Transformation mismatch plasticity in Pd induced by cyclic hydrogen charging
Introduction
Transformation mismatch plasticity can be induced in allotropic material by subjecting them to an applied stress during a phase transformation. Internal strains produced by the mismatch between the two coexisting allotropic phases are biased in the direction of the external stress, resulting in a strain increment after each transformation which is proportional to the biasing stress at low stresses. Strain increments created during cyclical transformation can be accumulated into large macroscopic strain (<100% in tension), a phenomenon called transformation superplasticity [1], [2], [3]. The phase transformation can results from thermal cycling in various metals and alloys [3], [4], [5], [6], [7], [8], [9] and has also been recently achieved by chemical cycling in titanium, i.e., by repeated hydrogen charging/discharging cycles under isothermal conditions [10], [11]. In the latter case, additional internal mismatch due to lattice swelling can be generated by hydrogen gradient within the sample without transformation, which can also produce strain under biasing by an applied stress [12].
Hydrogen-induced transformation mismatch plasticity has been studied exclusively in titanium and titanium alloys [2], [10], [11], [12], [13], [14], [15] at elevated temperature, where the diffusion of hydrogen in and out the sample is rapid. Palladium is unique among metals, as it exhibits, at ambient temperature, high solubility and diffusivity for hydrogen [16], [17], [18], [19]. Hydrogen dissociation on the Pd surface is highly not sensitive to “poisoned” due to exposure to air or humidity [19]. At ambient temperature, the equilibrium absorption and desorption pressure of H2 in Pd are ∼2.5 and 1.3 kPa, respectively [19], [20] and the hydrogen to palladium ratios (H/Pd) in saturated Pd and in the hydride are ∼0.015 and 0.6, respectively. At partial pressure of 100 kPa H2 (∼1 atm) the H/Pd ratio is reaching ∼0.7, resulting in lattice swelling of about 10% [17]. Upon heating to about 150 °C, the hydrogen desorption pressure is ∼100 kPa.
The mechanical properties of Pd and Pd alloys have been studied by various authors (see for example Ref. [21]). However, relatively little is known about the mechanical properties of Pd–H alloys [22]. The Pd mechanical properties after hydriding–dehydriding cycles have been studied by Goltsov et al. [23], [24], [25], [26], [27], [28] and by Goods and Guthri [22]. It was found that hydriding–dehydriding cycles increased the yield and tensile strength of Pd after as few as 1–2 cycles, boosting strength by a factors 2–4 with respect to the initial strength [28], but decreasing uniform elongation to value of less than 3% [22]. This strengthening effect was named HPN (hydrogen phase “naklep”) by Goltsov [23]. Additionally, Goltsov demonstrated that plastic deformation of Pd wire (90%) followed by hydrogen cycling at 100 °C can lead to transformation induced plasticity (TRIP), increasing of the total strain of cold-worked Pd from ∼5% to ∼25% [29], close to the value achieved for annealed sample [21], [22], [29]. This treatment combining plastic deformation and hydrogen cycling also increases the yield strength of the sample to values of 450–500 MPa [29].
Here, we demonstrate for the first time that Pd can be deformed by hydrogen-induced transformation mismatch plasticity through hydrogen cycling under a constant applied stress at near ambient temperature.
Section snippets
Experimental procedures
A schematic of the experimental apparatus is shown in Fig. 1. A 0.25 mm diameter Pd wire (99.99% purity from Alfa Aesar) was used in its as-received condition; the wire was not annealed as it is known [22] that swaged palladium that was annealed recovers its swaged mechanical properties already after 1–2 hydriding/dehydriding cycles. The wire was subjected to uniaxial tension through dead-loading using a basket with weights. The wire deformation was measured by a linear variable differential
Strain history during transformation mismatch plasticity experiments
The strain vs. time plot for an experiment performed at room temperature under variable stress is shown in Fig. 2. The experiment was started at a relatively low stress of ∼40 MPa. The first hydriding half cycle (Region A in Fig. 2) results in a strain of 2.8%, in good agreement with the value reported for lattice expansion during hydrogenation [17]. Upon desorption for ∼100 h in air (B), the wire shrunk by 2.0% thus accumulating 0.8% after this first cycle. The second hydriding half cycle
Conclusions
Swaged Pd wires were subjected to hydrogen charging–discharging cycles under a constant uniaxial stress. Strain increments are observed after each cycle, and they can be accumulated upon multiple cycles up to a total strain as high as ∼40%. As the number of hydrogen charging–discharging cycles increases, the desorption kinetics become more sluggish. Accelerating the desorption kinetics by Joule heating of the wire results in fracture after ∼17% tensile strain due to partial necking causing
Acknowledgements
The authors thank Eli Torgeman and Tzahi Erz-Kdosha for their technical assistance in performing the experiments, Arie Venkert for his help in the SEM work and Igal Alon for his help in the tensile tests.
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