Masters Theses

Date of Award

5-2000

Degree Type

Thesis

Degree Name

Master of Science

Major

Metallurgical Engineering

Major Professor

Peter K. Liaw

Committee Members

Raymond A. Buchanan, Thomas T. Meek, Terry M. Tiegs

Abstract

The purpose of the study is to create an NiTi/aluminum metal matrix composite (MMC) material which will have mechanical properties superior to those of the aluminum matrix. The goal is to fabricate a composite material by dispersing a shape memory alloy (NiTi) in the form of a powder, into an aluminum matrix, using isostatic or hot pressing, powder-metallurgy processing techniques. The author wishes to obtain a composite material with a satisfactory density (> 97% of theoretical density), greater strength, and improved fatigue resistance, relative to the aluminum matrix.

When the shape-memory alloy particles are embedded in the matrix, the shape memory effect is utilized by deforming the composite material below the Mg temperature, (around -20°C or -4°F), which will also deform each NiTi particle within the matrix (since the martensitic phase of NiTi has a much lower yield strength than aluminum at that temperature). Upon reheating to the austenite phase, the NiTi will return to it’s original shape, (within 8-9% of deformation), embedded within the aluminum matrix which has a much lesser degree of thermal strain. This action will create residual, internal stresses around each NiTi particle, tensile stresses in the longitudinal and transverse directions, and compressive stresses in the through-thickness direction, which will strengthen the material m a similar fashion as thermal stresses strengthen a ceramic particle reinforced metal matrix composite upon cooling from the manufacturing temperature.

In order to accomplish the objective, the NiTi powder may first be reduced in size to enhance bonding and deformation characteristics. This is done through a tedious and time-consuming mechanical milling process. Also, since the shape-memory effect is so strongly dependent on the composition of the NiTi, the powder must first be treated with a coating to resist the diffusion of the matrix into the NiTi particles during pressing. Oxide and nitride coatings have been investigated with moderate success. The process of hot pressing also lends difficulty because the time at elevated pressures and temperatures results in further diffusion. However, without a sufficient pressure and temperature in hot pressing, a good density cannot be achieved. Internal voids due to poor densification may become sites for crack initiation upon loading, thus weakening the material.

Several different NiTi/Al composites have been studied. The NiTi powders have been treated in a couple different manners. One method reduces the particle sizes and roughens their shape. A heat-treatment procedure has been developed in order to produce a surface oxide coating, which helps prevent diffusion during hot pressing. Different aluminum powders have been investigated also. The size of the aluminum particles has proved to be very influential in affecting the quality of composite materials produced. This finding is important because the failure of this kind of composite has been shown to be matrix-dominated.

Some physical and mechanical properties of the NiTi/Al composite material have been characterized by Digital Scanning Calorimetry (DSC), Scanning Electron Microscopy (SEM), and through tensile and fatigue testing. There is a considerable increase in the yield strength, the ultimate strength, and the fatigue resistance due to the addition of the NiTi powder into the matrix. However, the elastic modulus drops slightly in some materials, perhaps due to a phenomenon called stress-induced martensite, which is common in the NiTi shape-memory alloy.

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