Evaluation and Characterization of Alpha Case on Titanium Implants and Medical Devices - Test Methods

Lucideon offers detailed metallurgical services to accurately assess, characterize and measure the thickness of alpha case on titanium components. The results are compared to the requirements of the applicable ASTM and ISO standards.

Overview

Alpha case can form on the surface of titanium components due to thermal and chemical exposures during processing. This case is usually an oxygen rich surface layer that stabilizes the alpha phase of titanium in a normally properly mixed alpha/beta microstructure. Alpha case is generally brittle and can deleteriously affect the fatigue performance of the component.  Its presence on the surface of medical implants and devices is therefore controlled and limited by various standards for both conventionally processed material and additively manufactured components. The requirements of ASTM F136 and F3001 for the Titanium – 6% Aluminum – 4% Vanadium with extra low interstitials (Ti-64 ELI) grade and those of ASTM F1492 and F2924 for the Ti-64 alloy grade are assessed and evaluated in each case. The requirements of applicable ISO standards can also be assessed.

Expertise and Test Methodology

The evaluation of alpha case and its measurement can be difficult, complicated and subject to misinterpretation. This is particularly true when the requirement lists that no alpha case is allowed. Lucideon and its team of metallurgists have developed an appropriate test protocol that begins with the proper sectioning and sampling of the component in question to preserve the existing surface condition. Then the appropriate metallographic practices are employed to provide the best edge-retention for accurate measurement. Finally, a two stage etching process is applied with staining to accurately delineate the transition from the alpha surface layer to the normal alpha/beta microstructure. Additionally, if requested, other unusual surface microstructures can be documented such as alpha stabilized zones and other microstructural defects.

Form of Results

Reports are provided with the measurements of alpha case depth along with optical or scanning electron microscope (SEM) images of the surface layer and/or other defects or anomalies found. Assessments are then made to determine if the components meet any of the applicable standard requirements.

Related Tests

  • Preparation of Metallographic Specimens – ASTM E3
  • Microetching Metals and Alloys – ASTM E407.

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