Elsevier

Acta Materialia

Volume 49, Issue 5, 14 March 2001, Pages 903-911
Acta Materialia

Hardness and modulus of the lamellar microstructure in PST-TiAl studied by nanoindentations and AFM

https://doi.org/10.1016/S1359-6454(00)00375-XGet rights and content

Abstract

A nanoindenting atomic force microscope (NI-AFM) allows quantitative measurements of the modulus of elasticity and the hardness on a very local scale. This technique was used to study the elastic and plastic deformation properties of different lamellae in a polysynthetically twinned TiAl crystal. The results show, that the hardness determined at a maximum indentation force of 1000 μN is higher in α2 lamellae than in γ lamellae. Hardness variations were also observed between γ lamellae of different orientation. The shape of impressions left from indentations in γ lamellae deviate clearly from the shape of triangular Berkovich indenters. The irregular shape of these indents, as imaged with the atomic force microscope, indicate a strong anisotropic plastic deformation mode in γ domains. Pop-ins or yield-points, which mark the transition from elastic to plastic deformation, were observed frequently on γ lamellae and less frequently on α2 lamellae. The modulus in the investigated <11-20> direction of α2 lamellae is significantly lower than the modulus of γ lamellae. This corresponds with calculations of the indentation modulus from the elastic stiffness constants of these phases.

Introduction

The excellent topographic imaging characteristics of an atomic force microscope (AFM) allows quantitative investigations of very fine nanosized metallic microstructures [1]. Many currently used metallic alloys and composite materials consist of fine microstructures like superalloys [2] or intermetallic TiAl alloys. In these cases, the microstructural mechanical properties of different phases and the distribution of these phases in the alloys determine the bulk properties of the materials. The behavior of materials on a macroscopic scale is most often determined by the elastic and plastic properties of microscopically small constituents. Optimizing the macroscopic material behavior requires knowledge of the material parameters of these small microstructures. Therefore, direct nanomechanical measurements of hardness and elasticity are of great relevance for the development and optimization of improved alloys. The indentation method has the unique capability to determine the mechanical properties from very small indentations. With the nanoindenting atomic force microscope (NI-AFM) the mechanical properties of individual constituents in a multi-phase system such as γ TiAl alloys can be examined with a lateral resolution close to 50 nm. Since the indents are performed with the same diamond tip which is used for AFM imaging, indents can be positioned on a nanometer scale [3].

TiAl based alloys with a two-phase structure, consisting of the major γ TiAl phase with L10 structure and minor α2 Ti3Al phase with D019 structure are the most intensively studied materials among the promising intermetallic candidates for high temperature applications (see for example the review by Yamaguchi et al. [4]). Their low density and high modulus combined with some tensile ductility are very attractive for structural applications. The γ and α2 lamellae are stacked in the lamellar microstructure such that the {111} plane in the tetragonal γ phase is parallel to {0001} basal planes in the hexagonal α2 phase. Since the <-110]1 direction and the two other <10-1] and <0-11] directions are not equivalent in the tetragonal lattice a domain structure with six different γ variants results. The [-110] directions in γ are parallel to <11-20> directions in the α2 phase. Besides these γ/α2 interphase boundaries, many lamellar boundaries between two adjacent γ lamellae with different orientations are observed. Two lamellar γ plates rotated with respect to each other by an angle θ of n×60° (n=0–5) result in lamellar boundaries of true-twin type (θ=180°), rotational type (θ=120°, 240°) or pseudo-twin type (θ=60°, 300°). Domains of different variants can coexist within one γ lamella and boundaries between such domains are denoted domain boundaries. Such domain boundaries do not show any preference for a crystallographic orientation, whereas lamellar boundaries are always parallel to γ/α2 interphases (Fig. 1). The mechanical properties of the lamellar microstructure in TiAl-based alloys depend on the lamellar orientation, grain size, lamellar spacing and thickness. However, it was found that the lamellar orientation has far more influence than microstructural properties [5]. So called polysynthetically twinned (PST) crystals [6], where an entire crystal consists of a single lamellar grain are ideal objects to study the orientational dependences. Therefore, such technical single crystals were used in this study.

Section snippets

AFM imaging and indenting

The nanoindentation measurements shown in this paper were performed with an add-on force transducer from Hysitron Inc.. The transducer, mounted on a conventional AFM, controls the z-movement of the tip and measures the indentation force and displacement [7]. Nanoindentation measurements were done with diamond tips of different shape and radius. Three-sided pyramidal Berkovich tips are commonly used in nanoindentation experiments, since these indenters have the same area to depth relation as

Evaluation of load-displacement curves

Load-displacement curves are evaluated with the method proposed by Oliver and Pharr [8]. The hardness H is determined from the actual contact area AC and the maximum applied load, P.H=PAC

The modulus of elasticity E is calculated from a reduced modulus Er, which is determined from the elastic contact stiffness S by:Er=πSACwhere β is a constant, here assumed as 1.0, and the reduced modulus is given by the properties of the materials in contact.1Er=1−ν2tipEtip+1−ν2specimenEspecimen

The hardness

Elastic anisotropy

At very low indentation loads, say below 1 mN, single crystalline properties are always determined, even if a polycrystal with limited grain sizes was indented. Therefore, the elastic and plastic anisotropy of the indented material has to be considered. Vlassak and Nix [9], [10] developed a quasi analytical model to determine the influence of elastic anisotropy on indentation measurements. Though indentations are performed in specific crystalline directions, most often, the measured moduli are

Hardness of α2 and γ phase and depth dependence of hardness

A significant difference in hardness was found between the α2 and γ phase. The α2 phase is always harder than the γ phase, even though it is sometimes difficult to hit the α2 phase centrally. A relatively large height contrast of 100 nm between α2 and γ phase makes positioning of indents sometimes difficult. Figure 5 shows indents performed with a cube corner tip in α2 and γ lamellae. Different hardness values were obtained for α2 and γ phase. It should be noted, too, that the shape of indents

Yield points

Load-displacement curves obtained with nanoindentation techniques often show discontinuities or pop-ins during loading. Many examples are given in the literature for nearly all kinds of metals, although a complete understanding of this phenomenon is still missing [16], [17], [18]. While the yield points found in Si and some other ceramics may be associated with phase transformations, the yield points in most metals are clearly associated with the onset of dislocation plasticity. Evaluations of

Plastic anisotropy in the γ phase and pile-up effects

The impressions left from indents in the γ phase show a striking pile-up effect. Material is displaced during indenting against the sides of the triangular indenter in a very anisotropic manner. This pile-up effect can have an important influence on the measured mechanical properties since the contact area between the indenting tip and specimen is increased. Especially, strain hardened metals that exhibit a low strain-hardening rate will deform locally, thus creating large pile-ups around the

Discussion

The first direct investigation of the mechanical properties of PST-TiAl alloys by nanoindentation is presented. The hardness of γ phase is nearly half of that of the α2 phase, while the determined moduli are 200 and 160 GPa, respectively. However, the modulus data show a considerable scattering, especially for the values determined for the α2 phase, which may be attributed due to the fact that α2 lamellae are very thin and the measurements may be influenced by γ phase. The influence of

Acknowledgments

The authors are very grateful to H. Biermann and M. Riemer, University Erlangen–Nürnberg for preparation of the TiAl specimens and many fruitful discussions. Y. Umakoshi and T. Nakano, Osaka University, Japan are thanked for providing the PST-crystals. M. G. thanks the Alexander von Humboldt Stiftung, Bonn for financial support of a research stay at Stanford, CA, where some of the measurements were performed.

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