Bulk high-entropy nitrides and carbonitrides

High-entropy ceramics have potential to improve the mechanical properties and high-temperature stability over traditional ceramics, and high entropy nitrides and carbonitrides (HENs and HECNs) are particularly attractive for high temperature and high hardness applications. The synthesis of 5 bulk HENs and 4 bulk HECNs forming single-phase materials is reported herein among 11 samples prepared. The hardness of HENs and HECNs increased by an average of 22% and 39%, respectively, over the rule-of-mixtures average of their monocarbide and mononitride precursors. Similarly, elastic modulus values increased by an average of 17% in nitrides and 31% in carbonitrides over their rule-of-mixtures values. The enhancement in mechanical properties is tied to an increase in the configurational entropy and a decrease in the valence electron concentration, providing parameters for tuning mechanical properties of high-entropy ceramics.

Rule of Mixtures Model for 3D Printed Kevlar Reinforced Nylon: Determination of Volume Fraction Using Thermal Gravimetric Analysis

An experimental procedure to determine the elastic modulus and tensile strength of kevlar reinforced nylon composites is discussed. Thermal Gravimetric Analysis (TGA), has been performed to determine the volume fraction of fiber and matrix components. TGA is a robust method to determine the volume fraction. It is also less labor intensive as compared to other methods. Samples with varying kevlar-nylon layer ratio were additively manufactured using fused deposition modelling (FDM) based on ASTM D3039 standards. MarkForged Mark X7 3D printer was used to manufacture samples. Elastic and tensile tests of the samples were conducted. The relation between volume fraction and elastic modulus of the composite can indeed be fit into the rule of mixtures model. However, its applicability for ultimate tensile strength for high fiber ratio composites has been put to question. The direction of fibers in the additively manufactured samples has been kept parallel to the loading direction. In this paper we will give the readers a deeper understanding of how additively manufactured composite samples behave under loading, further facilitating the design process for materials produced by additive manufacturing.

The determination of thermal residual stresses in unidirectional and cross-ply titanium matrix composites using an etch removal method

Recent work has shown that a simple rule-of-mixtures approach may be used to predict the stress–strain behaviour of a cross-ply metal matrix composite laminate. However, the low-strain behaviour was not predicted accurately, probably because thermal residual stresses are obviously not included in such an approach. To increase the understanding of the limitations of the rule-of-mixtures approach for predicting the stress–strain response, the residual strain-state of the fibre reinforcement has been determined using an etching technique (henceforth referred to as the ‘total etch removal method’), and results have been compared both with finite element modelling and with thermal residual strain measurements derived from stress–strain curves. The results show that the residual strain distribution in a cross-ply composite may be more complex than previously thought, with the fibres in internal 0° plies having considerably higher thermal residual strains than fibres in external plies. The results confirm that the rule-of-mixtures approximation can be used, with some reservations with regard to the low-strain behaviour.

Determination of Mean Intrinsic Flexural Strength and Coupling Factor of Natural Fiber Reinforcement in Polylactic Acid Biocomposites

This paper is focused on the flexural properties of bleached kraft softwood fibers, bio-based, biodegradable, and a globally available reinforcement commonly used in papermaking, of reinforced polylactic acid (PLA) composites. The matrix, polylactic acid, is also a bio-based and biodegradable polymer. Flexural properties of composites incorporating percentages of reinforcement ranging from 15 to 30 wt % were measured and discussed. Another objective was to evaluate the strength of the interface between the matrix and the reinforcements, using the rule of mixtures to determine the coupling factor. Nonetheless, this rule of mixtures presents two unknowns, the coupling factor and the intrinsic flexural strength of the reinforcement. Hence, applying a ratio between the tensile and flexural intrinsic strengths and a defined fiber tensile and flexural strength factors, derived from the rule of mixtures is proposed. The literature lacks a precise evaluation of the intrinsic tensile strength of the reinforcements. In order to obtain such intrinsic tensile strength, we used the Kelly and Tyson modified equation as well as the solution provided by Bowyer and Bader. Finally, we were able to characterize the intrinsic flexural strengths of the fibers when used as reinforcement of polylactic acid.