A study of intergranular fracture in iron using auger spectroscopy

Low temperature impact tests on three fairly pure irons have shown that the propensity for intergranular fracture is largely independent of prior heat treatment. Furthermore, the effects of carbon and oxygen contents and carbon : oxygen ratio were found to be opposite to those previously reported. Examination of fracture surfaces by Auger Spectroscopy showed that sulfur was strongly segregated to grain boundaries but showed no evidence of oxygen segregation. The fracture behavior of specimens previously tested in creep or high temperature fatigue differed from that of untested specimens in that fracture was predominantly by transgranular cleavage. Formerly at the University of Waterloo     

The use of a boron addition to prevent intergranular embrittlement in Fe-12Mn

Fe-12 Mn alloys undergo failure by catastrophic intergranular fracture when tested at low temperature in the as-austenitized condition, a consideration which prevents their use for structural applications at cryogenic temperatures. The present research was undertaken to identify modifications in alloy composition or heat treatment which would suppress this embrittlement. Chemical and microstructural analyses were made on the prior austenite grain boundaries within the alloy in its embrittled state. These studies failed to reveal a chemical or microstructural source for the brittleness, suggesting that intergranular brittleness is inherent to the alloy in the as-austenitized condition. The addition of 0.002 to 0.01 wt pct boron successfully prevented intergranular fracture, leading to a spectacular improvement in the low temperature impact toughness of the alloy. Autoradiographic studies suggest that boron segregates to the austenite grain boundaries during annealing at temperatures near 1000 °C. The cryogenic toughness of a Fe-12Mn-0.002B alloy could be further improved by suitable tempering treatments. However, the alloy embrittled if inappropriate tempering temperatures were used. This temper embrittlement was concom-itant with the dissolution of boron from the prior austenite grain boundaries, which reestablishes the intergranular fracture mode.     

An acoustic emission investigation of microscopic ductile fracture

A high-strength 4340 steel fracture-toughness specimen was heat treated to give a ductile-rupture type of slow crack growth under rising load. For evaluation of the step-wise growth process, the specimen was instrumented with acoustic stress wave emission (SWE) detection equipment. The resulting crack area swept out by the advancing crack was correlated to the magnitude and number of the acoustic emission pulses. A crack growth model was developed which accounts for the direct relationship between crack area swept out and the sum of the individual SWE amplitudes, and for the experimentally observed bimodal distribution of the SWE amplitudes. The model postulates that slow crack growth takes place in a step-wise mechanism. This involves a repeated two-step process where the first step is the formation of a multitude of individual thumbnail cracks and the second step is the simultaneous interconnection of these thumbnail cracks to form a new continuous crack front.     

An acoustic emission investigation of microscopic ductile fracture

A high-strength 4340 steel fracture-toughness specimen was heat treated to give a ductile-rupture type of slow crack growth under rising load. For evaluation of the step-wise growth process, the specimen was instrumented with acoustic stress wave emission (SWE) detection equipment. The resulting crack area swept out by the advancing crack was correlated to the magnitude and number of the acoustic emission pulses. A crack growth model was developed which accounts for the direct relationship between crack area swept out and the sum of the individual SWE amplitudes, and for the experimentally observed bimodal distribution of the SWE amplitudes. The model postulates that slow crack growth takes place in a step-wise mechanism. This involves a repeated two-step process where the first step is the formation of a multitude of individual thumbnail cracks and the second step is the simultaneous interconnection of these thumbnail cracks to form a new continuous crack front. Formerly , Postdoctorate Associate, Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minn.     

In situ scanning auger analysis of hydrogen-induced fracture in Ti-6Al-6V-2Sn

Metallurgical and Materials Transactions A -     

Effect of boron on the grain boundary segregation of phosphorus and intergranular fracture in high-purity Fe-0.2 Pct P-B alloys

The effect of boron on the grain boundary segregation of phosphorus in a high-purity Fe-0.2 pct P alloy has been investigated by Auger electron spectroscopy (AES). The segregation of phosphorus decreases markedly with the segregation of boron; phosphorus atoms are replaced by boron atoms at grain boundaries. The free energy of segregation of boron at 1073 K is determined to be 100 kJ/mol. The effect of boron on the phosphorus-induced intergranular fracture (IGF) has been examined with impact testing, and the fractography has been studied with scanning electron microscopy (SEM). Addition of 12.5 wt · ppm boron completely prevents the IGF induced by the segregation of phosphorus and decreases the ductile-brittle transition temperature (DBTT) by about 170 K when quenched from 1073 K. The suppression of the IGF due to the addition of boron is caused by two mechanisms. One is the increased grain boundary cohesion of iron caused by the segregated boron as its inherent effect. The other is the decrease in the segregation of phosphorus caused by the segregation of boron. The former has been shown to be more effective than the latter in suppressing the IGF. Formerly Graduate Student, Postdoctoral Fellow Formerly Research Student, Institute for Materials Research, Tohoku University Formerly with the Institute for Materials Research, Tohoku University     

On the substructure of athermal and isothermal martensites formed in an Fe-21 Ni-4mn alloy

Transmission electron microscopy and diffraction analysis have been made on the athermal and isothermal martensites formed in an Fe-21Ni-4Mn alloy, with particular attention to details of the martensite crystallography and substructure. In general, a variation in the martensite substructure has been found, comparing adjacent regions of near proximity in the same specimen, but nevertheless general conclusions can be drawn. Plates of martensite were observed after both isothermal and athermal treatment. At an early stage of growth, the isothermal martensite plates exhibit similar crystallographic and substructural features compared to those found in the athermally formed plates. In both cases, the overall macroscopic martensite habit plane is near (252)_f, but such macroscopic plates actually consist of many small plates (termed subplates) of the same orientation with a (121)_f habit plane. In the case of isothermal plates an apparent rotation of the habit plane to ward ( 111 )_f occurs as the plates thicken. This rotation was not observed for athermally formed plates. The observations made in this work suggest that temperature dependent relaxation of transformation strains may affect the growth and autocatalytic nucleation of martensite subplates, thus causing the microstructural differences occurring between isothermal and athermal martensite plates. Formerly with the Department of Metallurgy and Mining Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801 Formerly with the Department of Metallurgy and Mining Engineering, University of Illinois     

High resolution electron microscopic study of the ordering processes in Ni4Mo alloy

We have studied the ordering processes of a Ni-20.1at.%Mo alloy using the many-beam imaging technique with the aid of an image processing method. No microdomain is detected in high resolution electron micrographs of the alloy quenched from 1000°C, but lattice modulations which probably be attributed to concentration waves appear parallel to the {210} planes at the initial ordering stage. It is revealed that the observed contrasts of D1_a-type and D0₂₂-type microdomains are apparently formed with superpositions of different pairs of the {210} lattice modulations. When the alloy is annealed at 650°C for 192 h, Ni₂Mo domains appear in between the Ni₄Mo domains, and their composition is determined to be the stoichiometry (32at.%Mo) by the energy dispersive X-ray (EDX) analysis. The superstructure images of Ni₄Mo and Ni₂Mo are discussed in comparison with computer simulations by the multislice method.     

Prevention of the intergranular fracture by addition of silicon and aluminum to a high-purity Fe-0.2 Pct P alloy with a trace of boron

The effect of addition of 0.25 pct silicon and aluminum on the intergranular fracture and the grain-boundary segregation of solutes in a high-purity Fe-0.2 pct P alloy with a trace of boron has been investigated by impact test, tensile test at low temperatures, optical and scanning electron microscopy, and Auger electron spectroscopy (AES). The addition of 0.25 pct silicon and aluminum remarkably reduces the susceptibility of an Fe-0.2 pct P alloy to the intergranular fracture and decreases the ductile-brittle transition temperature (DBTT). The addition of silicon and aluminum causes considerable segregation of boron to grain boundaries and, simulta-neously, remarkably decreases the segregation of phosphorus in the Fe-0.2 pct P alloy. The cause for these effects is discussed. Formerly Professor, Institute for Materials Research, Tohoku University     

High resolution electron microscopic studies of Ni-9.3 and 14.7 at.%Ti alloys

The early stages of decomposition in Ni-9.3 and 14.7 at.% Ti alloys are studied by means of high resolution electron microscopy. The microstructure of the as-quenched Ni-9.3 at.% Ti alloy shows contrast of broad circular regions, and local lattice images show some difference in lattice spacing suggesting compositional modulations. Electron and optical diffraction patterns indicate faintly ordered regions. After ageing for 15 min at 570°C, the ordered regions, approx. 50 Å in diameter, appear as clear lattice images surrounded by dark areas of the f.c.c. γ-solid solution. The Ni-14.7at.% Ti alloy aged for 15 min at 570°C shows ordered lattice images in wide areas of the matrix, apart from some large needle-like precipitates of the equilibrium η-phase formed during the homogenization at 1200°C. As the atomic arrangement in the ordered regions corresponds to the (110) projection of the L1₂ structure, the image is interpreted as the structure image of ordered L1₂, Ni₃(Ti_xNi_1−x). The lattice spacings locally change in the ordered regions, suggesting some modulation in composition and degree of order.     

Correlation of fracture toughness with the intergranular failure fraction in powder iron specimens

Powder Metallurgy and Metal Ceramics -     

The nucleation of high temperature brittle intergranular fracture in 2.25cr-1mo steel

An investigation of brittle intergranular crack nucleation at high temperatures in 2.25Cr-lMo steel is presented. A series of tests was designed to allow the independent variation of segregating impurities (e.g., sulfur and phosphorus) and locally applied stress levels to determine the effects of applied stress on impurity enrichment and fracture of grain boundaries in as-quenched material. The study utilized blunt notched specimens with suitable finite element stress analyses, together with high resolution Auger electron surface analysis techniques. It is shown that the imposition of a tensile hydrostatic stress enhances the segregation of elemental sulfur to grain boundariesprior to the nucleation of high temperature brittle intergranular cracks. The location of enriched grain boundaries relative to the peak stress regions in notched specimens is discussed, and possible mechanisms for the observed sulfur enrichment presented.     

The formation of water vapor bubbles in copper and their effect on intergranular creep fracture

Small, closely spaced water vapor bubbles are formed along grain boundaries in coarse grained copper by annealing first in air at 800°C and then in hydrogen at 600°C. Almost all of the cavities are formed at grain boundaries, primarily because oxygen strongly segregates to grain boundaries in copper. The average cavity diameter and planar spacing are found to be 0.8 and 3.2 μm, respectively. A study of the creep fracture properties of copper containing the above cavity microstructure indicates that intergranular cavity growth is controlled by the rate of self diffusion along the cavity surface in the manner first described by Chuang and Rice. The stress dependence of the rupture time is found to be of the form t_r ~ σ⁻³. The temperature dependence for fracture is characterized by an activation energy of 92 kJ/mol, which is close to that for surface self diffusion. These properties, together with the absolute rupture times are in close agreement with the Chuang-Rice theory in the limit of surface diffusion control. The determination of the controlling fracture mechanism is supported by metallographic and fractographic evidence.