivity about 36W/mK at room temperature, and it need to achieve to high porosity to decrease thermal conductivity to for application of refractory bricks. Therefore, high porosity mullite refractory brick is developed using GS (Gelation of Slurry) method that is already developed for production of high porosity metal foam. Appling this method to production of mullite foams, the ceramics foams from 93 to 97% porosity can be produced. Also, their thermal conductivities are proportional to densities and obey to Ashby-Glicksman model. Its thermal conductivity is about 0.07W/mK when density is 0.1 g/cm3. The high porosity mullite foams achieved enough thermal insulating properties for refractory brick.