Joints of continuous carbon fi ber reinforced lithium aluminosilicate glass ceramics matrix composites to Ti60 alloy brazed using Ti-Zr-Ni-Cu active alloy

Shengpeng HU,Dongdong FENG,Long XIA,*,Ke WANG,Renwei LIU,Zhento XIA,Hongwei NIU,Xioguo SONG

aSchool of Materials Science and Engineering,Harbin Institute of Technology,Weihai 264209,China

bShanghai Institute of Spacecraft Equipment,Shanghai 200240,China

KEYWORDS Brazing;Composites;Mechanical properties;Microstructure;Titanium alloys

Abstract Continuous carbon fi ber reinforced lithium aluminosilicate glass ceramics matrix composites(Cf/LAS composites)are joined to Ti60 alloy vacuum brazed using Ti-Zr-Ni-Cu brazing alloy.The effects of the brazing temperature on the interfacial microstructure and mechanical properties of brazed joints are investigated in details.The interfacial microstructure varies apparently with an increase of the brazing temperature.The thicknesses of the banded Ti solid solution(Ti(s,s))and the reactive layer between Cf/LAS composites and the interlayer grow gradually.The mechanical properties of brazed joints increase firstly and then decrease with an increasing temperature.In addition,a joint that is brazed at 980°C for 10 min shows the highest shear strength of～38.13 MPa.At the same time,the fracture paths of brazed joints also change as the temperature increases.When the brazing temperature is 950°C,the fracture position is in the TiC+ZrC+Ti2O+ZrSi2+Ti5Si3layer on the composite side.When the brazing temperature is 980°C,the fracture position is on the side of the braze seam(Ti,Zr)2(Ni,Cu),Ti2O+ZrSi2+Ti5Si3layer,and carbon fi ber in the composite material.When the brazing temperature is 990°C,the fracture position is in the Ti2O+ZrSi2+Ti5Si3layer on the composite side and the carbon fi ber in the composite material.

1.Introduction

Continuous carbon fi ber reinforced lithium aluminosilicate glass ceramics matrix composites(abbreviated as Cf/LAS composites)have drawn increasing attention owing to their great thermal shock resistance,high chemical durability,and low or even almost negative coefficient of thermal expansion(CTE),which are widely applied in many fields,such as the microelectronic industry,precision optics,and so on.1-3To achieve practical applications,Cf/LAS composites need to be joined to metallic parts,especially titanium alloys.As a near-α high-temperature Ti alloy,Ti60 alloy can be used under conditions of up to 600°C due to its good hightemperature oxidation resistance and mechanical properties,which is widely used in aerospace,chemical and medical equipment,and other industrial fields.4-6Reliable joining of Cf/LAS composites to Ti60 alloy is essential to expand their applications,for example,the use of a thermally stable platform made of Cf/LAS composites connected to a satellite made of Ti60 alloy can increase the resolution of space cameras on the platform.

Many methods have been developed to realize joining of ceramic matrix composites to metals,such as bolting,riveting,adhesive bonding,diffusion bonding,and brazing.However,bolting and riveting are inadvisable owning to the brittle nature of Cf/LAS composites.The applied field of adhesive bonding is limited in the ceramic system due to the high service temperature of joints.Meanwhile,the high bonding temperature(approximate 1700°C)restricts the utilization of the diffusion bonding technique.7,8So far,brazing is an effective approach to acquire excellent and robust quality joints of ceramic matrix composites and metals.9-15At present,research on brazing of ceramic matrix composites and metals has made great progress.In order to overcome the poor wettability problem of ceramic matrix composites,Ag-Cu-Ti16-18and Ag-Cu-Ti composite brazing alloys19-22have been widely used to join ceramic matrix composites to metals.Lin et al.joined carbon fi ber reinforced SiC composites to Ti-alloy successfully using Ag-Cu-Ti+short carbon fi bers,20Ag-Cu-Ti+(Ti+C)mixed powders,21and Ag-Cu-Ti+W mixed powders.22Liu et al.joined continuous carbon fi ber reinforced Li2O-Al2O3-SiO2ceramic matrix composites to TC4 alloy by using Ag-Cu-Ti active fi ller metal.The Ti element could react with carbon fi ber to produce a TiCxlayer,which ef fi caciously enhanced the wetting and spreading of molten brazing alloy on the surface of carbon fi ber.17However,joints of ceramic matrix composites and metals with Ag-Cu-Ti brazing alloys have limited mechanical properties at high temperature.To enhance the mechanical properties of brazing joints at high temperature,Ti-Zr-Ni-Cu brazing alloy,a sort of active brazing alloy,has been widely utilized for brazing ceramic matrix composites to metals.23-26Ti-Zr-Ni-Cu,23Ti-Zr-Ni-Cu+W mixed powders,24Ti-Zr-Ni-Cu+(Cu and Mo)composite interlayers,25and Ti-Zr-Ni-Cu+graphene nanoplatelets mixed powders26alloys have been researched to braze ceramic matrix composites to metals.Cui et al.joined carbon fi ber reinforced SiC composites to TC4 alloy successfully using Ti-Zr-Ni-Cu+W composite fi ller materials.Their results revealed that active Ti and Zr in the interlayer could react with the substrate,24which visibly enhanced the wetting and spreading of molten brazing alloy on the surface of ceramic matrix composites,and then sound joints were obtained.

In this work,Ti-Zr-Ni-Cu brazing alloy was applied to vacuum braze Cf/LAS composites to Ti60 alloy.The interfacial microstructures of Cf/LAS composite/Ti-Zr-Ni-Cu/Ti60 joints were investigated in details.Meanwhile,the mechanical properties and fracture locations of brazing joints were also analyzed.

2.Experimental section

The microstructures of Cf/LAS composites,Ti60 alloy,and Ti-Zr-Ni-Cu brazing alloy are shown in Fig.1(a)-(c),respectively.The chemical compositions of base metal and brazing alloy are shown in Table 1,and the fracture toughness values of Cf/LAS composites and Ti60 alloy were 25 MPa·m1/2and 41.3 MPa·m1/2,respectively.

The Cf/LAS composites mainly consist of carbon fi bers and lithium aluminum silicate glass-ceramic matrix,and the carbon fi bers are uniformly distributed in the lithium aluminosilicate glass-ceramic matrix as shown in Fig.1(a).The Ti60 alloy composes of α-Ti and β-Ti two-phase composition as shown in Fig.1(b).The Ti-Zr-Ni-Cu brazing alloy consists of a large number of polygonal powders as shown in Fig.1(c).

Before brazing,the Cf/LAS composites were cut to small pieces with a dimension of 4 mm×4 mm×4 mm using a ceramic slicer machine(carbon fi bers in the Cf/LAS composites were perpendicular to the faying surface),and the Ti60 alloys were sectioned into 10 mm×20 mm×3 mm pieces.The brazing surfaces of substrates were ground using SiC grit papers.Finally,experimental specimens were ultrasonically cleaned in acetone for 10 min and dried by air blowing.The prepared specimens and Ti-Zr-Ni-Cu brazing alloy were assembled into a sandwich model as demonstrated in Fig.2(a),and then set into a vacuum furnace cautiously.The weight of Ti-Zr-Ni-Cu brazing alloy required for each sample was～90 μg.

The brazing temperature adopted in this experiment was 950,960,970,980,and 990°C,respectively.The experiment was performed in a vacuum furnace of below 5.0×10-3Pa.During brazing,each specimen was firstly heated to 700°C at a heating rate of 20°C/min and maintained at 700°C for 10 min.The specimens were subsequently heated to the brazing temperature at a heating rate of 10°C/min,kept for approximate 10 min,and then cooled down to 200°C at a rate of 5°C/min.Eventually,the brazing joints were finally cooled down to room temperature in the vacuum furnace.

Fig.1 Microstructures of substrates and the brazing alloy.

Table 1 Chemical compositions of base metals and brazing alloy(wt%).

Fig.2 Schematic of brazing and shear tests.

The cross-sections of the brazed specimens were cut and prepared for microstructures observation.The microstructures of the brazed joints were characterized using a scanning electron microscope(FESEM,MERLIN Compact,ZEISS).The componential analysis of each phase in the brazed joints was conducted using an energy dispersive spectrometer(EDS,OCTANE PLUS,EDAX).Moreover,the microstructure of Ti60 alloy sides of the fracture surface was investigated using X-ray diffraction(XRD,DX-2700)aiming to identify the interfacial phases accurately.The room-temperature shear strength of brazed joints was gauged using a universal material machine(Instron 5967)at a constant speed of 1 mm/min,and a schematic picture of the shear test is demonstrated in Fig.2(b).For each experimental data,no less than four brazed joints were tested to achieve the average shear strength.After the shear test,SEM observation was utilized to examine the fracture surface.The interface phase on the fracture surface was characterized by EDS and XRD analysis.

3.Results and discussion

3.1.Typical interfacial microstructure of Ti60/Ti-Zr-Ni-Cu/Cf/LAS composites brazed joints

Fig.3 Interfacial microstructure and element distribution of Ti60/Ti-Zr-Ni-Cu/Cf/LAS composites joints brazed at 980°C for 10 min.

The BSE image in Fig.3 is a Ti60/Ti-Zr-Ni-Cu/Cf/LAS composites joint with a brazing temperature of 980°C for 10 min.Fig.3(a)distinctly demonstrates that the brazing interface has no holes or cracks.The brazed joint consists of three characteristic zones evidently in Fig.3(a):zone I(the dark gray phase and light gray phase mixed region),zone II(the irregular zonal and dark gray reaction layer with a thickness of～15 μm),and zone III(close to the right part of zone II,including a discontinuous thin reaction layer adjacent to Cf/LAS composites).As shown in Fig.3(b),the element Ti is continuously dissolved from the Ti60 substrate into the molten braze alloy during the brazing process.Meanwhile,the diffusion of Ti,Zr,Ni,and Cu elements in the molten fi ller toward the Ti60 alloy and Cf/LAS composites results in increases of these elements in the vicinity of the Ti60 substrate(zone I)and the Cf/LAS composites(zones III),as shown in Fig.3(b)-(e).Moreover,it is apparent that element Zr is enriched in front of the carbon fi bers in the Cf/LAS composites,as shown in Fig.3(e).The surface of the Cf/LAS composites contains large amounts of Si and C,as shown in Fig.3(f)and(g),which react with the active elements Ti and Zr of brazing alloy during the brazing process.

In order to further investigate the interfacial microstructure,the phase composition in the joint was studied.The joint mainly consists of four phases,as shown in Fig.4(a)and(b),which are marked by A,B,C,and D,respectively.According to the contrast of BSE images and chemical composition of each spot(Table 2),Zone I is mainly composed of two phases:light dark gray phase A and dark gray phase B.Phase A mainly consists of Ti,Zr,Ni,and Cu.The stoichiometry of Ti+Zr/Ni+Cu ratio in atomic percent is close to 2:1.According to the Ti-Zr-Cu ternary alloy phase diagram,Zr exhibits a certain level of solubility in Ti2Cu and TiCu.In addition,Ni can replace Cu in the Cu-Ti intermetallic compound lattice.25-27Phase A is supposed to be(Ti,Zr)2(Cu,Ni).Phase B mainly contains Ti and a small amount of Zr and Al elements,and is identified as a Ti solid solution(Ti(s,s)).Phase C is mainly composed of 7.69 at.%Ti,11.12 at.%Zr,and 76.47 at.%C.Since the Gibbs free energy of forming TiC and ZrC is negative at 980°C,elements Ti,Zr,and C can react with each other at 980°C.Therefore,phase C is supposed to be TiC+ZrC particles.A reaction layer of TiC+ZrC is formed by the reaction of reactive elements Ti and Zr with carbon fi bers in the brazing process.Ti and Zr in the interlayer diffuse toward the LAS ceramic and mainly react with oxides,such as SiO2,in the matrix of Cf/LAS composites.17Since the SiO2surface in the LAS ceramic is composed of oxygen atoms,and Ti and Zr have electrical negative values of 1.22 and 1.33,respectively,compared with Ni(1.91)and Cu(1.48),the electronegativity of O element in SiO2has bigger differences with Ti and Zr than that with Ni or Cu.Therefore,the O atoms in SiO2are selectively adsorbed on Ti and Zr.As Ti and Zr concentrations increase on the surface of the LAS ceramic substrate,the reaction of Ti and Zr with O atoms is more intense.Ti and Zr elements diffuse to the surface of the Cf/LAS composites and their vicinity,and especially the diffusion rate of Ti is faster and the enrichment degree is higher than those of Zr.When Ti reaches a certain amount,Ti reacts firstly with SiO2,leading to decomposition of SiO2.Ti reacts with SiO2,resulting in formation of Ti2O and Si.23The element Zr and the remaining Ti react with Si resultant to form compounds ZrSi2and Ti5Si3.Therefore,the D phase is possibly Ti2O,ZrSi2,and Ti5Si3phase.At 980°C,the Gibbs free energies of formation Ti2O,ZrSi2,and Ti5Si3are all negative.Therefore,elements O,Ti,Zr,and Si can react with each other at 980°C.Finally,TiC+ZrC and Ti2O+ZrSi2+Ti5-Si3compounds are formed at the adjacent carbon fi bers and the matrix in the composite material,respectively.Therefore,the typical interfacial microstructure of a Ti60/Ti-Zr-Ni-Cu/Cf/LAS composites joint consists of Ti60 alloy/Ti(s,s)+(Ti,Zr)2(Ni,Cu)+Ti(s,s)/(TiC+ZrC+Ti2O+ZrSi2+Ti5Si3)/Cf/LAS composites.

3.2.Effect of the brazing temperature on the microstructure of Ti60/Ti-Zr-Ni-Cu/Cf/LAS composite joints

Fig.4 High-magni fi cation BSE images of Ti60/Ti-Zr-Ni-Cu/Cf/LAS composites joints brazed at 980°C for 10 min.

Table 2 Chemical compositions and possible phases of each spot marked in Fig.4(at.%).

Fig.5 BSE images of the interfacial microstructures of joints brazed at different temperatures for 10 min.

Fig.5 demonstrates the interfacial microstructures of Ti60/Ti-Zr-Ni-Cu/Cf/LAS composite joints brazed at different temperatures to study the influence of the brazing temperature on the microstructure of brazed joints.The brazing temperature has important effects on the dissolution of the Ti60 alloy substrate into the molten braze alloy and the diffusion of the dissolved element to the Cf/LAS composite substrate.With an increase of the brazing temperature,the interfacial microstructure of the brazed joints varies obviously.It can be seen clearly from Figs.5 and 6 that the thickness of the reaction layer between the Cf/LAS composites and the interlayer and zone II increases gradually,while the thicknesses of zones I and III decrease.The brazing seam is nearly occupied by eutectic tissue(Ti(s,s)+(Ti,Zr)2(Ni,Cu)).The eutectic(Ti(s,s)+(Ti,Zr)2(Ni,Cu))is formed during the cooling process,which precipitates as eutectic components.With the extension of the cooling time,more eutectic tissue precipitates form a matrix.A higher brazing temperature accelerates the dissolution of the Ti element from the Ti60 alloy,resulting in an increase of the Ti content obviously.The high brazing temperature and the high content of Ti in the braze alloy lead to severe reaction,so the irregular shape of the dark gray Ti solid solution continues to grow,and the volume of the Ti solid solution gradually increases in the brazing seam,leading to an increase of zone II gradually.Meanwhile,large number of Ti dissolves into the molten braze alloy and reacts with Zr,Ni,and Cu to form a large amount of(Ti,Zr)2(Ni,Cu)phases.In the brazing process,the active element Ti from the Ti60 alloy and the braze alloy diffuses toward the Cf/LAS composites and reacts with the Cf/LAS composites.The active elements Ti and Zr react with the matrix and carbon fi bers of the Cf/LAS composites to form(Ti2O+ZrSi2+Ti5Si3)and(TiC+ZrC),respectively.The TiC+ZrC and Ti2O+ZrSi2+Ti5Si3reaction layer joints Cf/LAS composites with an interlayer.However,a low brazing temperature causes weak interfacial reaction,so the TiC+ZrC and Ti2O+ZrSi2+Ti5Si3reaction layer adjacent to Cf/LAS composites is very thin.As shown in Fig.6,more elements diffuse from the brazing seam to Cf/LAS composites with the brazing temperature increasing,and the TiC+ZrC and Ti2O+ZrSi2+Ti5Si3reaction layer between Cf/LAS composites and the interlayer thickens gradually.Therefore,the Cf/LAS composites are connected to the interlayer by the discontinuous reaction layer (TiC+ZrC+Ti2O+ZrSi2+Ti5Si3).

Fig.6 High-magni fi cation BSE images of the Ti-Zr-Ni-Cu/Cf/LAS composites interfaces brazed at different temperatures for 10 min.

3.3.Mechanical properties of Ti60/Ti-Cu-Zr-Ni/Cf/LAS composites joints

Fig.7 demonstrates the effect of the brazing temperature on the room-temperature shear strength of Ti60/Ti-Zr-Ni-Cu/Cf/LAS compositesjoints.Asthe brazing temperature increases,the interlayer reactant increases greatly,and the shear strength of the joints increases firstly and then decreases.

Fig.7 Room-temperature shear strengths of Ti60/Ti-Zr-Ni-Cu/Cf/LAS composites joints brazed at different temperatures.

The difference of thermal expansion coefficients between Cf/LAS composites and Ti60 is significant.In the process of a brazing experiment,the thermal stress is easily produced at a joint,which results in deterioration of the performance of the joint and difficulty of connecting the two kinds of base materials.When the brazing temperature is 950°C,the brazing joint has a shear strength of～9.8 MPa.The reaction between Cf/LAS composites and the brazing alloy is not sufficient,leading to a low shear strength of the joint.When the brazing temperaturereaches980°C,thedissolution,diffusion,and reaction between the brazing alloy and substrates are suf ficient,so the number of titanium solid solution increases,the brazing joint has good plasticity,the thermal stress is relieved,and then the shear strength of the joint reaches a maximum of～38.1 MPa.However,with further increasing the brazing temperature,the shear strength of the joint is obviously reduced.The excessive diffusion and reaction between the brazing fi ller and the substrates lead to a significant growth of a discontinuous and brittle reaction layer near the composite side(Fig.6).Meanwhile,the mismatch of CTEs between Cf/LAS composites and Ti60 is increased at higher temperature,which also deteriorates the joint strength.Therefore,it has a negative effect on the mechanical properties of brazed specimens.

To con fi rm the fracture locations of the brazed joints,the fracture morphologies of Ti60/Ti-Zr-Ni-Cu/Cf/LAS composites joints brazed at 950,980,and 990°C for 10 min on the Ti60 alloy side are shown in Fig.8,and the corresponding EDX results are shown in Table 3.With changes of brazing process parameters,the fracture position of a joint changes accordingly.The change of the fracture position of a joint with the brazing temperature is determined by the degree of interfacial reaction and the relative thickness of each reaction layer.

Fig.8 Fracture surfaces of Ti60/Ti-Zr-Ni-Cu/Cf/LAS composites joints brazed at different temperatures for 10 min after shear tests.

Table 3 Chemical compositions and possible phases of each spot marked in Fig.8(at.%).

Fig.9 Diagrams of fracture paths at different temperatures.

Fig.10 XRD patterns of fracture surfaces on Ti60 sides brazed at different temperatures after shear tests.

The diagrams of fracture paths at three temperatures are shown in Fig.9,and the XRD patterns of fracture surfaces on Ti60 sides after shear tests are shown in Fig.10.When the brazing temperature is 950°C,the atomic dissolution,diffusion,and reaction between the Cf/LAS composites and the Ti-Zr-Ni-Cu braze alloy are insufficient,and the reactive layer between Cf/LAS composites and the interlayer is too thin and discontinuous,so it is difficult to achieve strong metallurgical bonding,and the fracture position is in the reactive layer,as shown in Fig.9(a),which is in accordance with the XRD pattern(Fig.10).When the brazing temperature is 980°C,the interface dissolves,and the diffusion and the reaction degree increase obviously.In particular,the number of Ti(s,s)at the center of the brazing seam increases,which improves the plasticity of the joint obviously and can effectively relieve the thermal stress.While the thickness of the reactive layer between Cf/LAS composites and the interlayer is moderate,the joint shear strength reaches the maximum,and the fracture position is on the side of the braze seam(Ti,Zr)2(Ni,Cu),the Ti2O+ZrSi2+Ti5Si3interface layer,and carbon fi ber in the composite material,as shown in Figs.9(b)and 10.When the brazing temperature is 990°C,zone III gradually decreases,and the thickness of the reactive layer between Cf/LAS composites and the interlayer is gradually increased.Due to the high hardness,brittle nature,and poor toughness of the reaction layer,the joint strength decreases significantly,and the fracture position is in the Ti2O+ZrSi2+Ti5Si3layer and carbon fi ber in the composite material,as shown in Figs.9(c)and 10.In summary,it can be reckoned that the collapse of the joint chie fl y happens at the interlayer/Cf/LAS composites interface.

4.Conclusions

The interfacial microstructure of Ti60/Ti-Zr-Ni-Cu/Cf/LAS composite brazed joints is investigated in detail.Main conclusions are summarized as the following.

(1)The typical interfacial microstructure of a Ti60/Ti-Zr-Ni-Cu/Cf/LAS compositesbrazed jointbrazed at 980°C for 10 min consists of:Ti60 alloy/Ti(s,s)+(Ti,Zr)2(Ni,Cu)+Ti(s,s)/(TiC+ZrC+Ti2O+ZrSi2+Ti5Si3)/Cf/LAS composites.With increasing the brazing temperature,the thicknesses of zone II and the reactive layer between Cf/LAS composites and the interlayer in the brazed joint gradually increase,and the thicknesses of zones I and III gradually decrease.

(2)As the brazing temperature increases,the interlayer reactant increases greatly,and the shear strength of the joint increases firstly,followed by decreasing.The maximum shear strength of the joint is～38.1 MPa when brazed at 980°C for 10 min.

(3)The fracture position is in the TiC+ZrC+Ti2O+ZrSi2+Ti5Si3layer on the composite side at a brazing temperature of950°C.The fracture position changes to the side of the braze seam(Ti,Zr)2(Ni,Cu),Ti2O+ZrSi2+Ti5Si3interface layer for a specimen brazed at 980°C.To further increase the brazing temperature to 990°C,the fracture position is in the Ti2O+ZrSi2+Ti5Si3layer on the composite side and the carbon fi ber in the composite material.

Acknowledgments

This work was supported by the National Natural Science Foundation of China(51172050,51102060,51302050,and U1737205),the Natural scientific Research Innovation Foundation at Harbin Institute of Technology (HIT.NSRIF.2014129),the Shanghai Sailing Program of China(16YF1411100),and the China Aerospace Science and Technology Corporation Aerospace Science and Technology Innovation Fund of China.