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Materials Engineering
 


Materials Engineering

Materials Staff - Dr Martin Rist

 

Dr Martin Rist

Lecturer


It is with deep regret that we have to inform you that Dr Martin Rist passed away in December 2012 after a long illness. Our thoughts are with his family.

For Dr Rist's primary research publications please follow this link to my page at Open Research Online

Follow this link for all Publications

Martin Rist
Dr Martin Rist

 

Teaching:

·   T207 Engineering: mechanics, materials, design.

·   TXR220 Engineering in action

·   T357 Structural integrity: designing against failure

·   T198 Engineering at work

·   G18 Foundation Degree in Materials Fabrication and Engineering


Research:

Experimental and theoretical deformation mechanics of high-temperature engineering alloys and earth materials. Research examples:

·   Hot deformation of two-phase titanium alloys

·   Characterisation of quenching residual stresses

·   Creep of lead-free solders

·   Fracture mechanics of ice

Hot deformation of two-phase titanium alloys

A number of technologically important titanium alloys combine a hexagonal (alpha) phase with a body-centred cubic (beta) phase in a form that is optimised by heat treatment and hot working. Research is underway to develop a functional quantitative description of hot deformation in such alloys that takes account of both the two-phase nature of the material and its evolving microstructure. Hot compression testing is being used to determine the flow behaviour of single-phase alpha, single-phase beta and two-phase Ti-6Al-4V specimens at temperatures and strain rates relevant to thermo-mechanical processing. Semi-empirical, history-dependent constitutive equations, based on the evolution of internal microstructural variables, have been formulated to model the full stress-strain behaviour of the individual phases and of the two-phase material. The model, which is currently under development, takes account of temperature-dependent changes in phase volume fractions, flow properties and gross interaction mechanisms between the alpha and beta phases. As part of this work, methods for parameter optimisation in non-linear constitutive equations are also being investigated.

The key microstructural change during hot deformation of Ti-6Al-4V with an initial acicular microstructure (left, undeformed) is the coarsening and break up of the alpha lath structure (right, deformed at 950ºC, 0.3 /s to a true strain of 1).

Characterisation of quenching residual stresses

The production of modern gas turbine discs for use in aero-engines requires multiple manufacturing steps carried out to rigorous specification. A typical processing route involves casting, ingot refinement and shaping via hot deformation, heat treatment and then machining. To assist with quality control and cost savings, computational simulations of the various processing steps are often used to optimise processing parameters, and significant progress has recently been made with the development of integrated process modelling covering several manufacturing stages. To help validate such models, an assessment of quenching residual stresses created during post-forging heat treatment of a large, 40cm-diameter IN718 compressor disc has been carried out using neutron diffraction. Strain measurements were carried out using ENGIN-X, the engineering diffractometer at the ISIS pulsed neutron source in the UK, and three-dimensional stress components were derived for the first time in such a large superalloy specimen. Comparison with the results from a coupled thermal-mechanical finite-element model of the quenching process confirmed that near-surface residual compression is balanced by residual tension within the disc interior. The steepest stress and strain gradients were found to occur within the transition from compression to tension, about 1cm below the surface all around the disc. The largest stress component is in the tangential (hoop) direction and exceeds the effective yield stress because of the presence of significant hydrostatic stress.

 Two-dimensional finite-element model of an axi-symmetric turbine disc showing hoop residual stresses following heat treatment and water quenching.

Creep of lead-free solders

The new breed of environmentally-friendly solder alloys contain high proportions of tin, typically over 90%, and differ markedly from the traditional Sn-Pb eutectic solder alloy containing 37% lead. The key microstructural feature in the tin-based alloys is the presence of small intermetallic phase-particles, most notably those that form between the alloying components in the popular Sn-Ag, Sn-Cu and Sn-Ag-Cu systems. These alloys can be considered as precipitate-strengthened tin, and attempts are currently being made to incorporate  the associated matrix-particle effect into creep analyses using the concept of internal stress. Primary creep is simulated using constitutive equations that consider both the soft-matrix and hard-particle phases to deform in an elastic-viscoplastic manner. Stress redistribution leads to an internal back stress that evolves towards a steady-state under constant external stress. Tertiary creep is accounted for by incorporating a further internal damage parameter. This approach has been used to rationalise the creep-constitutive behaviour of Sn-3.8Ag-0.7Cu solder, emerging as the primary replacement for lead-containing alloys in general-purpose applications; an analysis of deformation in Sn-3.5Ag solder is nearing completion. Research is also underway using neutron diffraction experiments to identify active slip systems in tin-based solders and in pure tin.

Experimental creep dataset for Sn-3.8Ag-0.7Cu (symbols) compared to the power law-equation for steady-state creep, modified to include a temperature-dependent internal back stress Sss (solid lines).

Fracture mechanics of ice

Ice can be considered as a ‘high-temperature’ material because it normally exists on earth at temperatures very close to its melting point. Global warming is expected to lead to enhanced flow in land-based glaciers and ice sheets, but the recent rapid break-up of small floating ice shelves on the Antarctic peninsula has also highlighted the need for a better understanding of ice fracture mechanisms. As well as limiting the ice shelf extent by causing iceberg formation at the seaward margin, brittle fractures (crevasses) often penetrate to a significant portion of the total shelf thickness elsewhere and have an important effect on overall ice shelf dynamics. Fracture experiments on Antarctic ice core have been used in conjunction with linear-elastic ice-shelf fracture mechanics to predict the likely occurrence and propagation of surface and basal crevasses. In the case of ‘warm’ ice shelves, surface crevassing appears to be enhanced by meltwater -  a phenomenon that can play a significant role in promoting crack initiation. The meltwater not only applies an internal pressure to the crack faces, it also appears to promote crack blunting and slow, sub-critical crack growth, factors which are currently being incorporated into existing models.

Critical stress intensity factor for crack initiation in Antarctic shelf ice, measured using chevron-notched cylindrical ice core.

 

Recent Publications


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