Balancing negative and positive expansion effect in dual-phase La(Fe,Si)13/α -Fe composite with improved mechanical property

Abstract

Most materials exhibit a positive coefficient of thermal expansion (CTE), which leads to expanded lattices with temperature increases due to the population of higher energy levels of anharmonic lattice vibrations. However, small amount of materials contract upon heating and this phenomenon is called negative thermal expansion (NTE).Up to the present time, NTE has been observed in the well-known ZrW 2 O 8 family of materials $^{[{1-3}]}$, MnCoGebased alloys $^{[{4,5}]}$, PbTiO $_{3} -$based compounds $^{[{6,7}]}$, ScF $_{3} -$based compounds $^{[{8}]}$,(Bi,La)NiO $_{3} \quad ^{[{9}]}$ and antiperovskite manganese nitrides $^{[{10-12}]}$. NaZn 13 type La(Fe,Si $) _{13} -$based alloys are widely known to exhibit large negative thermal expansion during the magnetic transition. However, zero thermal expansion, which is more promising towards the utilization, has been rarely reported. Here, we introduce $\alpha -$Fe phase naturally to compensate the negative thermal expansion of 1:13 phase, and thus achieve zero thermal expansion in La(Fe,Si $) _{13}/ \alpha -$Fe composite. It is notable that the sample with $\mathrm {x}=0$ breaks itself during the magnetic transition for its poor mechanical property. In this case, it cannot be machined into regular shapes for the measurement of thermal expansion. Therefore, the average CTEs of the rest 4 samples are measured in this paper. The curves of ΔL/(L×ΔLT) are shown in Fig 1. Here, the reference temperature is 300 K and the average CTE is calculated by ΔL/(L×ΔLT). NTE is observed in the samples with $\mathrm {x}=4$ and 6. The average CTEs of samples with $\mathrm {x}=4$ and 6 reach $- 9.54 \times 10- 6 \mathrm {K}^{-1}(278 -304\mathrm {K})$ and $- 6.25 \times 10- 6 \mathrm {K}^{-1}(270 -291\mathrm {K})$. With the increase of extra Fe in ingredient, a large amount of $\alpha -$Fe phase appears and distributes in the 1:13 phase. NTE of 1:13 phase is offset by the PTE of $\alpha -$Fe phase. An extremely low CTE of $- 6.55 \times 10 ^{-7} \mathrm {K}^{-1}$ between 261 K and 282 K is obtained in sample with $\mathrm {x}=8$, leading to the establishment of ZTE. Besides the low CTE, good mechanical property is another crucial prerequisite for ZTE or NTE materials. La(Fe,Si)13based alloys with main 1:13 phase is typical brittle. In this experiment, the cuboid sample used for thermal expansion and stress-strain measurements are processed by diamond wire-sawing. During the machining process, the sample with $\mathrm {x}=0$ will break itself into some small bulks (right inset of Fig. 2, the small bulks are carefully polished for the microstructure observation on SEM), while the other sample are strong enough to be processed (left inset of Fig. 2). This indirectly proves that the increase of additional $\alpha -$Fe phase can improve the mechanical property. With increasing x, the values of largest yield compressive stress are sharply enhanced to 864, 900, 970 and 1004 MPa for $\mathrm {x}=4$, 6, 8 and 10, respectively (see Fig. 2). The improved mechanical property is highly related with the existence of $\alpha -$Fe phase. It is known that most brittle intermetallics display intrinsic weakness of grain boundaries and insufficient number of slip systems $^{[{13,14}]}$. Therefore, the applied stress will easily broke the weaker links between grains and leads to intergranular fractures. However, this situation will greatly changes when introducing a secondary ductile phase. In this experiment, the distributed ductile $\alpha -$Fe phase is helpful to prevent the movement and slipping of dislocation, thus hinder the cracks propagation along the weak grain boundaries and enhance the mechanical property. On the other hand, additional Fe atoms suppress the formation of La-rich phase, which is detrimental to the mechanical and corrosion properties. It also contributes to the improved mechanical property. In summary, we successfully fabricate dual-phase La(Fe,Si)$13/ \alpha -$Fe composite. The microstructures, magnetic properties, thermal expansion and mechanical properties of samples are investigated. With the increase of extra Fe in ingredient, a large amount of $\alpha -$Fe phase appears and distributes in the 1:13 phase. NTE of 1:13 phase is offset by the PTE of $\alpha -$Fe phase. An extremely low CTE of $- 6.55 \times 10 -7\mathrm {K}-1$ between 261 K and 282 K is obtained in sample with $\mathrm {x}=8$, leading to the establishment of ZTE. Additionally, as a reinforcing factor, $\alpha -$Fe phase is helpful to prevent the movement and slipping of dislocation, thus enhancing the mechanical property. Based on these improvement, ZTE and improved mechanical property are achieved simultaneously in La(Fe,Si)$13/ \alpha -$Fe composite.