Multiple crack healing of a Ti2AlC ceramic

A highly attractive self-healing material would be one which combines excellent mechanical properties with a multiple healing capability. Self-healing ceramics have been studied for over 40 years to obtain some performance recovery and to prevent material failure during service, but so far only materials with the capability of single healing event per damage site have been realized. Here we report on a self-healing Ti₂AlC ceramic capable of repeatedly repairing damage events. The Ti₂AlC ceramic achieves at least seven healing cycles after repeated cracking at a given location. The main healing mechanism at high temperature is the filling of the cracks by the formation well adhering α-Al₂O₃ and the presence of some rutile TiO₂. For healed samples, the flexural strength returned or even slightly exceeded the virginal strength. The fracture toughness recovery has been quantified for multiple healing cycles.     

Competitive Healing of Creep-Induced Damage in a Ternary Fe-3Au-4W Alloy

Metallurgical and Materials Transactions A - Autonomous healing of creep-induced grain boundary cavities by Au-rich and W-rich precipitates was studied in a Fe-3Au-4W (wt pct) alloy at a...     

Rating and ranking of multiple-aspect alternatives using fuzzy sets

A method is proposed to deal with multiple-alternative decision problems under uncertainty. It is assumed that all the alternatives in the choice set can be characterized by a number of aspects, and that information is available to assign weights to these aspects and to construct a rating scheme for the various aspects of each alternative. The method basically consists of computing weighted final ratings for each alternative and comparing the weighted final ratings. The uncertainty that is assumed to be inherent in the assessments of the ratings and weights is accounted for by considering each of these variables as fuzzy quantities, characterized by appropriate membership functions. Accordingly, the final evaluation of the alternatives consists of a degree of membership in the fuzzy set of alternatives ranking first. A practical method is given to compute membership functions of fuzzy sets induced by mappings, and applied to the problem at hand. A number of examples are worked out. The method is compared to another one proposed by Kahne who approaches the problem probabilistically.     

An algorithm for rating multiple-aspect alternatives using fuzzy sets

An improved algorithm for determining the (fuzzy) final rating of a multiple-aspect alternative according to a method proposed in an earlier paper (Baas ad Kwakernaak, 1977) is stated and proved.     

Minimax frequency domain performance and robustness optimization of linear feedback systems

It is shown that feedback system design objectives, such as disturbance attenuation and rejection, power and bandwidth limitation, and robustness, may be expressed in terms of required bounds of the sensitivity function and its complement on the imaginary axis. This leads to a minimax frequency domain optimization problem, whose solution is reduced to the solution of a polynomial equation.     

The effects of alloy microstructure refinement on the short-term thermal oxidation of NiCoCrAlY alloys

Three γ + β NiCoCrAlY alloys (a cast alloy, a laser-surface-melted (LSM) alloy, and a coating as deposited by electron beam-physical vapor deposition (EB-PVD)) with similar average composition (Ni-20Co-19Cr-24Al-0.2Y in at. pct), but with different microstructures prior to oxidation, were oxidized for 0.5 and 1 hours at 1373 K in an Ar 20 vol pct O₂ atmosphere (i.e., at a partial oxygen pressure of 20 kPa). It was found that on the alloy with β precipitates larger than 20 μm, the oxide layer was nonuniform in thickness, and had a laterally inhomogeneous composition and phase constitution. In this case, the oxide layer developed on top of the γ phase was thicker than that formed on top of the β phase and consisted of a NiCr₂O₄/Cr₂O₃ outer and an α-Al₂O₃ inner layer. For the thinner oxide formed on top of the β phase, the outer layer was constituted of a Cr and Co containing NiAl₂O₄ spinel and the inner layer also consisted of α-Al₂O₃. For the alloys with β precipitates smaller than 3 μm, a uniform and laterally homogeneous oxide formed, consisting of a Cr and Co containing NiAl₂O₄ outer layer on top of an α-Al₂O₃ inner layer. After oxidation, Y was distributed as numerous, small precipitates within the oxide layer for a homogeneous Y distribution prior to oxidation, or as a few, very large pegs along the γ/β phase boundaries of the alloy for an inhomogeneous Y distribution prior to oxidation. The performance of the alloys upon thermal cycling was improved for smaller β precipitates and for a more homogeneous Y distribution in the alloy prior to oxidation.