A small-angle x-ray scattering investigation of the zone formation of “maraging” type alloys

The small angle X-ray scattering from “Maraging” type alloys previously aged at temperatures below 450°C has been studied at room temperature. The behavior, during aging, of two high purity alloys containing Fe Ni Co and Fe Ni Co Mo is compared with an industrial Vascomax 300 type steel. The existence of G.P. zones is pointed out for the two Mo containing alloys. In the case of the quaternary alloy, the evolution of these zones during the aging is clearly defined. For the industrial steel, the interpretation of the observed phenomena is more complex due to the additional small amounts of Ti, Cu, Al and is discussed.     

Effect of heat treatment on the crystalline structure of martensite in iron-, nickel-, manganese- and silicon-doped CoW and CoMo alloys

X-ray investigation of the crystalline structure of martensite of CoWFe, CoMo(Fe, Ni, Mn, Si) single crystals is performed after quenching and preliminary aging in α and β phases. Continuous decomposition of α and β solid solutions at aging at temperatures 500–700°C is observed. In Fe- and Ni-doped alloys the multilayer martensile polytypes are formed. In CoMoFe and CoMoNi single crystals such polytypes are found at the satellite stages of decomposition.     

Control of ordered structure and ductility of (Fe,Co, Ni)3V alloys

This study is directed toward improvement of the ductility of long-range ordered alloys through control of their ordered crystal structure. A series of ordered alloys was prepared with a base composition of Co₃V, where Co was partially replaced with Fe and Ni. The stability and structure of the ordered phases in these (Fe,Co,Ni)₃V alloys were characterized by various metallurgical methods. The results indicate that the ordered structure in this alloy system can be controlled by adjusting the electron density, and that the L1₂ type cubic ordered structure (α′) is stable in the alloys with electron density less than 7.888. The phase relation in the cubic ordered alloys depends on the Fe concentration. For the alloys containing <20 pct Fe, the disordered α solid solution transforms to the cubic α′ ordered on the fcc lattice at temperatures below 1000°C. For the alloys containing >20 pct Fe, the α′ is formed through a peritectoid reaction, namely, α+σ→α′. Tensile tests indicate that the alloys with multilayered hexagonal ordered structure are very brittle, while the alloys with the cubic ordered structure are ductile at room temperature. The ductility of the cubic ordered alloys increases with decreasing Co content. The alloys with <55 pct Co showed dimple type ductile rupture with elongation over 40 pct at room temperature. The correlation of ductility with ordered structure is discussed.     

Co对高Ni-Co二次硬化钢微结构影响的穆斯堡尔谱研究

用穆斯堡尔谱研究了钴含量对高Ni-Co二次硬化钢微结构的影响。发现：随着钢中钴含量的减少,马氏体中的钴作为铁原子近邻的原子组态总量减少,钴在马氏体中的分布会影响镍,铬,钼原子在马氏体中的分布;钴促进奥氏体向马氏体转变,减少残余奥氏体量,钴促进合金碳化物的形成,增加其析出量。     

Effect of composition on the properties of strain-transformable β titanium alloys

A number of metastableβ titanium alloys were examined to determine the effects of composition on strain-transformation behavior and precipitation hardening response. Maximum ductility as solution heat treated was observed in alloys slightly richer in alloy content than the minimum alloy content required to retain an all-β microstructure on quenching. Such materials transformed to a martensitic structure upon straining and, as a result of strain transformation, developed room temperature ductility exceeding that found in unalloyed titanium. Uniform elongation of 35 to 45 pct was observed in a number of compositions of this type containing major additions of Mo, V, Cr, or Mn. Auxiliary alloy additions of Sn, Al, or Zr, or ternary alloying with molybdenum were necessary to preventω embrittlement during quenching in alloys containing V, Cr, or Mn. Alloying with Fe, Cu, Co, or Ni resulted in low ductility as solution heat treated, but it is probable that optimum amounts of these additions were not studied in this investigation. Oxygen above about 1200 ppm also had a detrimental effect on ductility. All alloys studied showed precipitation hardening when heat treated in the 800° to 1100°F range. Tensile strengths of 170 to 190 ksi were readily attainable in most alloy systems. Ductility as precipitation hardened appeared to be higher in alloys containing at least 6 pct Mo or V than in other alloys studied.     

Effect of cooling rate and alloying on the

The pearlitic hardenability of a high-purity Fe-0.8 pct C alloy and zone-refined iron binary alloys containing Mn, Ni, Si, Mo, or Co was studied by means of hot-stage microscopy. The binary alloys were carburized in a gradient furnace to produce eutectoid compositions, thus eliminating proeutectoid phases. A special technique based on hot-stage microscopy was used to study the effect of cooling rate (10°F/min to 25,000°F/min) on the transformation of austenite and provided data for the construction of continuous cooling-transformation diagrams. From these diagrams critical cooling rates were obtained for hardenability calculations. It was found that molybdenum is the most effective element, followed by Si, Ni, Co, and Mn, in suppressing the pearlite transformation,i.e., in increasing the hardenability of the alloys studied. The alloying additions were grouped into two classes according to their effect on hardenability: α-stabilizers (Mo and Si) and γ-stabilizers (Ni, Co, Mn), with the α-stabilizers being the more effective in improving hardenability. This paper is based on a presentation made at a symposium on “Hardenability” held at the Cleveland Meeting of The Metallurgical Society of AIME, October 17, 1972, under the sponsorship of the IMD Heat Treatment Committee.     

Effects of alloying additions and austenitizing treatments on secondary hardening and fracture behavior for martensitic steels containing both Mo and W

The effects of alloying additions and austenitizing treatments on secondary hardening and fracture behavior of martensitic steels containing both Mo and W were investigated. The secondary hardening response and properties of these steels are dependent on the composition and distribution of the carbides formed during aging (tempering) of the martensite, as modified by alloying additions and austenitizing treatments. The precipitates responsible for secondary hardening are M₂C carbides formed during the dissolution of the cementite (M₃C). The Mo-W steel showed moderately strong secondary hardening and delayed overaging due to the combined effects of Mo and W. The addition of Cr removed secondary hardening by the stabilization of cementite, which inhibited the formation of M₂C carbides. The elements Co and Ni, particularly in combination, strongly increased secondary hardening. Additions of Ni promoted the dissolution of cementite and provided carbon for the formation of M₂C carbide, while Co increased the nucleation rate of M₂C carbide. Fracture behavior is interpreted in terms of the presence of impurities and coarse cementite at the grain boundaries and the variation in matrix strength associated with the formation of M₂C carbides. For the Mo-W-Cr-Co-Ni steel, the double-austenitizing at the relatively low temperatures of 899 to 816 °C accelerated the aging kinetics because the ratio of Cr/(Mo + W) increased in the matrix due to the presence of undissolved carbides containing considerably larger concentrations of (Mo + W). The undissolved carbides reduced the impact toughness for aging temperatures up to 510 °C, prior to the large decrease in hardness that occurred on aging at higher temperatures.     

Elimination of precipitate free zones in an Fe−Nb creep-resistant alloy

Precipitation of the Fe₂Nb intermetallic compound has previously been found to cause substantial hardening during aging of Fe rich Fe-Nb alloys. However, the formation of a wide precipitate free zone adjacent to the grain boundaries caused a degradation of creep resistance. In an effort to decrease the precipitate free zone width, thereby improving the creep resistance, an extensive study was made of the precipitation behavior of an Fe-1.7 at. pct Nb(Cb) alloy quenched from the δ-phase field. The quenched alloy was found to decompose via a two step reaction during aging at temperatures below 550°C. The first step in the decomposition reaction is thought to occur by clustering of Nb atoms in the ferrite matrix, similar to the clustering of Mo atoms which is known to occur during aging of Fe-Mo alloys. The second step in the reaction is not well understood. The precipitate free zones were formed by solute depletion in the vicinity of the grain boundary and the subsequent difficulty of nucleation of the Fe₂Nb precipitates in the regions of lowered solute concentration. Using two step aging treatments, an initial low temperature step to develop the Nb atom clusters followed by a higher temperature step to cause Fe₂Nb precipitation, the precipitate free zones were eliminated from the aged alloys. The origin of this effect is thought to be the heterogeneous nucleation of Fe₂Nb precipitates on the clusters developed during the initial aging step.     

Role of Mo and W during sensitization of superaustenitic stainless steel—crystallography and composition of precipitates

This study concerns the crystallographic identification and compositions of precipitates formed in superaustenitic stainless steel. Three experimental alloys, all containing 24 wt pct Cr, 22 wt pct Ni, and 0.5 wt pct N but with varying amounts of Mo and W, were investigated after sensitization heat treatment (aging) at 900 °C. The contents of Mo and W in the three alloys were 7 wt pct Mo, (6 wt pct Mo + 2 wt pct W) and (5 wt pct Mo + 5 wt pct W), respectively. While σ and x were the main secondary phases found in the W-free alloy, replacement of Mo by W was found to promote the formation of Laves-related phases with high Mo + W content. The complex crystallographic nature of Laves-related precipitates was exemplified through the formation of intergrowing C14 Laves, μ, and C phases, all with closely related crystal structures. There was no difference in chemical composition between the three phases. Prolonged aging resulted in intragranular precipitation of different intermetallic phases, as well as formation of nitrogen bearing phases, π and Cr₂N, adjacent to previously formed intermetallic precipitates. The content of Mo + W was found to decrease with increasing aging time for all secondary phases.