Iron‐Rich Iron‐Titanium‐Silicon Alloys with Strengthening Intermetallic Laves Phase Precipitates

Two‐phase ternary Fe‐Ti‐Si alloys with Si contents from 2 to 16 at.% and Ti contents from 2 to 28 at.% were studied with respect to room temperature hardness, fracture strain and yield stress at room and higher temperatures up to 1150 °C. In addition oxidation was checked at temperatures between 400 and 1150 °C. The alloys are strengthened by precipitation of the stable Laves phase (Fe,Si)₂Ti which is a hard and brittle intermetallic phase. The yield stress as well as the brittle‐to‐ductile transition temperature (BDTT) increase with increasing Ti content. Yield stresses up to about 1400 MPa and BDTT between 100 °C and 600 °C with fracture strains of the order of 1 % below BDTT were achieved. The observed short‐term oxidation performance at temperatures up to 1150 °C compares favourably with that of Fe‐AI alloys with high Al contents.     

Microstructure and Deformation Behaviour of Iron‐Rich Iron‐Aluminium Alloys with Ternary Carbon and Silicon Additions

The microstructure, hardness, yield stress, fracture strain, and brittle‐to‐ductile transition temperature of Fe‐Al alloys with Al contents of 12‐18 at.% Al, which are in the range of the so‐called K‐state with possible short‐range ordering reactions, and with ternary additions of carbon and silicon were studied with respect to the effects of possible impurities on the hardening of Fe‐Al alloys. It was found that perovskite‐type Fe₃AlC carbide particles precipitate even in alloys with low C and Si contents; they are controlled by prior heat treatments and strongly affect the deformation behaviour.     

Deformation Behaviour of Iron‐Rich Iron‐Aluminium Alloys with Ternary Transition Metal Additions

The hardness and yield stress at room temperature and the brittle‐to‐ductile transition temperature of Fe‐Al alloys with 16 at.% Al, which is in the range of the so‐called K‐state with possible short‐range ordering reactions, and ternary additions of 0.5 and 4 at.% of the transition metals Cr, Mo, Mn, V, Ti and Ni were studied with respect to possible hardening effects of the ternary additions. The addition of Cr, Mo and Mn to the Fe‐Al alloys produce solid‐solution hardening which corresponds to the hardening effect of Al. Only Ti, V and Ni produce extra hardening effects which cannot be related to solid‐solution hardening. This extra hardening is attributed to possible fine NiAl precipitates in the Fe‐Al‐Ni case and to possible enhanced short‐range ordering and/or fine carbide precipitates in the cases of Fe‐Al‐V and Fe‐Al‐Ti.     

Mechanical properties of new stainless steels based on the system Fe‐30Mn‐5A1‐XCr‐0.5C

New stainless steels based on the system Fe‐30Mn‐5AI‐XCr‐0.5C (Cr mass contents of ≤ 9 %) were developed and evaluated as a replacement of conventional AISI 304 steel. The alloys were produced by induction melting and thermomechanically processed to obtain a fine equiaxed microstructure. A typical thermomechanical processing for AISI 300 austenitic stainless steels was used and included forging at 1200°C, rolling at 850 °C and final recrystallization at 1050 °C. A final fully austenitic microstructure with grains of about 150 μm in size was obtained in all the steels. Tensile tests at temperatures ranging from ‐196 to 400 °C showed similar results for the various alloys tested. In accordance with the values for the elongation to fracture, this temperature range was subdivided into three regions. In the temperature range of ‐196 °C to room temperature, elongation to fracture increases with decreasing temperature. At temperatures ranging from 100 to 300 °C, elongation to fracture increases with testing temperature and serrations on the stress‐strain curve were observed. Finally, higher testing temperatures were accompanied by a decrease in ductility. Examination of the microstructures after deformation led to the conclusion that mechanical twinning was the dominant mechanism of deformation at the tested temperatures.     

Effect of Ti Addition and Microstructural Evolution on Toughening and Strengthening Behavior of as Cast or Annealed Nb–Si–Mo Based Hypoeutectic and Hypereutectic Alloys

Room temperature fracture toughness along with compressive deformation behavior at both room and high temperatures (900 °C, 1000 °C and 1100 °C) has been evaluated for ternary or quaternary hypoeutectic (Nb–12Si–5Mo and Nb–12Si–5Mo–20Ti) and hypereutectic (Nb–19Si–5Mo and Nb–19Si–5Mo–20Ti) Nb-silicide based intermetallic alloys to examine the effects of composition, microstructure, and annealing (100 hours at 1500 °C). On Ti-addition and annealing, the fracture toughness has increased by up to ~ 75 and ~ 63 pct, respectively with ~ 14 MPa√m being recorded for the annealed Nb–12Si–5Mo–20Ti alloy. Toughening is ascribed to formation of non-lamellar eutectic with coarse Nb_ss, which contributes to crack path tortuosity by bridging, arrest, branching and deflection of cracks. The room temperature compressive strengths are found as ~ 2200 to 2400 MPa for as-cast alloys, and ~ 1700 to 2000 MPa after annealing with the strength reduction being higher for the hypoeutectic compositions due to larger Nb_ss content. Further, the compressive ductility has varied from 5.7 to 6.5 pct. The fracture surfaces obtained from room temperature compression tests have revealed evidence of brittle failure with cleavage facets and river patterns in Nb_ss along with its decohesion at non-lamellar eutectic. The compressive yield stress decreases with increase in test temperature, with the hypoeutectic alloys exhibiting higher strength retention indicating the predominant role of solid solution strengthening of Nb_ss. The flow curves obtained from high temperature compression tests show initial work hardening, followed by a steady state regime indicating dynamic recovery involving the formation of low angle grain boundaries in the Nb_ss, as confirmed by electron backscattered diffraction of the annealed Nb–12Si–5Mo alloy compression tested at 1100 °C.

     

On Phase Equilibria in Duplex Stainless Steels

The equilibrium conditions of four duplex stainless steels; Fe‐23Cr‐4.5Ni‐0.1N, Fe‐22Cr‐5.5Ni‐3Mo‐0.17N, Fe‐25Cr‐7Ni‐4Mo‐0.27N and Fe‐25Cr‐7Ni‐4Mo‐1W‐1.5Cu‐0.27N were studied in the temperature region from 700 to 1000 °C. Phase compositions were determined with SEM EDS and the phase fractions using image analysis on backscattered SEM images. The results showed that below 1000 °C the steels develop an inverse duplex structure with austenite and sigma phase, of which the former is the matrix phase. With decreasing temperature, the microstructure will be more and more complex and finely dispersed. The ferrite is, for the higher alloyed steels, only stable above 1000 °C and at lower temperatures disappears in favour of intermetallic phases. The major intermetallic phase is sigma phase with small amounts of chi phase, the latter primarily in high Mo and W grades. Nitrides, not a focus in this investigation, were present as rounded particles and acicular precipitates at lower temperatures. The results were compared to theoretical predictions using the TCFE5 and TCFE6 databases.     

A small-angle x-ray scattering investigation of the zone formation of “maraging” type alloys

The small angle X-ray scattering from “Maraging” type alloys previously aged at temperatures below 450°C has been studied at room temperature. The behavior, during aging, of two high purity alloys containing Fe Ni Co and Fe Ni Co Mo is compared with an industrial Vascomax 300 type steel. The existence of G.P. zones is pointed out for the two Mo containing alloys. In the case of the quaternary alloy, the evolution of these zones during the aging is clearly defined. For the industrial steel, the interpretation of the observed phenomena is more complex due to the additional small amounts of Ti, Cu, Al and is discussed.     

A Study of Microstructure and Performance of Tempered Fe‐V‐W‐Mo Alloy

Influences of tempering temperature, holding time and tempering times on the microstructure and performance of Fe‐5%V‐5%W‐5%Mo‐5%Cr‐3%Nb‐2%Co(Fe‐V‐W‐Mo) were investigated by means of metallography, optical microscopy, hardness measurements, impact tester and pin abrasion tester. The results show that the hardness of Fe‐V‐W‐Mo alloy remains constant when tempered below 350°C. The hardness decreases gradually as the tempering temperature increase until around 475°C and then it increases again to a peak at 525°C. The hardness of Fe‐V‐W‐Mo alloy reaches nearly the highest value after the first tempering and decreases after triple‐tempering. The toughness of Fe‐V‐W‐Mo alloy increases until the tempering temperature reaches 475°C and then decreases until the temperature reaches 525°C. However, it increases again when tempering is beyond that temperature. The excellent wear resistance can be obtained by tempering at 500‐550°C.     

Mössbauer spectroscopy of hexagonal iron-nitrogen alloys

In Fe?N alloys, the hexagonal-close-packed phase can be completely retained metastably at room temperature by rapid quenching from 700°C, with nitrogen contents ranging from about 17 to 27 at. pct N; (between the latter composition and 33 at. pct N, the hexagonal phase is stable at room temperature). The phase is ferromagnetic; the Curie temperature is a sharp function of nitrogen content, with the maximum Curie point (about 300°C) occurring at 24 at. pct N. The Curie point is below room temperature in the hexagonal phase for nitrogen contents less than about 17.5 at. pct N. For alloys of the Fe₃N composition quenched from various temperatures, Mössbauer spectroscopy indicates that the hexagonal phase undergeos ordering of nitrogen atoms on interstitial sites.     

Toughness variation with test temperature and

A two phase heavy alloy composite based on 95 W-3.5 Ni-1.5 Fe (wt pct) was fabricated from elemental powders by liquid phase sintering. Past reports on the heavy alloys indicate considerable disagreement concerning cooling rate effects on toughness. The present experiments determined the effect of both cooling rate and test temperature on the properties of the 95 W heavy alloy. This alloy undergoes a ductile to brittle transition with decreasing test temperature; the transition temperature is close to room temperature. The cooling rate from post-sintering anneals carried out at temperatures greater than 1000 °C has a large influence on toughness; rapid quenching gives superior toughness. These findings support an impurity segregation explanation for embrittlement in the heavy alloys.     

Structural and Magnetic Transformations in the Fe‐Mn Binary System

The phase transformations of five binary iron‐manganese (Fe‐Mn) alloys with manganese contents ranging from 1 to 21 weight percent have been characterized in the temperature range between room temperature and 1250 °C. Differential scanning calorimetry and dilatometry were used to experimentally characterize both the phases and magnetic transformation temperatures. X‐ray diffraction and light optical microscopy were employed for the room temperature microstructure characterization. Depending on the manganese content of the alloy, three different crystal structures can be found: body centered cubic (bcc) (α/α'), face centered cubic (fcc) (γ), and hexagonal compact (hcp) (?). At manganese contents lower than 10% the phases present are the α/α’ (bcc) and γ (fcc). Above ~10 weight percent manganese increasing amounts of ? (hcp) is formed at the expense of the body centered cubic structures, and no α/α’ (bcc) is observed for the 21 weight‐percent manganese alloy.     

High Temperature Materials Based on the Intermetallic Compound NiAl Reinforced by Refractory Metals for Advanced Energy Conversion Technologies

Advanced NiAl‐based high temperature materials are developed and characterized for structural applications in energy conversion systems. The intermetallic compound NiAl with B2 superlattice structure exhibits superior physical and high temperature mechanical properties, and excellent oxidation resistance. Disadvantages of polycrystalline pure NiAl are the lack in plasticity and fracture toughness at room temperature and insufficient high temperature strength at temperatures above 800 °C. The refractory metals Cr, Mo, and Re form with NiAl quasi‐binary eutectic systems which enable to produce metal fibres reinforced NiAl‐based alloys in the as‐cast condition and by performing directional solidification. These in‐situ composites show fine‐grained and thermally stable microstructures possessing high temperature strength, superior creep resistance and sufficient room temperature ductility.     

Development of an Aluminized Multi‐Phase Steel with Dual Phase Properties for High Temperature Corrosion Resistance Applications

A high strength, high Mn, Cr‐Mo containing multi‐phase steel grade was aluminized with a 90 wt% Al – 10 wt% Si alloy coating, using a laboratory hot‐dip simulator. The adhesion of the coating to the steel strip was evaluated and the microstructure of the as deposited material was assessed. The coated sheet steel was tested at high temperatures by monitoring the weight gain of the samples and their mechanical properties as a function of time. It was found that the thermal properties of the aluminized sheet were excellent. The analysis of the coating/substrate interface revealed the dissolution of brittle intermetallic phases, explaining the excellent high temperature resistance performance of the Al‐Si coating up to temperatures as high as 900°C. In addition, the use of a continuous annealing cycle common in current aluminizing lines, resulted in a dual phase microstructure.     

Enhancing the High Temperature Capability of Ti‐Alloys

Titanium is a widely used structural material for applications below approximately 500°C but right now it cannot be used at higher temperatures. Titanium forms a fast growing rutile layer under these conditions. Furthermore enhanced oxygen uptake into the metal subsurface zone leads to embrittlement which deteriorates the mechanical properties. To overcome this problem a combined Al‐ plus F‐treatment was developed. The combination of Al‐enrichment in the surface zone so that intermetallic Ti_xAl_y‐layers are produced which form a protective alumina layer during high temperature exposure plus stabilization of the Al₂O₃‐scale by the fluorine effect led to significantly improved resistance against increased oxidation and embrittlement in high temperature exposure tests of several Ti‐alloys. In this paper, the experimental procedures and achieved improvements are described. The results will be discussed for the use of Ti‐alloys at elevated temperatures.     

Mechanical behavior of thein situ composite alloys in the al-ni-ti system near the l12 phase field

The Vickers microhardness (VHN) test at room temperature and compressive tests at temperatures up to 1000 °C were carried out on the three-phase composite alloy, consisting of the Ll₂, facecentered cubic (fcc) Al₂TiNi, and Al₂Ti intermetallic phases, in the Al-Ti-Ni system. The microhardness tests indicated that the fcc Al₂TiNi phase was very hard and brittle. Comparatively, the L1₂ phase was softer and more crack resistant. A considerable hardening was noticed due to the precipitation of Al₂Ti within L1₂. In addition, the VHN of the L1₂ phase was found to increase with the combined content of nickel and titanium without the presence of any observable precipitates. Under compressive loading at room temperature, microcracks nucleated in the fcc Al₂TiNi phase. These cracks propagated catastrophically at a stress barely approaching yield stress, resulting in nil ductility. This behavior was observed up to 800 °C. Between 900 °C and 950 °C, brittle-to-ductile transition in compressive behavior was observed for the three-phase alloy. Compressive ductility of the order of 80 pct was observed at 1000 °C. The mechanism of dynamic recrystallization was found to be operative at 1000 °C. Metallographic investigation revealed new recrystallized grains in the primary L1₂ matrix. However, the oscillatory nature of the true stress-true strain curve could not be explained with the help of the existing model of dynamic recrystallization. S. BISWAS, formerly Graduate Student, Department of Mechanical Engineering, University of Waterloo     

On the fracture and fatigue properties of Mo-Mo<Subscript>3</Subscript>Si-Mo<Subscript>5</Subscript>SiB<Subscript>2</Subscript> refractory intermetallic alloys at ambient to elevated temperatures (25 °C to 1300 °C)

The need for structural materials with high-temperature strength and oxidation resistance coupled with adequate lower-temperature toughness for potential use at temperatures above ∼1000 °C has remained a persistent challenge in materials science. In this work, one promising class of intermetallic alloys is examined, namely, boron-containing molybdenum silicides, with compositions in the range Mo (bal), 12 to 17 at. pct Si, 8.5 at. pct B, processed using both ingot (I/M) and powder (P/M) metallurgy methods. Specifically, the oxidation (“pesting”), fracture toughness, and fatigue-crack propagation resistance of four such alloys, which consisted of ∼21 to 38 vol. pct α-Mo phase in an intermetallic matrix of Mo₃Si and Mo₅SiB₂ (T₂), were characterized at temperatures between 25 °C and 1300 °C. The boron additions were found to confer improved “pest” resistance (at 400 °C to 900 °C) as compared to unmodified molybdenum silicides, such as Mo₅Si₃. Moreover, although the fracture and fatigue properties of the finer-scale P/M alloys were only marginally better than those of MoSi₂, for the I/M processed microstructures with coarse distributions of the α-Mo phase, fracture toughness properties were far superior, rising from values above 7 MPa √m at ambient temperatures to almost 12 MPa √m at 1300 °C. Similarly, the fatigue-crack propagation resistance was significantly better than that of MoSi₂, with fatigue threshold values roughly 70 pct of the toughness, i.e., rising from over 5 MPa √m at 25 °C to ∼8 MPa √m at 1300 °C. These results, in particular, that the toughness and cyclic crack-growth resistance actually increased with increasing temperature, are discussed in terms of the salient mechanisms of toughening in Mo-Si-B alloys and the specific role of microstructure.     

Stress‐Temperature‐Transformation and Deformation‐Temperature‐Transformation Diagrams for an Austenitic CrMnNi as‐cast Steel

Stress‐Temperature‐Transformation (STT) and Deformation‐Temperature‐Transformation (DTT) diagrams are well‐suited to characterize the TRIP (transformation‐induced plasticity) and TWIP (twinning‐induced plasticity) effect in steels. The triggering stresses for the deformation‐induced microstructure transformation processes, the characteristic temperatures, the yield stress and the strength of the steel are plotted in the STT diagram as functions of temperature. The elongation values of the austenite, the strain‐induced twins and martensite formations are shown in the DTT diagram. The microstructure evolution of a novel austenitic Cr‐Mn‐Ni (16%Cr, 6% Mn, 6% Ni) as‐cast steel during deformation was investigated at various temperatures using static tensile tests, optical microscopy and the magnetic scale for the detection of ferromagnetic phase fraction. At the temperatures above 250 °C the steel only deforms by glide deformation of the austenite. Strain‐induced twinning replaces the glide deformation at temperatures below 250 °C with increasing strain. Below 100 °C, the strain‐induced martensite formation becomes more pronounced. The kinetics of the α'‐martensite formation is described according to stress and deformation temperatures. The STT and DTT diagrams, enhanced with the kinetics of the martensite formation, are presented in this paper.     

Fatigue deformation of TiAl base alloys

The fatigue behavior of Ti-36.3 wt pct Al and Ti-36.2 wt pct Al-4.65 wt pct Nb alloys was studied in the temperature range room temperature to 900°C. The microstructures of the alloys tested consisted predominantly of γ phase (TiAl) with a small volume fraction of γ phase (Ti₃Al) distributed in lamellar form. The alloys were tested to failure in alternate tension-compression fatigue at several constant load amplitudes with zero mean stress. Fracture modes and substructural changes resulting from fatigue deformation were studied by scanning electron microscopy and transmission electron miscroscopy respectively. The ratio of fatigue strength (at 10⁶ cycles) to ultimate tensile strength was found to be in the range 0.5 to 0.8 over the range of temperatures tested. The predominant mode of fracture changed from cleavage type at room temperature to intergranular type at temperatures above 600°C. The fatigue microstructure at low temperatures consisted of a high density of a/3 [111] faults and dislocation debris of predominantly a/2 [110] and a/2 [110] Burger's vectors with no preferential alignment of dislocations. At high temperatures, a dislocation braid structure consisting of all 〈110〉 slip vectors was observed. The changes in fracture behavior with temperature correlated well with changes in dislocation substructure developed during fatigue deformation. S. M. L. SASTRY was formerly NRC Research Associate in the Air Force Materials Laboratory, Wright-Patterson Air Force Base, OH     

A Study of Microstructure and Performance of Quenching Fe‐V‐W‐Mo Alloy

The critical points and time temperature transformation (TTT) curves of Fe‐5%V‐5%W‐5%Mo‐5%Cr‐3%Nb‐2%Co (Fe‐V‐W‐Mo) were measured, and the effects of quenching temperature and cooling modes on the microstructure and performance of Fe‐V‐W‐Mo alloy were investigated. The results showed that the hardness of Fe‐V‐W‐Mo alloy increased until the quenching temperature reached 1025°C and dropped down as the quenching temperature exceeded 1050°C in oil cooling. The hardness obtained in air cooling and spray cooling exhibited a similar tendency as that in oil cooling, but the temperature at which the highest hardness was obtained in these slower cooling processes changed to a higher range. The hot hardness and toughness of Fe‐V‐W‐Mo alloy increased with rising quenching temperature until it reached 1150°C, and from then on the toughness began to drop. The main reasons why the structures and properties of Fe‐V‐W‐Mo alloy obviously change under different quenching conditions are particularly analysed at last.