Phase Transformation, Microstructural Evolution and Tensile Properties of a TiH2-Based Powder Metallurgy Pure Titanium

Abstract

Multiple phase transformations were carried out during dehydrogenation process of TiH2-based powder metallurgy. The influence of phase transformation on the microstructure is highly worthy of attention. In situ synchrotron radiation was employed to investigate the phase transformation sequence of TiH2 powder compact during the vacuum sintering process. It was found that a transformation route TiH1.971 → TiH2 + TiH + TiH0.71 + α(H) → TiH + TiH0.71 + α(H) → TiH + TiH0.71 + α(H) + β(H) → α(H) → α took place, resulting in an equiaxed microstructure. Increasing heating rate and avoiding the intense dehydrogenation to retain hydrogen-rich β (β(H)) and TiHx aciculae at the interfaces is found to be a feasible method to fabricate hierarchical α-Ti structures. A fully dense fine martensitic microstructure was produced after fast heating the TiH2 powder compact to 1100 °C and an immediate hot extrusion. Subsequently, by vacuum annealing treatment at 700 °C, composite α/βt lamellar structures were generated and a simultaneously enhanced tensile strength of 746 MPa and excellent elongation to fracture of 33.8% were achieved. It is suggested that adjusting the dehydrogenation reactions of TiH2-based powder metallurgy is conductive to generating hierarchical lamellar structures with a highly promising combination of strength and ductility for pure Ti.

1. Introduction

Titanium and titanium alloys have always been important structural metal materials due to their excellent corrosion resistance, good bio-compatibility and high specific strength [1]. However, the ultimate tensile strength of pure titanium must be enhanced if it is used as a biomedical structure such as prosthetics. The common strengthening method is the introduction of alloying elements, such as Al and V. They are favorable for strengthening but toxic [2]. Based on the principles of “plainified materials” and “resource sustainability”, that is to say, improving the performance of materials by tailoring stable interfaces with fewer or no alloying elements [3], powder metallurgy technology (PM), especially TiH2-based PM, is recommended as a feasible method to fabricate pure titanium characterized by highly promising tensile strength.

It has been recognized that TiH2-based powder metallurgy technology contributes to improving the preparation efficiency and optimizing mechanical properties of titanium alloys. Firstly, titanium hydride (TiH2) powder is the intermediate product of the hydrogenated and dehydrogenated (HDH) Ti powder and is relatively cheaper than pure Ti powder. Second, hydrogen element is a strong β-phase stabilizer, so 0.3 wt% of hydrogen effectively reduces the phase transition temperature by about 100 °C [4] and is beneficial for decreasing the high-temperature deformation resistance of titanium alloys [5]. Third, it is demonstrated that hydrogen can be completely removed during the vacuum sintering process [6]. Meanwhile, during the dehydrogenation process, a high number of defects, such as dislocations and vacancies, are generated, which provide more nucleation sites for new α precipitations and are conducive to grain refinement [7].

The fine-grain strengthening resulting from complex phase transformations during the dehydrogenation process and the appropriate oxygen solution strengthening ascribed to the inherent oxygen-rich quality are the two major mechanisms of enhancing the tensile properties of TiH2-based powder metallurgy titanium alloys [8,9,10]. An ultra-fine microstructure characterized by excellent tensile and fatigue strength was obtained by the hydrogen sintering and phase transformation process of TiH2 and AlV60 blended powder compact [11]. The main refinement mechanisms were ascribed to the precipitation of intermediate α/α2 and the eutectoid reaction of β → α + δ at 210 °C [12,13]. A fine lamellar microstructure was achieved by induction heating TiH2 powder compact followed by hot extrusion, which exhibited good tensile strength of 692 MPa owing to fine α-lath and 0.32 wt% oxygen solid solution strengthening [14]. However, the phase transformation sequence from TiH2 to α-Ti is not mentioned.

It is well known that pure titanium with a lamellar microstructure is obtained by hot processing in the single β-phase field (>882 °C), while an equiaxed microstructure is prone to be obtained by hot deformation in the α + β two-phase field (650–882 °C) [15]. However, different microstructural characteristics of pure titanium were produced at the same sintering temperature by TiH2-based powder metallurgy. Fox example, a bimodal microstructure consisting of coarse α-grain and β-transformed domains (βt) containing a large number of interwoven α aciculae with a few retained β matrix was obtained by hot mechanical consolidation of milled TiH2 powder at 1050 °C [16]. An equiaxed microstructure was obtained by vacuum sintering TiH2 powder compact at 1050 °C for 3 h followed by hot extrusion [17]. The generation mechanisms of lamellar microstructures and equiaxed microstructures deserve to be revealed.

In this study, the phase transformation sequence of TiH2 converted into α-Ti after vacuum sintering was explored by an in situ synchrotron radiation experiment. The key mechanism of generating hierarchical lamellar structures was put forward. The microstructural evolution and tensile properties of hot-extruded pure titanium before and after vacuum annealing were investigated systematically. This work is of great theoretical significance for microstructure customization.

2. Materials and Experimental Methods

TiH2 powder with a hydrogen content of about 4.0 wt% was used in this work, which was fabricated by crushing TiH2 sponge particles using a planetary ball milling machine at a rotation speed of 300 rpm for 5 min. The TiH2 sponge particles were provided by Chaoyang Jinda Titanium Co., Ltd., Shenyang, China. Figure 1 shows the morphologies of powder and compact as well as corresponding powder size. The shape of the milled TiH2 powder is irregular. After sieving, the mean powder size of powder is measured to be 25 μm.

TiH2 powder was cold-pressed in a steel die at a maximum pressure of 960 MPa and held for 5 min. Well-shaped powder compacts with dimensions of φ53 × 45 mm³ were formed, and the relative density was measured to be 89%. The powder compact was induction-heated to 1100 °C at a rate of 80 °C/min, and held for 5 min to finish dehydrogenation and densification. Subsequently, the sintered billet was transferred into a steel die with a pre-heated temperature of 400 °C and hot-extruded with a ratio of 9:1 to obtain a rod of 18 mm in diameter. The sintering and hot extrusion were conducted in a glovebox filled with argon by controlling the oxygen content at less than 200 ppm. Subsequently, vacuum annealing was performed at a condition of 700 °C/6 h/FC, and the original vacuum value was 5 × 10⁻³ Pa. In addition, another small powder compacts with dimensions of φ25 × 15 mm³ and a relatively density of 80% were obtained at a pressure of 650 MPa and held for 5 min, and were used for phase transformation, thermal analysis tests and microstructural evolution under different sintered parameters.

The in situ synchrotron radiation experiment was conducted at the BW14 beamline of the Shanghai Synchrotron Radiation Facility, using LaB6 source with a wavelength of 0.687 Å. A TiH2 compact of 3 mm in diameter and 3 mm in height was cut out from the above small powder compact and placed in a graphite crucible, and then transferred to the vacuum chamber of the beamline. The sample was heated up to 1000 °C at a heating rate of about l0 °C/min. The initial vacuum value was 5 × 10⁻² Pa. The data were collected at a frequency of 2 HZ during the experiment. After that, the Debye rings were converted into corresponding diffraction patterns by Findit 2010 software.

A thermal dilatometry experiment (Netzsch DSC 404F3, Germany) was employed to monitor the linear shrinkage of the green compact under argon flowing at 80 mL/min. The tested sample with dimensions of φ5 × 2.5 mm³ were placed between two molybdenum sheets with a load of 10 g. After testing, an optical microscope (OM, ZEISS Axio Lab.A1, Germany) and scanning electron microscopes (SEM, JSM-7001F and JSM-6510A, Japan) were used to characterize the microstructure. The hydrogen and oxygen contents of some samples at different processing states were measured by a Bruker oxygen–hydrogen analyzer (G8 GALILEO, Bruker, Billerica, MA, USA).

Room-temperature tensile tests were performed on a Shimadzu AG/X Plus 100 KN machine at an initial strain rate of 5.0 × 10⁻⁴ s⁻¹ according to the common GB/T 228.1-2021 standard; Metallic materials tensile testing: method of test at room temperature, Bejing, China, 2022. Flat dog-bone-shaped specimens with a gauge length of 15 mm and cross-sectional area of 3 × 2 mm² were adopted for the tensile tests; tensile direction is parallel to the extrusion direction. Three tensile samples under each state were tested to obtain the average values of tensile properties with an extensometer.

3. Results and Discussion

3.1. Phase Transformations of TiH2 Power Compact

The phase transformations of TiH2 powder compact heated to 1000 °C at a heating rate of 10 °C/min were determined by in situ synchrotron radiation, as shown in Figure 2. The peaks marked by red dots belong to carbon resulting from the graphite crucible, which remained unchanged throughout the heating process, as shown in Figure 2a. From the XRD patterns in Figure 2b,c, the original titanium hydride is FCC-structured δ-TiH1.971 phase (PDF 25-0982, a = 4.44 Å).

It is found that the dehydrogenation reaction begins at 700 °C and the phase transition route is TiH1.971 → TiH2 + TiH + TiH0.71 + α(H), in which TiH2 is a face-centered tetragonal cell, and the hydrogen-rich α phase (α(H)) is a hexagonal closed-packed (HCP) cell, as shown in Figure 2d–f. The co-existence of many titanium hydrides is ascribed to the heterogeneous decomposition of different powder particles. TiH and TiH0.71 are the intermediate states characterized by an orthogonal cell of a = 4.176 Å, b = 4.233 Å, c = 4.585 Å and a = 4.34 Å, b = 4.18 Å, c = 4.02 Å, respectively. The detailed parameters of the cell structures are listed in

Table 1. Essential information of titanium hydrides detected by in situ synchrotron radiation.

Titanium Hydride	PDF	Crystal System	Crystallographic Parameter (Å)	Space Group Symbol/Number	Density (g/cm3)
TiH1.971	07-0370	FCC	a = b = c = 4.44	Fm$\overline{3}$m/225	3.87–3.94
TiH2	09-0371	Tetragonal	a = b = 3.12, c = 4.18	I4/mmm/139	3.88–4.074
* TiH	44-1217	Orthorhombic	a = 4.18, b = 4.22, c = 4.585	Cccm/66	4.02
* TiH0.71	40-0980	Orthorhombic	a = 4.34, b = 4.18, c = 4.02	-	-
β(H)	44-1288	BCC	a = 3.31	Im$\overline{3}$m/229	4.4
TiH(α(H))	mp-998969	HCP	a = 2.88, c = 5.26	P62/mmmc/194	4.3
α	11-1294	HCP	a = 2.95, c = 4.68	P63/mmc/194	4.5

* represents the metastable intermediate phases.

.

At 740–750 °C, the peaks of TiH₂ disappear and are replaced by small peaks of α(H) at 2θ = 15.46°, as marked by the red dots. Simultaneously, the α(H) peak intensity of 2θ = 17.5° increases with temperature, indicating the further conversion of TiH₂ into α(H). The path is TiH₂ + TiH + TiH_0.71 + α(H) → TiH + TiH_0.71 + α(H), as shown in Figure 3a,b. After that, the α(H) peak of 2θ = 16.74° gradually shifts to the right and the peak intensity gradually decreases with the increase in temperature, as shown in Figure 3c,d. At 790 °C, the broadened α(H) peak at 2θ = 23.67° splits into two small peaks of hydrogen-rich β phase (β(H)) and α(H) containing lower hydrogen content, a sign of the further dehydrogenation of α(H) and the transition of α(H) into β(H). The route is TiH + TiH_0.71 + α(H) → TiH + TiH_0.71 + α(H) + β(H). Between 790 °C and 820 °C, the peak of TiH at 2θ = 29.2° shifts to the right with increasing temperature. At 830 °C, the TiH_0.71, TiH and β(H) peaks disappear. The phase transition is TiH + TiH_0.71 + α(H) + β(H) → α(H), as shown in Figure 3e,f. At 840 °C, a small peak of α at 2θ = 16.9° is detected, which demonstrates that α(H) → α takes place, as displayed in Figure 3d.

It is worth noting that β(H) only remains between 790 °C and 820 °C, and then quickly transforms into α(H) or α. In this study, the predominant HCP-structured α-Ti phase is formed after 840 °C during vacuum sintering. The result agrees well with the report by Chen et al. [18], in which they considered the phase transition of TiH2 powder by neutron diffraction to be δ → δ + α(H) → δ + β(H) + α(H) → β(H) + α(H) + α → α. The diffusion rate of hydrogen in the BCC-structured β is 2~3 orders of magnitude higher than that in the HCP-structured α, so the hydrogen content in β(H) is rapidly reduced and is replaced by α(H) or α [19].

3.2. Microstructural Characteristics of As-Sintered TiH2 Power Compact

The micrographs of the sintered TiH2 samples are shown in Figure 4. After being vacuum-sintered at 700 °C and held for 30 min, an equiaxed microstructure containing α(H) and some acicular TiHx precipitated at the grain boundaries (marked by red arrows) is produced, as shown in Figure 4a,b.

The sintered sample after the in situ synchrotron radiation experiment is also characterized by an equiaxed microstructure composed of α phase, as shown in Figure 4c,d. According to the temperature curves inserted in Figure 4d, the preservation at 420 °C, 650 °C, 700 °C, 750 °C and 860 °C is, respectively, 5 min, 7 min, 25 min, 5 min and 5 min, in which the holding time of 700 °C is the longest, resulting in an intense dehydrogenation reaction. Compared with the titanium hydride compact in Figure 1a,b, it is established that complete dehydrogenation does not bring about obvious change in the morphology, except for the main phase changing from titanium hydrides to α-Ti phase. The microstructure is in accordance with the report that vacuum-sintered TiH2 powder compacts at conditions of 480 °C without holding, 480 °C/1 h, 580 °C/1 h and 700 °C/0.5 h also present equiaxed microstructures [20].

Under argon, the microstructure of the TiH2 powder compact heated directly to 1100 °C by a thermal dilatometry instrument without holding at 700 °C with a heating rate of 10 °C/min is shown in Figure 5a,b. The hydrogen content is measured to be about 0.19 wt%. A quasi-equiaxed microstructure composed of α(H), β(H) or β and TiHx is received. This directly confirmed that the dehydrogenation is not complete without holding at 700 °C. In addition, the release amount of hydrogen during the dehydrogenation reaction under argon is reported to be smaller than that of sintered samples under vacuum [20]. The above two sections suggest that a small amount of retained hydrogen content contribute to the stabilization of β. Furthermore, by controlling the heating rate to 20 °C/min and without holding at 700 °C, the microstructure of the sintered TiH2 powder compact at 900 °C is displayed in Figure 5c,d. The retained hydrogen content is about 0.30 wt%. It can be seen that a bimodal microstructure comprising α or α(H) lamellae, titanium hydride and α aciculae is produced, similar to the report in [14]. It was reported that by avoiding the complete hydrogen emission to retain hydrogen content ranging from 0.3 wt% to 0.6 wt%, the nucleation rate of α-laths was improved, and then coarse α plates and fine-grained α aciculae were created together. This is ascribed to the defects, such as vacancies and dislocations resulting from the Ti-H eutectoid reaction, accompanied by the partition of H between α-Ti and β-Ti phases [21].

XRD is employed to clarify the phase composition of the sintered sample, as shown in Figure 6. It is found that only the α(H) peak appears in the vacuum-sintered sample at 700 °C, while small peaks of titanium hydrides (TiH2) with a tetragonal structure are identified in the sample sintered under argon. It is worth noting that the TiH2 peaks of the sintered sample at 900 °C are more obvious than those of the sample sintered at 1100 °C, indicating that the proportion of retained titanium hydrides in the sample sintered at 900 °C is higher, consistent with the microstructural characteristic in Figure 5. No β or β(H) is detected in the XRD patterns. The reasons are either that the ratio of β(H) is too low to be detected by ordinary XRD or that β(H) has completely transformed into α(H) and TiH2. After all, the heating rate and surrounding atmosphere play a role in the retained hydrogen content, and hydrogen content influences the phase transition route of dehydrogenation reactions [22,23]. The construction of hierarchical α structures is the result of eutectoid transformation β(H) → α + δ [13].

3.3. Thermal Analysis of TiH2 Powder Compact

The dilatometric curves of the TiH2 powder compact are displayed in Figure 7 to identify the critical points of TiH2 decomposition and corresponding line shrink. Two discernible endothermic peaks present at about 590 °C and 700 °C, in which the intensity of the 590 °C peak at 10 °C/min heating rate is higher and the shrinkage rate is larger, as shown in Figure 7a,b. It implies that the starting temperature of dehydrogenation does not change with the heating rate, but the slow heating rate is conductive to the release of hydrogen. As reported, the faster the heating rate, the more delayed the dehydrogenation [24]. Obviously, compared Figure 2b with Figure 7a, it can be seen that the dehydrogenation temperature is lower in argon flow (587 °C) than under vacuum VS (700 °C), although there have been reports that dehydrogenation reactions were faster under vacuum [20]. This should be ascribed to the low vacuum value in the synchrotron radiation experiment.

In addition, the shrinkage rates of the samples sintered at 1100 °C and at 900 °C are 9.5% and 4.4%, respectively. The shrinkage before 900 °C results from the intense dehydrogenation reaction accompanied with a high density of dislocations and vacancies, while the shrinkage between 900 °C and 1100 °C is considered to be caused by the sintering densification, as demonstrated by a sintering neck beginning to form at 900 °C in Figure 5d. Notably, small endothermic peaks appear after 700 °C. Two small peaks are detected in Figure 7a, while only one small peak appears in Figure 7b. The first peak is considered to be related to the transformation from α(H) to β(H), and the second peak is ascribed to the phase transition of α → β, due to the temperature of 1050 °C and a lower heating rate conducive to the dehydrogenation reaction.

It can be deduced that increasing the sintering temperature to a single β-phase field is essential to improve relative density owing to the increase in the self-diffusion coefficient of titanium atoms during the α → β transition, as demonstrated by the higher density in the sample sintered at 1100 °C under argon than that of the sample sintered at 1000 °C under vacuum. Meanwhile, accelerating heating rate is favorable for fabricating hierarchical α structures, due to eutectoid reactions of β(H) to α and titanium hydrides favorable for the nucleation of fine α-laths, as proven by the microstructure of the sample sintered at 900 °C with a heating rate of 20 °C/min.

3.4. Microstructural Evolution of TiH2 Powder Compact After Thermal Mechanical Consolidation

The heating rate is further increased to 80 °C/min by induction heating and the relative density is enhanced to approximately 100% by virtue of hot extrusion at 1100 °C. The microstructural characteristics of hot-extruded bar are shown in Figure 8. After hot extrusion, the oxygen and hydrogen content are measured to be 0.28 wt% and 0.22 wt%, respectively. It suggested that most of the hydrogen is removed after sintering and hot extrusion. It can be seen that fine martensite microstructure composed of α(H) and β(H) accompanied by TiHx aciculae at the interface is obtained in the as-extruded sample, as shown in Figure 8a,b. The microstructure is uniform without obvious flow lines. This is because the extrusion temperature of 1100 °C and the defects resulting from the dehydrogenation reaction can promote the sufficient recrystallization of the β parent grains.

The α martensites are homogeneous and characterized by 2 μm width and 10–20 μm length, being thinner the Ti−0.85O fabricated by SPS sintering followed by hot extrusion [14], because of the fast cooling rate and relatively low oxygen content in this study. The grain boundaries of α (αGB) are difficult to distinguish, which is different from the traditional lamellar microstructure with visually articulated αGB produced by ingot metallurgy [25,26]. This indicates that the combined effect of phase transitions and recrystallization breaks through the traditional interface growth law, so the generation of continuous coarse αGB is inhibited and a fine lamellar microstructure is obtained.

After vacuum annealing, the hydrogen content decreases to 0.01 wt%. The retained TiHx phases and hydrogen-rich phases have been completely decomposed. The dark regions in the OM image are β-transformed (βt) regions composed of nanoscale acicular α obtained by the eutectoid reaction of β(H) → α + TiHx at 300 °C [27], in accordance with our previous report [28]. Consequently, another composite microstructure containing broadened α-laths and βt domains full of ultrafine interwoven α aciculae is generated in the as-annealed sample, as shown in Figure 8c,d. The mechanism of generation of such a novel microstructure after vacuum annealing was referred to the study in [16], except that the α-laths are more broadened rather than equiaxed in this study.

It is the multistage decomposition of titanium hydrides in combination with many defects resulting from the complex dehydrogenation reactions that is responsible for the generation of the novel lamellar microstructure containing βt domains and α-laths. Two keynote factors are necessary, as discussed in our previous study of near-alpha titanium alloys [28]. That is to say, firstly, and of crucial importance, a hydrogen-rich β phase β(H) is obtained at high temperatures by accelerating the heating rate to retain 0.22 wt% hydrogen content. During fast cooling after hot extrusion, α-laths grow preferentially and are accompanied by the distribution of hydrogen in α and β plates, so more and more hydrogen element is ejected into the β plates owing to the low solid solubility of hydrogen in the α-laths. Subsequently, the eutectoid reaction results in β plates and TiHx aciculae. At the same time, the oxygen solid solution and many defects such as vacancies and dislocations promote the precipitation of α in β plates. And then the growth of α precipitations is limited, but the nucleation rate of the α phase is promoted owing to the amount of dislocation and the fast cooling rate. Second, after further vacuum annealing at 700 °C, TiHx aciculae and hydrogen-rich phases are thoroughly decomposed, and then α precipitations in the β matrix grow rapidly to form βt domains accompanied by the broadening of previous α-laths to reduce the interface energy.

3.5. Tensile Properties and Fractural Morphologies of Hot Extruded Bar

The tensile properties of the as-extruded and as-annealed samples are displayed in Figure 9. The as-extruded sample exhibits a slightly high yield strength (YS) of 543 MPa, an ultimate tensile strength (UTS) of 723 MPa and a relatively low elongation to fracture (EL) of 19.8%. By contrast, the as-annealed sample has a YS of 536 MPa and higher UTS of 746 MPa, as well as a better EL of 33.8%, as listed in

Table 2. Tensile properties of pure titanium sample.

Sample	Yield Strength (MPa)	Ultimate Tensile Strength (MPa)	Elongation to Fracture (%)
As-extruded in this study	543 ± 8	723 ± 9	19.8 ± 1.3
As-annealed in this study	536 ± 7	746 ± 8	33.8 ± 1.7
Ti-SLM [29]	502	600	34.3
Ar-sintered TiH2 compact at 1100 °C for 4 h [7]	529	659	20.8

. It is speculated that hydrogen atoms in the solution and fine lamellae are beneficial for improving the yield strength, but the TiHx aciculae at the interface are not conductive to the ductility.

Synchronously enhanced strength and ductility are achieved in the as-annealed sample with a novel α/βt lamellar microstructure. Consequently, the elongation to fracture is much better than that of the TiH2 sample sintered under argon flow, and the tensile strength is 146 MPa higher than that of fine-grained equiaxed microstructure fabricated by selective laser melting (SLM) without sacrificing the tensile ductility [29], as listed in Table 2.

The tensile fractural characteristics are shown in Figure 10. It is found that many cracks were generated and they resulted from the TiHx aciculae at the interface in the as-extruded sample with fine lamellar microstructure. Notably, ductile fracture characterized by dense fine dimples and scattered large dimples was present in the as-annealed sample. The scattered large dimples are considered to correspond to the coarsen α-laths regions, while the dense small dimples are related to the region of the βt structures. This indicates that the βt structures contribute to the improvement in tensile ductility.

4. Conclusions

This study investigated the correlation of the phase transformation, microstructural characteristics during dehydrogenation reactions and corresponding tensile properties of TiH2-based powder metallurgy pure titanium. A novel composite α/βt lamellar microstructure was produced by fast induction sintering and hot extrusion, followed by vacuum dehydrogenation annealing treatment. Some key conclusions are summarized below.

1.

The phase transition sequence of TiH2 powder compact heated up to 1000 °C under vacuum at a heating rate of 10 °C/min is TiH1.971 → TiH2 + TiH + TiH0.71 + α(H) → TiH + TiH0.71 + α(H) → TiH + TiH0.71 + α(H) + β(H) → α(H) → α. Subsequently, an equiaxed microstructure is received owing to complete dehydrogenation after holding for 25 min at 700 °C.

2.

Increasing the heating rate and avoiding the intense dehydrogenation to retain hydrogen-rich β (β(H)) and TiHx aciculae at the interface is a feasible method of fabricating hierarchical α-Ti structures. A dense fine lamellar microstructure is produced after TiH2 powder compact induction-heated to 1100 °C accompanied with hot extrusion.

3.

After vacuum dehydrogenation annealing, α precipitations in β matrix grow rapidly to form βt domains accompanied with the broadening of previous α-laths owing to the thorough decomposition of β(H) and TiHx, and a novel composite α/βt lamellar microstructure is obtained.

4.

An enhanced strength of 746 MPa is received and elongation to fracture is increased from 19.8% to 33.8% in the α/βt lamellar microstructure. βt structures are proven to be beneficial for the improvement in tensile ductility.