Substructure of ausformed martensite in Fe-Ni and Fe-Ni-C alloys

The martensite substructure after ausforming has been studied for two different martensite morphologies: partially twinned, lenticular martensite (Fe-33 pct Ni, M_s =-105‡C) and completely twinned “thin plate” martensite (Fe-31 pct Ni-0.23 pct C, M_s = -170‡C), and in both cases ausforming produces a dislocation cell structure in the austenite which is inherited, without modification, by the martensite. In the Fe-Ni alloy, the dislocation cell structure is found in both the twinned (near the midrib) and untwinned (near the interface) regions, the latter also containing a regular dislocation network generated by the transformation itself and which is unaltered by the austenite dislocation cell structure. Similarly, in the Fe-Ni-C alloy, the transformation twins are unimpeded by the prior cell structure. These observations show that carbide precipitation during ausforming is not necessarily required to pin the austenite cell structure and that the martensite-austenite interface, backed by either twins or dislocations, does not exhibit a ”sweeping” effect. Although the martensite transformation twins are not inhibited by the ausforming cell structure, they do undergo a refinement with increased ausforming, and it is indicated that the transformation twin width in martensite depends on the austenite hardness. However, the relative twin widths remain unchanged, as expected from the crystallographic theory. T. MAKI, Formerly with the University of Illinois     

Substructure of ausformed martensite in Fe-Ni and Fe-Ni-C alloys

The martensite substructure after ausforming has been studied for two different martensite morphologies: partially twinned, lenticular martensite (Fe-33 pct Ni, M_s =-105?C) and completely twinned “thin plate” martensite (Fe-31 pct Ni-0.23 pct C, M_s = -170?C), and in both cases ausforming produces a dislocation cell structure in the austenite which is inherited, without modification, by the martensite. In the Fe-Ni alloy, the dislocation cell structure is found in both the twinned (near the midrib) and untwinned (near the interface) regions, the latter also containing a regular dislocation network generated by the transformation itself and which is unaltered by the austenite dislocation cell structure. Similarly, in the Fe-Ni-C alloy, the transformation twins are unimpeded by the prior cell structure. These observations show that carbide precipitation during ausforming is not necessarily required to pin the austenite cell structure and that the martensite-austenite interface, backed by either twins or dislocations, does not exhibit a ”sweeping” effect. Although the martensite transformation twins are not inhibited by the ausforming cell structure, they do undergo a refinement with increased ausforming, and it is indicated that the transformation twin width in martensite depends on the austenite hardness. However, the relative twin widths remain unchanged, as expected from the crystallographic theory.     

Magnetic investigation of martensite ⇌ austenite transformations in Fe-Ni-Co alloys

The martensite ⇌ austenite transformations were investigated in Fe-Ni-Co alloys containing about 65 wt pct Fe and up to 15 wt pct Co. A change in morphology of martensite from plate-like to lath-type occurred with increasing cobalt content; this change in morphology correlates with the disappearance of the Invar anomaly in the austenite. The martensite-to-austenite reverse transformation differed depending on martensite morphology. Reversion of plate-like martensite was found to occur by simple disintegration of the martensite platelets. Reverse austenite formed from lath-type martensite was not retained when quenched from much aboveA _s, with microcracks forming during theM→γ→M transformation.     

The effect of austenite grain size on microcracking in martensite of an Fe-1.22C alloy

Austenitic grain sizes of ASTM No. 9 and coarser were produced in an Fe-1.22 pct C alloy austenitized by immersion in molten lead at 1640†F (893°C), a temperature just above theA _cm for this alloy, for periods between 20 s and 1 h. Microcracking sensitivity,Sv, measured as crack area/unit volume martensite, was determined as a function of grain size in brine quenched specimens. Two locations of microcracks were observed in this investigation: 1) intragranular, resulting from the impingement of one martensite plate with another, and 2) grain boundary or intergranular resulting from the impingement of martensite plates at prior austenite grain boundaries. Intragranular microcracking sensitivity, the subject of previous investigations, increased and became the dominant type of cracking with increasing grain size, and reached a constant level for grain sizes of ASTM No. 4.5 and coarser. Total microcracking sensitivity, consisting of both intragranular and grain boundary microcracks, also increased with increasing grain size, then decreased to approach the intragranular value for grain sizes coarser than ASTM No. 3.5. On the other end of the scale, grain boundary microcracking made up a much larger proportion of the total microcracking in the fine grained specimens.     

The effect of Cu additions on the precipitation kinetics in an Al-Mg-Si alloy with excess Si

The microstructural evolution during age hardening of a Cu-bearing Al-Mg-Si alloy has been investigated by the three-dimensional atom probe (3DAP) and transmission electron microscope (TEM) techniques, in order to clarify the effect of Cu on the initial age-hardening response. After 30 minutes of artificial aging at 175 °C, the alloy shows a significant increase in hardness. The TEM observations have revealed that very fine, needle-shaped β″ precipitates are formed in addition to spherical Guinier-Preston (GP) zones, whereas only the spherical GP zones are observed in the Al-Mg-Si ternary alloy using the same aging condition. The number density of the precipitates is significantly affected by the preaging conditions. The 3DAP analysis shows that the distribution of Cu atoms is uniform after 30 minutes of artificial aging at 175 °C, whereas Cu atoms are incorporated into the needle-shaped β″ precipitates after 10 hours of aging at 175 °C. Based on these microanalytical results, the effect of Cu additions on the age-hardening response of Al-Mg-Si alloys is discussed.     

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.     

The effect of the first and second stages of tempering on microcracking in martensite of an Fe-1.22 C alloy

The effect of tempering on microcracking in the plate martensite of an Fe-1.22 C alloy was investigated by isothermal heat treatments in the temperature range between 180 and 225°C. The second stage of tempering, followed by X-ray measurement of retained austenite, was confirmed to depend upon the diffusion of C in austenite, and the transformation product was found to consist of very closely spaced cementite lamellae in ferrite. Microcracking, despite the volume expansion that accompanies the transformation of the retained austenite, decreased only slightly with time during the second stage. The major decrease in microcracking occurred during the first stage, a result attributed to the plastic deformation that accompanies the dimensional changes caused by the reduction of the lattice tetragonality of the high carbon martensite in the first stage. Metallographic observations of surface relief and etching effects associated with martensite plates provided evidence of the first-stage plastic flow. The authors were formerly Research Assistant and Professor, respectively at Lehigh University, Bethlehem, PA.     

Low temperature aging of the freshly formed martensite in an Fe-Ni-C alloy

Aging regime, below and at room temperature, of the martensite formed in an Fe-24.67Ni-0.87C (mass pct) alloy (M _s = 160 K) is studied by using a poly crystalline X-ray diffraction technique. Parameters such as tetragonal axial ratio, unit cell volume, intensity ratios of the tetragonal doublet peaks,etc. are obtained for various degrees of aging. At temperature below ∼200 K, where the carbon atoms can shift only into their neighboring sites, the variation of the parameters indicates some discrepancies with the existing models. At room temperature, a new peak appears in between the tetragonal doublet peaks. This peak and the variation of the parameters at room temperature are explained in terms of carbon cluster formation. Formerly Graduate Student, Tottori University.     

Effect of titanium additions on the aging characteristics of an al-zn-mg alloy

A minute titanium addition (0.04 pct) was added to an Al-Zn-Mg alloy in order to determine the effect of titanium on microstructure and aging characteristics of this alloy. Titanium was found to retard the kinetics of precipitation and to prevent solute segregation to grain boundaries on air quenching. Below G.P. solvus temperature, hardening of the alloy was due both to dislocation loop formation and to G.P. zone formation. Above G.P. solvus temperature, hardening was due only to formation of precipitate. These results have been explained in terms of titanium reducing the likelihood of the formation of solute-vacancy complexes, causing a reduction in zinc and magnesium mobility. Formerly Postdoctoral Associate, Rensselaer Polytechnic Institute, Troy, N. Y. 1 2181     

The tempering of martensite in an Fe-1.5 pct N alloy

The tempering of Fe 1.5 pct N martensite has been studied at temperatures up to 300°C using X-ray and electron microscope techniques. Stage 1 decomposition occurs below 270°C by the general precipitation, resembling spinodal morphology, of fine τa^" (Fe₁₆ N₂) lamellae on 001 habit planes in both matrix and twin crystals of the partially 112 twinned martensite plates. Yet, gaged by changes in the X-ray spectrum, the reaction is discontinuous, the tetragonal martensite doublets decaying in intensity without change in their Bragg positions. The anomaly and the failure to detect by electron microscopy regions exhibiting fractional stages of the fine scale α^’ → α + α^" reaction is attributed to its occurrence at different times in different martensite (or parts of martensite) plates. It is believed that transformation occurs in this manner because the nucleation of coherent α^" plates is controlled by the prevailing internal stress field. Thus the time exponent “n” for the reaction decays from a normal value between 1 and 0.67 to less than 0.3 as stress relief by recovery dominates the more protracted stages of the reaction. Above 200°C the more stable nitride γ^’ (Fe₄N) forms at an increasing rate as plates on 012 habit planes, accompanied by marked softening.     

Influence of Cu additions on the morphology of GeSi precipitates in an Al-Ge-Si alloy

Combined additions of Ge and Si to Al are known to produce higher precipitation hardening than that which occurs in the constituent binaries, when the total amounts of alloying atoms are the same for all the alloys investigated. In the resultant Al-Ge-Si alloys, the diamond cubic precipitates contain both Ge and Si and are designated as GeSi. During artificial aging at 160 °C, the GeSi precipitates are commonly present in three forms, i.e., equiaxed, 〈100〉_Al lath, and triangular plate. The equiaxed form is the dominant one of the three. This article examines the influence of varying amounts (i.e., 2 to 4 wt pct) of Cu additions on the morphology of GeSi precipitates formed in an Al-2.6 wt pct Ge-1.04 wt pct Si alloy during artificial aging at 160 °C. It is shown that Cu additions have the remarkable effect of maximizing the nucleation frequency of the 〈100〉_Al lath form and simultaneously suppressing the nucleation of the equiaxed and the plate forms of the GeSi precipitates. Increasing Cu additions also increase the homogeneity and cause refinement of the 〈100〉_Al laths. These results are discussed in light of (1) the critical requirement of vacancies for the nucleation and growth of GeSi precipitates having an atomic volume larger than Al and (2) the crystallographic nature of the negative dilation strains that develop locally in the Cu-rich regions of the Al matrix. It is further shown that, in the alloys containing increased levels (i.e., exceeding about 2.5 wt pct) of Cu, the precipitation of ϑ′ (metastable ϑ-Al₂Cu) phase occurs, and that the nucleation of Cu-rich ϑ′ precipitates occurs upon the 〈100〉_Al laths of GeSi. The latter effect is discussed in terms of the attainment of both the nucleation site and the necessary solute supersaturation at the 〈100〉_Al GeSi/α-Al interfaces.     

The effect of alloy additions on the rod-plate transition in the eutectic NiAl−Cr

Alloys of the eutectic NiAl-34 at. pct Cr with additions of Mo, V, W, and Fe were directionally solidified to determine the effect of the addition element and solidification rate on structure. The directionally solidified NiAl?Cr consists of 〈100〉-oriented columnar grains with long chromium rods of round cross section. The addition of 0.6 at. pct Mo or 0.7 at. pct W or 1.3 at. pct V causes a transition from the round rod grains to 〈111〉-oriented grains with faceted rods and plates of Cr(Mo), Cr(W), or Cr(V). At these compositions, the lattice parameters of the chromium-rich phase and the NiAl matrix phase are nearly equal. Additions of iron produced only the round rod structure as the difference of the lattice parameters was increased. High temperature anneals changed the nonequilibrium faceted rods to rods with regular hexagonal cross sections. Coherency strains near zero mismatch may influence the morphology and may be responsible for the effect of mismatch on structure. Further additions of molybdenum, tungsten, and vanadium resulted in a gradual reduction of the number of faceted rods and finally, the formation of cells with radial plates. Higher solidification rates formed cells and dendrites. A model relating cell and dendrite formation to freezing range of the eutectic alloy was derived.     

Effects of austenitizing temperature and austenite grain size on the formation of athermal martensite in an iron-nickel and an iron-nickel-carbon alloy

The effects of austenitizing conditions on the kinetics at the start of martensite formation in Fe-31Ni and Fe-31 Ni-0.28C alloys have been studied using electrical-resistance measurements during cooling of the specimens to follow the course of the transformation. The primary object of the study was to decide whether or not a change in austenitizing temperature, in the absence of a change in austenite grain size, has any effect on the Ms temperature or the burst characteristics of athermal martensite. It is concluded that it does not, suggesting that the potential nuclei (embryos) of martensite are mechanically stable crystal defects. Another interesting observation is that when the austenite grain size is small, the M_b temperature increases with increasing grain size and the burst is always small. When the austenite grains are coarse, the M_b temperature is independent of the grain size and the burst is large. It is suggested that this phenomenon is a result of the elastic shear stress concentration being related to the size of the first martensite plate and, in turn, to the size of the austenite grain. M. Umemoto, formerly a Graduate Student in the Department of Materials Science at Northwestern University W. S. Owen, formerly at Northwestern University     

Research on surface defects and continuous casting process of Fe-Ni alloy

Over the last decade,various Fe-Ni alloys have been developed at Baosteel using the EAF-AOD-LFVD-CC route. This paper first reveals the main cause of defects in Fe-Ni alloys,including surface edge cracks on hot-rolling strips and slivers on cold-rolling strips of Fe-36% Ni alloy. Then,the material properties and in-situ solidification behavior w ere experimentally investigated. The gas content and average diameter of the inclusions in Fe-36% Ni alloy that occur along the EAF-AOD-LF-VD-CC route w ere also investigated via potentiostatic electrolysis using a non-aqueous organic electrolytic. Furthermore,the heat transfer and solidification in a continuous casting mold w ere predicted based on an inverse heat transfer model using the measured mold temperature. Experimental results show that the gas content,w hich is 0. 001 5% in a continuous casting slab,and the average diameter of the inclusions both decrease during the metallurgical EAF-AOD-LF-VD-CC process. The average diameter of the inclusions in a continuous casting slab is ～ 18 μm,w hich tends to induce slivers during subsequent cold-rolling process. Experimental in-situ solidification results show that the mushy zone betw een the liquidus and solidus of Fe-Ni alloy is much narrow er than that of plain carbon steel. Stresses are generated during continuous casting,primarily due to the thermal contraction of a few percentage points,and any strain applied to the steel w ithin this temperature region w ill cause cracks to propagate outw ard from the solidification front betw een the dendrites. Numerical simulation results illustrate that heat flux and shell thickness are uneven across the w idth of the mold,particularly the shell thickness close to the edge of the slab surface in the fixed face is 6-mm thinner than that at the slab center. Based on these results,the incidence of surface defects in Fe-Ni alloy can be greatly reduced by the adjustment and optimization of its refining and continuous casting process.