Structures and tempering behavior of rapidly solidified high-carbon iron alloys

High-carbon iron alloys containing carbide formers of chromium and molybdenum were rapidly solidified by means of a single roller method. In the alloy containing a high level of both chromium and molybdenum (10Cr-5Mo) and a critical carbon content of about 4 pct, the metastable phases,ε phase and austenite, are retained after solidification. Theε phase could contain a large amount of carbon in solid solution so that during tempering at about 900 K, it decomposes to very fine ferrite and carbide, which bring about an enhanced hardness of 1300 DPN. Even after tempering at a high temperature around 1100 K, the hardness hardly deteriorates due to a remarkable dispersion of fine M₃C and M₇C₃ carbides. Thus, coaddition of chromium and molybdenum is effective in obtaining high hardness. Formerly Graduate Student, Kyushu Institute of Technology     

The tempering of low carbon steels containing tungsten

The tempering behavior of three steels each containing 0.20 pct C and having tungsten contents of 2.1, 3.9 and 5.9 pct has been followed by thermomagnetic analysis and electron microscopy. Using a Sucksmith Balance, the proportions of autotempered carbide and retained austenite in as-quenched specimens were estimated, and the amount of cementite precipitated upon subsequent tempering measured accurately. Solution of tungsten in cementite during tempering was monitored by observing changes in Curie temperature. The magnetic nature of alloy carbides precipitating at high temperatures allowed tentative identification and this directed and assisted the electron microscopy study which provided new information on the morphology of tungsten carbides. J. B. LUPTON, formerly Research Student, Metallurgy Department, University of Sheffield, England, S. MURPHY, formerly Research Fellow, Metallurgy Department, University of Sheffield,     

The microstructure of rapidly solidified Ti-Fe melt-spun ribbons

Ti-Fe binary alloys were rapidly solidified by the melt-spinning technique, and four compositions were examined: Ti-5 wt pct Fe, which is the critical composition for theβ to ω athermal transformation; Ti-10 wt pct Fe, which represents a hypoeutectoid composition; the eutectoid composition Ti-15 wt pct Fe; and Ti-20 wt pct Fe, as an example of a hypereutectoid alloy. The Ti-5 wt pct Fe rapidly solidified ribbons are composed of two different structures. The first consists of α′-martensite plates inβ matrix and the second, athermal ω particles inβ matrix. The Ti-10, 15, and 20 wt pct Fe alloys are also composed of two structures. These areβ grains and isothermal-like ω particles inβ matrix. A solidification model is suggested which explains the existence of two different microstructures at the same composition and the for-mation of two kinds of ω particles.     

The microstructure of rapidly solidified Al6Mn

The microstructure of rapidly solidified Al−Mn alloys containing 18 to 25.3 wt pct Mn was studied by transmission electron microscopy. One of the phases found in the microstructure exhibits icosahedral symmetry manifested in electron diffraction patterns having five-fold symmetry. A new structural concept is proposed to account for the observed electron diffraction patterns. The structure is assumed to be composed of many connected polyhedra. Although not forming a regular lattice, such structures are able to produce sharp diffraction peaks. The terminal stability and transformation of the icosahedral phase was also studied and reported.     

Phase distributions in rapidly solidified stainless steels

Phase distributions and the internal magnetic fields have been determined in rapidly solidified stainless steels (Fe-nCr-8Ni-0.05C, Fe-nCr-5Ni, and Fe-nCr withn in the range of 10 to 24) by transmission and conversion electron Mössbauer spectroscopy (TMS and CEMS). Based on these results, a modification of the phase boundaries in the Schaeffler diagram is suggested to account, in particular, for rapidly solidified stainless steels. The suggested modification is primarily an expansion of the austenite field toward higher Cr and lower Ni equivalent contents. Combining CEMS and TMS makes it possible to determine the phase distributions both in the near surface region (outmost 300 nm) and in the bulk of the ribbons. For the low-Cr alloys, the content of the bcc phase (martensite) in the surface region is higher than in the sample as a whole. In the high-Cr alloys, the content of the bcc phase (ferrite) is lower in the surface than in the bulk. This disparity is ascribed to the different mechanisms of formation of martensite (diffusionless) and ferrite (nucleation and growth) in relation to the higher cooling rates of the surface layers. The determinations of the internal magnetic field are in good agreement with earlier investigations on conventionally processed Fe-Cr steels, where it was found that the internal magnetic field decreases with increasing Cr content.     

The microstructure of rapidly solidifiedβ-phase Cu-Zn-Al alloys

The structure of theβ phase in rapidly solidified Cu-Zn-Al alloys was studied by transmission electron microscopy. The structure is observed to contain many unusual features not found in normal solid-solutioned material. The microstructure is very fine-grained, homogeneous in composition, only partially ordered, and contains an abnormally high density of dislocations. No metastable phases are seen. The grain structure is refined about two orders of magnitude in grain size and is quite variable in size and shape in a given sample. The grain boundaries are highly structured, with drastic curvatures and large steps. Dislocations are seen to be emitted by grain boundaries, contributing to the higher than usual dislocation density. Subgrain structures and low-angle boundaries are present in some regions and isolated martensite plates are observed well above the measuredM _stemperature.     

Phase transformation-induced grain refinement in rapidly solidified ultra-high-carbon steels

Superplastic forming offers a promising approach for reducing the cost of high-performance metal components with complex shapes. Severe thermomechanical deformation is one method for producing the fine grain structure needed to permit superplastic forming economically. Our approach to generating fine-grained microstructures is by cyclic heat treatment of rapidly solidified material. First, a metastable structure is produced by rapid quenching of the liquid metal. Then, solid-state phase transformations at modest temperatures are employed to refine this structure. In the ultra-high-carbon steels (UHCS) studied, the brittle as-cast structure of martensite and austenite was transformed, after cyclic heat treatment, to a ductile mixture of 1-μm ferrite and 0.25-μm carbide. Varying the heat-treat temperatures by 100 °C within the transformation range had little effect on the scale of the microstructure. Higher C resulted in coarser carbide spheroids, addition of Al refined the microstructure, and the finest mean carbide size was obtained with an intermediate level (5 pct) of Cr. Refinement of the martensite plates retained austenite via cyclic tempering and austenitization was found to be the key step in the overall mechanism for phase transformation-induced grain refinement in rapidly solidified UHCS.     

Solidification and microstructure analysis of rapidly solidified melt-spun Al-Fe alloys

Aluminum-iron alloys with Fe contents of up to 8 wt pct were rapidly quenched from the melt using the planar-flow melt-spinning technique. Microstructural changes that occur across the thickness of melt-spun ribbons were investigated as a function of initial melt composition and ribbon thickness. A transition exhibiting a dramatic change in the cell spacing from a microcellular to a coarse cellular region was observed for all the alloys examined. For a given alloy, the volume fraction of the microcellular region decreases significantly with increasing ribbon thickness. A theoretical explanation for the observed behavior is presented in which the heat flow and solidification characteristics are related to the undercooling achieved before the onset of nucleation.     

On the microstructure of rapidly solidified IN-100 powders

Powder particles generated by the RSR centrifugal atomization process exhibit at least two distinct types of microstructure. When a high melt superheat is maintained during atomization, the powder particles possess the familiar coarse grained dendritic microstructures. On the other hand, when the melt superheat is reduced by increasing the heat flow to the disc of the rotary atomizer, the powder particles are predominantly microcrystalline in character, with typically one small dendrite particle per grain. It is proposed that the unusual microcrystalline material has its origin in dendrite erosion occurring in a “mushy zone” of dynamic solidification on the disc of the atomizer. formerly with Government Products Division, is now with United Technologies Research Center.     

Phase transitions in rapidly solidified stainless steels cathodically hydrogen charged

The effects of cathodic hydrogen charging and subsequent aging on phase transitions and microstructures of rapidly solidified (RS) austenitic stainless steels (types 310 RS, 316 RS, and 316TiM RS) were investigated. The behavior of the martensitic phases,α′(bcc) andε(hcp), as well as the austenite phase,γ (fcc), of the RS steels during aging after charging was compared to the behavior of these phases of equivalent conventionally processed commercial solution-treated (ST) austenitic stainless steels (types 310 ST, 316L ST, and 316TiM ST) following identical cathodic-charging conditions by means of X-ray and electron diffraction techniques. The behavior of theα′ phase of both RS and ST steels (that formα′ phase) during aging was found to be very similar, while the behavior of bothγ andε phases during aging of all of the RS steels studied, as compared to the equivalent ST steels, was different. The development of lower internal stresses and minor lattice expansion of the RS steels, as compared to the ST steels, is probably due to a different distribution of hydrogen within the near-surface layer of the RS steels than that of the ST steels, which appears to be related to the markedly different microstructural characterizations of the RS steels from the ST steels. Scanning and transmission electron microscopy (SEM and TEM) observations indicated that the tendency toward cracking along the columnar-like structure is typical of all of the charged RS steels studied. Formerly Research Associate, Department of Materials Engineering, Ben Gurion University of the Negev     

Phase transitions in rapidly solidified stainless steels cathodically hydrogen charged

The effects of cathodic hydrogen charging and subsequent aging on phase transitions and microstructures of rapidly solidified (RS) austenitic stainless steels (types 310 RS, 316 RS, and 316TiM RS) were investigated. The behavior of the martensitic phases,α′(bcc) andε(hcp), as well as the austenite phase,γ (fcc), of the RS steels during aging after charging was compared to the behavior of these phases of equivalent conventionally processed commercial solution-treated (ST) austenitic stainless steels (types 310 ST, 316L ST, and 316TiM ST) following identical cathodic-charging conditions by means of X-ray and electron diffraction techniques. The behavior of theα′ phase of both RS and ST steels (that formα′ phase) during aging was found to be very similar, while the behavior of bothγ andε phases during aging of all of the RS steels studied, as compared to the equivalent ST steels, was different. The development of lower internal stresses and minor lattice expansion of the RS steels, as compared to the ST steels, is probably due to a different distribution of hydrogen within the near-surface layer of the RS steels than that of the ST steels, which appears to be related to the markedly different microstructural characterizations of the RS steels from the ST steels. Scanning and transmission electron microscopy (SEM and TEM) observations indicated that the tendency toward cracking along the columnar-like structure is typical of all of the charged RS steels studied. Formerly Research Associate, Department of Materials Engineering, Ben Gurion University of the Negev     

The microstructure and phase relationships in rapidly solidified type 304 stainless steel powders

The microstructure and relative amounts of fcc and bcc phases have been studied for rapidly solidified Type 304 stainless steel powders produced by vacuum gas atomization (VGA) and centrifugal atomization (CA). The VGA powder solidifies with a cellular microstructure while the CA powder has a dendritic microstructure. The volume fraction of fcc phase in the CA powder is found to increase from 40 Pct to 97 Pct with increasing particle size from 30 to 125 μm. In the VGA powder, the volume fraction of fcc phase is found to decrease from about 90 Pct to 77 Pct over the same range of particle sizes. The origins of the fcc and bcc phases in each powder are considered. It is concluded that bcc is present as both a primary crystallization phase in the smaller CA particles (<75 μm) and as compositionally stabilized eutectic ferrite at the cell walls of particles of both CA and VGA powders in which fcc was the primary crystallization phase.     

Formation of microstructure of the icosahedral phase in rapidly solidified aluminium-chromium alloys

Rapid solidification of AlCr alloys resulted in the nucleation and growth of the icosahedral phase in the 14–36 wt% Cr range. The highest volume fraction of this phase was obtained between 21–26 wt% Cr. It was found that the different alloy compositions contain a variety of morphologies and microstructures which also depend on location regarding the cross section of the ribbon. The icosahedral phase is not stoichiometric and its maximum Cr content is between 26–30 wt% Cr. In the higher alloy compositions the stable intermetallic ϵ phase grows epitaxial on the icosahedral phase with a distinctive orientation relationship.     

The effect of cooling conditions on the microstructure of rapidly solidified Ti-6Al-4V

The effect of cooling conditions, giving estimated cooling rates in the range 10⁴ °C per second to 10⁷ °C per second, on the microstructure of Ti-6Al-4V has been evaluated. The microstructures of as-solidified particulates were martensitic, with the martensite lath length decreasing with beta grain size,L, which in turn decreased with increasing cooling rate. For material alpha + beta heat-treated or vacuum hot pressed, the alpha morphology was dependent on the prior cooling rate. For materials cooled at <5 × 10⁵ °C per second martensite transformed to lenticular alpha, while material cooled at >5 × 10⁵ °C per second developed an equiaxed alpha morphology. This change in morphology was explained in terms of high dislocation density or grain size refinement, both of which result from the high cooling rate. When the beta grain size (L) was plottedvs section thickness (z), and estimated cooling rate (T), power law relationships analogous to those reported for secondary dendrite arm spacing were found:L = 1.3 ± 0.4z^089±006 (thin, chill-substrate quenched),L = 0.17 ± 0.05z^0.86±0.01(thick, convection-cooled material), andL = 3.1 × 10⁶ T^{−0.93±0.12} (all material), whereL and z are in μm andT is in K/s. The last relationship is in agreement with the 0.9 exponent predicted using a model developed for the effect of grain size on cooling rate assuming classical homogeneous nucleation and isotropic linear growth during solidification. The first two relationships were rationalized by assuming that the two materials cooled under near-Newtonian conditions.     

Effects of martensite morphology and tempering on dynamic deformation behavior of dual-phase steels

The effects of martensite morphology and tempering on the quasistatic and dynamic deformation behavior of dual-phase steels were investigated in this study. Dynamic torsional tests were conducted on six steel specimens, which had different martensite morphologies and tempering conditions, using a torsional Kolsky bar, and then the test data were compared via microstructures, tensile properties, and fracture mode. Bulky martensites were mixed with ferrites in the step-quenched (SQ) specimens, but small martensites were well distributed in the ferrite matrix in the intermediate-annealed (IA) specimens. Under a dynamic loading condition, the fracture mode of the SQ specimens was changed from cleavage to ductile fracture as the tempering temperature increased, whereas the IA specimens showed a ductile fracture mode, irrespective of tempering. These phenomena were analyzed in terms of a rule of mixtures applied to composites, microstructural variation, martensite softening and carbon diffusion due to tempering, and adiabatic shear-band formation.     

ZK60合金粉末烧结和热挤压后的组织与性能

采用雾化法制得ZK60合金粉末,并用掺胶法制备ZK60合金棒材,研究热挤压后ZK60合金的微观组织、相组成及力学性能.结果表明:合金粉末主要由α-Mg固溶体构成,呈枝晶与等轴晶混合组织,晶粒尺寸5～10μm;在后续热挤压过程中粉末之间结合良好,晶粒进一步细化,同时合金基体中大量析出MgZn_2球形纳米颗粒;经T5(175℃保温12h)热处理后,析出相密度呈增加趋势.挤压变形后材料的屈服强度(σ_(0.2))、最大抗拉强度(σ_(UTS))和伸长率(δ)分别为286.3MPa、337.7MPa及5.6%;随后T5处理可进一步提高强度((σ_(0.2))=300.1MPa,σ_(UTS)=340.5 MPa),增加塑性(δ=12.3%).     

An analysis of the microstructure of rapidly solidified Al-8 wt pct Fe powder

Rapidly solidified powders of Al-8 wt pct Fe exhibit four distinct microstructures with increasing particle diameter in the size range of 5 μm to 45 μm: microcellular α-Al; cellular α-Al; a-Al + Al₆Fe eutectic; and Al₃Fe primary intermetallic structure. Small powder particles (~10 μm or less) undercool significantly prior to solidification and typically exhibit a two-zone microcellular-cellular structure in individual powder particles. In the two-zone microstructure, there is a transition from solidification dominated by internal heat flow during recalescence with high growth rates (microcellular) to solidification dominated by external heat flow and slower growth rates (cellular). The origin of the two-zone microstructure from an initially cellular or dendritic structure is interpreted on the basis of growth controlled primarily by solute redistribution. Larger particles experience little or no initial undercooling prior to solidification and do not exhibit the two-zone structure. The larger particles contain cellular, eutectic, or primary intermetallic structures that are consistent with growth rates controlled by heat extraction through the particle surface (external heat flow).     

Mechanical behavior of rapidly solidified Al-Al2Cu and Al-Al3Ni composites

The eutectic alloys Al-Al₂Cu and Al-Al₃Ni have been unidirectionally solidified at rates from 1.05 to 6.80 in, per min by a semicontinuous casting technique, and then tested in tension at room temperature. In both alloys the flow stress and ultimate tensile strength increased with increasing solidification rate, except for the highest solidification rate. The increases in matrix work-hardening rate with solidification rate were too great to be accounted for by dislocation pileup mechanisms, but were found to correlate with elastic constraint effects of the matrix aluminum phase by the reinforcing phases. In the Al-Al₂Cu eutectic the strength of the Al₂Cu platelets increased as the platelet width decreased with increasing growth rate. Misalignment of the composite caused by either a cellular or a macroscopically concave solid-liquid interface resulted in a decrease in the ultimate strength, especially in the rod-like Al-Al₃Ni alloy. This has been related to the fracture behavior of the composites. The very low fracture toughness of the lamellar Al-Al₂Cu eutectic is consistent with models of composite materials, and seriously limits the alloy’s usefulness for engineering applications.     

Oxidation behavior of a fine-grained rapidly solidified 18-8 stainless steel

The cyclic oxidation behavior of a fine-grained, rapidly solidified 303 stainless steel was determined at 900 °C in pure oxygen. The rapidly solidified alloy exhibited superior resistance to oxidation compared with that of a wrought 304 stainless steel; its oxidation resistance was as good as that of a wrought 310 stainless steel, even though the latter alloy contained more Cr and Ni. The matrix of the rapidly solidified steel contained a uniform dispersion of fine MnS precipitates (0.2 to 0.5 μm), which were effective in inhibiting grain growth at elevated temperatures. The enhanced resistance to oxidation of the rapidly solidified alloy is attributed to two factors: (1) the formation and growth of protective Cr₂O₃ and SiO₂ scales were promoted by the fine alloy grain size (5 to 8 =gmm) and by the presence of the MnS dispersion, and (2) the adherence of the scale was increased by the formation of intrusions of SiO₂ from the external scale into the alloy, which formed around MnS precipitates and along closely-spaced alloy grain boundaries, and which acted to key the scale mechanically to the alloy.