Papers by Author: Mark R. Daymond

Abstract: Understanding the relationship between deformation mechanisms and microstructure is essential if one wants to fully exploit the potential of advanced nickel base superalloys and develop future alloys. In the present work, the influence of the lattice misfit between  and ’ has been studied by means of in-situ loading experiments using neutron diffraction in combination with crystal plasticity modelling on RR1000 and Alloy 720Li. Both alloys were processed to generate three simplified uni-modal γ’ microstructures to allow determination of γ’ responses and experiments were carried out at 750°C. The results showed that a positive misfit strain increases the level of load partitioning from  to ’ during plastic deformation introduced by uniaxial tensile loading.

Abstract: This paper reports results of an in-situ compression experiment carried out on a hot rolled Zircaloy-4 plate at ENGIN-X, ISIS. The experiment was aimed at characterizing the plastic anisotropy of the alloy, which can give rise to high intergranular stresses in the polycrystal. As expected from the crystal anisotropy, the various lattice reflections had very different behaviours. In the compression directions, the basal <0002> reflections appeared to bear much more load than the other planes. The resulting intergranular elastic strains could therefore reach up to 5000 microstrain after 10% total deformation, and were responsible for high type II residual stresses after unloading. Considering the macroscopic behaviour, the normal direction had higher mechanical properties than the other two processing directions. The strong texture measured from EBSD measurements suggest that the crystal anisotropy has been brought to a macroscopic level. The experiment also evidenced a significant change in texture for compression along the rolling direction which indicates twinning activation.

Abstract: We report neutron strain scanning experiments on archaeological bronzes, with the aim of identifying the original manufacturing techniques used. The specimens studied were a Picenan necklace from VI BC, and an Etruscan bucket handle from IV BC, exhibited at the Marches Museum of Archaeology, Ancona, Italy. Time-of-flight neutron diffraction and transmission experiments were performed at the ENGIN-X instrument, ISIS, UK. For the necklace, characteristic bending strain profiles and a small degree of preferred orientation indicate that the specimen had been cold worked. For the handle, broad diffraction peaks and highly distorted Bragg edges -typical of a columnar grain microstructure- suggested that this specimen was cast and did not undergo significant thermal or mechanical treatment. The relation between the experimental diffracted and transmitted time-of-flight spectra, and the microstructure of specimens are discussed.

Abstract: On ECRS-6 [1], we have presented first results of the researches of fatigue degradation and martensitic transformation of austenitic stainless steel AISI 321 by neutron diffraction stress analysis. A series of samples preliminary ex-situ cyclically fatigued at the frequency of 5 and 0.5 Hz was in-situ tested on the stress rig of the ENGIN instrument. In the high cycle fatigued (HCF) samples, the applied stress-elastic strain responses of austenite and martensite phases were find out to be strongly different as compared to the low cycle fatigued (LCF) samples, in which they are close. Moreover, the martensite Poisson ratio in the HCF-samples is almost twice to that of observed 0.28-0.30 in austenite and in both phases of the LCF-samples. With the purpose to search the reason of such unusual behavior of the martensite phase, one of the HCF-samples has been anew in-situ tested on the stress rig of the ENGIN-X in: 1) a LCF-mode at the frequency of 0.1 Hz to increase the fatigue level, and 2) a quasistatic mode to measure the applied stress-elastic strain responses of both phases. Also, two of the LCF-samples have been subjected to the ex-situ secondary HCF-testing at the frequency of 5 Hz and again in-situ measured on the ENGIN-X stress rig. Results of the mechanical characterization of phases in the twice fatigued austenitic stainless steel are presented and discussed.

Abstract: Plastic anisotropy can affect the strains measured by neutron diffraction. If this is not properly accounted for significant errors can result in the calculated stresses. This paper illustrates addresses this issue using measurements of the residual strain field around a 60 mm long single weld bead deposited on the surface of a 17 mm thick stainless steel plate. Measurements were made on ENGIN-X, the engineering spectrometer at the ISIS facility of the Rutherford Appleton Laboratory (UK). Diffraction spectra from these measurements have been fitted using both single and multi-peak fitting approaches. Both residual strain and stress results have been presented for (111) and (200) single peak fits and compared to a multi-peak fit (Rietveld) analysis. Results from these analyses have revealed significant anisotropy in the response of the individual lattice planes. This effect is most severe in measurements carried out in the normal direction and is shown to be predicted using an elasto-plastic self-consistent model.

Abstract: The influence of texture and anisotropy on the generation of intergranular stresses in clock-rolled zirconium is investigated using neutron diffraction and elastoplastic self-consistent modelling. Comparison between experimental data and model calculations indicates that the operation mainly of prismatic and basal slip explains the trends in intergranular stress evolution during in-plane tensile and through-thickness compressive deformation, whilst twinning plays a significant role during in-plane compression.

Abstract: Neutron diffraction has been used to study the progress of deformation twinning and intergranular strain evolution in extruded magnesium during cyclic and monotonic loading at two temperatures. Differences in the intergranular strains generated during tensile and compressive tests are attributed to the operation of twinning in compression. Twinning activity is reduced relative to slip at higher temperature, leading to greater similarities between tension and compression. During cyclic loading, a distinct Bauschinger effect is observed after each compressive loading stage. The origin of this effect is identified as the reversal of twinning during unloading and subsequent tensile loading.