Alloy powders in the Fe—Cu, Fe—Cr, Fe—Mn, and Fe—Cr—Mn systems

Studies have been done on iron-copper, iron-chromium, iron-manganese, and iron-chromium-manganese powder alloys, which have been made by dispersing the melts with high-pressure nitrogen. The use of such alloys in the preparation of low-alloy construction materials eases the stringent specifications for the oxidation potential of the controlled gas medium, and it also produces a more uniform distribution of the alloying elements in the sintered material.     

The reversible martensite transformation in iron-platinum alloys near Fe3Pt

A series of iron-platinum alloys containing 25 or 27 at. pct platinum and with ordering of the γ-phase varying from substantial disorder to nearly complete order have been thermally cycled between 25°C and - 196°C. The kinetics of the γ⇌α transformations, the hysteresis revealed by electrical resistanceJtemperature plots, the thermoelastic growth and the reappearance of an identical microstructure after thermal cycling (the microstructural memory effect) were studied as a function of the ordering of the γ-phase. Thermoelastic growth does not appear to be affected by changes in the degree of order of partially ordered specimens but the microstructural memory was imperfect in the most highly ordered specimen examined. In agreement with earlier observations by Dunne and Wayman,³ the difference between the A_s and M_s temperatures and the hysteresis decrease markedly as the order is increased. It is shown by transmission electron microscopy that in all but the most highly-ordered specimens the α- γ transformation produces plates of austenite with a high density of dislocations. These plates are separated from the surrounding untransformed parent austenite by arrays of dislocation loops lying in the interfaces between untransformed parent austenite and the original martensite plates. All the dislocations have a Burgers vector direction which is the same as that of the usual slip dislocation in austenite. Such dislocations lying in a habit plane must be sessile. In the well-ordered specimen dislocation pairs, typical of glide dislocations in a crystal with long-range order, were formed in the austenite formed by the reverse transformation. These dislocations were segregated into roughly plate-like clusters, but the number of clusters in unit volume was appreciably less than the number of original plates of martensite. In this case, no arrays of sessile loops of dislocations mark the locations of the original martensite-austenite interface. It is deduced from the microscopic and kinetic results that the inherited nuclei responsible for the microstructural memory effect are located in localized volumes of highly dislocated austenite formed by the α- γ transformation. No unique dislocation configurations which could be associated with specific nuclei were found. The effects of ordering on the various kinetic effects and the microstructural memory are discussed in terms of the concept of inherited nuclei, the change of the flow stress of the γ-phase with ordering and temperature and the variation of T_o and the transformation driving-force with ordering. Formerly at Northwestern University     

Spinodal decomposition in the Fe−Cr system

Fe?Cr binary alloys ranging from 20 to 50 pct Cr have been analyzed by Mössbauer spectroscopy, in the queached state and after aging respectively at 470° and 540°C. A clear distinction could be made in this way between the nucleation and growth, and the spinodal process of decomposition during aging. When the alloy lies inside of the miscibility gap but outside of the spinodal, a pramagnetic precipitate clearly appears right from the beginning of the aging process, while when the alloy lies inside of the spinodal, no paramagnetic phase appears even after 1050 hr of aging at 470°C. This certainly indicates that the spinodal decomposition is very slow. One more point is shown: aging outside of the miscibility gap shows clearly a clustering of the chromium atoms and a large departure from the concept of an ideal solid solution.     

Fe2O3、Cr2O3掺杂对4YSZ导电性能的影响

用柠檬酸溶胶-凝胶法制备了分别掺Fe2O3和Cr2O3的4YSZ前驱体凝胶,凝胶在500℃预烧,压制成圆片状后在1 300℃煅烧2 h得到所需试样;分别研究Fe2O3、Cr2O3的不同掺量对试样的烧结性能、电导率的影响。结果表明掺Fe2O3可提高试样的电导率和烧结性能;掺Cr2O3可提高试样的低温电导率,但使试样的烧结性能下降。     

Precipitate formation in low-temperature nitrided cold-rolled Fe94Ni4Ti2 and Fe93Ni4Cr3 films

Using various experimental techniques, the formation and evolution of precipitates, the dilation of the matrix, and the nitrogen uptake and release were investigated in cold-rolled Fe₉₄Ni₄Ti₂ and Fe₉₃Ni₄Cr₃ materials subjected to nitriding in a gaseous mixture of NH₃ + H₂ and to reduction in an H₂ atmosphere at low (300 °C) to moderate (600 °C) temperatures. Transmission electron microscopy (TEM) revealed that in both alloys the precipitates are thin platelets. In the Ti-containing alloy, the precipitates were also small in the lateral dimensions. The difference in size, aspect ratio of the precipitates, and misfit and coherency on the interface consistently explain the substantial differences in dilation of the matrix and nitrogen uptake for the Ti- and Cr-containing alloys under examination. The results provide evidence for the formation of mixed Fe-Ti-N precipitates (Guinier-Preston (GP) zones) in the early stages of nitriding.     

Kinetics of the pearlite reaction in Fe−C−Cr

Velocity and spacing measurements on the pearlite reaction in three eutectoid alloys of Cr (0.4, 0.9 and 1.8 wt pct) have been recorded and compared with growth and spacing equations based on a variety of thermodynamic and kinetic models. At high temperatures the reaction is controlled by phase boundary diffusion of Cr while at lower temperatures, a local equilibrium no-partition model with kinetic control by carbon diffusion best represents the data. At intermediate temperatures there is a smooth transition rather than a sharp break betwewn the two regimes. There is a discrepancy between theory and experiment on the slope of the spacingvs inverse under cooling plot which can be removed by assuming that the ferrite-cementite epitaxy improves with the addition of chromium. R. C. SHARMA, formerly with McMaster University