Pre-precipitation Stage Research Articles

Nitriding of low-carbon stainless steels

1. The maximum case hardness is attained in brief nitriding of steels 000Kh18 and 000Kh18N9. An increase of nitriding time leads to a lower case hardness to a value depending on the temperature, after which it remains almost constant. 2. The case hardness varies with the dispersity of the nitrides, and the maximum hardness is attained when the nitrides are in the preprecipitation stage, ensuring the largest distortion of the matrix lattice. 3. Brief nitriding of low-carbon stainless steels makes it possible to obtain an even high-quality case with a higher hardness than after prolonged nitriding. 4. Cold treatment of nitrided steel 000Kh18 increases the case hardness due to precipitation (preprecipitation) of CrN. 5. High-temperature heating with quenching and tempering at 500° of nitrided steel 000Kh18N9 increases the case hardness due to the precipitation or preprecipitation of dispersed nitrides with decomposition of γ phase to α phase and CrN by the intermediate mechanism. 6. The case hardness reaches the desired value not only with nitriding but also in the process of cooling, and varies with the dispersity of the nitrides formed.

A study of pre-precipitation in Al-Zn alloys with X-rays and neutrons

Al–Zn alloys of various concentrations in the final pre-precipitation stage have been studied with X-ray and neutron techniques. Reasonable agreement in absolute integrated intensities and in the characteristic parameters of the scattering pattern has been obtained. By optimizing the resolution in the neutron experiments an ordering parameter and new information on pre-precipitation-size and size distribution could be deduced from the scattering data.

The Precipitation of Carbides during Tempering of High Carbon Martensite

The tempering process of high carbon martensite was investigated by means of the electrical resistivity measurement, dilatometry, and transmission electron microscopy. The first stage of tempering is not the precipitation of ε-carbide, but is probably the pre-precipitation stage such as the segregation of carbon atoms to the lattice defects or the carbon cluster formation. The second stage is the precipitation of ε-carbide which is related to the martensite by Jack’s orientation relationship. The retained austenite decomposed into lower bainite in the temperature range between 230° and 330°C. Tempering up to 330°C produced χ-carbide on the twin interfaces and this exhibited the monoclinic Pd5B2 type structure determined by Duggin. Further tempering up to 400°C, the in-situ transformation of χ-carbide into cementite occurred.

Resistivity and Electron Microscopy Study of Precipitates in Fe-Ni-Co-Mo

The details of the precipitation hardening mechanisms have been studied previously. Peters and Cupp have measured the kinetics of precipitation in an 18 pet Ni-Co-Mo steel. Hardy and Heal have studied the reversion processes caused by annealing for short times at temperatures above the aging temperature. This paper reports studies of the pre-precipitation and reversion stages in an 18 pct. Ni-Co-Mo steel (maraging with .46 pct Ti).The annealed microstructure of the parent steel consists of BCC martensitic grains. Within the matrix lies a fairly high density of dislocations whose configurations appear irregular. These dislocations are nucleation sites and are shown in Figure 1.

Mise en evidence d'un stade de pre-precipitation au cours du vieillissement, vers 400°C, d'alliages martensitiques Fe-Ni-Mn-Ti

Some martensitic Fe-Ni-Mn-Ti alloys have been studied. It was found that they hardened considerably during aging experiments in the temperature range 400–500°C. The increase of hardness was found to come from the formation of coherent domains which were deduced from the presence of side lines in X-ray diffraction diagrams. Consequently, a pre-precipitat ion stage appears before the stage during which Fe 2Ti and Ni 3Ti equilibrium phases are formed.

The X-Ray Investigation of Preprecipitation in Supersaturated Solid Solutions

AbstractThis paper briefly traces the study of the preprecipitation or cold-aging stage of the decomposition of supersaturated solutions by means of diffuse scattering of X-rays.This part of the overall precipitation process is of considerable practical as well as theoretical interest because it is during preprecipitation that changes in many physical and mechanical properties take place without the formation of a precipitate phase.Examples will be given to show the types of zones encountered during this stage in various age-hardening alloys. These will attempt to illustrate the contribution which the study of diffuse scattering of X-rays, guided by present-day concepts of crystal imperfections, has made toward a better understanding of the precipitation process in alloys.

Study on Precipitation Phenomena in Thin Films of Supersaturated Gold-Nickel Alloy

Structure changes of supersaturated gold-nickel alloy after successive agings were studied by means of electron diffraction and electron microscopy. Specimens were prepared by vacuum evaporation and subsequent quenching (Y. Fukano and S. Ogawa, J. Phys. Soc. Japan 14 (1959) 1671). After aging at temperatures lower than 165°C, side maxima appear beside the main spots from the matrix. The side maxima disappear after aging at 250–300°C, indicating that they correspond to pre-precipitations. The side maxima were interpreted by assuming a modulated structure similar to that found in Au-Pt alloy by Tiedema et al. (T.J. Tiedema, J. Bouman and W.G. Burgers, Acta Metallurgica 5 (1957) 310). This implies that gold-nickel alloy at the pre-precipitation stage contains a modulated structure similar to that in Au-Pt alloy. After aging at temperatures higher than 300°C, two stable phases with parallel lattices appear. In this state, Widmanstatten structure was found in electron micrographs.

Pre-precipitation in AlMgGe and AlMgSi

Single crystals of high-purity aluminium-base alloys (Al-0.7% Mg 2Si, Al-1.0% Mg 2Ge and Al-1.4% Mg 2Ge) have been studied by a crystal monochromated X-ray technique. The pre-precipitation stage of these alloys has been investigated in detail during isothermal ageing at 135, 150, 200, 250 and 300°C. Diffuse X-ray scattering observed during the early ageing period shows that the needle-shaped G.P. zones are probably formed in at least two steps. When first detected these segregates do not seem to possess any internal periodicity whatever. X-ray scattering effects are also observed which seem to indicate that these primitive zones possess a high vacancy concentration. The second part of the process is associated with the establishment of a linear periodicity along the segregate axis which lies parallel to one of the matrix cube directions. The periodicity is identical to that of the matrix—4.04 Å—along this direction. During this stage, the vacancy concentration of the primitive zones abruptly decreases. Hardness measurements of single crystal specimens of these alloys isothermally aged at the same temperatures as above reveal that the G.P. zones are the only hardening agent.

Study on Precipitation Phenomena during Aging of Aluminium-Silver Alloy in Thin Evaporated Films

Precipitation phenomena during aging have been observed on aluminium-20 wt. per cent silver alloy in thin evaporated films by electron micryscopy and electron diffraction. A super-saturated solid solution has successfully been obtained in thin films by a new quenching method. In a preprecipitation stage aggregates composed of random clusters have been found in electron micrographs, no appreciable diffraction effect having been perceived. In a true precipitation stage precipitates have been found in electron micrographs to change from a needle-like to a three dimensional shape with increasing aging, the axial ratio obtained from diffraction patterns changing from 1.63 3 to 1.59 passing 1.61 2 . It is a merit of the present study that a hexagonal precipitate with the ideal axial ratio has been detected in the course of aging. Except this point, the results obtained confirm generally those reported by X-ray studies. So called “equal-thickness interference fringes” have been found in images of precipitates aged at 250°C.

Contribution a l'etude de la precipitation dans un groupe d'alliages a base de nickel

The investigation of the pre-precipitation phenomenon in the alloy Ni-80%-Cr 20%, hardened with aluminum and titanium, has led to the study of precipitation in the following nickel-base alloys: Ni-Al, Ni-Al-Ti, Ni-Cu-Al, Ni-Si, Ni-Cu-Si and Ni-Mo-Si. All these alloys precipitate an ordered face-centred-cubic phase in a f.c.c. matrix. The ageing sequence, identical for all the alloys, is as follows: pre-precipitation stage after short ageing periods there is the presence of anomalous X-ray scattering in the form of satellites near the matrix Bragg spots; intermediate phase-slightly quadratic as a result of its coherency with the matrix; the visible stable precipitate processing a f.c.c. structure. Two classes of alloys exist: those in which the precipitates form platelets and those which form cubic particles. The second category appears when the concentration provokes a difference in parameters between the matrix and the precipitate of the order of 0.01 Å.