Production of Neutron-Absorbing Zirconium-Boron Alloy by Self-Propagating High-Temperature Synthesis and Its Refining via Electron Beam Melting

Abstract

The paper presents the results of the study of the processes of self-propagating high-temperature synthesis of Zr-1%B alloy and its refining by electron beam melting. Experiments on the influence of boron’s amorphous and crystalline modifications on the safety parameters of the synthesis process of Zr-1%B alloy necessitated the conversion of amorphous boron into crystalline form by electron beam melting, with an increase in its purity from 94% to 99.9%. High efficiency of vacuum induction and electron beam equipment was demonstrated, which provided a high purity of the Zr-1%B alloy of at least 99.9%. The alloy ingots had a uniform distribution of the alloying element (boron) all over the volume. The obtained alloy is suitable for the production of materials with thermal neutron capture cross-sections of up to 40 barns for neutron protection.

1. Introduction

The known methods of obtaining binary alloys of refractory metals are vacuum arc, electron beam, and induction melting of individual pure elements. These processes require high-purity metals, the price of which is high. Methods of obtaining metals of nuclear purity, such as iodide, zone melting, distillation, etc., allow reduction in the content of impurities to 10^–2–10^–4%, but they are energy-consuming and low-productive [1,2]. In the XXI century, the processes of self-propagating high-temperature synthesis (SHS) of alloys and powders of zirconium, hafnium, and other refractory metals were developed [3,4,5,6].

A more economical and productive process for the production of binary refractory alloys, such as zirconium, is the reduction melting of zirconium tetrafluoride (ZrF₄) with high-purity calcium in the presence of alloying elements. This process was developed to produce high-purity zirconium-niobium alloys, which are used in VVER-type reactors [7,8]. The purity of ZrF₄ at the level of 99.9% was achieved by processing the pure zirconium oxynitrate ZrO(NO₃)₂ after hafnium extraction in the process of their separation by solvent extraction [9].

The method of obtaining binary zirconium alloys directly in the SHS process by the calcium-thermal reduction method allows obtaining, for example, a binary alloy Zr-10%Hf, which has a thermal neutron absorption degree at the level of 10 b [10]. The increase in thermal neutron absorption degree of corrosion- and radiation-resistant zirconium alloys doped with boron is of practical interest [11]. Pure boron has a large thermal neutron capture cross-section of 3840 b. This is much higher than that of hafnium (115 b), so it is used in the control systems for the protection of nuclear reactors [12].

Thus, the introduction of boron, which is more effective than hafnium, into the zirconium alloys makes it possible to significantly increase neutron capture. According to calculations, the addition of 1% boron into the Zr alloy can increase the thermal neutron capture cross-section from 0.18 to 40 b, which makes it possible to use this alloy for protection against neutron radiation [13]. ZrB₂ alloy is used to produce heat-resistant and neutron-absorbing coatings by sputtering, but cannot be used for deformation processes to produce products such as sheets [14].

Zirconium alloying with boron up to 1% provides a high degree of deformation of the alloy to obtain a wide range of products: sheet, tube, and bar [15]. In industry, boron is obtained in the form of amorphous powder in the process of reducing boron oxide with magnesium [16]. Boron is known to have a more positive effect on the physical and mechanical properties of steel, in particular, the microstructure, compared to traditional alloying components [17]. Boron is introduced into iron-based alloys as a ligature since its solubility in melts is low due to its high melting point [18].

Usually, boron is in an amorphous state because when it is obtained by the metal-thermal method, the heat of the reduction reaction is not enough to obtain a homogeneous ingot. Amorphous boron can contain up to 6% of impurities [19], which significantly complicates the alloying process. The addition of boron increases the strength of steel without reducing its ductility, but the efficiency of alloying depends on the chemical composition of the alloy. To increase the efficiency of boron as an alloying component, it is advisable to purify it from impurities. Taking into account the melting point and elasticity of boron vapor, such purification can be carried out by refining in the process of electron beam melting (EBM). However, there is practically no information about such studies in the literature.

SHS processes are usually used to obtain new valuable materials by solid-phase interaction of pure elements [20]. SHS reactions proceed at a rate of up to 10 cm/s, which makes it possible to obtain oxygen-free refractory compounds: borides, silicides, carbides, intermetallides, and several refractory alloys. Industrial alloys of hafnium with nickel, zirconium with niobium, nickel, and aluminum were first obtained in Ukraine by SHS in the process of reduction in ZrF₄ or HfF₄ by calcium in the presence of powdered alloying components [10]. Boron is known to be used to obtain the magnetic alloy Nd-Fe-B, where the boron content does not exceed 1% [21]. The alloying of iron and nickel with niobium, rare earth elements (REE), and boron allows for the formation of a ligature, which is used in the production of high-strength cast iron [22].

The present work aimed to study the SHS process of Zr-1%B alloy by calcium-thermal reduction of ZrF4 in the presence of boron powder before and after its refining by electron beam melting. The secondary goal was to study the influence of boron purity on the alloy quality and process conditions and to determine the uniformity of boron distribution over the ingot volume.

2. Materials and Methods

2.1. Characteristics of Materials

Chemical reagents of “chemical purity” degree were used as initial materials for the production of Zr-1%B alloy: sublimated ZrF₄ of 99.9% purity in β-phase, with a particle size ≤ 3 mm; calcium chips of 99.9% purity obtained by cutting monolithic calcium in argon atmosphere; and amorphous and crystalline boron powders. The content of the main impurities in the initial components of the charge is given in

Table 1. Content of impurities in initial materials.

Component	Impurity (wt.%)
	N	C	O	Si	Al	Ni	Fe	H2O	B
ZrF4	0.003	0.005	0.050	0.005	0.005	0.008	0.050	-	-
Ca	0.002	-	0.080	-	0.003	0.020	0.003	-	-
B (amorphous)	-	-	-	0.100	-	-	0.200	0.300	94.0
B (crystalline)	-	-	0.050	0.001	-	-	0.001	-	99.9

.

2.2. Methodology of Zr-1%B Alloy Production Using Amorphous Boron on Vacuum Induction Unit IPHT-200

The flow sheet diagram of the process of obtaining Zr-1%B alloy using amorphous boron is shown in Figure 1.

The quality of the alloy was ensured by the purity of the starting materials, inert atmosphere, and cooling of the copper wall of the collecting crucible, which prevented the interaction of molten ingot with copper. The purity of ZrF₄ in terms of introduction impurities (O₂, N₂) was ensured by sublimation purification in a vacuum at 700–750 °C.

The reaction of ZrF₄ reduction by calcium is exothermic, and the released heat is sufficient for the complete melting of products and reaching the temperature of 2200 °C. The completeness of separation of alloy and slag is based on the difference in their densities, which are 6.5 and 2.7 g/cm³, respectively.

SHS melting to obtain Zr-1%B alloy (1 wt.%) was carried out on the IPHT-200 unit (Figure 2), which is a vacuum induction furnace with a copper water-cooled crucible of an inner diameter of 200 mm.

Main technical characteristics of the IPHT-200 unit:

-

power consumption—up to 563 kW;

-

inductor power—up to 460 kW;

-

inductor voltage—up to 800 V;

-

current frequency—2400 ± 5 Hz:

-

vacuum in the furnace chamber—up to 5·10^–4 mm Hg.

Chromel–alumel thermocouples (T₁–T₇) were used to control the temperature in the furnace, the scheme of their arrangement is shown in Figure 3.

The reduction melting process was carried out as follows: A copper water-cooled tray was inserted inside the “cold” crucible and installed at a distance of 30 mm below the bottom coil of the inductor. The gap between the wall of the cold crucible and the tray was sealed with an asbestos cord. Amorphous boron powder was thoroughly mixed with ZrF₄ powder and the calculated amount of charge components was loaded into the crucible. Chromel–alumel thermocouples were installed along the height of the charge to control the temperature during the charge heating (Figure 3). The proposed scheme of thermocouple arrangement allowed for control of the charge heating and the rate of SHS of the alloy.

At a distance of 0–10 mm above the charge, a graphite igniter was connected to the mechanism of “firing”. That scheme allowed the removal of the igniter from the charge to a distance of 100 mm before the initiation of the SHS reaction. The furnace chamber was closed with a lid, sealed, and vacuumed to a residual pressure of 1·10⁻² mm Hg, after which the chamber was filled with argon to an overpressure ≤0.1 atm.

Heating of the charge and initiation of the reduction reaction was carried out at 300 kW. The reaction products were cooled in the furnace for 1 h. Then, they were removed from the crucible, and the metal ingot with alloy garnissage was separated from the slag (CaF₂). The yield of metal in the ingot was determined by the gravimetric method. Samples were taken from the ingot to analyze for boron content. The composition of alloy samples was identified using a scanning electron microscope (SEM, Zeiss, Oberkochen, Germany) with energy-dispersive spectroscopy (EDS).

2.3. Methodology for Obtaining Crystalline Boron by Electron Beam Refining on the UE-177RL Unit

Technical characteristics of the UE-177RL unit are given in

Table 2. Technical characteristics of the UE-177RL.

Characteristics	Unit	Value
Electron beam heater wattage	kWt	≤50
Accelerating voltage	kV	≤18
Electron beam current	A	≤2.5
Cathode current	A	60
Residual gas pressure:	mm Hg
- in the electron beam heater chamber		1·10–4–1·10–5
- in the working chamber		1·10–2–1·10–4
Vacuum pumping speed:	L/s
- from the electron beam heater chamber		630
- from the working chamber		500
Working chamber volume	m3	1
Crystallizer internal dimensions	mm	270 × 50 × 50
Preparatory cycle duration	min	60
Overall dimensions of the unit	mm	3020 × 1350 × 2100

.

Obtaining crystalline modification of boron from amorphous powder was carried out on the UE-177RL unit according to the following procedure: A portion of amorphous boron powder weighing 0.6–0.8 kg was poured into a copper water-cooled crystallizer. Then the chamber of the unit was vacuumed to the residual pressure of 1·10^–3 mm Hg, after which the powder was degassed by heating it with an electron beam of low power (7–10 kW) to a temperature not exceeding the melting point of boron (2348 K). The duration of degassing was 15–20 min.

The end of the degassing process was controlled by decreasing the pressure in the melting chamber to the initial value. Then the power of the electron beam was increased up to 22–29 kW and the liquid bath of boron was melted for 30–40 min. After that, the boron ingot was cooled for 60 min in argon, after which the next portion of amorphous boron powder weighing 0.3–0.5 kg was poured on the surface of the ingot in the crystallizer and the melting cycle was repeated until the ingot weighing 1.5 kg was obtained. The total mass of boron powder loaded for melting was 1.1–1.9 kg. To ensure uniformity of the composition of crystalline boron powder, the ingot was milled to 0.1–1.0 mm.

2.4. Methodology of Calcium-Thermal Reduction of ZrF₄ in Vacuum Induction Furnace ISV-1.0

The pilot batch of Zr-1%B alloy was produced on an industrial induction vacuum furnace of ISV-1.0 type with a split copper water-cooled crucible with a diameter of 500 mm. The power supply capacity of the unit was 1500 kW.

Sublimed ZrF₄ powder of 99.9% purity with a grain size of 0.1–1.0 mm, monolithic calcium chips, and crystalline boron powder with a grain size of 0.1–1.0 mm were used as initial components of the charge. The boron powder was thoroughly mixed with ZrF₄ powder, and the charge of the initial materials was carried out layer by layer on a copper water-cooled tray. To control the process of preheating, four chromel–alumel thermocouples (T₁–T₄) in covers of zirconium tubes were installed along the height of the charge (Figure 4).

Dosing of crystalline boron into the initial charge was carried out based on its maximum theoretical content in the alloy of 1.2 wt.%. The furnace was vacuumed to a residual pressure of 5·10^–3 mm Hg, the charge was flushed with argon, vacuumed anew, and then filled with argon to an overpressure ≤0.1 atm. The charge was heated in discrete mode until the spontaneous onset of the reaction. After the reaction, the products were cooled to 40 °C, the ingot was separated from the garnissage, and the yield of metal in the ingot was determined by gravimetric method.

2.5. Methodology of Electron Beam Melting of Zr-1%B Alloy on the UE-177RL Unit

Electron beam unit UE-177RL (Figure 5) consists of the following main assemblies: working chamber, electron beam heater chamber, flat beam electron gun with quadrupole lens, crystallizer, mechanism for moving the workpiece (ingot), ingot cart, vacuum system, water-cooling system, electron beam heater power supply system, and electron beam control system.

Refining melting of the alloy was carried out as follows: The billet consisting of three crude ingots, cut in half for convenience of feeding into the melting zone, was placed on the loading table of the feeding mechanism. The total mass of the billet was 4.92 kg. The chamber was evacuated to a residual pressure of 1·10^–3 mm Hg. Then, the billet was heated by a 6 kW electron beam for degassing. The duration of the degassing process was 10 min. The fusion of the billet was started after increasing the power of the beam to 27 kW. The metal flowing from the end of the billet fell into a water-cooled copper crystallizer with an internal size of 270 mm × 70 mm × 30 mm. The electron beam was used to form a flat rectangular ingot.

2.6. Methodology of Zr-1%B Alloy Refining at the UE-178M Electron Beam Unit

Electron beam unit UE-178M with the copper water-cooled intermediate vessel is designed for the melting of refractory metals and alloys in a vacuum to ensure their deep refining. Technical characteristics of the unit are given in

Table 3. Technical characteristics of the UE-178M unit.

Characteristics	Unit	Value
Set the wattage of the electron beam heater	kWt	500
Number of electron beam cannons	-	6
Set the wattage of the accelerating voltage source of the power supply and control system	kWt	500
Accelerating voltage	kV	18
Dimensions of alloyed workpieces:	mm
- length		600
- diameter		300
Dimensions of melted ingots:	mm
- length		580
- diameter		180; 200; 230
Operating pressure:	mm Hg
- in the heating chamber		1·10–4–5·10–5
- in the melting chamber		1·10–2–1·10–4

.

Refining of Zr-1%B alloy in the UE-178M electron beam unit was carried out as follows: The ingot with a diameter of 500 mm was preliminarily cut in half by the electron beam in the UE-177RL unit. The cut parts were stacked on top of each other and joined using a zirconium stud with a diameter of 10 mm. The assembled blank was suspended on a molybdenum wire from the hook of the blank feeder mechanism. The furnace was vacuumed to a residual pressure of 5·10^–4 mm Hg. Degassing of the billet was carried out by heating it in a vacuum by electron beams at a power of about 50–60 kW. The duration of the degassing was 10–15 min, after which increasing the power of the beams up to 100–140 kW started melting the billet into a copper water-cooled intermediate container.

As the billet heated up and the melting process stabilized, the power of electron guns heating the intermediate vessel and the billet was increased to 180–210 kW. The metal was poured into a copper water-cooled crystallizer with a diameter of 180 mm. At the same time, a bath of liquid metal was constantly maintained in the mold with the help of electronic guns operating at a power of 90–110 kW. As the alloy was melted, the ingot was pulled out by 20–30 mm. The operating specific power of the beams per unit surface area of the bath was in the range of 0.22–0.26 kW/cm², and in the crystallizer, it was 0.35–0.43 kW/cm². The melting rate was maintained in the range of 30–55 kg/h. The melted ingots had a diameter of 177 mm and a height of 490–540 mm.

To determine the uniformity of boron distribution over the diameter and height of the refined ingots, they were cut according to the scheme shown in Figure 6. Numbers 1 to 9 represent the cut locations.

This sampling scheme for analyzing alloy impurities was worked out in the industrial production of ZrNb alloys at the electron beam melting stage.

3. Results and Discussion

3.1. Calcium-Thermal Reduction of ZrF₄ with Boron Addition in IPHT-200 Unit

The SHS process for the production of Zr-1%B alloy can be described by the following exothermic reaction:

ZrF₄ + B + 2Ca → ZrB + 2CaF₂ + Q.

(1)

Three series of experiments of 9 melts each with different modifications of boron were carried out. In the first series of experiments on obtaining Zr-1%B alloy by calcium-thermal reduction, amorphous boron was used, which was introduced into the charge in the amount of 1 wt.% of the alloy mass. The results of the melts are given in

Table 4. Results of reduction melting with amorphous boron powder (first series).

Melt No.	Load (g)			Charge Temperature in Control Points before Initiation (°C)							Calcium Excess (%)	Ingot Yield (%)	Reaction Mode
	ZrF4	Ca	B	T1	T2	T3	T4	T5	T6	T7
1	8623	4235	47.4	400	410	115	120	205	215	450	4.7	54.8	With a slight noise
2	8498	4261	46.7	-	340	-	-	-	-	-	4.7	33.1	Violently with the flames shooting out
3	9175	4550	50.5	140	315	405	435	95	170	325	3.5	38.4	Violently with significant gas emissions
4	8840	4447	48.6	300	310	400	170	20	50	355	5.0	40.8	Violently with significant gas emissions
5	9194	4520	50.6	496	590	205	226	120	105	680	2.6	37.9	Calmly
6	8438	4399	46.4	410	525	390	245	155	130	675	8.8	45.2	Calmly
7	9600	4910	52.8	237	310	448	518	70	108	295	6.8	34.1	Rampantly gas emissions
8	6561	3321	36.1	220	268	335	285	280	315	415	5.6	50.4	Calmly
9	10,668	5366	70.6	120	158	290	272	141	105	-	5.0	46.2	Spontaneously, calmly

.

Excess calcium in the range of 3.5–8.8% was chosen to determine its effect on the efficiency of melting. As can be seen from Table 4, some melts were very violent, with a significant release of gases. The turbulent nature of the reactions is most likely because the finely dispersed amorphous boron contained moisture up to 0.3 wt.%. In addition, amorphous boron had a high specific surface area (2.2 m²/g), which contributed to the sorption of gases from the air, such as oxygen and nitrogen. Moisture and oxygen actively interacted with calcium, releasing the reaction products.

When the charge was heated to higher temperatures (melting no. 1, 5, 6, and 8), the reduction proceeded smoothly and the average yield of metal in the ingot increased from 36.6 to 46.9 wt.%. The temperature at points T5 and T6 was at least 120 °C and 105 °C, respectively. Conducting reduction melting at this temperature regime causes the spontaneous onset of the reaction and release of gases from the charge. Thus, the use of amorphous boron powder for zirconium alloying proved low efficiency.

The results of the first series of melts with the use of amorphous boron show that at all heating modes, the SHS proceeds violently with partial release of charge. The average yield of alloy in the ingot in 9 melts amounted to 42.7%. This unambiguously showed that the amorphous boron does not provide the specified process parameters at the level of more than 80%. From nine melts, only two (fifth and sixth) passed quietly at temperature values in points T5 and T6—120–155 °C when the melting temperature of calcium was reached in point T7. The reaction front moved from top to bottom.

The second series of melts was carried out with amorphous boron, which was pre-degassed at 1000 °C in a vacuum for 90 min. The calculated amount of boron in the charge was 1.2%. This allowed us to ensure its content in the ingot at the specified level of 0.9–1.1%. The yield of alloy in the ingot did not exceed 50% in all trials, which proceeded rather vigorously. The result of the second series of melting required changing the structure of the boron from amorphous to crystalline. The detailed technique of boron transforming into crystalline modification is given in Section 3.2.

In the third series of experiments, we used crystalline boron powder obtained by melting amorphous boron in the UE-177RL electron beam unit with subsequent grinding of the material. The results of this series of experiments are shown in

Table 5. Results of reduction melting with crystalline boron (third series).

Melt No.	Load (g)			Charge Temperature in Control Points before Initiation (°C)							Calcium Excess (%)	Ingot Yield (%)	Reaction Mode
	ZrF4	Ca	B	T1	T2	T3	T4	T5	T6	T7
1	8000	4024	53.0	420	540	120	155	99	89	- 1	5.0	34.5	Vigorously
2	10,668	5366	70.6	155	212	126	175	150	102	- 1	5.0	62.2	Calmly
3	10,668	5366	70.6	88	-	205	235	50	45	- 1	5.0	53.7	Calmly
4	8000	4024	53.0	120	180	160	280	55	43	250	5.0	51.5	Calmly

¹ These melts were conducted without a fuse.

.

In the case of using crystalline boron powder, reduction melting proceeded smoothly and calmly, without gas and reaction mass emissions from the crucible. The average yield of metal in the ingot increased to 55.8%, and the share of side garnissage from the alloy reached 44%. The temperature of the charge at the controlled points before the initiation of the reaction was lower than in the first series of experiments.

3.2. Preparation of Crystalline Boron from Amorphous Boron by Electron Beam Refining Method

Two variants of obtaining crystalline boron were studied:

1. Melting of amorphous boron powder in the UE-177RL electron beam unit in a copper water-cooled crystallizer.

2. Melting of amorphous boron powder in vacuum induction furnace IPHT-200 in graphite crucible.

Implementation of the second option was complicated by the possibility of the formation of refractory boron carbide (melting point = 2723 K), which necessitated the use of a crucible made of other materials, for example, boron nitride, a more resistant but expensive material.

The results of melting show the principal possibility of obtaining crystalline boron by melting amorphous boron in an electron beam furnace and the feasibility of using this process in industrial technology. However, the experiments showed the low productivity of the process in the existing machinery due to the need to several times carry out additional loading of boron powder, which is due to its low bulk mass. This disadvantage can be eliminated in two ways:

1. Application in the electron beam furnace of a loading hopper with a dozer. This allowed the ingot smelting without intermediate additional loads causing depressurization of the furnace.

2. Application of amorphous boron powder pressed into briquettes.

The second option showed lower efficiency than the first one. The results of electron beam melting of amorphous boron are shown in

Table 6. Results of electron beam melting of amorphous boron.

No.	Melting Duration (min)	Boron Loading (g)	Boron Yield (%)	Boron Purity (%)
1	10	800	94.0	99.8
2	20	810	91.9	99.9
3	10	800	90.3	99.9
4	15	805	78.9	99.9
5	35	800	83.6	99.9
6	6	800	93.1	99.1

.

The transformation of boron from the amorphous state to the β-rhombohedral crystalline modification is accompanied by purification from impurities (H₂, Be, K, and Ca), as can be seen in Table 1. For the most effective removal of impurities, the initial beam power should be in the range of 0.02–0.04 kWh/cm², and the value of residual pressure should be no more than 7·10^–2 mm Hg. As impurities are removed, the residual pressure decreases to 8·10^–3 mm Hg.

Boron melting occurs when the specific power is increased to 0.1–0.2 kWh/cm². Boron melting is kept for 10–30 min, which allows for the removal of impurities Fe, Si, and Cr. Thus, melting of boron occurs in two stages: removal of impurities and melting of boron.

When heating is provided, boron crystallizes in β-modification. The purity of the ingot meets the requirements for such metals as zirconium, hafnium, and titanium. The remaining impurities do not affect the physical properties of the obtained alloys.

Thus, electron beam melting of amorphous boron allows us to obtain a β-rhombohedral modification with a purity of 99.9%. The specific yield of crystalline boron averaged 90%.

3.3. Calcium-Thermal Reduction of ZrF₄ in Industrial Vacuum Induction Furnace ISV-1.0

The results of melting on the ISV-1.0 unit are presented in

Table 7. Results of zirconium tetrafluoride melting on the ISV-1.0 unit.

Melt No.	Load (kg)			Charge Temperature at Control Points before Initiation (°C)				Weight of Ingot (kg)	Weight of Garnissage (kg)	Boron Content 1 (wt.%)	Ingot Yield (%)
	ZrF4	Ca	B	T1	T2	T3	T4
1	120	60.4	0.796	295	125	520	80	51.4	11.6	c 0.80p 1.10	77.6
2	120	60.4	0.796	140	105	125	135	55.1	8.8	c 0.98p 0.80	83.2
3	120	60.4	0.796	340	120	85	80	56.4	11.1	c 0.96p 0.96	85.2
4	150	75.5	0.995	550	90	95	50	62.9	19.9	c 1.09p 1.17	76.0
5	150	75.5	0.995	400	125	40	100	62.3	15.5	c 0.80p 0.85	75.2
6	150	75.5	0.995	370	55	80	75	63.8	13.5	c 1.15p 1.20	77.1
7	150	75.5	0.995	440	90	40	60	58.2	18.5	c 1.20p 1.20	70.3

¹ Analyses for boron content were taken from: the c—center; p—periphery of the ingot.

.

In the course of melting, a cylindrical garnissage with a wall thickness of 2–5 mm was obtained along the lateral surface of the top part of the ingot. It was formed as a result of gases escaping from the molten alloy from the peripheral zone of the crucible at the end of the reduction melting. Rapid crystallization of the melt on the walls of the water-cooled copper crucible did not allow the alloy to join the ingot.

The garnissage was removed mechanically, re-melted into a cylindrical ingot 200 mm in diameter on the IPHT-200 unit, and used for repeated electron beam melting. The total yield of ingot and garnissage was 88.8–96.7%. The yield of the garnissage depended on the gas content in the charge and varied from 8.5 to 18.5%.

Samples for boron content in the alloy were taken from the center (C) and periphery (P) of the ingot. Vacuum conditions of melting, the presence of argon atmosphere, and the absence of interaction of the melt with the wall of the copper crucible preserved the purity of the alloy in terms of impurities.

The average yield of the alloy in the ingot amounted to 77.3%. The reason for this is, apparently, insufficient and uneven heating of the charge before the reaction. The charge was heated in pulse mode: the number of pulses varied from 10 to 30 generator starts. The low alloy yield corresponded to 10 starts, the maximum—30.

It should also be noted that there is a slight decrease in the yield of metal in the ingot after increasing the loading of the charge, starting from the fourth melting, which is explained by an increase in the height of the charge. As can be seen from the results of the analyses (Table 7), the boron content in the alloy is in the range of 0.80–1.20 wt.% with a fairly uniform distribution of boron in the ingot.

3.4. Refining of Zr-1%B Alloy Ingots by Electron Beam Melting in the UE-177RL Unit

The obtained alloy ingot had the dimensions 270 mm × 70 mm × 30 mm and a mass of 3.7 kg. The yield of the alloy in the ingot was 53% after 60 min of the melting. To study boron distribution in the ingot, four samples were taken from the central and peripheral parts of the ingot, on the surface and at a depth of 10 mm. The boron content in these points and the crude ingots included in the billet are presented in

Table 8. Boron content in crude and refined Zr-1%B alloy ingots.

Ingot	Boron Content (wt.%)	Average
Crude-1	0.75	0.78
Crude-2	0.80
Crude-3	0.80
Refined:		0.93
- center (surface)	0.88
- center (10 mm depth)	0.80
- periphery (surface)	0.94
- periphery (10 mm depth)	1.10

.

Next, an alloy sampling from each zone of ingots was carried out following the scheme in Figure 6. As can be seen from

Table 9. Boron distribution in the refined ingot.

Zone	Boron Content (wt.%)	Regulatory Requirement (wt.%)
1	1.00	0.80–1.20
2	1.01
3	0.94
4	0.92
5	0.98
6	0.92
7	0.80
8	1.08
9	0.80

, the distribution of the boron in the ingots obtained by electron beam melting is quite uniform and meets the requirements. The analysis of boron content in the refined ingot showed that boron is distributed in it in the specified range of 0.8–1.2%. The uniform alloy quality was confirmed during 5 consecutive melts.

As can be seen from Table 9, the boron content in the refined ingot exceeds its content in the original ingots. Apparently, in the process of electron beam refining there is some evaporation of the alloy base (zirconium), and as a consequence, an increase in the boron concentration in the ingot.

3.5. Refining of Zr-1%B Alloy Ingots by Electron Beam Melting on the UE-178M Unit

The results of refining of alloy ingots are given in

Table 10. Technological parameters of Zr-1%B alloy melting.

Melt No.	Workpiece Weight (kg)	Weight of Ingot (kg)	Melting Duration (h)	Melting Rate (kg/h)	Metal Yield in Ingot (%)
1	90.0	86.0	1.67	53.9	96.2
2	83.3	77.5	2.42	34.4	93.0
3	86.8	80.0	2.67	32.5	92.2
4	94.9	92.6	2.33	40.7	97.5
5	98.2	91.4	2.90	33.9	93.1

.

The average product yield during the melting was 92.2–97.5%. It was observed that during electron beam melting of zirconium with specific power in the range of 0.4–1.0 kW/cm², refining of zirconium from boron does not occur. This can be explained by two reasons:

1. The melting temperature of boron (2349 K) is more than 200 K higher than the melting temperature of zirconium (2125 K).

2. The partial vapor pressure of boron at 2300 K is much lower than that of zirconium (P_Zr = 10^–9 at, P_B = 10^–8 at) [3].

However, there is evidence [23] that zirconium boride (ZrB₂) in the temperature range of 1500–3000 K decomposes into Zr and B in the gas state, but the rate of this vaporization is quite low.

Electron beam melting of Zr-1%B alloy occurred in stable mode. We found that the optimal melting rate was 5 kg/h, as no boron liquation or localization of boron concentration was found. Refining of the alloy from boron at a specific power of 0.2 kW/cm² did not occur, on the contrary, its concentration increased due to evaporation of the alloy base (Table 7). At the same time, the mentioned increase in concentration is insignificant and does not require adjustment of boron content in the initial alloy. The obtained ingots had a height of 490–540 mm and a diameter of 177 mm. The total content of impurities in the ingots was less than 0.1%.

The obtained alloy is characterized by the thermal neutron capture cross-section in the range of 40–45 b and can be used in neutron-absorbing materials production. The only known industrial method [24] for producing neutron-absorbing materials with similar properties (the In-Cd-Ag alloy used in PWR and BWR nuclear reactors) is vacuum arc melting of pure components (metals). This method is very energy-consuming and expensive [25]. The SHS process proposed in this work is more economical because it allows the use of more available materials (zirconium salt and inexpensive calcium) instead of scarce high-purity metals (In, Cd, and Ag of nuclear purity). Nevertheless, the Zr1%B alloy is experimental and is proposed to produce pilot batches of products for protection against neutron radiation. If necessary, the production of this alloy can be scaled up to the level of tens of tons per year. For this purpose, additional studies under the operating conditions of nuclear reactors are required.

4. Conclusions

1. The influence of boron quality on the technological indicators of the ZrF₄ calcium-thermal reduction was experimentally determined. It was found that the use of amorphous boron powder, which contains a large amount of adsorbed gases, leads to a violent SHS reaction. This deteriorates the quality of the alloy and reduces its yield in the ingot.

2. Electron beam refining of amorphous boron allows for a crystalline modification with a purity of 99.9%. The use of crystalline boron instead of amorphous ensures the process safety and increases metal yield in the ingot by 80%.

3. The research proposes the novel process of SHS of Zr-1%B alloy via calcium-thermal reduction of ZrF₄ in a vacuum induction furnace with cold crucibles. The refining of Zr-1%B ingots in the industrial electron beam unit UE-178M ensures a uniform distribution of boron at a level of 1 ± 0.2%.

4. SHS of alloys directly from the compounds of refractory elements can become an alternative to the energy-consuming method of producing these alloys from individual elements.

5. Crystalline boron obtained by electron beam refining can be used for the production of refractory compounds such as ZrB₂, TiB₂, and HfB₂.