G. Lütjering

Property optimization through microstructural control in titanium and aluminum alloys

Abstract In this paper the microstructure/property relationships of Ti-alloys (α+β and β alloys) and high strength, age-hardened Al-alloys are discussed. Common features in these microstructure/property relationships can be found for α+β Ti-alloys and Al-alloys in the underaged condition as well as for β Ti-alloys and Al-alloys in the overaged condition. Basic differences are the presence of dispersoids and large Fe and Si containing inclusions in Al-alloys and the alloying element partitioning effect in bi-modal structures of α+β Ti-alloys.

Influence of grain size and heat treatment on the microstructure and mechanical properties of the nickel-base superalloy U 720 LI

Abstract The purpose of the current investigation was to develop an optimized balance between creep resistance and LCF-strength of U 720 LI, a high-strength nickel-base superalloy. While a fine grain size is beneficial for improved LCF-strength, it is detrimental to high-temperature creep resistance. Therefore, microstructures with medium grain sizes in the range of 50–150 μm were produced by appropriate recrystallization treatments. The coarse grain size showed improved creep resistance and lower yield stress as well as a tendency for lower LCF-strength. By increasing the amount of fine γ′-precipitates by modified aging treatments, it was possible to increase the yield stress considerably with a concomitant increase in creep strength.

Fatigue properties of Al-Mg alloys with and without scandium

Abstract The fatigue properties, e.g. HCF strength, crack growth of small surface and long-through cracks, of two wrought Al-5Mg-0.3Mn-0.06Zr alloys without and with scandium (0.25%) were investigated in vacuum, air and salt water. The alloy with scandium showed a higher tensile yield stress, higher fatigue strength values and a higher resistance against microcrack growth, due to the presence of coherent Al 3 (Sc, Zr) precipitates and a very fine subgrain structure. The scandium free alloy exhibited a slightly higher resistance against macrocrack growth, due to the coarser grain structure.

Processing, microstructure, and properties of β-CEZ

Abstract For the β-CEZ material, some of the important processing parameters to establish a necklace type of microstructure by through-transus deformation were evaluated with respect to optimizing fracture toughness and yield stress level. In addition, bi-modal microstructures which can be produced by conventional α + β processing were evaluated. For the necklace microstructure a high fracture toughness value of 68 MPa m1/2 at a yield stress level of 1200 MPa was reached in forgings, but the fracture toughness was anisotropic dropping to 37 MPa m1/2 in the short transverse direction. For the bi-modal microstructure an isotropic fracture toughness value of 37 MPa m1/2 at the same yield stress level of 1200 MPa was reached without major difficulties.

Effect of Periodic Overloads on Fatigue Crack Propagation in Aluminium Alloys

The influence of variable amplitude loading on fatigue crack propagation was investigated for two high purity versions of the alloy Al 2024 in sheet form, one with fine equiaxed grains, and the other with coarse elongated grains. Fatigue tests on center cracked specimens were conducted in vacuum at constant amplitude (R-ratio of 0.1) and with periodically applied single tensile overloads with an overload ratio of 1.5. The number of intermittent baseline cycles between consecutive overloads was varied (n=100 and n=10.000). Detailed fractographic investigations were carried out for the identification of changes in the fracture surfaces due to the overloads. Crack closure measurements were performed in all cases. The results revealed a strong influence of the overloads on the crack propagation rate. Whether overloads are retarding or accelerating the fatigue crack propagation depends on the crack propagation mechanism at constant amplitude loading and the number of intermittent baseline cycles. For n=100 retardation occurred for the fine grained alloy exhibiting homogeneous slip at constant amplitude while acceleration was observed for the alloy with coarse elongated grains showing pronounced slip band fracture at constant amplitude. For n=10.000, the formation of steps parallel to the direction of crack propagation by overloads is assumed to be the reason for the observed increase in fatigue crack propagation resistance resulting in retardation for both alloys compared to constant amplitude and n=100. The influence of crack closure on the overload effects was minor. This was verified by additional tests at R=0.5.

The microstructure and mechanical properties of the intermetallic compound Super Alpha 2

Abstract This paper focuses on the modification of the microstructure of the Ti3Al-based intermetallic compound Super Alpha 2. Owing to the high content of β-stabilizing elements, this alloy has a two-phase microstructure of α2 and β2, offering the possibility to modify the geometrical arrangements of the two phases in a wide range of appropriate thermomechanical treatments. Three basically different microstructures were investigated: lamellar, equiaxed and bimodal. The influence of these microstructures on the relevant mechanical properties (yield strength and ductility at ambient temperature and 600°C, high cycle fatigue and creep at 600°C) was investigated. It was found, that for the lamellar and equiaxed structures the mechanical properties can be improved by the precipitation of fine α2 plates in the β phase. For the bimodal structure, the yield stress can be improved by increasing the cooling rate from the recrystallization temperature without noticeable loss of ductility and creep resistance. The best combination of properties was found for a bimodal structure with an intermediate cooling rate. The room temperature ductility of this condition (5% tensile elongation) is considered sufficient for technical applications.