Applications of Laser Ultrasonics (lus) to Studies of Microstructural and Mechanical Properties of Metals

During the last three years the Swedish Institute for Metals Research (SIMR) has done extensive work on applications of laser generated ultrasound (LUS). This paper reports results from several experiments mainly aimed at industrial on-line applications. LUS has been used to determine grain sizes in commercial low-carbon (LC) and stainless steels. The analysis method employs a scattering theory that yields grain sizes in absolute values, without any free-parameter fits or calibration curves. The correlation is linear to grain sizes determined with optical microscopy. Dynamic changes during annealing of cold rolled ferritic steels, such as phase transformation and recrystallisation, have been evaluated using the LUS technique. These phenomena are clearly revealed from changes in ultrasonic velocity due to changes in crystallographic texture. LUS has also shown to be an alternative method to determine elastic properties of metals. By applying an isotropic approximation to the material symmetry, parameters like Young’s modulus and Poisson’s ratio may be calculated for rolled sheets. The results are compared with a method based on mechanical resonance and standard tensile testing. Introduction: Ultrasound is a stress wave and the characteristic features of its propagation depend on several mechanical properties of the propagation medium. Characteristics of the ultrasonic source together with specimen geometry and position of the detection spot with respect to the generation spot determine which waves will be registered. Common ultrasonic wave types are bulk (P and S) waves, surface (Rayleigh) waves and plate (Lamb) waves. Ultrasonic waves that travels through a metal sample interact with the microstructure and the registered wave spectra will carry information about the microstructural state of the penetrated volume. An ultrasonic spectrum is mainly characterised by the time of arrival for various echoes, i.e. the wave velocities, and the amplitude of the successive declining echoes (attenuation). For certain applications more detailed information can be extracted using a frequency analysis which reveals velocity or attenuation for specific frequency components within a specific wave. Two principal types of microstructure information can be obtained. The wave velocity depends on the material type – its phase constitution and crystallographic texture. The wave attenuation depends mainly on grain size and on the dispersion of second phases if these are present. Table 1 lists various parameters that can be measured from ultrasonic velocity and attenuation. Wave characteristics: Depends on: Features that can be registered: VELOCITY (time of arrival) Elasticity of single crystals Crystallographic texture Recrystallisation Phase transformation Dimension Temperature ATTENUATION Scattering and absorption from various microstructure components Grain size Second phases Precipitation Dislocations Table 1: Parameters that can be measured from ultrasonic velocity and attenuation Ultrasonic velocity is determined by the elasticity of the polycrystalline aggregate and is hence dependent on a combination of the elastic properties of the crystal structure together with as the crystallographic texture. The ultrasonic velocity will as a consequence differ in different directions of measurement for a textured sample. In general, metals always show more or less pronounced textures and, in principal, all processing steps such as deformation and annealing will alter the microstructure and also the texture, so affecting the ultrasonic velocity. Ultrasonic waves can penetrate several centimetres in metals although the wave amplitude becomes progressively attenuated by the microstructure as it travels over longer distances. A propagating wave is scattered at grain boundaries and absorbed by dislocations, grain boundary scattering being the dominant effect. The wave will also be attenuated by dispersions of second phases or defects. The attenuation experienced is dependent on grain size such that large grains have a dominating influence; also higher frequencies are more strongly affected and will decline more rapid within the spectra. Depending on the wavelength/grain size ratio (λ/D), the attenuation α can be described as for λ>>D or for λ≈D [4]. 4 3 ~ f D α 2 ~ Df α An ultrasonic spectrum will hence contain various amounts of a specific frequency component depending on the grain size. The literature contains many examples where ultrasonic attenuation shows correlation with grain size. Theories relating grain size and ultrasonic attenuation can be found in the literature e.g. [4]. Microstructural changes that occur during annealing recrystallisation, phase transformations and grain growth are among the most important metallurgical processes occurring in metals from an industrial point of view and extensive efforts are spent around the world trying to describe mechanisms and kinetics for these processes. For physically based models it is extremely important to have relevant microstructure data in order to verify proposed models or hypotheses. Information about microstructure development during annealing is generally collected from interrupted annealing trials followed by sample sectioning, preparation and microscopy work. This is very time consuming and some structures like austenite are very difficult or even impossible to maintain at room temperature. Several non-destructive techniques do exist to monitor recrystallisation continuously but they all suffer from limitations that restrict their use in an industrial environment. A continuous measure of the microstructural state online during industrial annealing would be of great importance for maintaining high quality and uniformity of properties in the final products. Today, material that does not fulfil correct specifications has to be downgraded or even reprocessed causing considerable economic losses. The laser ultrasonic device at SIMR was designed and built by Accentus, UK and was installed at SIMR in February 2001. The design of the system allows high flexibility together with a high safety standard so the system is fully enclosed and most measurement set-ups are done from a control unit and a PC. Figure 1 show an overview of the system and Table 2 gives the most important technical specifications. A lens system facilitates the IR beam to be positioned either on opposite or the same side of the sample as the detection laser. There is also a loopcontrolled device that controls the power of the incident detection laser beam. This device helps the system to manage variations in amount of light reflected from the sample surface during for example oxidation due to heating or geometric changes during compression. Figure 1: Overview of the LUS system at SIMR. Excitation laser Continuum “Surelite I”, YAG. Wavelength: 1064nm. Pulse duration: 5ns. Pulse energy: 450mJ (ablative regime). Repetition rate: 20Hz. Detection laser Coherent “Verdi”, Nd:YVO4. Wavelength: 532nm. Line width: <5MHz. Maximum power: 5W Interferometer Confocal Fabry-Pérot (working in transmission or reflection mode) Frequency range: 1-100MHz. Data aquisition 1 GHz oscilloscope and PC Table 1: Technical specifications of the LUS system at SIMR Results: The elastic modulus is possible to calculate using LUS since the characteristic features of the propagation of ultrasonic waves depend on the elastic properties of the propagation medium, together with its density. In particular, propagation velocity is a function of the elastic tensor. A relation between phase velocities and tensor components can be found by solving Christoffel’s equation [1]. Exact analytical solutions to Christoffel’s equation exist only for simple cases of isotropic solids and some low index directions in high symmetry crystals. For all other cases approximations or numerical solutions must be used. For the isotropic case, a simple expression relates the elastic modulus to two bulk wave (P, S) velocities. In turn the Rayleigh wave velocity may be simply expressed in terms of the same bulk wave velocities. This applies for Lamb waves as well, but only for the very first symmetric and asymmetric modes (S0 and A0) and in regimes where λ>>d or λ<>d or λ<<d) by using the isotropic approximation. However, a better method is to compare Lamb waves from experiment to the theoretic dispersivity of Lamb waves. For an isotropic approximation this dispersivity may be calculated if P and S velocity are known. So, if the P velocity is known the S velocity may be derived by finding the S velocity that fits experimental Lamb waves to theoretic Lamb waves. This is shown in Figure 2 above. Figure 3 summarises the LUS measurements of the elastic modulus. There is also a comparison to results from other methods such as tensile testing and a method based on mechanical resonance [2,3]. All measurements consider hot or cold rolled sheets where P wave velocity was measured in the normal direction (ND, to rolling plane) and S wave velocity was measured in either rolling direction (RD) or perpendicular to RD. Recrystallisation was studied in stainless steel plates that had been annealed industrially where different degrees of recrystallisation would have resulted in a large scatter in the mechanical properties. SEM-BSE micrographs were used to measure the volume fraction of recrystallisation and P-wave velocities were determined through the plate. In this case it is seen that both wave velocity and tensile strength vary almost linearly with the fraction of recrystallisation, Figure 4.