Introduction

Intensive development of industry enforces the need for the development of materials engineering and continuous improvement of the existing materials and the development of entirely new classes of materials. Requirements for the resulting materials are increasingly high and precise. To improve the mechanical properties of alloys, a modification of the chemical composition is used, which allows alloy strengthening as a result of reinforcing phases, by fragmentation of the structure as a result of heterogeneous nucleation of phases with a higher solidification temperature than the matrix of the alloy, as well as the result of plastic forming allowing the grain to grind, but also as a result of the disadvantages of the point and line construction created in the alloy, which increase the possibility of dissolving the supersaturating phase and then separating the strengthening phases in the grain increasing the strength properties as a result of the heat treatment [1,2,3].

Knowledge of the phenomena occurring in solidifying copper alloys is of great importance because it allows to create the microstructure and thus the functional properties of alloys and also affects the technological parameters of heat and plastic treatment, including temperature and time. Increasing the cooling rate will support avoiding segregation (block and dendritic), obtaining a beneficial phase dispersion (fragmentation of multi-component eutectics and more even distribution at the interface) [2,3,4,5,6,7].

The durability and operational reliability become a priority objective and force the need to search for materials with more favourable and more precise sets of usable features and high and stable mechanical, electrical or thermal properties during operation. A significant part of these properties is fulfilled by the alloys from precipitation-hardened copper alloys, such as Cu–Fe, Cu–Cr, Cu–Co, Cu–Ni–Si and Cu–Ni–Si–Cr [2,3,4].

The subject of research is increasingly becoming the CuNi2Si1 alloy, whose mechanical properties can be shaped by heat and plastic treatment [5,6,7,8,9]. In the study of CuNi2Si1 alloys, Cr and sometimes Cr and Mg or Cr and Ti are often introduced into the alloy. In the obtained Cu–Ni–Si–Cr, Cu–Ni–Si–Cr–Mg or Cu–Ni–Si–Cr–Ti alloys, the research results indicate the occurrence of phase transformations and change of morphology and, as a result, achieve more favourable mechanical properties and in some part also changes in conductivity [10, 11].

Analysis of the CuNiSiCrMg alloy showed that the stability of mechanical properties is determined by their microstructure which consists of inter alia the primary precipitation of the Cr3Si phase and Ni2Si formed during decomposition of the supersaturated solid solution [2, 10, 11]. Study of the CuNi2Si1 alloy revealed that the formation of coherent Ni2Si precipitates is formed during solution and ageing process of the alloy. Too long ageing time causes coagulation of these precipitates [11, 12].

Material and investigation methodology

The purpose of the study was to determine the effect of Re addition on the kinetics of crystallization and alloy structure CuNiSiCr.

The interdependence between the chemical composition and the structure of the Cu–Ni–Si–Cr and Cu–Ni–Si–Cr–Re is shown in Table 1. The cast alloys were made at 1500 °C in the high-temperature vacuum furnace HT-2100-G-Vac-Graphite-Special type—in graphite crucibles ∅25. Alloy elements were placed in a crucible in a pure form.

Table 1 Chemical composition of the analysed copper alloys
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In order to determine the relationship between the crystallizations kinetics of the alloy and the chemical composition and microstructure of Cu alloy the following investigations were performed:

  • Thermo-derivative analysis using the UMSA device equipped with a computer-controlled cooling system, which allows to set flexible the cooling rate applied to the Cu–Ni–Si–Cr and Cu–Ni–Si–Cr–Re alloys. The samples for thermo-derivative analysis were prepared with a diameter of ∅8 mm and a height of 10 mm. There have also been made holes for thermocouples, in the samples were the thermal node occurs for this type and arrangement of the sample geometry. The investigated samples were characterized with a mass of 8.16 g ± 3 g for this type of geometrical dimension.

  • The supersaturation temperature of Cu–Ni–Si–Cr and Cu–Ni–Si–Cr–Re alloys was 900 °C, with a heating time of 1 h in a protective argon atmosphere.

  • Measurement of electrical conductivity was taken on Sigmatest Foerster device.

  • Microstructure and chemical composition investigations were carried out using EDS microanalysis on the scanning electron microscope Zeiss Supra 25.

  • Alloy structure using MEF4A optical microscope supplied by Leica together with the image analysis software as well as scanning electron microscope using Zeiss Supra 25 device within high-resolution mode.

  • The microhardness test was performed on the hardness tester Vickers FUTURE-TECH (FM-ARS9000) with a load of 100 gf for 15 s.

  • Samples prepared for observation of the microstructure were ground and polished mechanically and then etched:

  • Electrochemically in an electrochemical reagent consisting of ethanol, phosphoric acid, propanol, urea and distilled water.

  • Or in a reagent with the following composition: iron chloride, hydrochloric acid and ethyl alcohol.

Investigation results

In the microstructure of CuNiSiCr alloys, there are two types of precipitates: primary precipitations of Cr3Si and Ni2Si phase resulting from decomposition of solution [2, 3, 11].

The results of microstructure observation performed using the Olympus light microscope are shown in Figs. 1 and 2. It can be observed that the α-matrix and the phases with chromium are solidified.

Fig. 1
figure 1

Microstructure of the Cu–Ni–Si (a, b) as cast (#1—separation with Ni)

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Fig. 2
figure 2

Microstructure a of the Cu–Ni–Si–Cr (#2—as cast separation with Ni); b of the Cu–Ni–Si–Cr as supersaturated (#1—phases Cr3Si); c of the Cu–Ni–Si–Cr–Re as cast; d of the Cu–Ni–Si–Cr–Re as supersaturated

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Primary phases of Cr3Si (10 μm) (Fig. 2 #1) do not melt and do not solidify during the crystallization process of the alloy. The Ni-containing phase is flushed through the front of crystallization of the α phase into the interdendritic areas (Figs. 2, 9).

This is unconfirmed by ATD studies, but the EDS analysis shows that Re is concentrated at areas with higher content of Cr and Ni (Figs. 1, 2, 3, 4, 9).

Fig. 3
figure 3

Cooling and derivative curves of Cu–Ni–Si–Cr alloy

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Fig. 4
figure 4

Cooling and derivative curves of Cu–Ni–Si–Cr–Re alloy

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However, on the derivative curve, no clear bends indicating the presence of the Re phase were observed.

Cooling and crystallization curves of the investigated Cu–Ni–Si–Cr and Cu–Ni–Si–Cr–Re alloys freely cooled are shown in Figs. 3, 4. As a result of the thermal derivative analysis, TL and TSOL temperatures were determined (Table 2).

Table 2 Results from thermal derivative analyses of Cu–Ni–Si–Cr and Cu–Ni–Si–Cr–Re alloys
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Comparison of the presented graphs allows observing the shift of the solidus temperature of analysed copper alloys. As a result of the Re addition, the melt temperature was supercooled and the TSOL temperature decreased by 160 °C. The determination of TSOL allowed to determine the supersaturation temperature and to state that the supersaturation temperature of the alloys with the addition of Re is lower than the temperature of the alloys without the Re.

The thermal derivative analysis also allowed the determination of the fraction solid of analysed alloys (Fig. 5). An evident change in the course of the solid fraction curve for alloys with the addition of rhenium is visible. It can be noticed that in a non-Re alloy the curve has a mild course and crystallization phases that contain the Cr and Ni elements can be observed. However, the analysis of the course of the alloy curve with the addition of Re showed a rapid increase in the fraction solid in the temperature range of 1060–1020 °C.

Fig. 5
figure 5

Fraction solid during the crystallization of the particular compounds of the Cu–Ni–Si–Cr and Cu–Ni–Si–Cr–Re alloys

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The saturation of alloys at designated temperatures (Table 3) was performed as a result of a thermal derivative analysis. Then the conductivity and microhardness were measured. The results of the electrical conductivity of the analysed Cu–Ni–Si–Cr and Cu–Ni–Si–Cr–Re alloys in as-cast condition and after saturation are presented in Fig. 6, which shows that in Cu–Ni–Si–Cr and Cu alloys Ni–Si–Cr–Re, saturated at 900 °C, the average electrical conductivity decreased. The results of microhardness are shown in Fig. 7, where it can be observed that the alloy with a small addition of Re (0.3%) is characterized by higher microhardness about 45HV.

Table 3 Parameters of the supersaturation process of Cu–Ni–Si–Cr and Cu–Ni–Si–Cr–Re alloys
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Fig. 6
figure 6

Average electrical conductivity of Cu–Ni–Si–Cr and Cu–Ni–Si–Ce–Re alloys

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Fig. 7
figure 7

Microhardness of tested alloys after supersaturation from temperature determined by the thermal derivative analysis

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Analysis of chemical composition in microareas and scanning electron microscopy showed that in the test alloy the α-phase and eutectics occur (Figs. 8, 9).

Fig. 8
figure 8

Microstructure of the Cu–Ni–Si–Cr–Re

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Fig. 9
figure 9

Microstructure of the Cu–Ni–Si–Cr–Re. The X-ray diffraction pattern of Cu–Ni–Si–Cr–Re alloy (point #1: Si = mas. 2.9%, Ni = mas. 3.7%, Cu = mas. 61.3%, Cr = mas. 30.2%, Re = mas. 1.6%—phases with Cr and Re; point #2: Si = mas. 1 2.67%, Ni = mas. 21.82%, Cu = mas. 65.51%—phases with Ni)

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Figure 10 represents the distribution of the elements which are the main components of the Cu–Ni–Si–Cr–Re alloy.

Fig. 10
figure 10

Structure of the Cu–Ni–Si–Cr–Re alloy (a—#1 phases with Ni, #2 phases with Ni)—image obtained using the secondary electron (SE) and the distribution of elements: b Cr, c Ni, d Si

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Conclusions

The application of thermal derivative analysis allowed to determine the temperature of the beginning of the solidification (TL) and the beginning and end of the eutectic and phase crystallization as well as the solidus temperature (TSOL) of the tested alloys.

The results can be summarized as follows:

  1. 1.

    The addition of Re to the alloy resulted in increased undercooling of the alloy and decreasing of the TSOL temperature from 1060 to 896 °C.

  2. 2.

    Decreasing the TSOL temperature is associated with the Ni–Si–Re phase coagulation most likely at the boundaries of α-phase.

  3. 3.

    In the tested alloys supersaturated at the temperature determined as a result of the thermal derivative analysis, it was found that the conductivity of the alloy with the Cr addition decreased by 5.25 MS m−1, while the Re caused a decrease of 2.5 MS m−1.