J.W. Martin

High-resolution measurement of crack-tip plastic zone sizes by selected area channelling patterns

Abstract Selected area channelling patterns (SACPs) have been used to measure plastic zones (PZ) associated with fatigue cracks in a nickel-based superalloy MA6000. Refinement of the technique has enabled the PZ dimension to be measured to within ±0.5 μm, which represents an order of magnitude improvement on previously reported work. It has been shown, in a series of fatigue tests at various frequencies and temperatures, that the size of the plastic zone at a given value of ΔK varies with the square root of the crack propagation rate.

The study of local plastic strain in nickel-based superalloys by selected area channeling patterns in the STEM

Abstract It is shown that selected area channeling patterns (SACPs) of high quality may be obtained from two nickel-based superalloys (namely Nimonic 90 and Mar-M246) by employing, in the scanning mode of operation, a JEM 100C electron microscope with TEMSCAN attachment, using an accelerating voltage of 100 kV. Both differentiated signal processing and mode dc were used to observe the channeling patterns. The effect of specimen plastic strain upon the SACPs has been analyzed, and by using a plot of (h 2 + k 2 + l 2 ) for the highest order line indexable in the Nimonic 90 material, a systematic deterioration in SACP quality with strain has been demonstrated. For unidirectionally solidified Mar-M246 the variation in line width [for different (hkl) lines] was followed for specimens subjected to increasing strain. This demonstrated a linear variation for the lines measured. The technique has been used to map the plastically strained area around a fatigue crack in Nimonic 90.

The study of local plastic strain distribution in aged Al alloys by selected area channelling patterns in the SEM

Abstract It is shown that SACPs can be obtained from fully aged At-Mg-Si alloys, and a semiquantitative relationship has been obtained between the quality of the patterns and the percent plastic strain in the material. The technique has been used to compare the distribution of strain adjacent to a propagating crack (a) in a ternary alloy, when the fracture path is intergranular and (b) in an alloy containing Mn, when transgranular fracture takes place. A marked difference in the dimensions of the plastic zone associated with the crack was recorded

A microanalytical study of M23C6 carbides as a function of ageing time in Nb-bearing austenitic steels

Abstract A microanalyzing electron microscope (an AEI EMMA 4 instrument) has been used to measure the Nb content of intergranular M23C6 particles as a function of ageing time at 750°C in a series of 20Cr25Ni austenitic stainless steels of differing C/Nb ratio. It is found that in a given alloy the amount of Nb in M23C6 increases as the particle size and/or ageing time increases. For a given size of carbide, it is found that an increase in the ageing time leads to a greater increase in Nb content the higher the C/Nb ratio of the alloy. The results may be understood when the overall precipitation behavior of the alloys is considered.

The effect of plastic deformation on the formation of sigma phase in an austenitic stainless steel

Abstract A 24% Ni, 25% Cr austenitic steel containing 0.27% Ti has been solution-treated and quenched from 1150°C. The effect of prior cold-rolling on the aging processes at 750°C has been studied by transmission electron microscopy, comparative tests having been made on a Ti-free steel. Acceleration of ferrite and sigma-phase formation on recrystallization of the matrix was observed. It is suggested that the effect arises from the segregation of Ti atoms to dislocations, which element would otherwise promote the formation of these phases. The presence of serrated stress-strain curves in tensile tests at 704°C in the Ti-containing alloy supports this hypothesis. Only when the dislocation density is reduced by recrystallization is the free Ti content sufficient to promote the formation of the ferrite and sigma phases.

Study of the crack tip plastic strain in a nickel-based superalloy

Abstract Selected area channeling patterns (SACPs) of high quality have been obtained from a single crystal nickel-based superalloy, and the effect of plastic strain upon the patterns has been analyzed. Analysis of the pattern quality enables strains of approximately 1% to be readily detected, thus permitting the mapping of a crack tip plastic zone associated with a fatigue crack. A systematic deterioration in SACP quality with strain has also been demonstrated and the technique applied to the determination of the strain distribution within the plastic zone.

A localized melting phenomenon in 20% Cr-25% Ni austenitic stainless steels

Abstract A series of 20% Cr-25% Ni austenitic stainless steels, stabilized by small additions of niobium and/or titanium have been studied. With the aid of an electron microprobe it is possible to show that at temperatures near the melting point, incipient melting occurs due to microsegration of niobium and titanium. The process is the formation of Fe2Nb and Fe2Ti from NbC and TiC, via a metastable molten phase. An orientation relationship between Fe2Nb and austenite was established.

Precipitation of HfC in MM-002

The effect of prolonged exposure to high temperature (1050°C) upon the microstructure of the nickel-based superalloy MM-002 has been investigated. The “script” (Ta, Ti, Hf)C carbides initially present were seen to be unstable, and precipitation occurred both of M6C and of an Hf-rich MC phase. The orientation relationship with the matrix of the HfC phase was determined as [001]m[210]HfC; [110]m[12o]HfC. A habit plane of the precipitate is suggested, namely (111)m(210HfC.

Structure and strength of an Al-Zn-Mg alloy after duplex aging treatment

Abstract Multiple aging has been applied to an Al-7.5 wt.% Zn-2.5 wt.% Mg alloy with a view to producing a duplex distribution of precipitate particles. A combination of a coarse distribution of η phase, together with finer η′ and GP zones, has been obtained. Transmission electron microscopy shows that coarse, well-defined slip bands occur on deformation of the alloy after a single aging treatment to peak hardness. A more homogeneous dislocation distribution is observed in the material after double aging treatment because of the presence of the incoherent phase in the alloy. Tensile tests indicate that no improvement in ductility occurs with duplex aging, but this may be owing to the larger grain boundary PFZ present.

The growth kinetics of NbC particles in 20Cr-25Ni austenitic steels

Abstract Four steels of differing Nb/C ratio have been solution-treated then aged directly for periods up to 107 sec at temperatures between 650°C and 1100°C. Parabolic growth of the intragranular NbC particles was observed, and from the measured particle sizes, the time to form a particle of diameter 1000A was calculated for each alloy and ageing temperature. The results are compared with calculated growth curves using an equation employing the invariant field approximation to simplify the analysis of the diffusion problem. Although the Nb diffusion data necessarily employed relate to a pure γ-iron matrix, the correlation with experiment appears adequate to encourage the further use of the analysis.