Georg Frommeyer

High strength Fe-Mn-(Al, Si) TRIP/TWIP steels development - properties - application

Abstract Deformation twinning, martensitic phase transformation and mechanical properties of austenitic Fe-(15–30) wt.%Mn steels with additions of aluminium and silicon have been investigated. It is known that additions of aluminium increase the stacking fault energy γfcc and therefore strongly suppress the γ→e transformation while silicon decrease γfcc and sustains the γ→e transformation. The γ→e phase transformation takes place in steels with γ fcc ⩽20 mJ m 2 . For steels with higher stacking fault energy twinning is the main deformation mechanism. Tensile tests were carried out at different strain rates and temperatures. The formation of twins, α- and e- martensite during plastic deformation was analysed by optical microscopy, X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The developed light weight high manganese TRIP (“transformation induced plasticity”) and TWIP (“twinning induced plasticity”) steels exhibit high flow stress (600–1100 MPa) and extremely large elongation (60–95%) even at extremely high strain rates of about 103 s−1. Recent trends in the automotive industry towards improved safety standards and a reduced weight as well as a more rational and cost effective manufacturing have led to great interest in these high strength and “super tough” steels.

Microstructures and properties of high melting point intermetallic Ti5Si3 and TiSi2 compounds

Physical and mechanical properties of high melting point Ti5Si3 and TiSi2 intermetallics with hexagonal D88 and orthorhombic C54 structure have been investigated. Youngs moduli of about 160 GPa (Ti5Si3) and 250 GPa (TiSi2) were recorded at room temperature. At 1000°C the elastic moduli are 143 and 215 GPa respectively. Flow stresses of about 1050 MPa for Ti5Si3 and 230 MPa for TiSi2 at 1000°C were measured. With increasing temperature an exponential decrease of the flow stresses occurs. The low density of 4.3 g cm−3 and the pronounced creep resistance are important for high temperature applications of this material.

Flow stress anomaly and order-disorder transitions in Fe3Al-based Fe–Al–Ti–X alloys with X=V,Cr,Nb, or Mo

Abstract The stress–strain behaviour as a function of temperature as well as the critical temperatures of the D0 3 ↔B2 and B2↔A2 structural transitions were studied on quaternary Fe 3 Al-based Fe-26 at.% Al–4 at.% Ti–X alloys containing 2 or 4 at.% of X=V, Cr, Nb, or Mo. The microstructures of the alloys were characterized by light optical microscopy, transmission electron microscopy (TEM), and electron-probe microanalysis (EPMA). The alloys containing V, Cr, or Mo form solid solutions, whereas additions of 2 and 4 at.% Nb result in the formation of Laves phase precipitates. A comparison of the temperatures of the maximum of the flow stress anomaly and the D0 3 ↔B2 transitions determined in compression tests and by differential thermal analysis (DTA), respectively, clearly reveals that there is no correlation between the stress anomaly and the degree of ordering. The effect of the different alloying elements on the order-disorder transition temperatures of the quaternary alloys is discussed and compared with that of respective ternary Fe 3 Al-X alloys without Ti.

Microstructure and mechanical properties of nickel based superalloy IN718 produced by rapid prototyping with electron beam melting (EBM)

Abstract A nickel alloy of a composition similar to that of the nickel based superalloy Inconel alloy 718 (IN718) was produced with the electron beam melting (EBM) process developed by Arcam AB. The microstructures of the as processed and heat treated material are similar to that of conventionally produced IN718, except that the EBM material showed some porosity and the δ phase did not dissolve during the solution heat treatment because the temperature of 1000°C apparently was too low. Mechanical testing of the layer structured material, parallel and perpendicular to the built layers, revealed sufficient strength in both directions. However, it showed only limited elongation when tested perpendicular to the built layers due to local agglomerations of pores. Otherwise, data for the hardness, Young’s modulus, 0·2% yield tensile strength and ultimate tensile strength match those recommended for IN718.

Mobile dislocations at the α2/γ phase boundaries in intermetallic TiAl/Ti3Al-alloys

Abstract Ti 3 Al/TiAl interfaces of four titaniumaluminium alloys with and without chromium additions were examined in detail by tilting experiments using conventional transmission electron microscopy (TEM). Careful adjustment of weak beam conditions showed that the TiAl- as well as the Ti 3 Al- phase contain interfacial dislocations which accommodate the lattice misfit between both phases. In the most ductile TiAl47Cr1Si0.2 alloy only one set of 〈10〉 interfacial dislocations in the TiAl phase was found. Most of them possess screw character and will leave the interface during plastic deformation. Consequently, this alloy exhibits remarkable ductility at room temperature.

Oxidation behaviour of a Ti–46Al–1Mo–0.2Si alloy: the effect of Mo addition and alloy microstructure

Abstract The influence of molybdenum addition and alloy microstructure on the oxidation behaviour of a Ti–46.8Al–1Mo–0.2Si alloy was studied in air at temperatures ranging from 600 to 900°C. The alloy produced by arc melting exhibited a structure of coarse lamellar grains in as-cast condition that transformed to a duplex microstructure after hot extrusion at 1300°C. Oxidation rate and scale spallation resistance were not affected by the type of microstructure. The effect of molybdenum addition was related to the formation and thickening of a protective continuous nitride layer. The low mass gain measured can be associated with the low oxidation rate of the nitride layer. Finally, the poor spalling resistance of the oxide scale at 900°C was attributed to the mismatch in thermal expansion coefficients of the oxide scale and the nitride layer.

Intermetallic TiAl(Cr,Mo,Si) Alloys for Lightweight Engine Parts

The microstructure of this new class of lightweight titanium aluminide alloys is controlled by the processing parameters for manufacturing semifinished products or components, and by minor modifications of the alloy composition. These materials exhibit excellent physical and mechanical properties for the use as components in automotive and jet engines, especially in rotating and oscillating parts.

Creep behavior of intermetallic FeAl and FeAlCr alloys

Abstract A study of creep behavior of several alloys based in the Fe Al and Fe Al Cr systems ranging in aluminium from 21.7 to 48 at.% was undertaken. The alloys were produced by induction melting and possess a coarse and equiaxial microstructure, with a grain size of about 500 μm. Compression tests at rates from 10 −5 to 10 −2 s −1 were conducted at temperatures ranging from 700 to 1000°C under a protective atmosphere of argon to minimize oxidation. Analysis of the stress-strain data revealed in the binary alloys a stress component of about 3 that suggest that creep is controlled by viscous glide of dislocations. For an aluminium content above 30 at.%, the activation energy for creep does not vary very much with the aluminium concentration and values ranging from 360 to 395 kJ mol −1 were obtained. On the other hand, the ternary alloys present an improvement in strength in the high temperature compressive creep. A stress exponent of 4–5 is observed in this case that suggest that creep is controlled by dislocation climb. An activation energy for creep of about 505 kJ mol −1 was deduced for the alloy containing 30 at.% Al and 10 at.% Cr.

Creep mechanisms in particle strengthened α-Titanium–Ti2Co alloys

Abstract Quasi-eutectoid Ti–12Co–5Al and Ti–10Co–4Al alloys consist of an α-titanium solid solution matrix, a fine dispersion of intermetallic Ti2Co particles—topologically closed packed CF96 structure, prototype: Ti2Ni—in large volume fractions of about 30 and 22% and Ti3Al (α2) precipitates. These alloys exhibit high flow stress of ≈900 MPa, high elastic stiffness of ≈130 GPa and sufficient fracture toughness KIC equal to 21–35 MPa m−0.5 at room temperature. Because of the effective particle reinforcement, these alloys show good creep properties up to 600°C. The microstructure of the α-Ti/Ti2Co alloys is thermally stable and not sensitive to surface embrittlement due to α-case formation by oxygen penetration in contrast to high temperature resistant near α-titanium alloys. The creep properties of both alloys have been investigated in the temperature range of 500–700°C and in the strain rate range of 10−7–10−4 s−1. High threshold stresses and stress exponents n=8–10 and an activation energy for creep of ≈300 kJ mol−1 at temperatures up to 550°C are attributed to the strong dispersion strengthening effect caused by the high volume fraction of intermetallics. At 600 and 650°C, creep is controlled by dislocation climb (solid solution class II behavior) in the α-titanium matrix and the intermetallic Ti2Co phase (n=4–5). The activation energy of 305–410 kJ mol−1 is in between the values for self diffusion of titanium in the α-Ti(Al) solid solution and for chemical diffusion of Ti and Co in Ti2Co.

Microstructural characterization of an ultrahigh carbon and boron tool steel processed by different routes

Abstract A newly developed ultra high carbon and boron tool steel containing 0.8wt.%B-1.3wt.%C-1.6wt.%Cr was processed to obtain a fine grain microstructure following two routes. In the first route, several thermomechanical treatments of the as-cast material, including extensive warm rolling, were used to refine the microstructure. A microstructure consisting of large and small borocarbides in a ferritic matrix with a grain size of about 2 μm was obtained. In the second route, powder metallurgy techniques, including consolidation of rapidly solidified powders by extrusion and hot isostatic pressing (HIP), were used. The powders were produced by argon atomization and exhibit a dendritic microstructure, which remains unchanged after consolidation by HIP at temperatures up to 900 °C. The microstructure after consolidation by extrusion at 1050 °C is coarser and consists of spherical borocarbide particles, 2 μm in size, in a fine-grained ferritic matrix. In addition, the shear forces developed during the extrusion process improve the bonding between the powder particles. In comparison to the microstructures obtained by thermomechanical processing, the powder metallurgy material possesses a better homogeneity in the size and shape of borocarbide particles and a finer microstructure.