Non-acid, alcohol-based electropolishing enables high-quality electron backscatter diffraction characterization of titanium and its alloys: Application to pure Ti and Ti-6Al-4V

Highlights

• A new electropolishing sample preparation procedure for electron backscatter diffraction of Ti and its alloys is developed.

• The procedure involves a non-acid, ethanol-ethylene glycol-NaCl electrolyte along with low voltage at room temperature.

• The procedure eliminates the need for HF, HClO₄, H₂SO₄ hazardous acids and for high voltages and low temperature solutions.

• High-quality EBSD datasets are collected for undeformed and deformed pure Ti and additively manufactured Ti-6Al-4V.

• The developed procedure can streamline and make safer future metallographic studies of Ti and its alloys.

Abstract

This paper conceives an electropolishing sample preparation procedure aimed at characterization of titanium (Ti) and its alloys using electron backscatter diffraction (EBSD). Specifically, it is shown that a single non-acid, ethanol-ethylene glycol-NaCl electrolyte solution can be used to electropolish Ti and its alloys for obtaining high-quality EBSD datasets. As a result, the use of hazardous acids common in standard Ti metallographic preparation procedures, such as hydrofluoric, perchloric, and sulfuric acid, can be circumvented. Moreover, electropolishing Ti with an ethanol-ethylene glycol-NaCl electrolyte is performed at room temperature with low voltage, as opposed to the low temperatures and/or high voltages used when electropolishing Ti in traditional solutions. The utility of the novel procedure is demonstrated on samples of pure α-Ti before and after plastic deformation and on samples of alloy Ti-6Al-4V created by additive manufacturing (AM) in stress-relived and heat-treated conditions. EBSD scans performed on undeformed pure α-Ti had high-quality diffraction patterns, which allowed for large areas to be scanned at fast speeds without sacrificing indexing accuracy. The electropolishing created excellent surfaces on the deformed pure α-Ti sample despite elevated lattice strains and numerous deformation twins in its microstructure. It also allowed for detailed EBSD mapping of fine sub-grain features in AM Ti-6Al-4V despite the alloying additions of aluminum and vanadium, the inhomogeneous AM microstructure, and the different processing conditions. It is anticipated that the major breakthrough achieved in this work will streamline and make safer future metallographic studies of Ti and its alloys.

Introduction

Titanium (Ti) is a transition metal that in its metallic form has a range of desirable mechanical and chemical properties that make it a structural material in high demand [[1], [2], [3]]. Ti has a high specific strength, melting point (1688 °C), biocompatibility, and corrosion resistance in oxidizing environments [1,[4], [5], [6], [7], [8]]. Metallic Ti has three equilibrium phases: a hexagonal close-packed (hcp) phase (α-Ti), a body-centered cubic (bcc) phase (β-Ti), and a hexagonal phase (ω-Ti). Through the use of alloying and heat-treating, major alterations can be made to Ti microstructures, which can be tailored to alter and enhance specific material properties. This flexibility has led Ti and many of its alloys, such as Ti-6Al-4V, to become excellent structural materials, particularly in aerospace, marine, and medical industries [[9], [10], [11], [12]].

During the research and development of Ti alloys, comprehensive microstructural analysis is performed to study the relationship between observed structure and material properties. Throughout this process, metal samples can be examined by various characterization techniques. Depending on these techniques, specific surface finishes are required to reveal specific microstructural features. To achieve proper surface quality, samples are prepared metallographically. Often, samples are ground with silicon carbide (SiC) or alumina (Al₂O₃) abrasive paper, followed by a fine polish using an alumina, silica (SiO₂), or diamond suspension on a non-abrasive pad [13,14]. Chemical etchants or acidic/alkaline polishing suspensions can also be used as a final step to reveal certain microstructural features or when a specific material responds better to chemical dissolution versus mechanical removal [13].

Ti is a difficult material to mechanically polish, which makes it difficult to achieve pristine surface finishes for certain scanning electron microscope (SEM) techniques such as electron backscatter diffraction (EBSD) [13]. Therefore, when preparing Ti for EBSD, samples are often etched with a solution that utilizes a strong and/or oxidizing acid such as hydrofluoric acid (HF), perchloric acid (HClO₄), or sulfuric acid (H₂SO₄) [13,[15], [16], [17], [18]]. Another method of surface treatment common with Ti is electropolishing. Electropolishing can be favorable over etching because of its conformability, speed, and ability to remove large amounts of material autonomously [19,20]. That being said, for Ti, the electrolyte solutions include combinations of the previously mentioned acids, which are particularly toxic and generally require the use of high voltages and variable working temperatures in order to be effective [20,21]. Fushimi et al. [22,23] investigated the electropolishing of commercially pure (CP) Ti at room temperature in a 1 M ethylene glycol (C₂H₆O₂)-sodium chloride (NaCl) electrolyte solution. Furthermore, Kim et al. [20] expanded on this methodology and investigated the effect of ethanol (C₂H₆O) additions on the anodic dissolution of Ti in a 1 M ethylene glycol-NaCl solution. It was determined that 20% ethanol by volume in solution produced the best surface finish (2.341 nm), with no noticeable sign of surface oxides [20].

The purpose of this study was to adopt the electropolishing technique explored in Kim et al. [20] to undeformed, ultra-high purity (UHP) α-Ti and semi-quantitatively test its effectiveness as an EBSD preparation by observing the fraction of EBSD patterns indexed with a confidence index (CI) > 0.10 as a function of bin size and imaging gain. Furthermore, samples of deformed UHP α-Ti and an additively manufactured (AM) Ti-6Al-4V alloy with two different processing histories were subsequently electropolished and scanned to test the effectiveness of the ethanol-ethylene glycol-NaCl electrolyte solution on pure, alloyed, undeformed, deformed, and heat-treated Ti with regard to EBSD preparation. The collection of high quality UHP α-Ti and Ti-6Al-4V EBSD datasets, made possible via electrolytic surface preparation with a single non-acid, electrolyte solution using low voltage at room temperature, would eliminate the need for HF, HClO₄, H₂SO₄, and/or low temperature solutions. The methods presented here eliminate the hazards of working with such acids and provide highly polished surfaces for electron microscopy with a simple apparatus.