Optimierung der mechanischen Eigenschaften von Beta-Titanlegierungen durch die Verwendung von Wasserstoff als temporäres Legierungselement

The use of hydrogen as a temporary alloying element as a part of the thermomechanical process is considered being a promising approach for improving mechanical properties especially of titanium cast alloys.
The present study shows, that this so-called thermohydrogen processing (THP) improves monotonic and cyclic strength even of complex metastable beta titanium alloys by taking advantage of the reversibility of the titanium-hydrogen interaction and by exerting an selective influence on phase equilibria, kinetics of phase transformation and the distribution of alloying elements. Thermohydrogen processing is applied on two beta titanium alloys differing considerably with respect to beta stability.
Thermohydrogen processing of highly beta stabilized titanium alloy Ti-3Al-8V-6Cr-4Mo-4Zr (Ti-38644) increases yield strength by more than 4 % and fatigue limit by 8 % as compared to a duplex aged (reference) microstructure. The 5-step Hi-Read-process (Hydride-induced Rearrangement of Dislocations) optimizes the precipitation behavior of the secondary alpha phase in a completely recrystallized beta microstructure, coming from hydride-induced multiplication of dislocations within beta solid solution. Designating the THP contributes to the mechanism of dislocation formation as initially described by Frank Read.
The 4-step HiRe-Beta process (Hydride-induced Recrystallization of beta Phase) applied on high-strength beta titanium alloy Ti-10V-2Fe-3Al (Ti-1023) provides the best way to improve strength by maximizing the driving force for precipitation of the secondary alpha phase. Despite complete destabilization of globular primary alpha phase, hydride-induced recrystallization of the beta phase simultaneously preserves a fine beta microstructure. As compared to the conventionally heat-treated material the THP increases yield strength by more than 8 %. Fracture of samples occurred within the elastic region of the stress-strain curve is attributed to the existence of process-related crack formation.
Both THP designed for beta titanium alloys fundamentally differ with respect to hydrogen uptake. The Hi-Read process utilizes time-independent Sieverts-type hydrogenation behavior at high temperature. In contrast, hydride formation already occurs during hydrogenation at low temperature in the HiRe-Beta process. Therefore, the volume fraction of precipitated hydrides is immediately affected by hydrogenation time (time-dependent hydrogenation behavior).
Determination of THP-strategies for both metastable beta titanium alloys relies on studies on the relationship between microstructure and mechanical properties, exhibiting the detrimental effect of precipitate-free zones and grain boundary alpha phase, both decreasing yield strength and fatigue limit and additionally promoting the propagation of long cracks.
Studying the kinetics and thermodynamics of hydrogen uptake and the effect of hydrogen on phase stability enables to establish specified hydrogen concentrations in metastable beta titanium alloys Ti-38644 and Ti-1023.
Evaluation of hydrogen concentration profiles emerged from diffusion annealing of electrochemically hydrogenated small titanium bars enables the determination of hydrogen diffusion coefficients. Assuming an exclusively diffusion-controlled (ideal) hydrogen uptake, the hydrogen diffusion coefficients are used to calculate necessary hydrogenation times by means of numerical methods.
Volumetric measurements reveal that even at temperatures above the stability limit of the titanium oxide an exclusively diffusion-controlled hydrogen uptake cannot be facilitated. The impact of the surface on the kinetics of hydrogen uptake is therefore considered by means of correction factors, which are used to estimate the times being necessary for complete hydrogen uptake and release. Specified hydrogen concentrations and homogeneous hydrogen distributions are established within the cross-sections of the samples in this way.
Coating samples of both beta titanium alloys electrochemically with the hydrogen absorber palladium considerably accelerates the kinetics of hydrogen uptake, thereby facilitating feasible process times even at lower temperatures.
Both beta titanium alloys studied exhibit a distinct reduction of the transition temperature (beta transus) with increasing hydrogen concentration. In case of Ti-38644 the modified beta transus decreases until the eutectoid transformation of the beta phase into alpha phase and hydride begins. For Ti-1023 the modified beta transus decreases continuously and remains constant over a wide range of hydrogen concentration after reaching the eutectoid temperature. Since hydride formation (eutectoid transformation) is considered to play the key role for microstructure optimization via thermohydrogen processing the location of the solubility limit of beta solid solution for hydrogen is determined roughly within the phase diagrams of both beta titanium alloys.
Knowing the modified beta transus and the hydrogen solubility limit of beta solid solution is essential since it enables specification of thermohydrogen process routes within the phase diagrams of beta titanium alloys Ti-38644 und Ti-1023.
Thermohydrogen processing of metastable beta titanium alloys Ti-38644 und Ti-1023 led to microstructures which cannot be facilitated through conventional thermomechanical processes. Additionally, the Hi-Read and HiRe-Beta thermohydrogen process provide the opportunity for adjusting the microstructures of both beta titanium alloys gradually according to prevailing load situations. Therefore, both THP show all features of an innovative heat treatment.