Anales AFA Vol. 33 Nro. 1 (Abril 2022 - Julio 2022) 24-30

https://doi.org/10.31527/analesafa.2022.33.1.24

Materia Condensada

EFECTO DEL TRATAMIENTO TÉRMICO SOBRE LA SOLUBILIDAD DEL

HIDRÓGENO EN LA ALEACIÓN Zr-2.5Nb

EFFECT OF HEAT TREATMENT ON HYDROGEN SOLUBILITY IN Zr-2.5Nb ALLOY

C. García1, V.P. Ramunni*2,3

1Instituto Sabato - UNSAM/CNEA, Av. Gral. Paz 1499, (1650) San Martín - Argentina.

2CONICET Godoy Cruz 2390 (C1425FQD) - Argentina.

3Gcia. Materiales-CNEA, Av. Gral. Paz 1499, (1650) San Martín - Argentina.

Autor para correspondencia: * vpram@cnea.gov.ar

Recibido: 02/06/2021; Aceptado: 24/12/2021

ISSN 1850-1168 (online)

Resumen

Estudiamos la solubilidad sólida terminal (SST) del hidrógeno (H) en muestras de Zr-2.5Nb tratadas

térmicamente a 470◦C duramte 10 h, seguidas de una carga gaseosa de hidrógeno y un tratamiento

de homogeinización a 470◦C durante 6 h para redistribuir el H en el volumen de la muestra. La

microestructura fue caracterizada por microscopía óptica, microscopía electrónica de barrido y por

rayos X. Ensayos de calorimetría diferencial fueron realizados para determinar las temperaturas de

disolución y precipitación de hidruros. Nuestros resultados revelaron que comparativamente con los

tratamientos a 380 ◦C realizados por otros autores, el tratamiento térmico aquí empleado no afecta

signiﬁcantemente la microestructura; no obstante la SST resultó ser ligeramente mayor.

Palabras clave: hidrógeno, hidruros, solubilidad, aleaciones Zr-2.5Nb, ensayos experimentales.

Abstract

We have studied the hydrogen terminal solid solubility (TSS) on Zr-2.5Nb. The samples were aged

at 470 ◦C for 10 h, plus a hydrogen gaseous charge followed by a homogenization treatment at 470

◦C - 6 h in order of distributing the hydrogen throughout the sample’s bulk. The microstructure was

characterized by Optical Microscopy, Scanning Electron Microscopy, and X-Rays. Differential Scan-

ning Calorimetry (DSC) test were carried out to determine the hydrides dissolution and precipitation

temperatures from which the hydrogen TSS curves were obtained. Our results reveal that the thermal

treatment employed has not signiﬁcantly affected the microstructure compared to treatments at 380

◦C by other authors; however, the hydrogen TSS was slightly higher.

Keywords: hydrogen, hydrides, solubility, Zr-Nb alloys, experimental tests.

1. INTRODUCTION

The Delayed Hydride Cracking phenomena (DHC), is a fracture mechanism which occurs in ma-

terials where hydrides are formed, as is the case of zirconium and its alloys. In the particular case

of Zr-2.5Nb, which is used in the manufacture of the pressure tubes of CANDU type nuclear power

plants, DHC can lead to the catastrophic rupture of the component. Hydrogen can be present as a

remnant impurity of the manufacturing process or enter in the alloy during the material service life.

It is very important to evaluate the alloy properties with the hydrogen concentration. The DHC phe-

nomena [1] depends on the microstructure, the hydrogen solubility and the hydrogen diffusion. For

example, Zr-2.5Nb is a bi-phasic alloy with grains in the αphase (hcp, with 0.6% Nb) surrounded

by plates in the β-Zr phase (bcc, with 20% Nb). During thermal treatments, TT, the original plates

of β-Zr lose continuity and the fast H-diffusion paths are interrupted while their Nb concentration

changes [2]. Here we have studied the hydrogen’s terminal solid solubility (TSS) for an intermediate

temperature between the two thermal treatments performed by Parodi et al. [3] on Zr-2.5Nb. With

this purpose we have submitted the samples to ageing, gaseous hydride charge and a homogenization

thermal treatment; and characterized the microstructure and hydride structure by optical microscopy,

scanning electron microscopy and X-rays. In order to obtain the hydrogen’s TSS we have determi-

ned the hydride’s dissolution (TD) and precipitation (TP) temperatures. The hydrogen Terminal Solid

Solubility for Dissolution (TSSD) and Precipitation (TSSP) curves were measured by Differential

Scanning Calorimetry (DSC). The present paper is structured as follows: Section 2describes the hys-

teresis effect on the hydrogen solubility. Sections 3and 4-6, are devoted to summarize the material

and the experimental treatments performed on the samples, respectively. Section 7summarizes our

experimental results. The last section presents some conclusions.

2. HYDROGEN SOLUBILITY IN Zr BASED ALLOYS

Zirconium and its alloys show an important hysteresis between terminal solid solubility of hydro-

gen during heating or dissolution (TSSD) and cooling or precipitation (TSSP). This behavior has been

ascribed to plastic deformation produced in a α-Zr matrix during hydride precipitation [1,2] by the

misﬁt between matrix and hydride. In two-phase Zr-2.5Nb alloy, TSS curves depend on the fraction

and composition of both, α-Zr and β-Zr phases and also on thermal cycles [1]. Fig. 1shows the Zr-H

system phase equilibrium diagram [4] and the hydride compositions in Zr.

FIG. 1: Equilibrium phase diagram of the Zr-H system taken from Ref. [4].

The Terminal Solid Solubility (TSS) is deﬁned as the maximal hydrogen concentration in solu-

tion without forming hydrides [5]. As shown in Fig. 2[5], the TSS curve for hydrogen dissolution

vs. temperature presents the hysteresis phenomenon for the heating and cooling processes. As the

temperature increases, the hydrogen concentration, CH, in α-Zr increases following the TSSD disso-

lution curve along the segment AB. When the temperature decreases, BC, CHdoes not change until

the TSSP curve is reached (C point). When the temperature decreases, CHdecreases, segment CD.

CHdoes not change until the TSSD curve is reached. The hysteresis has been ascribed to plastic de-

formation produced in α-Zr matrix during hydride precipitation [6,7] by the misﬁt between matrix

and hydride. In two-phase Zr alloys, TSS curves depend on the fraction and composition of αand β

phases [1,2] and also on thermal cycles [8]. The TSSD and TSSP curves are obtained from the dif-

ferential scanning calorimetry technique, DSC, which allows to determine the hydride’s dissolution

FIG. 2: Hydrogen solubility hysteresis effect in a thermal cycle on a Zr-2.5Nb sample [6].

(TD) and precipitation (TP) temperatures. TDhas only one value, while TPdepends in general on the

maximum temperature to which the sample was previously heated, the time at which that temperature

is holding and the cooling speed [7].

3. MATERIAL AND Zr-2.5Nb SAMPLES

Zr2.5Nb samples come from a section of an extruded and cold worked pressure tube, autoclaved

at 400 ◦C for 24 h, with approximate size of (40 ×30 ×4.3) mm3. These samples were prepared by

an ageing thermal treatment, hydrogen charge and a homogenization treatment as described below.

Table 1summarizes the chemical composition of the samples.

TABLE 1: Samples chemical composition.

Nb (%wt) H (ppm) O (ppm)

Sample 2.7±0.1 16.1±4.4 820±82

The hydrogen TSS on samples of Zr-2.5Nb have been previously studied by Parodi et al. [3]. They

have performed two thermal treatments, TT, namely: (i) 168 hours at 500◦C and (ii) 24 hours at 380◦C.

These TT have resulted in the ageing of the β-Zr phase and in the increase of the Nb concentration

up to 93% and 59%, respectively. We have here studied an intermediate TT between the two carried

out in the work of Parodi [3] obtaining then a Nb concentration of 77% in the β-Zr phase. Fig. 3

shows the Time-Temperature transformation diagram of Zr-2.5 Nb [9]. The samples of 10 ×10 ×5

mm3, were treated at 470◦C for 10 h in vacuum to decompose the β-Zr phase in order to produce

a variation in the Nb concentration. After the ageing TT, the samples were polished to eliminate

the superﬁcial defects, and cleaned with trichlorethylene to remove all possible dirt on the sample

surface. Afterwards, an electrolytic oxide ﬁlm deposition with a 4% SO4H2solution was performed.

The oxide ﬁlm was then removed with a 600-grit silicon carbide paper, except on the edges of the

surface. This anodizing process was carried out with the purpose of allowing the hydrogen enters into

the sample during the gaseous charge only through the oxide free surfaces. Then, hydrogen diffuses

homogeneously throughout the thickness of the sample. The process is shown in Fig. 4. Finally, the

samples were submitted to a gaseous hydrogen charge procedure. Hydrogen is incorporated into the

sample using the SIEVERT equipment in the Fig. 5. The charging temperature was set at 350◦C so that

the oxide layer is not removed. The amount of hydrogen moles, n, which enters into the sample was

calculated with the ideal gas equation, PV =nRT , by measuring the difference between the initial and

ﬁnal pressure, P, and the room temperature, T,Vis the chamber volume and Rthe ideal gas constant.

FIG. 3: Time-Temperature Transformation diagram of Zr-2.5 Nb [9].

FIG. 4: Steps before the hydrogen gaseous charge process.

After the gaseous charge the hydrogen remains mostly on the sample surface. Since, it has no time

enough to diffuse homogeneously a hydride gradient was generated through the sample thickness.

Then, we perform a ﬁnal TT at 470◦C - 6 h for hydrogen homogenization. The maximum temperature

reached and the duration of the treatment are factors that deserves some cares due to their importance

regarding the transformation of the β-Zr phase during the ageing treatment.

4. SAMPLES CHARACTERIZATION

In order to characterize the microstructure and hydride distribution the samples were observed by

Optical Microscopy (OM) and Scanning Electron Microscopy (SEM). The microstructure was reveal

by etching in a solution of 45 ml of nitric acid, 45 ml of distilled water and 6 ml of hydroﬂuoric acid,

while a solution of 45 ml of nitric acid, 45 ml of lactic acid and 7 ml of hydroﬂuoric acid, to reveal

the hydride particles. For a sample with 85 ppm of H, Figs. 6and 7show the micrograph obtained

by SEM, on the Circumferential-Radial plane (CR) and on the Axial-Radial plane (AR), respectively.

The presence of dark and light grey phases, correspond respectively to α- and β-Zr.

Fig. 8, shows the orientation and morphology of the hydrides precipitated on the CR plane of the

FIG. 5: Scheme of the gaseous hydrogen charge equipment.

TABLE 2: The total H concentration on samples, CT

H=CH+C0

H, with C0

Hthe initial hydrogen concentration

and the incorporated amount, CH.

Sample CHCT

M146 46 + 12 = 58

M273 73 + 12 = 85

M3107 107 + 12 = 119

M4126 126 + 12 = 138

pressure tube. In all cases, the hydrides form plates whose orientation is perpendicular to the radial

direction. These so-called çircumferential"hydrides are oriented according to the crystalline texture

and residual stresses of the tube. We have observed that, hydrides quantity and length increase with

the hydrogen concentration.

5. X-RAY DIFFRACTION

X-ray diffraction tests were performed with a conventional diffractometer from the position 2θ=

33.0◦to 2θ=39.90◦; with a step of 0.01◦and 33 seconds per time step, and Cu radiation. These

initial parameters were used in order to observe the peak corresponding to the reﬂection on the (110)

plane of the βphase. The degree of β-Zr phase transformation was evaluated through its Nb con-

centration by using Eq. (1), which correlates%at Nb with the lattice parameter aβof the β-Zr phase

[10]

aβ() = 3.5878−0.00288×CNb(%atNb).(1)

6. DIFFERENTIAL SCANNING CALORIMETRY

Differential Scanning Calorimetry (DSC) measurements were performed to obtain the hydride’s

dissolution, TD, and precipitation, TP, temperatures. The terminal solid solubility curves (TSSD/TSSP)

are determined from TDand TPusing the maximum slope temperature (MST) [3] criteria and the to-

tal hydrogen concentration. Before DSC measurements samples of approximately 2.5×2.5×4 mm,

were cleaned in ultrasonic bath containing acetone. All experiments were performed with an empty

crucible as reference, under an Ar (99.9997%) dynamic atmosphere of 25 ml/min. The equipment

was calibrated using the melting point of In, Sn, Al, Fe elements. Each cycle consisted of a heating

up to a maximum temperature (Tmax), hold time at this temperature and cooling to a minimum tem-

perature. Two runs were performed at Tmax =380◦C and two others at Tmax =450◦C in order to

analyse the effect of the maximum temperature on TPand to obtain TSSP2 and TSSP1, respectively.

In these cycles, hold times at maximum temperatures were 10 min, with heating and cooling rates of

FIG. 6: (a) Left: SEM of the CR plane (see Fig. 4) of a sample with 85 ppm of H. (b) Right: The presence of

hydride is emphasized.

FIG. 7: SEM of the AR plane (see Fig. 4) of the same sample.

10 ◦C/min. Dissolution and precipitation temperatures result from the average of the values obtained

after each Tmax.

7. EXPERIMENTAL RESULTS

7.1. X-Ray diffraction

Zr-2.5Nb X-ray diffraction patterns for two TT are shown in Fig. 9. The position of the (110)-β

and (0002)- and (1011)-αpeaks, are also presented.

In Table 3we summarize the values of aβparameter and Nb concentration of the β-Zr phase

according to the according to the thermal treatment performed.

TABLE 3: Lattice parameter, aβ, and Nb concentration in β-Zr phase according to the thermal treatment.

T.T. (◦C/h) aβ(Å) %Nb (%at) %Nb (%wt)

470 - 10 3.377 73.0 73.4

470 - 16 3.367 76.3 76.6

380 - 24 3.397 66.1 66.5

500 - 168 [3] 3.319 93.1 93.2

7.2. Thermal solid solubility curves (TSS)

Table 4shows the results obtained from the heating and cooling calorimetry curves. It lists the

total hydrogen concentration CH(in weight ppm), the dissolution temperatures TDfor both, γand δ,

hydrides and the precipitation temperature for delta hydrides (TP) with different maximum tempera-

tures. The ﬁrst heating cycle corresponds to the dissolution of hydrides precipitated during cooling

carried out after the homogenization treatment. In this cycle, samples with 85 and data 118 ppm of

FIG. 8: Hydride distribution along the CR plane of samples heat treated at 470 ◦16 h, respectively with (a) 58,

(b) 85, (c) 119 and (d) 138 ppm of H.

FIG. 9: X-ray diffraction pattern for two heat treatments.

H present two dissolution peaks, one of γ-hydrides and another of δ-hydrides. For 58 ppm the sen-

sitivity of DSC could not be enough to detect the minority γphase. Regarding the sample wit 138

ppm the γ-hydride was not detected either; in Ref. [11] was observed that the precipitation of the

γ-hydride decreases as the hydrogen concentration increases. The γ-hydrides were no detected during

the subsequent DSC cycles of heating and cooling with a rate of 10 ◦C/min. This may be due to the

fact that the γ-hydride precipitates by a peritectoid transformation, as reported in Ref. [11,12], thus

a cooling rate lower than 10 ◦C/min is required. Hydrogen Terminal Solid Solubility for Dissolution

(TSSD) and Precipitation (TSSP) curves were calculated by ﬁtting the hydrogen concentration CH

versus TD/TPof δ-hydrides reported in Table 4, according to the Arrhenius equation (2),

CH=C0exp(−QD,P

RT ).(2)

Where C0is the pre-exponential factor, QD,P, are the transformation hydrides enthalpy for dissolution

or precipitation respectively, and Ris the gas constant. C0and QD,Pvaries according to the alloy and

the hydride phase. So, three equations can be obtained, for dissolution (3) and for precipitation (4),

where i=1 corresponds to Tmax =450◦C and i=2 to Tmax =380◦C.

T SSD =C0exp(−QD

RT ),(3)

T SSP

i=C0exp(−QP

RT ).(4)

The enthalpy of dissolution, QD, and precipitation, QP

i, are expressed as,

QD=Qq−wel,D+wpl,D(5)

+wint,D,

QP=Qq+wel,P+wpl,P(6)

+wint,P.

In Eqs. (5) and (6), Qqis the chemical energy, Wel and Wpl the elastic and plastic energies associa-

ted with the mismatch between hydride and matrix (due to the difference in speciﬁc volume between

both phases), and Wint the interaction energy with the stresses in the environment of the hydride (such

as residual stresses, applied external stresses and stresses between the microplates). The term Wpl,D

implies that the plastic deformation produced during hydride precipitation is recovered, which de-

pends on the temperature attained during heating. Fig. 10 displays the experimental data reported in

Table 4and the TSSD, TSSP1 and TSSP2 curves. For samples with 58 and 138 ppm of H, no preci-

pitation signal was detected during the cooling cycle from Tmax =380◦C, so the TSSP2 curve could

not be obtained with more data. Table 5shows the adjustment parameters for each curve with and

without considering the hydrogen concentration of 58 ppm.

Sample CHCicle Tmax TDTDTmax TP

(ppm) γ δ δ

1 58 1 470 - - 380 -

2 380 - - 380 -

3 380 - - 450 -

4 450 - 226 450 164

2 85 1 470 204 318 380 255

2 380 - 310 380 254

3 380 - 313 450 238

4 450 200 310 450 236

3 119 1 470 243 279 380 283

2 380 . 340 380 284

3 380 - 336 450 265

3 450 - - 450 264

4 138 1 470 - - 380 -

2 380 - - 380 -

3 380 - 375 450 308

4 450 - 377 450 308

TABLE 4: The precipitation and dissolution temperatures, TP, TDfor different H concentrations at a hea-

ting/cooling rate of 10◦C/min.

TABLE 5: Parameters obtained from the ﬁt with/without CH=58ppm.

Parameters TSSD TSSP1

A (ppm) 3300 / 6951 2300 / 3410

Q (J/mol) 16967 / 20785 13631 / 15396

Fig. 11 includes TSSD results of this work together with those measured by Parodi et al., [3] with

different thermal treatments. In Ref. [3], the samples with 93.1% at Nb in β-phase (treated at 500◦C

- 168 h) showed a lower TSS than the samples with 66.1% at of Nb (treated at 380◦C - 24 h). It was

expected that the material treated at 470◦C with 76.3% Nb in β-phase would have an intermediate so-

lubility between the two treatments evaluated by Parodi since the beta phase contains an intermediate

FIG. 10: Hydrogen concentration CHand dissolution/precipitation temperatures reported in Table 4. The ﬁtting

lines were obtained with the Arrhenius equation (2).

FIG. 11: Comparison of the TSSD δ-hhydride results measured in this work and those reported in ref 3, with

thermal treatments at three different temperatures.

percentage of Nb. Nevertheless, its TSSD is higher as shown in Fig.11. On the other hand, Müller [13]

used the material treated at 380◦C - 24 h and measured the solubility after a quenching treatment and

thermal cycling with heating / cooling rates of 10◦C/min. He have found that, although the βphase

has the same ageing before and after quenching, the solubility of hydrogen is affected by the different

distribution of hydrides, this result was interpreted by means of the Eqs. (5) and (6). The quenching

treatment promotes a ﬁner hydride distribution, which means that they retain greater elastic defor-

mation, thus reducing the value of Q and increasing the TSSD and TSSP. Figs. 12 and 13 show our

TSSD, TSSP1 ﬁtted curves with cooling of 10◦C/min and those obtained by Müller [13] and Parodi

[3].

Fig. 14(a) and (b), show the microstructure and hydride distribution for respectively 119 and 122

ppm of H in samples of Parodi [3] for TT at 380◦C - 24 h and 500◦C - 168 h. The continuity of the

βphase treated at 470◦C in this paper (Fig. 6(a)) is similar to that of the material treated at 380◦C,

while at 500◦C the βphase is remarkably spheroidized. Thus the increment in TSS observed in the

samples treated at 470◦C could not be attributed to the βphase ageing. On the other hand, the hydride

distribution observed in the samples of Parodi [3] is similar to that obtained in the sample with 119

ppm used in this work (Fig. 14(c)). So the unexpected higher TSS obtained with the TT at 470◦C

cannot be explained nor by the beta phase ageing neither by hydride distribution as it is observed by

FIG. 12: Comparison of TSSD curves from present experiments with those obtained in [3,13].

FIG. 13: Comparison of TSSP1 curves from present experiments with those obtained in [3,13].

optical microscopy.

In Zirconium pressure tubes, hydride precipitates observed with low magniﬁcation are usually de-

picted like plates resting on the circumferential - axial plane, the so called circumferential hydrides.

But, electron microscopy studies reveal that these plates are compose of stacks of smaller particles

with habit plane (111)δclose to the α-Zr basal plane (0002)α[6]. More recent studies with X- Rays

Synchrotron show that, the majority of δparticles precipitate in α-Zr grains having (0002)αpoles

tilted 20 - 30 degrees from circumferential direction of the tube, and a minor fraction parallel to the

circumferential (hoop) direction [14,15]. This trend can be reverted by tensile stresses applied along

hoop direction [14] or fast cooling rates [13,15]. Under these conditions Terminal Solid Solubility in-

creases [13,14]. As inferred from Eqs. (5) and (6), several factors affects the hydride precipitation, the

solubility of present phases, the ductility of the alloy, the presence of internal (such as intergranular)

or external stresses. More studies are necessary, such as electron microscopy and X Ray synchrotron

or neutron diffraction, to clarify the result obtained in this work.

FIG. 14: Microstructure and hydride distribution of Zr-2.5Nb with (a)-(b) 122 ppm of H and (c)-(d) 119 ppm

of H, respectively in Ref. [3].

8. CONCLUSIONS

In summary, we have performed heat treatment on a Zr-2.5Nb pressure tube samples to be com-

pared with other heat treatments reported in literature. Experimentally, the percentage of Nb in the

beta phase was analysed and the dissolution / precipitation solubility of hydrogen in the alloy was

calculated by differential scanning calorimetry tests.

Our experimental remarks are:

Thermal treatment carried out at 470◦C for 10 - 16 hours does not signiﬁcantly affect the conti-

nuity of beta phase ﬁlaments compared to that of the same material treated at 380◦C - 24 hours

of Parodi [3] unlike the treatment at 500◦C in which the βphase is spheroidized. A percentage

of 73-77% Nb in phase βwas measured.

The following curves were calculated for terminal solid solubility of dissolution and precipita-

tion: T SSD =3300exp(−16867.3/RT and T SSP1=2300exp(−13630.5/RT ).

Contrary to our expectative the TSS of the 470◦C treatment is slightly higher than the measu-

rements for the 380◦C treatment.

ACKNOWLEDGEMENTS

This work was partially ﬁnanced by CONICET PIP-11220170100021CO.

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