On the microstructure and mechanical behavior of melt-spun Fe-24 pct Ni-0.5 pct C Ribbons: Effect of austenite grain size

The role of carbon on the retention and decomposition of austenite in a melt-quenched Fe-24 wt pct Ni-0.5 wt pct C alloy made by the melt-spinning method has been investigated, using a combination of X-ray diffractometry, optical and TEM metallography, microhardness measurements, and tensile tests. It is found that the addition of 0.5 wt pct C to Fe-24 wt pct Ni alloy leads to retention of austenite to a temperature close to -196 °C, when the alloy is quenched from the melt. The austenite grain size varies from ∼0.2 μm to ∼2 μm on going from the wheel to the gas side. The cooling rate, accordingly, changes from 5 × 10⁷ to 4 × 10⁴ Ks^-1. The changes in the mechanical properties have been correlated with the accompanying changes in the ribbon microstructure. The Central Metallurgical Research and Development Institute, National Research Centre, Dokki, Cairo, Egypt     

The thermal activation energy for fatigue of Fe- 1 pct Cr- 0.5 pct Mo

The temperature dependence of fatigue crack propagation is considered in an Fe-1 pct Cr-0.5 pct Mo alloy steel. This material was tested at temperatures between 425 and 550 °C, a frequency of 1 Hz, and anR-ratio of 0.1. It is shown that the effect of temperature can be explained in terms of a thermal activation energy for fatigue. The magnitude of this activation energy is a function of ΔK and varies from more than 150 kJ/mole at 15 MPa√m to 30 kJ/mole above 30 MPa√m. The magnitude of these activation energies supports the idea that oxidation, and not creep, is the rate-controlling time-dependent process for the test conditions studied.     

The tensile properties of an Fe-4 pct Mo-0.2 pct C martensite tempered under applied stress

In Fe-4 pct Mo-0.2 pct C martensite which is a typical secondary hardening steel, premature failure o°Curred in tensile test at 600 °C to 700°C where solute atoms could diffuse easily. To clarify this phenomenon, the quenched specimens were tempered under applied stress and tensile-tested at room temperature. The following results were obtained: (1) Typical intergranular fracture was observed in specimens tempered in a temperature range of 600 °C to 650 °C with tempering times of five minutes to 10 minutes and applied stress (70 MPa to 140 MPa). (2) Based on Auger analysis, this phenomenon was considered to be caused by segregation of P, S, and Mo on prior austenite grain boundaries due to applied stress. (3) The direction of applied stress was found to be very significant. Namely, when the tensile direction was parallel to the applied stress during tempering, the specimen was more brittle, and when tensile direction was normal to the applied stress, the specimen was not so brittle. (4) To reduce this embrittlement, solution treatment temperature was adjusted, and it was found that the embrittlement was considerably reduced both in specimens with fine prior austenite grains and with some ferrite phase on prior austenite grain boundaries. TAKATOSHI OGAWA, formerly with Kyoto University. YOSHIFUMI OHMURA, formerly with Kyoto University. This paper is based on a presentation made at the “pcter G. Winchell Symposium on Tempering of Steel” held at the Louisville Meeting of The Metallurgical Society of AIME, October 12-13, 1981, under the sponsorship of the TMS-AIME Ferrous Metallurgy and Heat Treatment Committees.     

The relationship between austenite strength and the transformation to martensite in Fe-10 pct Ni-0.6 pct C alloys

The effect of austenite yield strength on the transformation to martensite was investigated in Fe-10 pct Ni-0.6 pct C alloys. The strength of the austenite was varied by 1) additions of yttrium oxide particles to the base alloy and 2) changing the austenitizing temperature. The austenite strength was measured at three temperatures above theM _s temperature and the data extrapolated to the experimentally determinedM _s temperature. It is shown that the austenite yield strength is determined primarily by the austenite grain size and that the yttrium oxide additions influence the effect of austenitizing temperature on grain size. As the austenite yield strength increases, both theM _s temperature and the amount of transformation product at room temperature decrease. The effect of austenitizing temperature on the transformation is to determine the austenite grain size. The results are consistent with the proposal¹ that the energy required to overcome the resistance of the austenite to plastic deformation is a substantial portion of the non-chemical free energy or restraining force opposing the transformation to martensite.     

High-temperature deformation behaviour and microstructural evolution of low carbon steel based on processing map

The high-temperature deformation behaviours of low carbon steel QD08 were investigated by hot compression tests over temperatures from 1000 to 1200°C and strain rates from 0.1 to 10 s^?1. The processing map was obtained by superimposition of the power dissipation and the instability maps and the regions having the lowest strain rate sensitivity added for more clarification of low and high workability regions. The results show that the security domain mainly of hot deformation with a higher powder dissipation factor and maintain a smooth variation, by the metallographic observations, the grain refinement by DRX under the stable deformation conditions. On the basis of processing map and microstructure evolution, the optimal deformation processing parameters are the hot deformation temperature range from 1070 to 1100°C, and strain rates range from 5 to 10 s^?1.     

The morphology and substructure of highly tetragonal martensites in Fe-7 pct Al−C steels

This investigation reports on an optical and electron microscope study of bct martensite formed in Fe-7 pct Al-1.5 pct C and Fe-7 pct Al-2.0 pct C alloys. In each case the martensite is plate-like containing \((112)[\bar 1\bar 11]\) transformation twins 100 to 200Å in width. The particular twin plane variant \((112)[\bar 1\bar 11]\) corresponds to the martensite habit plane variant (3, 15, 10)_F, which is predicted by the crystallography theory. The twins are uniformly spaced and extend completely from one martensite-austenite interface to the other as would be theoretically expected. The martensite plates are ideally lenticular in the 2 pct C alloy but those in the 1.5 pct C alloy frequently exhibit irregular interfaces which are attributed to impingement effects. All observations are in accordance with the phenomenological crystallography theory as applied to ferrous martensites with a {3, 15, 10}_F habit plane.     

Effect of silicon content and carbon addition on primary recrystallization of Fe-3 pct si

A study was made on the effect of the increase of silicon content and the addition of carbon on the primary recrystallization of silicon steel sheet containing MnS and AlN as inhibitors. The increase of silicon content led to the reduction of the size of primary grains and the intensity of {110} that is considered to represent the amount of {110}〈001〉-oriented nuclei, to the gain of the size of precipitates, and to the coarse dispersion of the precipitates. The carbon addition promoted primary recrystallization and decreased the size of primary grains. From the points mentioned earlier, the increase of silicon content is considered disadvantageous, and the carbon addition is advantageous to the secondary recrystallization of the {110}〈001〉 texture.     

The effect of structure on the deformation of as-quenched and tempered martensite in an Fe-0.2 pct C alloy

As-quenched and tempered martensite in an Fe-0.2 pct C alloy were subjected to tensile testing and structural characterization by light and transmission electron microscopy. The light temper, 400°C-l min, did not change packet morphology, but did reduce dislocation density, coarsen lath size and cause the precipitation of carbides of a variety of sizes. The yield strength of the as-quenched martensite was strongly dependent upon packet size according to a Hall-Petch relationship, but tempering significantly diminished the packet size dependency, a result attributed to packet boundary carbide precipitation and the attendant elimination of carbon segregation present in the as-quenched martensite because of autotempering. Examination of thin foils from strained tensile specimens showed that a well-defined cell structure developed in the as-quenched martensite, but that the random distribution of jogged dislocations and carbide particles produced by tempering persisted on deformation of the tempered specimens. The authors were formerly Research Assistant and Professor, respectively, at Lehigh University, Bethlehem, PA.     

Microstructure and tensile properties of Fe-40 At. pct Al alloys with C,Zr, Hf,and B additions

The influence of small additions of C, Zr, and Hf, alone or in combination with B, on the microstructure and tensile behavior of substoichiometric FeAl was investigated. Tensile prop-erties were determined from 300 to 1100 K on powder which was consolidated by hot extrusion. All materials possessed some ductility at room temperature, although ternary additions generally reduced ductility compared to the binary alloy. Adding B to the C- and Zr-containing alloys changed the fracture mode from intergranular to transgranular and restored the ductility to ap-proximately 5 pct elongation. Additions of Zr and Hf increased strength up to about 900 K, which was related to a combination of grain refinement and precipitation hardening. Fe₆Al₆Zr and Fe₆Al₆Hf precipitates, both with identical body-centered tetragonal structures, were iden-tified as the principal second phases in these alloys. Strength decreased steadily as temperature increased above 700 K, as diffusion-assisted mechanisms, including grain boundary sliding and cavitation, became operative. Although all alloys had similar strengths at 1100 K, Hf additions significantly improved high-temperature ductility by suppressing cavitation.     

The microstructural evolution of near beta alloy Ti-10V-2Fe-3Al during subtransus forging

The microstructural evolution of titanium alloys during subtransus isothermal forging (IF) has been effectively demonstrated using a testing methodology developed at Imperial College London. Double truncated cone specimen geometries were isothermally deformed at near β transus temperatures to obtain microstructural information for a range of strains within a single specimen. The methodology was applied to the near β alloy, Ti-10V-2Fe-3Al, to determine the effect of strain, strain rate, and IF subtransus temperature on microstructural evolution. An erratum to this article is available at .     

A study on the microstructural evolution of Al-25 at. pct V-12.5 at. pct M (M = Cu,Ni, Mn) powders by planetary ball milling

The alloying behavior of Al-25 at. pct V-12.5 at. pct M (M = Cu, Ni, Mn) by planetary ball milling of elemental powders hours as been investigated in this study. In Al₃V binary system, an amorphous phase was produced after 6 hours and the amorphous phase was mechanically crystallized after 20 hours. The large difference in the diffusivities between Al and V atoms in Al matrix results in the formation of the amorphous phase when the homogeneous distribution of all the elements in a powder was achieved at 6 hours. According to thermal analyses, the amorphous phase in the binary Al₃V was crystallized at 350 °C. The addition of ternary elements (Cu, Ni, Mn) increased the activation energy for the crystallization to D0₂₂ phase by interfering with the diffusion process. Therefore, ternary element addition improved the thermal stability of the amorphous structures. The amorphous phase in the 12.5 at. pct Ni added Al₃V was crystallized to D0₂₂ phase at 540 °C. The mechanical crystallization of the amorphous phase in the ternary element-added Al-V system either occurred later or was not observed during ball milling up to 100 hours. It is thought that the amorphous intermetallic compacts could be produced more easily in ternary element-added alloys by using an advanced consolidation method.     

Microstructural studies and micromagnetic analysis of nanocrystalline NdFeCoMB (M = Ga, Ge) melt-spun ribbon

The effects of Ge and Ga additions on the microstructure and coercivity of melt-spun ribbons of Nd_13.5 Fe_(74.9−z) Co_5.5 M _z B₆ (z=0.0 to 0.54; M=Ge, Ga) alloys have been investigated. For the first time, a detailed microstructural study of nanometer-sized Nd-rich grain boundary phase enriched with small amounts of Ga and Ge was carried out using a three-dimensional atom probe (3DAP) technique. Ga partitions to thin uniform Nd-rich grain boundary phase, causing effective isolation of Nd₂Fe₁₄B grains. As a result, the coercivity increased. On the other hand, Ge caused the formation of Ge- and Nd-rich precipitates, resulting in the formation of dispersoids along the grain boundaries. Due to the poor isolation of the Nd₂Fe₁₄B grains, the coercivity decreased.     

The morphology,crystallography, and mechanism of carbide precipitation in an Fe-0.12 Pct C-3.28 Pct Ni alloy

Carbide precipitation during the eutectoid decomposition of austenite has been studied in an Fe-0.12 pct C-3.28 pct Ni alloy by transmission electron microscopy (TEM) supplemented by optical microscopy. Nodular bainite which forms during the latter stages of austenite decomposition at 550 °C exhibits two types of carbide arrangement: (a) banded interphase boundary carbides with particle diameters of about 20 to 90 nm and mean band spacings between 180 and 390 nm and (b) more randomly distributed (“nonbanded”) elongated particles exhibiting a wide range of lengths between 33 and 2500 nm, thicknesses of approximately 11 to 50 nm, and mean intercarbide spacings of approximately 140 to 275 nm. Electron diffraction analysis indicated that in both cases, the carbides are cementite, obeying the Pitsch orientation relationship with respect to the bainitic ferrite. The intercarbide spacings of both morphologies are significantly larger than those previously reported for similar microstructures in steels containing alloy carbides other than cementite (e.g., VC, TiC). Both curved and straight cementite bands were observed; in the latter case, the average plane of the interphase boundary precipitate sheets was near {110}_α//{011}_c consistent with cementite precipitation on low-energy {110}_α//{111}_γ ledge terrace planes (where α,β, andc refer to ferrite, austenite, and cementite, respectively). The results also suggest that the first stage in the formation of the nonbanded form of nodular bainite is often the precipitation of cementite rods, or laths, in austenite at the α:γ interfaces of proeutectoid ferrite secondary sideplates formed earlier. Although these cementite rods frequently resemble the “fibrous” microstructures observed by previous investigators in carbide-forming alloy steels, they are typically much shorter than fibrous alloy carbides. The bainitic microstructures observed here are analyzed in terms of a previously developed model centered about the roles of the relative nucleation and growth rates of the product phases in controlling the evolution of eutectoid microstructures.     

The effect of temperature and carbon content on the cavitation behavior of a 1.5 pct Cr-0.5 pct V steel

Constant strain rate tests at 10^-6 s^-1 have been carried out at 823 K and 923 K on a vacuum melted 1 1/2 pct Cr 1/2 pct V ferritic steel containing 3 different carbon contents. After straining to various elongation values specimens were unloaded, cooled and fractured at 77 K. This gave fracture surfaces consisting almost entirely of intergranular facets, enabling a quantitative study to be made of the different stages of cavity nucleation and growth. It was found that cavity growth rates were independent of carbon content but were higher at 923 K than at 823 K. Subsequent grain boundary sliding measurements, using a surface offset technique showed that sliding increased with increasing carbon content and that cavity nucleation occurred selectively at large grain boundary carbides. Formerly of the Department of Metallurgy, University of Manchester.     

The fracture toughness and toughening mechanisms of wrought low carbon arc cast,oxide dispersion strengthened,and molybdenum-0.5 pct titanium-0.1 pct zirconium molybdenum plate stock

The high-temperature strength and creep resistance of low carbon arc cast (LCAC) unalloyed molybdenum, oxide dispersion strengthened (ODS) molybdenum, and molybdenum-0.5 pct titanium-0.1 pct zirconium (TZM) molybdenum have attracted interest in these alloys for various high-temperature structural applications. Fracture toughness testing of wrought plate stock over a temperature range of −150 °C to 1000 °C using bend, flexure, and compact tension (CT) specimens has shown that consistent fracture toughness results and transition temperatures are obtained using subsized 0.5T bend and 0.18T disc-CT specimens. Although the fracture toughness values are not strictly valid in accordance with all ASTM requirements, these values are considered to be a reasonable measure of fracture toughness. Ductile-to-brittle transition temperature (DBTT) values were determined in the transverse and longitudinal orientations for LCAC (200 °C and 150 °C, respectively), ODS (<room temperature and −150 °C), and TZM (150 °C and 100 °C). At test temperatures > DBTT, the fracture toughness values for LCAC ranged from 45 to 175 MPa√m, TZM ranged from 74 to 215 MPa√m, and the values for ODS ranged from 56 to 149 MPa√m. No temperature dependence was resolved within the data scatter for fracture toughness values between the DBTT and 1000 °C. Thin sheet toughening is shown to be the dominant toughening mechanism, where crack initiation/propagation along grain boundaries leaves ligaments of sheetlike grains that are pulled to failure by plastic necking. Specimen-to-specimen variation in the fraction of the microstructure that splits into thin sheets is proposed to be responsible for the large scatter in toughness values at test temperatures > DBTT. A finer grain size is shown to result in a higher fraction of thin sheet ligament features at the fracture surface. As a result finer grain size materials such as ODS molybdenum have a lower DBTT.     

The workability of commercial and experimental 0.6 pct carbon low alloy steels in the temperature range of 650 °C to 870 °C

The torsional strength and ductility of commercial AlSI 4063 steel along with four 0.6 pct carbon, low alloy experimental steel compositions were determined with temperature. These parameters were then related to the workability of the steels. The influence of initial lamellar or spheroidal microstructures, as well as of vacuum or air-melt practices, were studied in the deformation temperature range of 650 °C to 870 °C and strain-rate range of 0.71 to 2.13 s⁻¹ The experimental steels showed increased ductility and lower peak flow stresses over the entire temperature range when compared to the commercial alloy. Lamellar microstructures resulted in higher maximum flow stresses and subsequent work softening in the ferritic regime. Initial carbide morphology did not influence the maximum flow stresses in the austenitic range. Improved ductility of the experimental steels over the entire working temperatures could possibly be attributed to the combination of a reduced amount of oxides and sulfides, reduced particleto-matrix decohesion, improved grain-boundary cohesion, or the ability to annihilate or heal microcracks which may form during deformation. Constitutive equations were developed for the ferritic and austenitic conditions with both spheroidized and lamellar carbides.