| The intrinsic brittleness of bulk metallic glasses(BMGs)during deformation has severely impeded their rapid development and possible engineering application。The in-situ fabrication of shape-memory type BMG composites with the precipitation of the shape-memory crystalline phase in the glassy matrix has been proven to be one of the most effective and acceptable solutions.So far,many studies have focused on the microstructural control during fabrication and the transformation-induced-plasticity(TRIP)effect during deformation for CuZr-based shape-memory amorphous composites.For Be-bearing Ti-based BMG composites,by tailoring alloy compositions,the stable β-Ti dendrites can be transformed into metastable.However,the Be addition in these metastable β-type Ti-based BMG composites severely impedes their wide applications as structural materials due to the hypertoxicity of beryllium oxides originating from melting.Hence,in order to retain the TRIP effect,the study on Ti-based BMGs without Be additions becomes more popular.However,due to the preferred precipitation of brittle intermetallic compounds at B2 TiCu crystal boundaries and the highly inapparent martensitic transformation within B2 TiCu crystals,TiCu-based alloy system does not exhibit better mechanical properties than CuZr-based(B2-type)and Be-bearing Ti-based BMG composites(metastable β-type),thus which needs in-depth investigation in the future.Firstly,as cast Ti38Cu38Zr10Pd14,Ti40Cu36Zr10Pd14,Ti45Cu32Zr10Pd14 and Ti56Cu20Zr10 Pd14 rod samples were prepared by vacuum arc suction-casting device.The phase composition and microscopic properties were analyzed by SEM and EDS,and the mechanical properties were also tested.According to the test,the Ti38Cu38Zr10Pd14 alloy is mainly consist of amorphous phase and B2 Ti(Cu,Pd)phase,the total crystal volume fraction is 47 vol.%。The samples display strong work-hardening ability with yield strength of 2009 ± 5 MPa and maximum plasticity is 15.7± 1.0%.As Ti cotent increase to 45 at.%,the amorphous phase gradually decreases and the crystalline content increases to 52 vol.%,β-Ti phase appeared,the B19’Ti(Cu,Pd)martensite phase and Ti2(Cu,Pd)phase gradually increase,which reduced the strength and plasticity.When Ti content increased to 56 at.%,the main phase is metastable βTi phase and Ti2(Cu,Pd)crystal phase with a small amount of amorphous phase,which further reduces the strength,but the appearance of a large amount of β phase makes the plasticity improved,indicating that the type of the second crystal phase has a great influence on the mechanical properties of the amorphous composites,By adjusting Ti content,the transformation from B2 type amorphous composites to metastable βtype amorphous composites can be achieved.Secondly,by substituting Co element with Cu to induce the precipitation of B2 phase during the formation,Ti40Cu36-xZr10Pd14Cox(x=1,2,3,4 and 5 at.%)metalic glass composites were prepared using a vacuum arc melting suction casting equipment.It was found that the glass forming ability(GFA)decreasing with increasing of Co content,and the B2 Ti(Cu,Pd)crystal volume fraction increased from 0.9 ± 0.5%to 93.0±6%.The compressive plasticity and fracture strength at room temperature first increased and then decreased.When the Co content is 3 at.%,the samples display best comprehensive mechanical properties and strong work-hardening ability with yield strength of 1353±10 MPa,the compressive strength of 2039± 10 MPa,and maximum plasticity of 15.7 ± 1.0%.During deformation,the martensitic transformation occurred,and a large number of dislocation cells were formed,which provide strong work-hardening ability.Therefore,by inducing the precipitation of B2 phase and controlling its volume fraction through composition modulation,the amorphous composites with excellent properties can be prepared.Thirdly,in order to further explore TiCu-based amorphous composites,on the basis of Ti40Cu36Zr10Pd14 alloy composition,Cr element was introduced to replace the Cu element content in Ti40Cu36Zr10Pd14 alloy composition,and Ti40Cu36-xZr10Pd14Crx(x=0.5,1.0,1.5 and 2.0 at.%)amorphous composites were prepared using a vacuum arc melting suction casting equipment.The effects of Cr addition on the organization and mechanical properties of the amorphous composites were investigated,and their deformation behaviors were analyzed.The results showed that the compressive plasticity and fracture strength of the amorphous composites first increased and then decreased with the increase of Cr content at room temperature.When the Cr content is 1.0 at.%,the best comprehensive mechanical properties were obtained and showed strong work-hardening behavior,a new Be free TRIP-reinforced TiCu-based amorphous composite was successfully prepared,with a yield strength of 1741±5 MPa,compressive strength of 2256±5 MPa and maximum plasticity of 11.9±0.5%.During deformation,the specimens exhibit high fracture strength,high plasticity and strong workhardening ability due to the proliferation of multiple shear bands within the softer B2 crystals and the martensitic phase transformation of the B2 crystals.However,as the Cr content increases,brittle intermetallic compounds precipitate inside the B2 crystals,thus leading to a decrease in material plasticity.Lastly,since single solid solution tends to form in medium-or high-entropy alloys(MEAs or HEAs),it offers an opportunity to obtain shape-memory type bulk metallic glass(BMG)composites when the selected system belongs to not only BMG but also MEAs or HEAs.Herein,through phase prediction parameters used in MEAs and HEAs,the phase formations for the medium-entropy Ti40Cu36-xZr10Pd14Cox(x=2,4,and 6 at.%)glass-forming alloys were evaluated before experiments.According to prediction,both fully amorphous samples and BMG composites were fabricated by rapid solidification.To observe the phase formations and the transformation-induced plasticity(TRIP)effect,fully crystalline MEAs were obtained by annealing.With increasing Co addition,B2 Ti(Pd,Cu)phase can be stabilized to room temperature.All the samples exhibit excellent mechanical properties mainly due to different contributions from the transformation strengthening,second phase strengthening,and dislocations strengthening effects. |
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