1998
This minor improvement may be related to the improved efficiency of the passivation layer that was attributed to the increased β-phase content and the reduction of α/β interfaces .
 be, passivation, layer, reduction
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: manufacturing standard, machanical property, manufacturing process, material, engineering feature, process parameter, machine or equipment, application, process characterization, enabling technology, biomedical, concept or principle 
Text: {0} 
Answer:
 be, passivation, layer, reduction