Fatigue crack propagation behavior of K40S cobalt-base superalloy under ambient atmosphere at 700 ℃ and 900 ℃ was investigated. The detailed fatigue crack propagation and fracture mechanism under the alternating loa...Fatigue crack propagation behavior of K40S cobalt-base superalloy under ambient atmosphere at 700 ℃ and 900 ℃ was investigated. The detailed fatigue crack propagation and fracture mechanism under the alternating loads were studied. The results show that, there is a defined threshold for K40S alloy at elevated temperatures. The fatigue threshold is 23.9 MPa·m 1/2 at 700 ℃ and 12 MPa·m 1/2 at 900 ℃. The significant decrease of the threshold with increasing temperature is associated with the oxidation induced embrittlement at crack tip. Observation on the fatigue fracture surfaces indicates a ductile fracture mechanism related to the fatigue crack growth.展开更多
In situ fatigue tests in special designed SEM were conducted to trace the whole process of crack initiation and propagation fill to fracture in mckel-base P/M superalloy seeded inclusions. The experimental results sho...In situ fatigue tests in special designed SEM were conducted to trace the whole process of crack initiation and propagation fill to fracture in mckel-base P/M superalloy seeded inclusions. The experimental results show that non-metallic inclusions can induce crack initiation. When the inclusion size is larger than the critical one, the crack can propagate as the main crack that induces the specimen to fracture. As a result, the LCF life of the specimen decreases.展开更多
An investigation was made on the creep crack growth behaviour under static load at 1023 K for Ni-base superalloy GH30 exposed to air and air+10% SO_2.The results showed that in the region of low stress intensity facto...An investigation was made on the creep crack growth behaviour under static load at 1023 K for Ni-base superalloy GH30 exposed to air and air+10% SO_2.The results showed that in the region of low stress intensity factor,the creep crack growth rate is higher in air+10% SO_2 than in air only,while in the high region,it is reverse.The fractograph of specimens has been analyzed,and the mechanism of creep crack growth together with the influences of sul- phur and oxygen has been discussed as well.展开更多
Time-dependent Fatigue Crack Propagation (FCP) behaviors of five Ni-base superalloys were investigated at various temperatures under fatigue with various holding times and sustained loading conditions. The new concept...Time-dependent Fatigue Crack Propagation (FCP) behaviors of five Ni-base superalloys were investigated at various temperatures under fatigue with various holding times and sustained loading conditions. The new concept of damage zone is defined and employed to evaluate the alloys' resistance to hold-time FCP. A special testing procedure is designed to get the maximum damage zone of the alloys. Udimet 720 and Waspaloy show shorter damage zones than alloys 706 and 718. The fractographical analyses show that the fracture surfaces of the specimens under hold-time fatigue conditions are mixtures with intergranular and transgranular modes. As the extension of holding time per cycle, the portion of intergranular fracture increases, The effects of loading stress intensity, temperature, holding time, alloy chemistry, and alloy mi-crostructure on damage zone and the crack growth behaviors are studied. Hold-time usually increases the alloy's FCP rate, but there are few exemptions. For instance, the steady state hold-time FCP rate of Waspaloy at 760℃ is lower than that without hold-time. The beneficial effect of hold-time was attributed to the creep caused stress relaxation during the hold-time.展开更多
Surface crack of components of the cast nickel base superalloy was repaired with twin laser beams under proper technological conditions. One laser beam was used to melt the substrate material of crack, and the other t...Surface crack of components of the cast nickel base superalloy was repaired with twin laser beams under proper technological conditions. One laser beam was used to melt the substrate material of crack, and the other to fill in powder material to the crack region. The experimental results show that the surface crack with the width of 0.1 ～ 0.3?mm could be repaired under the laser power of 3?kW and the scanning speed of 6 ～ 8?mm/s. The repaired deepness of crack region is below 6.5?mm. The microstructure of repaired region is the cellular crystal, columnar crystal dendrite crystal from the transition region to the top filled layer. The phases in repaired region mainly consisted of supersaturated α Co with plenty of Ni, some Cr and Al, Cr 23 C 6, Co 2B, Co Ni Mo, Ni 4B 3, TiSi and VSi. The hardness of filled layer in repaired region ranged from HV 0.2 450 to HV 0.2 500, and the hardness decreases gradually from the filled layer to joined zone.展开更多
An in situ study of twinning at crack tip in a TiAl-base alloy has been performed.The result shows that twinning with long shear vector(2/6) [112] can generate on(111) plane, even though usually it is very difficult t...An in situ study of twinning at crack tip in a TiAl-base alloy has been performed.The result shows that twinning with long shear vector(2/6) [112] can generate on(111) plane, even though usually it is very difficult to occur because of the high energy barrier. It was further shown that (1/6) [112](111) twinning is considerably easier to generate. Furthermore,(1/2)<110) ordinary dislocations were very active and dominated nearly the whole plastic zone, in spite of low Schmid factors. On the other hand, however <101) and (1/2) <112] superdislocations with higher Schmid factors can hardly be observed.展开更多
The creep crack growth(CCG)and the time to carck initiation and rupture of specimen (t_(rc)and t_r)were measured by means of electrical potential method on single edge notched specimens at 700℃.The field near the cra...The creep crack growth(CCG)and the time to carck initiation and rupture of specimen (t_(rc)and t_r)were measured by means of electrical potential method on single edge notched specimens at 700℃.The field near the crack tip under steady-state creep was represented by energy rate integral(C~*),and the CCG rate as a function of C~* has been obtained.The agreement between the predicted and observed t_(rc)values is quite good.It was found that the addition of small amount of Mg and Zr in the alloys causes t_(rc)and t_r of the specimens to in- crease significantly.展开更多
The effect of lamellar orientation on crack paths in PST crystals of y-TiAl based alloys was investigated by in-situ SEM technique. The results indicate that the crack paths in PST crystals of y-TiAl based alloys are ...The effect of lamellar orientation on crack paths in PST crystals of y-TiAl based alloys was investigated by in-situ SEM technique. The results indicate that the crack paths in PST crystals of y-TiAl based alloys are strongly dependent on lamellar orientation ofPST crystals, and the differently oriented PST crystals show different nucleation and propagation mechanisms of crack, resulting in different levels of fracture toughness.展开更多
文摘Fatigue crack propagation behavior of K40S cobalt-base superalloy under ambient atmosphere at 700 ℃ and 900 ℃ was investigated. The detailed fatigue crack propagation and fracture mechanism under the alternating loads were studied. The results show that, there is a defined threshold for K40S alloy at elevated temperatures. The fatigue threshold is 23.9 MPa·m 1/2 at 700 ℃ and 12 MPa·m 1/2 at 900 ℃. The significant decrease of the threshold with increasing temperature is associated with the oxidation induced embrittlement at crack tip. Observation on the fatigue fracture surfaces indicates a ductile fracture mechanism related to the fatigue crack growth.
文摘In situ fatigue tests in special designed SEM were conducted to trace the whole process of crack initiation and propagation fill to fracture in mckel-base P/M superalloy seeded inclusions. The experimental results show that non-metallic inclusions can induce crack initiation. When the inclusion size is larger than the critical one, the crack can propagate as the main crack that induces the specimen to fracture. As a result, the LCF life of the specimen decreases.
文摘An investigation was made on the creep crack growth behaviour under static load at 1023 K for Ni-base superalloy GH30 exposed to air and air+10% SO_2.The results showed that in the region of low stress intensity factor,the creep crack growth rate is higher in air+10% SO_2 than in air only,while in the high region,it is reverse.The fractograph of specimens has been analyzed,and the mechanism of creep crack growth together with the influences of sul- phur and oxygen has been discussed as well.
文摘Time-dependent Fatigue Crack Propagation (FCP) behaviors of five Ni-base superalloys were investigated at various temperatures under fatigue with various holding times and sustained loading conditions. The new concept of damage zone is defined and employed to evaluate the alloys' resistance to hold-time FCP. A special testing procedure is designed to get the maximum damage zone of the alloys. Udimet 720 and Waspaloy show shorter damage zones than alloys 706 and 718. The fractographical analyses show that the fracture surfaces of the specimens under hold-time fatigue conditions are mixtures with intergranular and transgranular modes. As the extension of holding time per cycle, the portion of intergranular fracture increases, The effects of loading stress intensity, temperature, holding time, alloy chemistry, and alloy mi-crostructure on damage zone and the crack growth behaviors are studied. Hold-time usually increases the alloy's FCP rate, but there are few exemptions. For instance, the steady state hold-time FCP rate of Waspaloy at 760℃ is lower than that without hold-time. The beneficial effect of hold-time was attributed to the creep caused stress relaxation during the hold-time.
文摘Surface crack of components of the cast nickel base superalloy was repaired with twin laser beams under proper technological conditions. One laser beam was used to melt the substrate material of crack, and the other to fill in powder material to the crack region. The experimental results show that the surface crack with the width of 0.1 ～ 0.3?mm could be repaired under the laser power of 3?kW and the scanning speed of 6 ～ 8?mm/s. The repaired deepness of crack region is below 6.5?mm. The microstructure of repaired region is the cellular crystal, columnar crystal dendrite crystal from the transition region to the top filled layer. The phases in repaired region mainly consisted of supersaturated α Co with plenty of Ni, some Cr and Al, Cr 23 C 6, Co 2B, Co Ni Mo, Ni 4B 3, TiSi and VSi. The hardness of filled layer in repaired region ranged from HV 0.2 450 to HV 0.2 500, and the hardness decreases gradually from the filled layer to joined zone.
文摘An in situ study of twinning at crack tip in a TiAl-base alloy has been performed.The result shows that twinning with long shear vector(2/6) [112] can generate on(111) plane, even though usually it is very difficult to occur because of the high energy barrier. It was further shown that (1/6) [112](111) twinning is considerably easier to generate. Furthermore,(1/2)<110) ordinary dislocations were very active and dominated nearly the whole plastic zone, in spite of low Schmid factors. On the other hand, however <101) and (1/2) <112] superdislocations with higher Schmid factors can hardly be observed.
文摘The creep crack growth(CCG)and the time to carck initiation and rupture of specimen (t_(rc)and t_r)were measured by means of electrical potential method on single edge notched specimens at 700℃.The field near the crack tip under steady-state creep was represented by energy rate integral(C~*),and the CCG rate as a function of C~* has been obtained.The agreement between the predicted and observed t_(rc)values is quite good.It was found that the addition of small amount of Mg and Zr in the alloys causes t_(rc)and t_r of the specimens to in- crease significantly.
文摘The effect of lamellar orientation on crack paths in PST crystals of y-TiAl based alloys was investigated by in-situ SEM technique. The results indicate that the crack paths in PST crystals of y-TiAl based alloys are strongly dependent on lamellar orientation ofPST crystals, and the differently oriented PST crystals show different nucleation and propagation mechanisms of crack, resulting in different levels of fracture toughness.
