A Review of Hydride Precipitates in Titanium and Zirconium Alloys: Precipitation, Dissolution and Crystallographic Orientation Relationships

Egle Conforto,X Feaugas

doi:10.1051/matecconf/202032111042

Abstract

This work proposes a review of recent results on the formation and dissolution of hydrides in HCP alloys (Ti and Zr alloys) correlated to the nature of crystallographic hydride phases and their ORs. The crystallographic coherence observed between the surface hydride layer and the substrate is very important for many applications as for biomaterials devices. Five particular orientation relationships (OR) were identified between titanium/zirconium hydride precipitates and the oc-Ti and a-Zr substrates. In addition, the nature of hydrides have a large implication on the ductility, the strain hardening, and the local plastic strain accommodation in the Ti alloys. Our studies using XDR, TEM and SEM-EBSD have been demonstrating that the nature of the hydride phase precipitates depends on the hydrogen content. DSC has been used to obtain the hydride dissolution and precipitation energy values at the bulk scale, whose difference can be associated to misfit dislocations. Local in-situ TEM dissolution observations show the depinning of part of misfit dislocations during dissolution process. Hydride reprecipitation is thus possible only if hydrogen is not driven away during heating by misfit dislocations depinning.

Highlights

Hydrogen diffuses very rapidly into materials with close-package hexagonal (HCP) structure as titanium, zirconium and their alloys
The equilibrium phase diagrams of Ti- and Zr- show that H solubility is very low at ambient temperature, which leads to the spontaneous precipitation of hydride phases TiHx and ZrHy [3]
Hydrogen can favor the refinement of some microstructures of Ti alloys [7], and improve the biocompatibility at the surface of biomaterials [8]

Summary

Introduction

Hydrogen diffuses very rapidly into materials with close-package hexagonal (HCP) structure as titanium, zirconium and their alloys. Hydrogen can favor the refinement of some microstructures of Ti alloys [7], and improve the biocompatibility at the surface of biomaterials [8]. This can lead to possible embrittlement shown in several industrial applications [9]. The dependence of the nature of the precipitate phases on the H content and on the temperature stability will define the crystallographic relationships existing between the hydride and the matrix ( phase) We shall discuss these points with reference to our experimental work [11]. Their role on the dissolution/reprecipitation process is will be discussed in the present review, as well as their influence on the crystallographic relationship with the substrate after many thermal cycles

The precipitation of hydrides in HCP structures

Conclusions