J.H. Root

Kinetics of the δ to γ zirconium hydride transformation in Zr-2.5Nb

Abstract When the concentration of hydrogen exceeds the solubility limit in a metal matrix, metal hydrides may appear as precipitates that degrade the performance of the material. Neutron diffraction was combined with microscopy to study the δ to γ phase transformation of zirconium hydride precipitates in Zr-2.5 wt.% Nb. Specimens were heated to dissolve all hydrides, then cooled to holding temperatures ranging from 17–100 °C, to investigate the kinetics of transformation from the high-temperature δ -hydride to the low-temperature γ -hydride. The δ to γ transformation proceeds over a period of many hours, with a rate that increases as the holding temperature is decreased. Transmission Electron Microscopy images indicate that the boundary regions of hydride precipitates transform to the γ -phase, leaving a shrinking core of the δ -phase. The crystallographic orientations of the hydrides appear to be determined by the texture of the α -Zr matrix, even after complete dissolution and re-precipitation.

Determination of the temperature dependence of the lattice parameters of cementite by neutron diffraction

The structure and behavior of the cementite phase has a profound effect on the properties of steels and cast irons. For example, the size and distribution of cementite particles is known to influence the fracture and fatigue behavior of steels. Cementite plays a critical role in various phase transformations in the system, for example the pearlite and bainite reactions. Recently, it has been shown that cementite plays an important role in the process of austenite formation which occurs when the eutectoid temperature is exceeded. Despite the importance of the cementite phase in the context of ferrous metallurgy, it appears that little emphasis has been placed on determining the dependence of the lattice parameters on temperature. The results which are available have been determined using X-ray diffractometry, which can be subject to systematic errors arising from (i) specimen height misalignments, and (ii) temperature inhomogeneities. The purpose of the present paper is to report data from a high purity Fe-C hypereutectoid steel, which were obtained using high-resolution neutron diffractometry. The cementite is thus in equilibrium with the matrix phase, even above the eutectoid temperature, and this allows measurements to be obtained with a considerable degree of precision.

Observation of kinetics of γ zirconium hydride formation in Zr–2.5Nb by neutron diffraction

Abstract Neutron diffraction was employed to observe an isothermal transformation among zirconium hydrides in a commercial zirconium alloy. A specimen of Zr–2.5Nb, which contained about 200 mg deuterium/kg of alloy, was heated to 450°C for almost 17 h, then cooled directly to 17°C and held at this temperature for an extended time. A series of neutron diffraction patterns was collected during this thermal cycle. The diffraction patterns show that a small amount of the tetragonal γ-phase zirconium hydride appears soon after cooling, along with a predominant quantity of the cubic δ-phase hydride. Over three subsequent days at 17°C, the amount of γ-phase hydride increases while there is a corresponding reduction in the amount of δ-phase hydride.

Effects of Multiple Scattering and Wavelength-Dependent Attenuation on Strain Measurements by Neutron Scattering

Spatial maps of lattice strain inside bulk structures can be obtained by neutron diffraction. In this paper, we describe a systematic investigation of multiple scattering and wavelength-dependent attenuation effects on such measurements deep inside steel plates. The experimental results suggest that wavelength-dependent attenuation can generate peak shifts that correspond to apparent tensile strains of order 1 part in 104 at depths of order 10 mm in ferritic steel. Moreover, near the wavelength corresponding to the (21 1) Bragg cut-off, even larger apparent compressive strains of the order 1 part in lo3 were observed. The experimental results are consistent in sign with a theoretical calculation but there is a discrepancy in magnitude. Multiple scattering was found to have no observable influence on measurements of strain at depth.

Thermally and plastically induced residual strains in textured Zircaloy-2 plate

The residual intergranular strains in textured Zircaloy-2 plate samples induced by cooling from 823 K to ambient temperatures, by cold-rolling by 1.5% and 25% and by deforming in tension by 1.5% were measured by neutron diffraction. The strong rolling texture, which gives rise to two ideal orientations, permitted the interpretation of much of the data in terms of strain tensors for the two orientations. The experimental results were compared with calculations based on the elasto-plastic self-consistent model with no adjustable parameters. Close agreement was obtained for samples in the as-cooled state and deformation in tension by 1.5% but the agreement is less satisfactory for cold-rolling.

NEUTRON DIFFRACTION IN THE MATERIALS SCIENCE TOOLBOX

Abstract Neutron diffraction provides information on residual stress, crystallographic texture, composition and phase transformation in engineered components. The unique power of this analysis tool is provided by the ability of neutrons to penetrate deeply inside most materials. Three dimensional maps of the residual stresses within intact components and true bulk-averaging of material properties are readily achieved by this non-destructive probe. In this paper, two examples of the use of neutron diffraction as a tool in the development of mathematical models of industrial metallurgical processes are presented: (1) the measurement of residual stresses generated by forced convective quenching of steel bars and (2) the measurement of texture evolution during recrystallization of hot rolled aluminum.

Plastic Deformation of Magnesium Alloy Subjected to Compression-First Cyclic Loading

In-situ neutron diffraction has been employed to study the deformation mechanisms in a precipitation-hardened and extruded Mg-8.5wt.% Al alloy subjected to compression followed by reverse tension. The starting texture is such that the basal poles of most grains are oriented normal to the extrusion axis and a small portion of grains are oriented with the basal pole parallel to the extrusion axis. Diffraction peak intensities for several grain orientations monitored in-situ during deformation show that deformation twinning plays an important role in the elastic-plastic transition and subsequent plastic deformation behavior. Significant non-linear behavior is observed during unloading after compression and appears to be due to detwinning. This effect is much stronger after compressive loading than after tensile loading.

Application of neutron diffraction in materials science and engineering

Abstract In materials science and engineering, neutron diffraction has continued to be developed and today is applied mainly to the study of crystallographic texture, micromechanics, residual stresses, and in situ studies of microstructural kinetics. Of key importance to the application of the technique has been an ongoing interaction between researchers at neutron laboratories with an interest and understanding of materials science and leaders in the research community, both in universities and national laboratories and in manufacturing industries. We have found that flexibility in approaching this interaction has been of great benefit and has enabled far more work to be performed with a greater range of collaborators more rapidly than would otherwise be the case. This interaction has lead to many important studies of materials and to the evolution of a service of relevance to materials engineering, the most recent of which will be reviewed briefly.

Neutron scattering at Chalk River laboratories

Abstract The end of the twentieth century was a tough time for Canadas national neutron beam program. National agencies increasingly narrowed their focus on primary missions and tended to cut other activities. In 1996, following a decade of shrinking budgets and increasing constraints, Atomic Energy of Canada Limited (AECL) announced its intention to terminate several of its basic science programs, including the neutron scattering program, which was centered on the National Research Universal (NRU) reactor at Chalk River. With the leadership of Bill Buyers and support from other members of the neutron scattering staff, an agreement was reached between AECL and the National Research Council of Canada (NRC) to transfer the neutron program into the NRCs Steacie Institute for Molecular Sciences. The three-year agreement preserved Canadas neutron scattering team until the newly named Neutron Program for Materials Research (NPMR) could be settled into the long-term framework of the NRC.

Neutron diffraction for industry: optimized processing, failure analysis and regulations

Abstract A vast array of industrial problems can potentially be addressed by neutron diffraction techniques. While it is desirable to strengthen the links between industry and neutron diffraction facilities, the worldwide scarcity of neutron resources dictates that projects be selected carefully. Throughout the 12 years of operation, the Chalk River program of Applied Neutron Diffraction for Industry (ANDI) has undertaken a spectrum of industrial projects: (1) characterizing the effects of processing parameters on material properties, (2) participating in failure analyses on specific engineering components and (3) acquiring new information that could influence industry regulations, such as fitness-for-service guidelines. Examples will be shown of ANDI projects that include phase analysis, stress-scanning and real-time measurements of hydride precipitation.