Deformation behavior of a Ni−30Al−20Fe−0.05Zr intermetallic alloy in the temperature range 300 to 1300 K

The deformation behavior of an extruded Ni-30 (at. pct) Al−20Fe−0.05Zr intermetallic alloy was studied in the temperature range of 300 to 1300 K under initial tensile strain rates varying between about 10⁻⁶ and 2×10⁻³ s⁻¹ and in constant load compression creep between 1073 and 1300 K. The deformation microstructures of the fractured specimens were characterized by transmission electron microscopy (TEM). Three deformation regimes were observed: Region I consisted of an athermal regime of low tensile ductility (less than 0.3 pct) occurring between 400 and 673 K, where the substructure consisted of slip bands in a few grains. Exponential creep was dominant in region II between 673 and 1073 K, where the substructure changed from a mixture of dislocation tangles, loops, and dipoles at 673 K to a microstructure consisting of subgrains and dislocation tangles at 973 K. The tensile ductility was generally about 2.0 to 2.5 pct below 980 K in this region. A significant improvement in tensile ductility was observed in region III, which occurred between 1073 and 1300 K. An analysis of the data suggests that viscous glide creep with a stress exponent,n, of about 3 and high-temperature dislocation climb withn≈4.5 where the two dominant creep mechanisms in this region depending on stress and temperature. The average activation energy for deformation in this region was about 310±30 kJ mol⁻¹ for both processes. In this case, a transition from viscous glide creep to dislocation climb occurred when σ/E<1.7×10⁻⁴, where σ is the applied stress andE is the Young’s modulus.     

Creep behavior of an Al-2. 0 wt pct Li alloy in the temperature range 300 °C to 500 °C

The elevated temperature deformation behavior of an Al-2. 0 wt pct Li alloy in the temperature range 300 °C to 500 °C was studied using constant extension-rate tension testing and constant true-stress creep testing under both isothermal and temperature cycling conditions. Optical microscopy and transmission electron microscopy (TEM) were employed to assess the effect of deformation on microstructure. The data showed that the stress exponent,n, has a value of about 5. 0 at temperatures above theα +δAlLi solvus (approximately 380 °C) and that subgrains form during plastic deformation. Models for dislocation-climb and dislocation-glide control of creep were analyzed for alloys deformed in the temperature range of stability of the terminal AlLi solid solution. A climb model was shown to describe closely the behavior of this material. Anomalous temperature dependence of the activation energy was observed in this same temperature range. This anomalous behavior was ascribed to unusual temperature dependence of either the Young’s modulus or the stacking fault energy, which may be associated, in turn, with a disorder-order transformation on cooling of the alloy. Formerly with the Materials Engineering Section. Department of Mechanical Engineering, Naval Postgraduate School.     

Fe -30Ni -20Cr合金连铸板坯纵裂纹的研究

采用立式连铸生产Fe-30Ni-20Cr合金（N08810）,开展了连铸坯纵裂纹试验分析和高温力学拉伸试验等研究。金相试验结果表明,纵裂纹两侧晶粒度不均匀,裂纹四周已经被氧化;电子探针能谱分析发现,裂纹内部含有Na、K等保护渣典型成分元素;高温力学性能试验结果表明,Fe-30N i-20Cr合金连铸坯纵向裂纹敏感性明显高于横向铸坯,且比不锈钢304和310S更易出现纵裂纹。将连铸冷却强度由强冷改为弱冷、采用合适熔点和黏度的保护渣将有利于降低N08810连铸坯纵裂纹的出现几率。     

Elevated temperature deformation behavior of an Al-8.4 wt pct Fe-3.6 wt pct Ce alloy

The elevated temperature deformation characteristics of a rapidly solidified Al-8.4 wt pct Fe-3.6 wt pct Ce alloy have been investigated. Constant true strain rate compression tests were performed between 523 and 823 K at strain rates ranging from 10⁻⁶ to 10⁻³ s⁻¹. At temperatures below approximately 723 K, the alloy is significantly stronger than oxide dispersion strengthened (ODS) aluminum. However, at higher temperatures, the strength of the Al-Fe-Ce alloy falls rapidly with increasing temperature while ODS aluminum exhibits an apparent threshold stress. It is shown that particle coarsening cannot fully account for the reduction in strength of the Al-Fe-Ce alloy at elevated temperatures. The true activation energy for deformation of the Al-Fe-Ce alloy at temperatures between 723 and 773 K is significantly greater than that for self-diffusion in the matrix. This is unlike the behavior of ODS alloys, which contain nondeformable particles and exhibit true activation energies close to that for self-diffusion in the matrix. Since abnormally high true activation energies for deformation are also exhibited by materials containing deformable particles, such as γ^′ strengthened superalloys, it is concluded that elevated temperature deformation in ythe Al-Fe-Ce alloy involves deformation of both the matrix and the precipitates. The loss of strength of the Al-Fe-Ce alloy appears to be related to a reduction in strength of at least some of the second phase particles at temperatures above 723 K. Formerly Research Assistant, Department of Materials Science and Engineering, Stanford University.     

Dynamic recrystallization during creep in a 45 Pct Ni-35 pct Fe-20 pct Cr alloy system

A combined 3.5 wt pct Mo + 1.2 wt pct Ti imparted dynamic recrystallization in a 35 wt pct Fe-45 wt pct Ni-20 wt pct Cr alloy system during creep at 700 °C, whereas 3.5 wt pct Mo addition alone did not initiate recrystallization. Dynamic recrystallization substantially increased the creep elongation and produced a high ductile fracture topography in the present alloy system. A subgrain coalescence nucleation mechanism for dynamic recrystallization mechanism was operative during creep. The critical initiation strain requirements are also discussed.     

Fracture behavior of thick-section weldment in Fe-12Cr-12Ni-10Mn-0.24N stainless steel at liquid helium temperature

Type JJ1, a nitrogen-strengthened austenitic stainless steel (Fe-12Cr-12Ni-10Mn-0.24N), is emerging as the preferred structural material for superconducting fusion magnet casings to operate at liquid helium temperature (4 K). This article describes an examination of the cryogenic fracture behavior of thick-section (200 mm) weldment in JJ1 forged stainless-steel plate. The compact tension geometry was used for the elastic-plastic fracture toughness tests at 4 K. The through-thickness fracture toughness was investigated with particular attention to the influence of the nitrogen and inclusion contents. The magnitude of the 4-K fracture toughness decreased with increasing inclusion content. In addition to fracture toughness data, temperature rise near the crack tip during fracture was measured. Significant adiabatic heating occurred, and internal temperature rises up to 52 K were reported.     

Effect of yttrium on the oxidation behavior of cast Ni-30Cr alloy

Cast Ni-30Cr and Ni-30Cr-0.5Y alloys were oxidized at 1000°C in pure O₂ for various times, then were either furnace cooled to room temperature, or thermally cycled between 1000°C and different lower temperatures. The isothermal oxidation rate of the Ni-30Cr alloy was reduced by about a factor of 3.6 by the addition of 0.5% Y. Acoustic emission signals, which are generated by scale fracture events, were collected during isothermal oxidation, during continuous furnace cooling and during thermal cycling. These data showed, as others have shown, that the scale formed on Ni-30Cr-0.5Y was significantly more resistant to fracture than that on Ni-30Cr. This advantage of the Y-containing alloy was evident for comparisons based on equal oxidation times, and more importantly, at equal scale thicknesses. SEM and EDAX analyses show that continuous Cr₂O₃ scales were formed on both Y-bearing and Y-free alloys after a short time of oxidation (2 h), but after a longer period of oxidation and thermal cycling, a NiO or NiCr₂O₄ outer layer was found. This outer scale created a new interface with the Cr₂O₃ scale where thermal stresses will be generated during cooling due to the thermal expansion difference between Cr₂O₃ and NiO or NiCr₂O₄. Spallation at the inner scale/outer scale interface, as well as at the metal/scale interface, was observed. X-ray measurements of scale strains at equal scale thicknesses showed that the growth strains (at the end of the isothermal oxidation period) were larger on the Y-containing alloy, and that this alloy also sustained larger residual strains upon cooling to room temperature. Using a model based on elastic strain energy, estimates of the surface energy for scale fracture (a measure of scale adhesion) were significantly higher for the Y-containing alloy at equal scale thicknesses. Both the AE and the strain measurements are consistent with the proposal that Y improves the inherent strength of the metal/scale interface. The smaller rate of scale cracking for Y-containing alloys, combined with their slower scale growth rate, offers the further benefit of delaying the onset of NiO or NiCr₂O₄ overgrowth layers, which themselves may degrade the integrity of the scale.     

Thermal fatigue of a heat-resistant Fe-0.45C-26Cr-33Ni-2Si-2Nb alloy

The thermal fatigue of a heat-resistant Fe-0.45C-26Cr-33Ni-2Si-2Nb alloy is studied during thermal cycling in the temperature range 50–900°C up to 1000 cycles. The alloy is investigated in the initial as-cast state and after isothermal annealing during 1000 h at a temperature of 800, 900, 1000, or 1100°C; these conditions imitate the temperature conditions of operation and the structural state of various layers in a reaction pipe in the radiant furnace coils of ethylene production installations. After isothermal annealing, the thermal fatigue life of the alloy is found to decrease by a factor of 1.7–1.2 as compared to the initial as-cast state. It is shown that isothermal annealing and subsequent thermal cycling lead to the formation of carbide precipitates of various sizes in the alloy structure that affect the thermal fatigue life of the alloy. Thermal fatigue cracks are shown to form and grow predominantly at the sites of accumulation of fine carbide precipitates. Coarse (>10 μm) precipitates retard crack growth, and cracks branch near such precipitates.     

The effects of ce additions on the high temperature corrosion behavior of Fe-30Cr-5Al alloy in 80 Pet Na2SO4-20 Pct NaCI molten salt

High temperature corrosion tests in 80 pct Na₂SO₄-20 pct NaCI were made on five Fe-30 wt pct Cr-5 wt pct Al alloys containing various amounts of cerium up to 0.68 pct. After the corrosion tests, samples were examined metallographically and by X-ray diffraction and X-ray microanalysis. It was found that the surface scale formed on each sample decreased in thickness as the Ce content increased. Electron probe microanalyses suggest that this retarding behavior is related to the early formation of a Ce-induced protective scale of α-Al₂O₃. The role of Ce in promoting the formation of the α-Al₂O₃ scale is discussed.     

Aging effects in a Cu-12Al-5Ni-2Mn-1Ti shape memory alloy

The isothermal aging effects in an as-quenched Cu-11.88Al-5.06Ni-1.65Mn-0.96Ti (wt pct) shape memory alloy at temperatures in the range 250 °C to 400 °C were investigated. The changes in the state of atomic order and microstructural evolutions were traced by means of in situ X-ray diffraction and electrical resistivity measurements, as well as transmission electron microscopy (TEM) and optical observations. The kinetics of the aging process, i.e., the temperature and time dependence of the properties including hardness, resistivity, martensitic transformation temperatures, and shape memory capacity were characterized, and at least three temperature-dependent aging stages were distinguished: (1) D0₃ or L2₁ atomic reordering, which causes the martensitic transformation temperatures to shift upward and leads the M18R martensite to tend to be a N18R type structure; (2) formation of solute-depleted bainite which results in a drastic depression in martensitic transformation temperatures and loss of the shape memory capacity, accompanied by the atomic disordering in both the remaining parent phase and bainite; and (3) precipitation of the equilibrium α and γ ₂ phases and destruction of the shape memory capacity.     

A study of retained austenite in a fine-grained Fe-12Ni-0.25Ti alloy

The behavior of retained austenite in a fine-grained Fe-12Ni-0.25Ti cryogenic alloy system was investigated. When the alloy was held at a temperature within the two phase (α + γ) field austenite reversion occurred by a diffusion-controlled process. The redistribution of solute elements appeared to control the stability of the reverted austenite. The microstructural appearance of the retained austenite was examined using transmission electron microscopy. A preferential distribution of the austenite phase along martensite lath boundaries was observed. A precipitate-correlated austenite was also found to occur. The beneficial effect of introducing retained austenite appeared in the improvement in the tensile elongation as well as in the Charpy impact toughness at low temperatures. The retained austenite in this system did not improveK _IC at ?196°C. On the contrary, samples containing retained austenite showed increased susceptibility to unstable crack propagation in low temperature fracture toughness tests.     

An investigation of the fatigue and fracture behavior of a Nb-12Al-44Ti-1.5Mo intermetallic alloy

This article presents the results of a study of the fatigue and fracture behavior of a damage-tolerant Nb-12Al-44Ti-1.5Mo alloy. This partially ordered B2 + orthorhombic intermetallic alloy is shown to have attractive combinations of room-temperature ductility (11 to 14 pct), fracture toughness (60 to 92 MPa√m), and comparable fatigue crack growth resistance to IN718, Ti-6Al-4V, and pure Nb at room temperature. The studies show that tensile deformation in the Nb-12Al-44Ti-1.5Mo alloy involves localized plastic deformation (microplasticity via slip-band formation) which initiates at stress levels that are significantly below the uniaxial yield stress (∼9.6 pct of the 0.2 pct offset yield strength (YS)). The onset of bulk yielding is shown to correspond to the spread of microplasticity completely across the gage sections of the tensile specimen. Fatigue crack initiation is also postulated to occur by the accumulation of microplasticity (coarsening of slip bands). Subsequent fatigue crack growth then occurs by the “unzipping” of cracks along slip bands that form ahead of the dominant crack tip. The proposed mechanism of fatigue crack growth is analogous to the unzipping crack growth mechanism that was suggested originally by Neumann for crack growth in single-crystal copper. Slower near-threshold fatigue crack growth rates at 750 °C are attributed to the shielding effects of oxide-induced crack closure. The fatigue and fracture behavior are also compared to those of pure Nb and emerging high-temperature niobium-based intermetallics.     

Punch-stretching behavior of a 2014 aluminum alloy in the temperature range 250° to 500 °C

The influence of forming temperatures in the range 250° to 500 °C on the performance of a 2014 aluminum alloy in punch stretching has been investigated. In tests at moderate strain rates within the 250° to 450 °C range, the biaxial stretching limits of annealed sheets were greatly superior to the room-temperature values. Limiting depths of pressing increased with increasing temperature in the range 250° to 400 °C as a result of improvement in both limit strains and strain distribution, but increasing deformation temperature above ~400 °C caused the limit strains to decrease as a result of cavitation at high strains. Under comparable conditions of temperature and extension rate, such cavity growth was more rapid in equibiaxial stretching than in uniaxial or plane-strain stretching. At 500 °C with a punch speed of 0.083 mm s^-1, the thickness strain which could be applied in biaxial stretching without significant cavitation damage was less than 0.4. Thus, susceptibility to cavitation imposes an important restriction on opportunities for combining solution treatment with a hot stretch-forming operation when using an alloy based on Al-4 pct Cu.     

Elimination of inclusions and evaporation control during melting of Ti-48Al-2Cr-2Nb-1B intermetallic alloy

Titanium aluminide intermetallic compounds have an excellent capability for use in engineering structures at high temperatures. In the present work the formation of Nb rich inclusions in microstructure and evaporation of Al during melting of γ-TiAl based alloy (Ti-48Al-2Cr-2Nb-1B (at %)) was studied. The results show that the inclusions cannot be removed even with a four-stage melting process, when elemental Nb is used as raw material. However, by replacing Nb with NbAl₃ and using a three-stage melting process, the inclusions were removed from microstructure and also evaporation of Al was reduced remarkably. Otherwise, with removing elemental Al from raw material by using TiAl compound, evaporation of Al will be very low. Increasing vessel pressure from 400 to 600 mbar will not influence evaporation of Al. The article is published in the original.     

Room-temperature deformation behavior of a directionally solidified β (B2)-(Ni-Fe-Al) intermetallic alloy

The room-temperature mechanical behavior of a directionally solidified columnar-grained, single-phase β (B2)-(Ni-20 at. pct Fe-30 at. pct A1) intermetallic alloy deformed along the “hard” 〈001〉 direction has been characterized. The 0.2 pct offset compressive yield stress was found to be comparable to that of 〈001〉 single crystals of stoichiometric NiAl. The dislocation substructure consisted of a preponderance of long, straight a〈111〉 screw dislocations on {112} planes, with cross-slip on {123} and {110} planes. The superpartials were not resolved by weak-beam imaging conditions, indicating that the antiphase boundary (APB) energy of NiAl is not reduced significantly by the Fe addition. The dislocation substructure was analyzed as a function of strain and compared to the dislocation substructure in 〈001〉 NiAl and body-centered cubic (bcc) metals deformed at low homologous temperatures. The debris left behind by a〈111〉 screw dislocations consisted of prismatic edge dipole loops 5 to 25 nm in diameter.